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CONTRIBUTED P A P E R

High-Power Wind Energy Conversion Systems: State-of-the-Art and Emerging Technologies Wind energy installed capacity increased exponentially over the past three decades,

and has become a real alternative to increase renewable energy penetration

into the energy mix.

By Venkata Yaramasu, Member IEEE, Bin Wu, Fellow IEEE, Paresh C. Sen, Life Fellow IEEE,

Samir Kouro, Member IEEE, and Mehdi Narimani, Member IEEE

ABSTRACT | This paper presents a comprehensive study on the state-of-the-art and emerging wind energy technologies from

the electrical engineering perspective. In an attempt to de-

crease cost of energy, increase the wind energy conversion

efficiency, reliability, power density, and comply with the strin-

gent grid codes, the electric generators and power electronic

converters have emerged in a rigorous manner. From the mar-

ket based survey, the most successful generator-converter

configurations are addressed along with few promising topol-

ogies available in the literature. The back-to-back connected

converters, passive generator-side converters, converters for

multiphase generators, and converters without intermediate

dc-link are investigated for high-power wind energy conver-

sion systems (WECS), and presented in low and medium voltage

category. The onshore and offshore wind farm configurations

are analyzed with respect to the series/parallel connection of

wind turbine ac/dc output terminals, and high voltage ac/dc

transmission. The fault-ride through compliance methods used

in the induction and synchronous generator based WECS are

also discussed. The past, present and future trends in megawatt

WECS are reviewed in terms of mechanical and electrical tech-

nologies, integration to power systems, and control theory. The

important survey results, and technical merits and demerits of

various WECS electrical systems are summarized by tables. The

list of current and future wind turbines are also provided along

with technical details.

KEYWORDS | ac-ac; ac-dc; dc-ac; dc-dc power conversion; doubly fed induction generator (DFIG); fault-ride through (FRT);

grid codes; low voltage (LV); medium voltage (MV); multilevel

converters; permanent magnet synchronous generator (PMSG);

power electronics; squirrel cage induction generator (SCIG);

wind energy conversion systems (WECS); wind farms; wound

rotor induction generator (WRIG); wound rotor synchronous

generator (WRSG)

I . I N T R O D U C T I O N

Due to depleting fossil fuels and environmental concerns

about global warming, renewable energy sources have

emerged as a new paradigm to fulfill the energy needs of

our society. In recent years, electricity production from the

hydro, solar, wind, geothermal, tidal, wave and biomass

energy sources has come under increasing attention [1],

[2]. By 2012, the power production from renewable energy sources worldwide exceeded 1470 gigawatt (GW) repre-

senting approximately 19% of global energy consumption

[3]–[5].

Manuscript received May 7, 2014; revised September 10, 2014; accepted November 26,

2014. Date of publication May 18, 2015; date of current version May 22, 2015. This

work was supported by the Natural Sciences and Engineering Research Council

of Canada (NSERC) through Wind Energy Strategic Network (WESNet) Project 3.1,

by Fondecyt 1131041, and by SERC Chile (FONDAP/15110019) and AC3E (FB0008)

of Conicyt.

V. Yaramasu, B. Wu, and M. Narimani are with the Department of Electrical and Computer Engineering, Ryerson University, Toronto, Ontario, Canada, M5B 2K3

(e-mail: [email protected]; [email protected]; [email protected]).

P. C. Sen is with the Department of Electrical and Computer Engineering, Queen’s University, Kingston, Ontario, Canada, K7L 3N6 (e-mail: [email protected]).

S. Kouro is with the Electronics Engineering Department, Universidad Técnica Federico Santa Marı́a, Valparaı́so, Chile, 2390123 (e-mail: [email protected]).

Digital Object Identifier: 10.1109/JPROC.2014.2378692

0018-9219 � 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

740 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

Among all the renewable energy sources, wind energy is increasingly becoming mainstream and competitive with

conventional sources of energy [6], [7]. The cumulative

installed wind power capacity increased exponentially from

6100 megawatt (MW) in 1996 to 282.6 GW by 2012. It is

anticipated that, following the current trend, the cumula-

tive wind capacity would reach 760 GW by 2020. In 2012,

approximately 45 GWs of new wind power was added

which represents investments of about euros 56 billion. The wind energy industry is also providing many direct or

indirect job opportunities leading to a significant stimulus

to economic development. This industry demonstrated an

excellent growth rate of more than 19%, and represents

1.9% of the world’s net electricity production. Currently

83 countries are using wind energy on a commercial basis

to generate electricity [3]. Approximately 24 countries

have more than 1 GW cumulative installed wind power capacity, including 16 in Europe, 4 in the Asia-Pacific area

(China, India, Japan, Australia), 3 in North America

(Canada, Mexico, US) and 1 in Latin America (Brazil) [4].

The cost reduction, government incentive programs, and

technological advancements are some of the key reasons

behind this impressive growth rate.

The energy from the wind has been harnessed by

mankind for millennia to carry ships across oceans and later to pump water and grind grain. The conversion of

wind kinetic energy to electrical energy has started during

1887 with an automated wind turbine equipped with a

12-kW dc generator. To generate electricity from wind

turbines more efficiently and reliably, and to compete

against fossil fuel based power plants, many improvements

have been made in the design of wind turbine mechanical

and electrical components. The wind turbine technology has reached a sufficient maturity level by 1980s leading to

the commissioning of first 50-kW utility-scale wind

turbines [8], [9].

According to the aerodynamic properties, the power output of a wind turbine is proportional to the square of a

rotor diameter and a cubic of wind speed [6], [7]. The large

turbines can capture higher wind power with lower instal-

lation and maintenance costs compared to the group of

small turbines. Owing to this fact, the size of commercial

wind turbines has exponentially increased over the past

30 years as demonstrated in Fig. 1. The turbine size has

increased from 50 kW in 1980 to 7.5 MW in 2010 [10], [11]. The wind turbine rotor diameter also increased from

15 m in 1980 to 126 m in 2010. The largest wind turbine

reported by 2014 is 8 MW with a diameter of 164 m (Vestas

V164), and it is currently in testing stage at Osterild,

Denmark [12], [13]. The 10 MW wind turbines have been

announced by the Clipper, Sway Turbine AS, and Windtec-

AMSC, and the GE Energy has ambitious plans to develop

15 MW turbines (refer to Table 11 for details). The offshore technology is another important driving force behind this

amazing growth size in wind turbines [14]. The market

survey indicates that the rotor diameter and power ratings

of offshore (located in the sea) wind turbines are higher

compared to the onshore (located on the land) wind

turbines. In 2013, the average size of onshore and offshore

wind turbine are reported as 1.926 and 3.613 MW,

respectively [5]. The market trend also indicates that 10– 20 MW turbines will be operational in near future with

rotor diameters exceeding 150 m, which is approximately

twice the length of a Boeing 747 airplane.

The wind energy industry has gone through much

technological advancement in terms of aerodynamic

design, mechanical systems, electric generators, power

electronic converters, integration to power systems and

control theory. From the electrical engineering perspec- tive, the electric generators and power electronic con-

verters are two major components in the operation of wind

energy conversion systems (WECS). Since the beginning

Fig. 1. Evolution in the size of commercial wind turbines.

Yaramasu et al.: High-Power Wind Energy Conversion Systems

Vol. 103, No. 5, May 2015 | Proceedings of the IEEE 741

of grid-connected operation in 1980s, various combina- tions of electric generators and power electronic con-

verters have been developed in commercial wind turbines

to achieve fixed-speed, semi-variable-speed and full-

variable-speed operation [15], [16].

A group of wind turbines are often placed over an ex-

tended area to form the wind farm, and they are connected

collectively to a national electric grid. The wind farms can

be located on the land (onshore) or in the sea (offshore) [14]. Traditionally onshore wind farms have been devel-

oped to take the advantage of easy access, lower initial and

maintenance costs and better proximity to the transmis-

sion lines [17]. The initial and maintenance costs of off-

shore wind farms are higher compared to the onshore

farms for same power levels because stronger foundations

are needed and the connection to the onshore grid is per-

formed by submarine cables. To connect the onshore and offshore wind farms to electric power system, various

series/parallel and ac/dc configurations, and high voltage

ac and dc (HVAC and HVDC) transmission systems have

been developed by the wind turbine manufacturers, off-

shore operators and academic researchers [18]–[22].

Due to the rapid integration of wind power into the

electric grid, many concerns emerged related to the stable,

secure and efficient operation of the existing electric power system. The grid codes have been updated and enforced in

many countries on the grid-connection of large-scale wind

turbines and wind farms [23]–[27]. To increase wind

energy conversion efficiency, reduce mechanical stress on

wind turbines, improve grid power quality and to meet the

grid codes, the high-power wind turbine technology has

upgraded from fixed-speed to full-variable-speed operation.

As of 2013, more than 90% of global wind turbine manu- facturers have announced their future projects based on

semi-variable-speed and full-variable-speed technology

[12], [13], [28], [29].

Reflecting the research and development activities by

both industry and academia, many books or book chapters

[30]–[44]; reports [45]–[48]; and excellent survey papers

on wind energy conversion systems [6]–[11], [14]–[18],

[49]–[69] have been published over the past few decades. These works have discussed some specific aspects of the

wind energy, electric generators, power converters, grid

integration and grid code compliance. The complete list of

electrical technologies applicable for the high-power wind

turbines have not been addressed yet. Moreover, in recent

years, researchers all around the world contributed through

large number of research articles to enable the wind energy

further. This paper is aimed to extensively review the state- of-the-art and emerging MW wind generator-converter

configurations, wind farm configurations and grid code

compliance methods with respect to the previously pub-

lished research articles, survey papers, books and industrial

repots. To aid our discussion, in this paper, the generator-

converter configurations are classified into four categories

and compared based on component count, modularity, re-

liability, converter/control complexity, device voltage stress, operation voltage, and achievable power levels.

The organization of paper is shown in Fig. 2. The first

three sections are introductory in nature, while Sections IV

to X contain in depth technical details, and the last two

sections provide concluding remarks.

I I . O V E R V I E W O F H I G H P O W E R W E C S

In this section, an overview of high power WECs is pre-

sented. The major components of grid-connected WECS,

classification of WECS operating voltages and grid code

requirements are presented in detail. The information pro-

vided in this section will be used in the subsequent sections.

A. Major Components of Grid-Connected WECS The basic configuration of grid-connected MW-WECS

is depicted in Fig. 3. The WECS is composed of several

components that convert wind kinetic-energy into electric-

energy in a controlled, reliable and efficient manner. The

major components of a WECS can be broadly classified as

mechanical, electrical and control systems. The mechan-

ical components include tower, nacelle, rotor blades, rotor

hub, gearbox, pitch drives, yaw drives, wind speed sensors,

drive-train, and mechanical brakes [42]. The electrical components include electric generator, possible power

electronic converter along with generator- and grid-side

harmonic filters, step-up transformer and three-phase grid

or collection-point [30]. The control related components

are used with both the mechanical and electrical energy

conversion systems [7], [37], [70]. The most visible parts in

the large wind turbines are tower, nacelle and rotor blades,

and rest of the components are housed inside the wind turbine.

1) Mechanical Components: The wind kinetic-energy is first converted to mechanical-energy with the help of

airfoil-shaped rotor blades. For the current MW wind

turbines, three-bladed design is most efficient and common

[71], [72]. The tower, nacelle and rotor hubs provide me-

chanical support to the rotor blades. The kinetic to mecha- nical energy conversion efficiency depends on many factors

such as the shape of rotor blades, angle of blades, wind

speed velocity, air density, etc. [36]. The wind velocity and

direction are measured with the help of sensors, and a yaw

drive is used to move the rotor blades along with nacelle

towards the wind to extract the maximum possible energy.

As per the aerodynamic properties, any particular turbine

generates maximum electricity at or above the rated wind speed only. When the wind speed is more than the rated

value, the angle of blades is changed such that the electric

power output is limited to the rated value [16], [73], [74].

The MW wind turbines usually run at very low speed

(typically 6–20 rpm) and high torque.

To couple the low-speed, high-torque wind turbine

shaft with the high-speed, low-torque generator shaft, a

Yaramasu et al.: High-Power Wind Energy Conversion Systems

742 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

multistage gearbox is used. A three-stage gearbox

containing one planetary stage and two helical stages is

usually employed in high-speed generators. For a three-

stage gear box, the gear ratios usually vary between 60 and

120. The gearbox presents with several serious issues such

as high initial cost, high audible noise, extensive wear and

Fig. 2. Organization of content in this paper.

Fig. 3. Basic configuration of a grid-connected megawatt wind turbine.

Yaramasu et al.: High-Power Wind Energy Conversion Systems

Vol. 103, No. 5, May 2015 | Proceedings of the IEEE 743

tear, reduced life span, reduced efficiency and need for regular maintenance [30], [50]. By matching the generator

speed with the wind turbine speed, the need for the

gearbox can be eliminated. The omission of gearbox (often

referred to as gearless or direct-drive technology) helps to

overcome the aforementioned problems, especially in off-

shore wind turbines [49], [75]. This concept was first in-

troduced in 1992 by a German manufacturer, Enercon

through E-40/500 kW wind turbine model. In recent years, many turbine manufacturers (Avantis, GE Energy,

Goldwind, Vensys to name a few) have used direct-drive

technology in their commercial products.

On the other hand, the direct-drive operation leads to

some drawbacks in the design of generator such as large

diameter and more weight [66]. To make a compromise

between the high-speed and low-speed operation, a

medium-speed generator can be used in conjunction with the single or two-stage gear box. The single-stage

gearbox with a gear ratio of 10 was first introduced by

Multibrid (now Areva Wind) in their M5000 wind turbine

models [76]. Few other turbine manufacturers such as

MingYang and WinWinD developed two-stage gearboxes

with a gear ratio of 20–30 [13]. The list of commercial

turbines along with the gearbox details are given in

Appendix. The commercial wind turbines have many al- ternative designs for the drive-trains and they will be dis-

cussed in Section III. The mechanical brakes are mounted

directly on the generator drive-train (high-speed shaft)

to stop the wind turbine during fault conditions or high

wind gusts.

2) Electrical Components: An electric generator is used to convert rotational mechanical-energy into electric-energy. Over the past 30 years, many generators such as the

squirrel-cage induction generator (SCIG), wound rotor

induction generator (WRIG), doubly-fed induction gener-

ator (DFIG), permanent magnet synchronous generator

(PMSG) and wound rotor synchronous generator (WRSG)

have been developed for wind turbines [46], [47], [63]–

[65]. The first generation of wind turbines were based on

SCIG only, but the present generation turbines incorpo- rates both induction and synchronous generators. The in-

duction generators (IGs) usually operate at high rotational

speeds, while the synchronous generators (SGs) can ope-

rate at low, medium, or high speeds [68], [77], [78]. In

order to achieve lower operational speed, the generator

needs to be equipped with a large number of poles, which

is a feasible solution with SG’s. In accommodating the

large number of poles, the stator radius becomes 6 times larger and 4.5 times heavier compared to the three-stage

gearbox based induction generators [50], [54].

The generator output voltage and frequency change

with respect to the wind speed. The generator can be di-

rectly coupled to the grid or it can be interfaced through a

power electronic converter. By arranging the power switch-

ing devices in different ways, possibly with the dc-link

elements such as capacitors or inductors, numerous power converter topologies can be derived. As shall be detailed in

Section III, these power electronic converters can be com-

bined with the electric generators to form a wide variety of

WECS configurations. Again by connecting the wind tur-

bines in different manner, various wind farm configura-

tions can be obtained. The switching harmonics are

inevitable when using power converters, and to solve this

issue, harmonic filters are used in generator- and grid-side converters [57]. The harmonic filter on the generator-side

helps to reduce harmonic distortion of the generator cur-

rents and voltages. This leads to a reduction in harmonic

losses incurred in the generator’s magnetic core and wind-

ing. The harmonic filter in the grid-side converter helps to

meet strict harmonic requirements specified by the grid

codes [79]–[81]. The output of the grid-side harmonic filter

is connected to a three-phase grid (or collection-point) through a step-up transformer, electric switch gear and a

circuit breaker. By operating the power electronic con-

verter at collection-point voltage level, the need for the

step-up transformer can be avoided.

3) Control System: The wind turbine system also consists of several slave control systems for the mechanical/

electrical components and a master control system (not shown in Fig. 3) to achieve desired dynamic and steady-

state performance for the WECS. The controller usually

monitors various variables such as wind speed velocity,

wind direction, generator voltages/currents, filter/dc-link

voltages if any, grid voltages and currents, and adjusts the

system operating states or variables at the reference value

or in the set boundaries [7], [70]. For example, when the

wind speed is more than the rated value, the master con- trol system initiates passive stall, active stall or pitch con-

trol systems to respond and change the angle of blades

such that the turbine output power can never exceed the

rated value [16], [73], [74]. The control systems have been

with limited functions in the first generation of wind

turbines, and now they perform large number of functions

with respect to the turbine, generator and power converter

operation, grid integration, protection standards, and wind farm operation, to name a few [32], [37], [82]. The control

systems are usually implemented using a computer, micro-

controller, digital signal processor (DSP) or field program-

mable gate array (FPGA) [83], [84]. With the modern

control platforms, the control actions can be taken very

fast (in less than 100 microseconds) and repeatedly.

B. WECS Operating Voltages The definition of WECS operating voltages in the North

American and European market is summarized in Table 1

[45]. These operating voltages are further classified accord-

ing to low voltage (LV) and medium voltage (MV) opera-

tion. The LV class includes voltages below 1000 V, where as

voltages in the range of 1–34.5 kV belong to the MV class.

The most standard low voltages used for electric generators

Yaramasu et al.: High-Power Wind Energy Conversion Systems

744 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

and power converters in the North American and European

market are 575 and 690 V, respectively. The current MV

generators and power converters are in the range of 3–4 kV.

Due to the participation of European manufacturers in

North America and vice versa, these regional classifications

are becoming less important. The reason is that the com-

mercial wind turbines can be connected to the collection points or transmission lines through step-up transformers

irrespective of the regional voltage classes. The most com-

mon collection point voltages are 34.5 and 33 kV in North

America and Europe, respectively.

C. Grid Code Requirements The steady growth in the power levels of wind turbines

and wind farms have led to significant penetration of wind

energy systems in the existing electric power system. To

ensure the grid stability and consumer power quality, many

specific technical requirements often called as ‘‘grid codes’’

have been developed and regularly updated [23], [24]. The main elements in grid codes include active power control so

as to adjust the grid frequency, reactive power control to

regulate the grid voltage, grid power quality, flickers, har-

monic oscillations, fault ride-through (FRT) operation, and

system protection. The correct interpretation of these

codes is crucial for wind turbine manufacturers as well as utility operators.

1) FRT: The grid disturbances might lead to disconnec- tion of large-scale wind power generation units. The sud-

den disconnection of generation units stimulates instability

of the utility network. The grid codes have dictated some

special requirements such as FRT operation to overcome

the aforementioned scenario. The FRT requirement is a broad category covering zero voltage ride-through (ZVRT),

low-voltage ride-through (LVRT) and high-voltage ride-

through (HVRT). The ZVRT and LVRT requirements are

essentially same: during grid faults, the grid voltage be-

comes zero in ZVRT profile, while in LVRT profile the grid

voltage becomes 15%–25% of its nominal value [24].

Among all the grid codes, the FRT is a major concern for the

wind turbine and power converter manufacturers. A detailed discussion on various methodologies to comply

with the FRT requirements will be discussed in Section X.

The transmission and distribution system operators

(TSOs and DSOs) of diverse countries issued different FRT

profiles [23]–[26]. Among all the FRT profiles, the German

Transmission and Distribution Utility (E.ON) regulation

introduced in early 2003 is likely to set the standard [27].

The ZVRT and HVRT profiles according to E.ON regulation are shown in Fig. 4. They specify that the wind turbines

must ‘‘ride-through’’ instead of ‘‘trip off’’ during transmis-

sion faults. According to this code, the FRT function should

start when the grid voltage falls below 90% of its nominal

value. The wind turbine must be connected to electric

network if the grid voltage profile is above the ZVRT limit

line specified by the utility operator. It is allowed to dis-

connect from grid if the magnitude of grid voltage falls below the ZVRT limit line. Similar interpretation can be

applied to the HVRT function. Recently considerable re-

search has been carried out addressing this issue [85]–[91].

2) Reactive Power Generation: Apart from the FRT ope- ration, another important requirement for WECS is that it

should perform ‘‘reactive power control’’ similar to the

Table 1 Regional Classification of Low and Medium Voltages [Source: NREL (2012)]

Fig. 4. Voltage ride-through requirements according to the E.ON regulation.

Yaramasu et al.: High-Power Wind Energy Conversion Systems

Vol. 103, No. 5, May 2015 | Proceedings of the IEEE 745

conventional power plant [16]. The reactive power control helps to compensate the transmission equipment such as

cables and transformers in addition to maintaining voltage

stability. Thus it became an important grid code regulation

to maintain reliable and efficient transmission and distri-

bution grids.

Many reactive power profiles are defined by diverse

TSOs similar to the FRT profiles [23]–[26]. As an example,

according to Danish grid code, when the WECS delivers rated (1.0 p.u.) active power, it should be able to supply

�0.33 p.u. reactive power to support grid voltage. This case implies that the WECS should be able to adjust

the grid power factor anywhere from 0.95 lagging to

0.95 leading when it delivers rated active power. Though

this is a stringent grid code, it can be easily fulfilled in

variable-speed WECS by properly designing the power

converter and its control system [92].

I I I . C O M M E R C I A L C O N F I G U R A T I O N S O F M W - W E C S

The major electrical components in WECS are the gene-

rator and power electronic converter. As shown in Fig. 5,

using different designs and combinations with these two

components, a wide variety of WECS configurations can be

achieved such as [15], [48], [51], [60], [65], [67]:

• Type 1: Fixed-speed (�1%) WECS with SCIG, • Type 2: Semi-variable speed (�10%) WECS with

WRIG,

• Type 3: Semi-variable speed (�30%) WECS with DFIG,

• Type 4: Full-variable speed (0–100%) WECS with SCIG, PMSG or WRSG, and

• Type 5: Full-variable speed (0–100%) WECS with WRSG.

The above five configurations have been analyzed, do- cumented and commercialized extensively over the past

three decades.

A. Type 1 WECS Configuration A fixed-speed SCIG-based WECS without power con-

verter interface (Type 1 turbine) is illustrated in Fig. 5(a),

where the generator is connected to the grid through a soft

starter and step-up transformer [49], [66], [93]. This is the

oldest and very first technology (‘‘Danish’’ concept) devel-

oped for the wind turbines. In high-power WECS, the

SCIG contains 4 or 6 poles for 50- or 60-Hz operation,

respectively. The generator speed varies within 1% around the corresponding synchronous speed at different wind

speeds, and thus this configuration is called fixed-speed

WECS. A gearbox is normally required to match the speed

difference between the turbine and generator. After the

start-up procedure, the soft-starter is bypassed by a switch,

and the system essentially works without any power con-

verter. The SCIG draws reactive power from the grid and

to compensate for this, three-phase capacitor banks are

usually employed [30], [94]. This configuration features simplicity, low initial

costs, and reliable operation. The major drawbacks include:

(i) lower wind energy conversion efficiency; (ii) changes in

the wind speed are reflected to the grid; and (iii) the grid

Fig. 5. State-of-the-art electric generator and power converter configurations for the commercial WECS.

Yaramasu et al.: High-Power Wind Energy Conversion Systems

746 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

faults cause severe stress on the mechanical components of the wind turbine [95]. The fixed-speed wind turbines are

equipped with additional hardware, such as STATCOM, to

comply with the grid codes [96], [97]. Despite its draw-

backs, this configuration has been accepted by the wind

industry and commercial solutions are available in MW

range such as: (i) Vestas V82, 1.65 MW; and (ii) Siemens

SWT 2.3–101, 2.3 MW. It should be noted that the fixed-

speed turbines were popular until a decade back and this technology is slowly becoming seldom due to its inherent

disadvantages. The fixed-speed turbines which have been

installed already are still in operation to generate the

electricity.

B. Type 2 WECS Configuration The variable-speed operation of the wind turbine in-

creases the energy conversion efficiency, and reduces mechanical stress caused by wind gusts, reduces the wear-

and-tear of gearbox and bearings, reduces the maintenance

requirements, and thus increases the life cycle. The semi

variable-speed WECS using WRIG and partial rated (10%)

power converter is shown in Fig. 5(b) (Type 2 turbine).

The change in the rotor resistance affects the torque/

speed characteristic of the generator, enabling variable-

speed operation of the turbine, and this configuration is often called Optislip control [98]. The rotor resistance is

normally made adjustable by a power converter composed

of a diode-rectifier and chopper [30]. The speed adjust-

ment range is typically limited to about �10% of its rated speed. With variable-speed operation, the system can cap-

ture more power from the wind, but also has energy losses

in the rotor resistance. This configuration also requires a

gearbox, soft starter, and reactive power compensation. The WRIG with variable rotor resistance has been on

the market since the mid 1990’s with a power rating up to a

couple of megawatts. A few examples of commercial solu-

tions are: (i) Vestas V66-2.0 MW; and (ii) Suzlon Energy

S88-2.1 MW. This configuration is also becoming less

important among the wind turbine manufacturers due to

limited speed range and low energy conversion efficiency.

C. Type 3 WECS Configuration Another semi variable-speed WECS using DFIG is shown

in Fig. 5(c) (Type 3 turbine) [99]–[102]. As the name im-

plies, the power from the generator is fed to the grid through

both stator and rotor windings. A partial rated (30%) power

converter is employed in the rotor circuit to process the slip

power, which is approximately 30% of the rated generator

power. Similar to those in Type 1 and 2 turbines, this con- figuration also uses the gearbox, but there is no need for a soft

starter and reactive power compensation [103].

The use of the power converters allows bidirectional

power flow in the rotor circuit and increases the speed

range of the generator. This system features improved

overall power conversion efficiency by performing maxi-

mum power point tracking (MPPT) [104], [105], extended

speed range (�30%), enhanced dynamic performance and robustness against power system disturbances compared to

the Type 1 and 2 turbines [106]–[108]. These features have

made the DFIG WECS one of the dominating technologies

in today’s wind industry with a market share of approx-

imately 50% [54], [58].

The FRT capability is limited due to the partial scale

power converter. The gearbox increases overall turbine

cost, weight and as well demands regular maintenance. Moreover, the power converter is connected to the rotor

windings through slip rings and brushes. The average life

time of brushes is 6–12 months only, and thus regular

maintenance is essential in these turbines. These major

drawbacks impeded these turbines being applied in off-

shore wind farms where maintenance cost is quite expen-

sive. A few high power DFIG turbines are: (i) Repower 6M,

6.0 MW; (ii) Bard 5.0, 5 MW; and (iii) Acconica AW-100/ 3000, 3 MW.

D. Type 4 WECS Configuration The performance of WECS can be greatly enhanced

with the use of full-scale (100%) power converters as

shown in Fig. 5(d) (Type 4 turbine) [109]–[113]. The

PMSG, WRSG, and SCIG have all found applications in this

type of configuration with a power rating of up to several megawatts. Since the power converters must be rated same

as generator capacity, the size, cost and complexity of

overall system increases. Moreover the losses in power

converter are higher leading to lower efficiency. However,

with the full-scale power converter, the generator is fully

decoupled from the grid, and can operate at full speed range

(0 to 100%).

The power converters also enable the system to per- form reactive power compensation and smooth grid con-

nection. The wind energy conversion efficiency is highest

in these turbines compared to other types of turbines

[114]–[116]. The best FRT compliance can also be achieved

without any external hardware. Though the cost of power

converter is high, it only a small fraction (approximately

7%–12%) of total wind turbine cost [53]. The need for the

gearbox can be eliminated by using a high-pole number PMSG/WRSG. This configuration is more robust against

power system faults compared to the Type 1, 2, and 3

turbines [117]. The typical commercial turbines include:

(i) Enercon E126, 7.5 MW; (ii) Multibrid M5000, 5 MW;

and (iii) Vestas V-112, 3 MW.

The distributed drive-train concept is used in recent

megawatt Type 4 wind turbines. Though SCIG and WRSG

can be used in this concept, the PMSG is most suitable because it eliminates the need for slip rings/brushes and

there by gives simple design [68]. The gearbox drives

multiple generators at higher speeds. Due to the distrib-

uted drive-train and multiple generators, a higher power

density can be achieved [118]. One of the commercial ap-

plications is Clipper Liberty which uses a quantum drive-

train, 4 generators and 4 converters as shown in Fig. 6 [46].

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The higher torque is distributed among the four drive trains. The power rating of the converters is one-fourth of the

system rating. This configuration also offers effective fault-

tolerant operation. When one converter fails, the other three

converters can still deliver the power to the grid [30]. To

minimize the circulating currents, multi-winding trans-

former is used on the grid-side. The main disadvantage with

this configuration is complicated drive-train.

E. Type 5 WECS Configuration The Type 5 wind turbine with direct grid-connected

WRSG with speed/torque converter is shown in Fig. 5(e).

This is rather an old concept for wind turbines where the

variable speed operation is achieved by mechanical con-

verter rather than the electrical converter [46], [65]. The

torque/speed converter, also known as variable ratio trans-

mission (VRT) converts variable speed of wind turbine to constant speed. The generator operates at a fixed-speed

and it is directly connected to the grid through a synchro-

nizing circuit breaker.

The overall system cost and space becomes lower than

Type 4 turbine as no power electronic converter is needed.

The generator can be directly connected to MV collection

point without any step-up transformer as there is no re-

striction imposed by the power electronic converter unlike in Type 4 turbine. Despite the advantages of this configu-

ration, it is rarely used in the wind energy industry due to

the limited knowledge, and issues related to the mechan-

ical converter. The commercial solutions using this

technology are: (i) DeWind D8.2, 2.2 MW, 4.16/13.8 kV,

(ii) AMSC-Windtec SuperGear (SG), 2.0 MW, 11 kV, and

(iii) Wikov W2000, 2.0 MW, 6.3/11 kV.

F. Comparison of WECS Configurations The top 10 wind turbine manufacturers as of December

2012 and their main turbine configurations are summa-

rized in Fig. 7. They account for approximately 77% of the

45 GWs installed wind power capacity in 2012 [3]–[5]. The

details about the turbine configurations are obtained from

the respective company product brochures [12], [13], [28],

[29] and details from the survey papers listed before in Section I. The Type 3 turbines (DFIG) hold the highest

market share and this technology have been used by 7

manufacturers among the top 10. Approximately 100 dif-

ferent DFIG turbine models are available from all the wind

turbine manufacturers. The Type 4 turbines are produced

by 6 manufacturers, while 4 of them are offering direct-

drive solutions. This implies that the best selling wind

turbines in the present market use Type 3 and 4 technol- ogies. The future projects announced by the wind turbine

manufacturers indicate that the Type 4 technology would

take over the wind energy market in coming years.

The summary of all five types of turbines is given in

Table 2. They are compared using generator, power con-

verters employed; capacity of power converter; speed-range

achievable; requirement for soft-starter, gearbox and exter-

nal reactive power compensation; and maximum power point tracking (MPPT) ability; aerodynamic power control,

compliance with the fault ride-through requirement; tech-

nology status; and market penetration. Overall, the Type 3

and 4 turbines are most favorable for MW-level application.

In this paper, the generator-converter configurations are

investigated in detail for these two types of wind turbines.

I V . O V E R V I E W O F P O W E R C O N V E R T E R S F O R M W - W E C S

The five types of WECS discussed earlier show that, since

1980s, the power electronics technology has an important

collaboration with the commercial grid-connected wind

turbines [32]. This technology has gone through much ad-

vancement, and the state-of-the-art solutions are available

in the form of full-scale converters. The current technol- ogy uses power electronics at wind turbine and wind farm

level for the energy conversion and grid integration. This

section is dedicated to discuss the power electronics

technology briefly. The general overview of power

converters, technical requirements for power converters

in WECS and classification of power converters for MW-

WECS are addressed.

Fig. 6. Type 4 WECS with distributed drive-train and quantum generators.

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A. General Classification of Power Converters The general classification of power converters which

are more relevant to the wind energy application is shown

in Fig. 8. The objective of the power converters is to enable

variable-speed operation in Type 3 and 4 WECS, while

eliminating the need for soft-starter and reactive power

compensation. To enable the grid connection of these

wind turbines, the variable voltage/frequency of the wind

Table 2 Summary of Five Types of Wind Energy System Configurations

Fig. 7. Summary of top 10 wind turbine manufacturers and their market share by December 2012. [source: REN21 and GWEC] (2S/3S ¼ two-/three-stage gearbox, DD ¼ direct-drive).

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generator should be converted to fixed voltage/frequency.

To accomplish this, a wide variety of power conversion

stages can be employed as summarized in Fig. 8 [70]. Most

of the power conversion stages have found commercial

applications, and some have been proposed in literature

with promising features for future development. The converter topologies are broadly classified as direct

and indirect according to the power conversion performed.

The direct conversion uses single-stage ac/ac converters,

while indirect conversion uses two-stage (ac/dc+dc/ac) or

three-stage (ac/dc+dc/dc+dc/ac) conversion. Some of

these converters are adopted from the electric drives in-

dustry, while some other are solely developed for wind

power application. The direct ac/ac converters and current source converters are also shown in the classification be-

cause they are main competitors to the voltage source

converters in the electric drives industry [119]–[121]. The

successful converters from the electric drive industry are

also presented even though they have not been used in

wind industry yet.

B. Technical Requirements for MW-WECS Power Converters

The power converters in the first generation of wind

turbines (Type 1) were used for smooth grid connection only. They are disconnected from the circuit once the tur-

bine is tied to the grid. But the modern Type 3 and 4 WECS

demand the power converters to meet several technical

and operational requirements [30], [33], [69], [122]. The

most important requirements are listed below:

• Initial Cost: This factor has prime importance in achieving low cost of energy (COE) and competing

with other energy sources [37]. The initial cost of

power converter is only a fraction (approximately

7%–12%) of overall wind turbine cost [53]. Though

it is a small part, huge cost savings can be accom-

plished for a wind farm which consists of hundreds

of wind turbines. • Reliability, Modularity and Maintenance Cost:

In addition to the initial cost, the maintenance cost

(replacement cost for components and salary for

technicians) must also be very less to achieve low

COE. According to the latest reports on wind tur-

bine faults, the electric generators and power con-

verters are on the top of list with an average failure

rate of 13%–20% [123], [124]. The power convert- er faults increase the downtime of wind turbine

operation, and overall cost of energy. For this rea-

son, the power converters for wind turbines, espe-

cially offshore turbines must have high reliability

[52]. The power converters having a modular

structure are preferable because even one power

converter fails, the wind turbine can still work

with reduced capacity, and thus the downtime can be mitigated [125].

• Efficiency: At MW power level, efficiency is an important factor in reducing COE [126]. Even 1%

improvement in the efficiency of power converter

can save millions of dollars at the wind farm level,

which consists of hundreds of power converter

based wind turbines. The power losses which di-

rectly affect the efficiency should be minimized by using highly efficient switching devices, optimal

arrangement of switching devices (also called

Fig. 8. General classification of state-of-the-art high power converters (LCI: load commutated inverter, PWM: pulsewidth modulated).

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750 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

power converter topology), cooling system, modulation/control schemes, etc.

• Power Quality: The power quality can be attrib- uted to various parameters of power converter. The

output voltage waveform must be close enough to

sinusoidal waveform. This is also described as

number of steps in the output voltage waveform

ðdv=dtÞ. As the number of steps in the waveform increases, the dv=dt decreases and thus the require- ment for output filter also decreases. Moreover,

the electromagnetic interference becomes lower as

dv=dt decreases [127], [128]. The total harmonic distortion (THD) of generator and grid currents

must be lower to decrease generator shaft oscilla-

tions and to feed quality currents to the grid, re-

spectively [70].

• Grid Code Compliance: It is one of the important requirements for the grid-connected MW wind

turbines [33]. The power converters must feed

currents to grid with low THD (less than 5%),

provide reactive power whenever requested by the

grid operator, ride-through during grid faults, and

provide voltage/frequency support among other

requirements. These requirements must be accom-

plished by the power converter itself, without re- questing support from the external hardware/

components such as STATCOM or FACTS.

• Footprint and Weight: Unlike in the electric drives, a limited space is available in the nacelle of

wind turbine. The power converter (also electric

generator) must have high power density to achieve

small footprint and weight. This is an important

requirement especially for the offshore wind turbines.

• Cable Size and Losses: The generator/converter ac output is connected to the step-up transformer/

substation through ac cables. The typical hub

heights for the modern wind turbines are in the

range of 60–150 m, and thus the cost of cables and

associated losses become high. The power conver-

sion system must take this factor into account and decrease the cable cost and losses to the maximum

possible extent.

An ideal power converter must possess all the above features. In practice it is impossible to design a power

converter embedding all the above technical merits. The

engineering approach would be to satisfy most crucial

requirements while sacrificing the least important condi-

tions. A best-selling power converter obviously incorpo-

rates most of the above features for wind turbines.

C. Classification of Power Converters for MW-WECS In pursuit of achieving the previously mentioned tech-

nical requirements, various power converter configura-

tions have been developed by wind turbine manufacturers

and their supporting power converter companies. The

classification of state-of-the-art WECS power converters is

of a complex subject matter and it is not possible to classify

all the converters based on one parameter/operation. In

this paper, we have classified the generator-converter con- figurations into four different groups as summarized in

Fig. 9 to facilitate easier discussion.The power converter

topologies which have been commercialized by several

wind turbine manufacturers, and also proposed in litera-

ture with promising features, belong to these four distinct

categories. The main features and drawbacks of each con-

figuration are discussed in the following sections with

important survey results being tabulated (refer to Table 8 in the Appendix). To simplify the diagrams of various

WECS configurations in this paper, the generator-side fil-

ters are not shown, and the grid-side filters are represented

by equivalent block diagram.

V . B A C K - T O - B A C K C O N N E C T E D P O W E R C O N V E R T E R S

The power converters, which are identical on both the

generator- and grid-side, and linked through a dc-link, are

classified as back-to-back (BTB) connected converters.

Different BTB converters which can be used in the com-

mercial WECS are summarized in Fig. 10. They perform a

conversion of variable voltage/frequency output of the

generator to dc, and then dc to ac, with fixed voltage/ frequency for the grid connection. The power flow is bi-

directional, and thus the BTB converters can be used with

SCIG, DFIG, PMSG, and WRSG. The BTB converters are

Fig. 9. Classification of power converters for MW-WECS.

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classified as low voltage (G 1 kV) and medium voltage (1– 35 kV) converters according to IEC 60038 standard given

in Table 1.

A. LV Converters In this subsection, four LV converter configurations are

analyzed as follows. The most standard voltages used by

many commercial wind turbine manufacturers for the LV

grid connection are 690 and 575 V.

1) Full-Scale BTB 2L-VSCs: A typical Type 4 WECS using BTB connected full-scale two-level (2L) voltage source

converters (VSCs) is shown in Fig. 11. The voltage source

rectifier (VSR) and voltage source inverter (VSI) are linked by a dc-link capacitor. The VSR and VSI are realized by LV

Insulated Gate Bipolar Transistors (LV-IGBTs) arranged in

a matrix form. The dc-link unit is usually realized using

series/parallel string of capacitors to achieve required

voltage and capacitance level. The dc-link provides decou-

pling between the generator and grid, and thus the

transients in the generator do not appear on the grid-side.

The wind generators, PMSG [109], [110], WRSG [129],

and SCIG [82], [113] can be used with this configuration. In terms of technology status and market penetration, it is

a mature power converter topology and being used by 90%

Type 4 wind turbines rated below 0.75 MW.

The power rating of the converter is usually equal to the

generator output power. For example, a 0.75-MW electric

generator is connected to the grid through a 0.75 MW power

converter. The VSR controls the generator torque and speed,

while the VSI controls the net dc-bus voltage and grid reactive power. The net dc-bus voltage is maintained higher

than the peak of grid line-line voltage to ensure proper

operation of grid-side converter. The switching frequency of

VSR and VSI is maintained at 1–3 kHz to achieve lower

switching losses and higher power density [130], [131].

The grid current contains higher total harmonic distor-

tion, and to meet the grid codes, LCL filters are used on the grid-side [79]. The generator-side harmonic filter is not shown. The entire power converter including generator-

side filters, VSR, dc-link, VSI, grid-side harmonic filter are

packed in a cabinet and placed in the nacelle. The output

of grid-side LCL filter is connected through three-phase ac cables to the step-up transformer which is located at the

Fig. 10. Classification of back-to-back connected converters. (VSC: voltage source converter, NPC: neutral-point clamped, FC: flying capacitor, 2L: two-level, 3L: three-level, 4L: four-level).

Fig. 11. Type 4 WECS with two-level BTB voltage source converters. (Mainstream commercial power converter configuration).

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752 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

bottom of tower. The ac cables present significant cost and

losses as they are rated for low voltage and high current

operation. Due to the mass production of three-phase con-

verter modules, the cost of these converters is low. One of the most widely used commercial VSC modules is SKiiP by

Semikron which is equipped with its own heat sink, semi-

conductor switching devices and gate drivers, and thus they

feature compact design with high power density [132].

2) Partial-Scale BTB 2L-VSCs: The Type 3 semi-variable speed WECS using BTB voltage source converters is shown

in Fig. 12. The stator of the generator is directly connected to the grid, while rotor of the generator is connected

through power converter [99]–[101], [133]. The power

rating of the converter is usually 30% of the generator

rated power [58]. For example, a 2.5 MW DFIG requires

only a 0.75 MW power converter. Similar to the full-scale

converter, the partial scale converter in DFIG WECS can

control the generator active/reactive power, dc-link volt-

age and grid power factor [103], [134]–[137]. The speed range achievable is decided according to the

rating of power converter. A power converter with 30%

rated capacity can control the DFIG speed in �30% which is sufficient to perform variable-speed operation. The BTB

converters in rotor circuit enable bidirectional power flow

and thus power can flow from rotor to grid or vice versa.

The use of partial-scale converter decreases the size and

weight of Type 3 turbine in comparison to Type 4 turbine.

The lower size and weight of power converter allows more

room in nacelle and thus step-up transformer can be placed in nacelle to decrease the cable costs and losses. This ap-

proach has been applied in ECO110 model of Alstom wind

turbines, among few others. The FRT operation of Type 3

wind turbines is complicated and challenging compared to

the Type 4 wind turbines, and a significant research has

been carried out in this area [58], [138]–[140].

3) Parallel BTB 2L-VSCs With Common DC-Link: For power ratings greater than 0.75 MW in Type 4 turbines

(2.5 MW in Type 3 turbines), the current carrying capabi-

lity can be increased by connecting the three-phase VSC

converters along with harmonic filters in parallel. For

example, as shown in Fig. 13, two BTB VSC modules can be

connected in parallel to achieve a power rating of 1.5 MW

and 5 MW in Type 4 and 3 turbines, respectively. The

dashed lines represent the connection of Type 3 turbine, where stator is directly connected to the grid, and the

power converter is connected to the rotor windings. For

higher power ratings, more converter modules can be

connected in parallel. The dc-link is configured as a

common element for all the converters to reduce cost and

space [141]. This configuration offers energy efficiency and

Fig. 12. Type 3 WECS with two-level BTB voltage source converters. (Mainstream commercial power converter configuration).

Fig. 13. Type 3 and 4 WECS with parallel connected BTB 2L-VSCs and common dc-link. (Mainstream commercial power converter configuration).

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redundancy. For example, when the wind speed is low, one

or more converters can be turned-off leading to higher

system efficiency. When a converter fails, other converters

can still deliver the power, but with reduced capacity. Moreover, by operating converters in interleaving mode,

the equivalent converter switching frequency can be in-

creased, therefore producing less total harmonic distortion

in generator- and grid-side currents.

Due to a mismatch in converter and grid-side filter pa-

rameters, the circulating currents exist in both the generator

and grid-side converters and this issue should be considered

in the design of the controller. On the generator side, L filters are connected between each converter to reduce the

circulating currents. On grid-side, in addition to the L filters, a CL filter is used to reduce the THD of grid currents. Since the dc-link is common, the generator-side converters along

with harmonic filters can be placed close to the generator in

nacelle, while the dc-link, grid-side converters and step-up

transformer can be placed at the bottom of tower. The

generator-side converter is connected to the dc-link through dc cables leading to lower cables costs and losses, in addition

to the reduced nacelle weight. However, this configuration

leads to lower reliability as dc-link capacitors are more prone

to failure. In Enercon E-126 model, more than 10 power

converters are connected in parallel to reach a power rating

of 7.5 MW. The large number of modules leads to complex

control as well.

4) Parallel BTB 2L-VSCs With Individual DC-Links: To solve the issue of circulating currents, as well as reliability

issue, the dc-links can be configured as individual elements

as shown in Fig. 14 without losing the best qualities such as

power handling capability, modularity, redundancy and

efficiency. But the individual dc-link in each converter

module leads to higher system cost. Moreover, the com-

plete power converter must be placed in nacelle to decrease the voltage drop in generator-side cables, and thus overall

nacelle space requirement increases. Despite these dis-

advantages, this configuration is still dominant technology

in Type 3 and 4 WECS.

To minimize the circulating currents, open winding

transformers can also be used at the grid-side. The open-

winding transformer provides isolation between the con-

verters, but with a drawback of high cost and size. The configuration of harmonic filters on grid-side is simpler

compared to the previous topology in Fig. 13. The LCL filters are used separately for each grid-side converter. The

circulating currents still exist on the generator-side for

which open-winding generators should be used. The dis-

cussion on open-winding generators will be presented in

Section VII.

B. MV Converters The LV converters discussed before are efficient and

cost effective at power levels lower than 3 MW in Type 4

turbines. As the power rating increases, the number of

converter modules increases and as a result, the size, cost, and complexity of the system increases [61], [131]. A

summary is provided between the LV and MV operation of

a 6- MW Type 4 wind turbine in Table 3, where it has been

noticed that the MV operation of WECS is the most suitable

and economical approach for power ratings greater than

3 MW [70], [142]. The MV operation is a mature technol-

ogy in the electric drives industry [122], but wind turbine

manufacturers are reluctant to move from LV to MV tech- nology due to the limited availability of MV generators and

less knowledge available for the MV operation of turbines.

In [45], a detailed cost analysis has been carried out be-

tween the LV and MV operation of wind turbines, and the

results show that the cost of energy production can be

decreased by 2%–4% with MV operation. Currently only

4-MV wind turbines are operational, but 7 manufacturers

have announced their future projects based on MV tech- nology (refer to Table 11 given at the end of this paper).

The power converter configurations for the MV operation

of wind turbines are discussed as follows.

1) Series Connected Switches: The two-level VSC shown in Fig. 11 can also be used for MV applications by connect-

ing the switching devices in series [69]. This is a simple

Fig. 14. Type 3 and 4 WECS with parallel connected BTB 2L-VSCs, individual dc-link’s and open winding transformer. (Mainstream commercial power converter configuration).

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754 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

solution for MV operation, but due to the mismatch in

IGBT characteristics, the converter capacity decreases.

The Converteam VDM5000 with a maximum power and

voltage rating of 7.2 MW and 4.16 kV, respectively, fea- tures this converter for the MV drives application [122].

2) BTB Neutral-Point Clamped Converters: As an alterna- tive solution, the three-level diode clamped converter (3L-

DCC) also known most popularly as neutral-point-clamped

(NPC) converter has been widely studied in literature for

Type 4 turbines [90], [111], [143]–[145]. In this configu-

ration, two 2L-VSC’s are stacked one over the other using

split dc-link capacitors and clamping diodes [146]. With

this arrangement, the converter output phase voltage

contains three levels leading to reduced dv=dt and electromagnetic interference than 2L-VSCs [126], [127],

[146], [147]. As shown in Fig. 15, the NPC converters

enable MV operation, and commercial wind turbines have

reached 6 MW power rating without connecting switching

devices in series or parallel [143]. In the commercial

solutions offered by ABB, the switching device is realized

by Integrated Gate-Commutated Thyristor (IGCT) with a

Table 3 Comparison of LV and MV Operation for a 6-MW Wind Turbine [142]

Fig. 15. Type 4 WECS with BTB connected neutral-point clamped (NPC) converters. (Commercial power converter configuration).

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voltage rating of 4.5–6.5 kV [143]. The other manufac- turers such as Converteam and Ingeteam use MV-IGBT as

the switching device [144].

In the current MV wind turbines, offered by the Areva,

Shandong, XEMC-Darwind and Zephyros, the NPC con-

verters have been used with the PMSG, but they can also

be used with WRSG [145], SCIG [111], [148] and DFIG

[149]–[151]. One of the future wind turbine projects,

Condor6 proposed to use MV SCIG in conjunction with the BTB NPC converters. In the current WECS market, DFIGs

with MV stator voltages (6.6 to 12 kV) and LV rotor voltages

are being offered by Acciona, China Creative Wind Energy

(CCWE) and Senvion. The MV stator voltages can eli-

minate the wind turbine step-up transformers (and asso-

ciated losses), and contributes to significant savings in the

collector system costs. As stated by these manufacturers,

this configuration is ideal for wind farms which are in close proximity to the substation (less than 5 km). The rotor

circuit voltage ratings are lower and thus BTB NPC con-

verters can be employed with LV switching devices, or

simply BTB 2L-VSCs can be used.

The switching actions of the semiconductor

switches lead to the drift in the capacitor voltages. If the

capacitor voltages are not balanced, it leads to higher stress

on the semiconductor switches and damages them. This issue has been extensively studied by industry and

academia [152]–[154]. The use of external hardware can

mitigate the problem of capacitor voltages imbalance

[152]. It is also possible to use carrier-based pulse with

modulation (PWM) with zero-sequence voltage injection

[155] or space vector modulation (SVM) with redundant

switching states selection [153], [154] for the balancing of

capacitor voltages. Since the high-power NPC converters have been marketed by many manufacturers, it can be

assumed that this problem has been solved [126]. To mini-

mize the switching losses and also to allow proper heat

dissipation, the semiconductor device switching frequency

is limited to few hundred Hertz [143], [144]. The outer

switching devices (those connected to the positive and

negative dc-bus) operate at higher switching frequency

while the inner switches (those connected to the split-dc bus) work with low switching frequencies. This phenom-

enon leads to uneven power losses, heat dissipation, and

possible derating of converter. This leads to a difficulty in

the design of the mechanical layout of the semiconductor

switches [128].

3) Other Voltage Source Multilevel Converters: The uneven power loss challenge associated with the NPC can be solved by using active neutral-point clamped (ANPC) con-

verters as shown in Fig. 16(a). In this configuration the

clamping diodes are replaced by the IGBT switches giving

more redundancy to maintain equal switching frequency

(and thus switching losses) among all the IGBT’s [126],

[156]. Under similar operations, the BTB 3L-ANPC con-

verters are capable of handling of 32% higher power (up to

7.12 MW) and 57% higher switching frequency (1650 Hz) compared to the BTB 3L-NPC converters [157]. This con-

figuration is applied more recently in the MV drives in-

dustry, and it can be used in the WECS industry as well.

One of the top manufacturers, Vestas is currently re-

searching this power converter topology.

The flying capacitor converter configuration is similar

to the NPC converter, where the clamping diodes are re-

placed by the flying capacitors (FC) as shown in Fig. 16(b). This configuration offers a simplified structure and more

redundant switching states in order to achieve easier con-

trol for the capacitors voltage balancing [130], [158], [159].

The power distribution among the switching devices be-

comes more even in FC converters compared to the NPC

converters [160]. This configuration requires a large num-

ber of capacitors and each of them requires precharging

circuit [16], [69]. The clamping capacitors in FC are less reliable compared to the diodes in NPC converter. The

control scheme requires a greater number of sensors to

send feedback signals from the FC’s and these add cost and

complexity to the system. Moreover, the average switching

frequency of the FC converter should be high (1200 Hz) to

ensure the balancing of capacitor voltages and this causes

higher switching losses [128]. This configuration has not

found its commercial application in the wind energy industry yet, even though it was commercialized in the MV

drives industry (with less market penetration).

The NPC converters are most suitable for 3–4 kV class

MV operation. To connect to the MV collection point of

6.9 kV (North America Standard) or 6.6 kV (Europe

Standard) without using a step-up transformer, the

switching devices in a NPC converter should be connected

in a series [126], [161], but this approach leads to derating the converter. As shown in Fig. 16(c), a four-level (4L)

diode-clamped converter (DCC) can be used to achieve

higher levels of MV operation [70], [162]. In comparison

to the 3L-DCC (NPC converter), the 4L-DCC offers added

benefits such as: MV operation with greatly reduced de-

vice voltage rating, increased equivalent switching fre-

quency, lower values for the grid-side filter, grid friendly

waveforms, and better grid code compliance [92]. How- ever, the dc-link capacitor voltages balancing becomes

more complex and sophisticated, and therefore control

techniques or external hardware should be used [163],

[164]. The semiconductor device count also increases. In

particular, the clamping diodes increase from 12 to 36 in

comparison to the BTB NPC converters. Another disad-

vantage is that the uneven power loss and heat dissipation

among outer and inner switching devices becomes predo- minant compared to the NPC converters. The four-level

converters are not commercialized yet, but the basic

power semiconductor switches are readily available in the

market.

To achieve higher operating voltages, decrease grid-

side filter size and eliminate the wind turbine transformer,

multilevel ANPC converters can be used as shown in

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Fig. 16(d). For a 5L-ANPC converter, the 3L-FC converter

is connected between the internal switching devices of

the 3L-ANPC converter, there by producing high number

of levels in the output voltage waveform. Though the

power handling capacity remains same as 3L-ANPC con-

verter, the higher number of output voltage levels can be

achieved by adding more number of FC cells [126]. Com- pared to the 3L-NPC, 4L-DCC and 3L-ANPC converters

discussed earlier, the multilevel ANPC converters feature

more modular design without connecting switching or

clamping devices in series. Compared to the standard 5L

diode-clamped converter, the number of switching devices

and clamping diodes are lower in 5L-ANPC converter

[165]. The disadvantages for this converter includes com-

plex structure, pre-charging circuits for the FC voltages, complicated control scheme to regulate the FC voltages in

addition to the split dc-link capacitor voltages.

The high grid current quality is more important (to

comply with the grid codes) compared to the generator

current quality. For this reason, multilevel ANPC converters

can be used at the grid-side, while converters which produce

moderate power quality can be used at the generator-side.

This leads to hybrid configuration with cost savings in the

wind turbine power converters. Note that the dc-link in multilevel ANPC converters consists of two split dc-link

capacitors, therefore they can be integrated easily with the

generator-side 3L-NPC or 3L-ANPC converters. At grid-

side, the size for the filters can be further decreased by

using 7L and 9L ANPC converters as proposed in [166].

Although these converters can increase the number of

output levels and decrease output filter size, the voltage

stress of the main switches remain as the same as three- level converters which limits the power rating of these

converters.

Fig. 16. Per-phase representation of promising multilevel converters for medium voltage WECS: (a) BTB three-level active NPC (ANPC) converters. (b) BTB three-level flying capacitor (FC) converters. (c) BTB four-level diode-clamped converters. (d) Multilevel ANPC converters.

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4) BTB Current Source Converters: The previously dis- cussed topologies belong to voltage source converters. The

current source converters (CSC) can also be used in wind

turbines as shown in Fig. 17. The CSCs are duality of VSCs. A

comprehensive summary between the VSC and CSC con-

verters for MV-WECS is given in Table 4 [70], [161], where it

has been observed that the CSC configuration is most favo-

rable for power ratings greater than 5 MW. The topology consists of PWM current source rectifier (CSR) and current

source inverter (CSI). The active switches are realized by

Symmetric Gate-Commutated Thyristor (SGCT). The CSR

and CSI are linked by a dc-choke and thus, similar to the

VSCs, decoupling between the generator and grid can be

achieved [167], [168]. Three-phase capacitor banks are used

on the ac-sides of CSR and CSI to assist the commutation of

semiconductor devices and also to mitigate the switching

harmonics [161], [169].

This topology features a simple structure, and reliable

short-circuit protection. In VSCs, the dc-link capacitors are bulky components, whereas in CSC, the dc-choke is

the bulky component. The dynamic response of the CSC

converters is slower compared to the VSC converters due

Fig. 17. Type 4 WECS with BTB connected current source converters. (Promising power converter configuration).

Table 4 Comparison of Voltage and Current Source Converters for MV-WECS [70], [161]

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to bulky dc-choke and lower switching frequency opera-

tion. The net dc-bus voltage is maintained lower than the

peak of grid line-line voltage to ensure proper operation

of current source inverter. In MV drives industry, the CSCs are competent to the multilevel converters due to

transformerless operation, but in WECS a step-up

transformer is mandatory. The cost of CSC production

is 1%–2% lower compared to the VSC converters [45].

The CSC technology is successfully applied to the multi-

megawatt MV drives [121], but it is not yet used in the

MV-WECS.

C. Comparison of BTB Power Converters The summary of comparison between BTB converters

is given in Table 5 with respect to power/voltage rating,

semiconductor/passive component count, voltage stress of

switches, reliability, power quality, converter and control

complexity, grid code compliance, technology status and

market penetration. The commercially practiced power

converters in the present wind industry are limited to 2L-VSC, parallel 2L-VSCs and 3L-DCC only. The analysis

given here summarizes the feasibility of applying various

power converters in LV and MV WECS. The advantages

and disadvantages of these converters are listed in Table 8

in the Appendix. The multilevel converters are promising

for next-generation wind turbines due to the technical

merits they exhibit.

V I . P A S S I V E G E N E R A T O R - S I D E C O N V E R T E R S

The previously discussed BTB topologies enable a four-

quadrant operation. But in the WECS, the power flow is unidirectional i.e., from the generator to the grid. For this

reason, passive (diode-bridge) converters can be employed

on the generator side instead of pulse width modulated

(PWM) active converters [170], [171]. The diode-bridge

rectifiers are less expensive and inherently more reliable

compared to the PWM converters. In the PMSG and

WRSG, the rotor flux is generated by permanent magnets

and rotor field excitation, respectively. For this reason, the generator-side power conversion system in the PMSG/

WRSG wind turbines can be realized using passive con-

verters [172]. The induction generators (SCIG/DFIG) re-

quire magnetizing current during its operation and thus

they cannot allow passive converters on the generator side.

The use of passive generator-side converters is asso-

ciated with a few disadvantages. The generator currents

contain significant 5th (14%) and 7th (7%) harmonics and this leads to 6th harmonic distortion (10%) in the electro-

magnetic torque [30], [173]. However, due to the decou-

pling offered by a second dc-link, these torque ripples and

generator current distortion do not cause any conflict to

the grid-codes. The passive generator-side converters have

been used in practical WECS ranging from a few kilowatts

(kWs) to megawatts (MWs). Few examples of MW wind

Table 5 Comparison of Back-to-Back Connected Power Converters for Megawatt Wind Turbines

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turbines include the Enercon E82, Clipper Liberty C89,

Vensys V70/77, and Gold Wind GW70/77. The summary of

comparison between the active and passive generator-side

converters for MW-WECS is given in Table 6 [70], [85].

1) Diode Rectifier + 2L-VSC: The power converter con- figuration for PMSG/WRSG WECS with diode rectifier and

2L-VSC is shown in Fig. 18 [46]. This topology offers low

cost, light weight solution compared to the BTB 2L-VSCs.

The generator output voltage is converted to dc by the

diode-rectifier which is then converted back to ac by 2L-

VSC. During low wind speeds, the diode rectifier output

voltage becomes significantly lower. As discussed earlier, to

transfer the generated power to grid, the dc-link voltage must be higher than the grid line-line voltage. To ensure

this condition, the generator should be over rated [174].

This configuration is being used in the Clipper Liberty

2.5 MW wind turbines along with the quantum drive-train

shown in Fig. 6 [118]. The absence of intermediate dc/dc

converter decreases the degree of control freedom by one.

Table 6 Comparison of Active and Passive Generator-Side Converters for LV-WECS [70], [85]

Fig. 18. Type 4 WECS with diode rectifier and 2L-VSI. (Commercial power converter configuration).

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In other words, the control system cannot incorporate

either MPPT operation or regulation of dc-link voltage. In

the Clipper Liberty wind turbines, MPPT is achieved by controlling the grid-side inverter, while dc-link voltage is

allowed to vary with respect to the wind speed. The grid-

side inverter is then designed according to the maximum

possible dc-link voltage.

2) Diode Rectifier + 2L-Boost Converter + 2L-VSC: To increase the control freedom by one, a boost converter can

be employed as an intermediate stage. The WECS with diode-rectifier, boost converter and PWM inverter is

shown in Fig. 19. The variable output voltage of the gene-

rator is converted to dc by the diode-rectifier, and the

boost converter increases the unregulated dc-link voltage

to a higher level that is suitable for the PWM inverter. The

capacitor in the first dc-link filters the ripple in diode

rectifier output dc voltage, and it is an optional component

[53], [175]. The boost converter also enables variable- speed operation for the PMSG/WRSG WECS by perform-

ing maximum power point tracking (MPPT) [176], [177].

With this scenario, the wind energy conversion efficiency

improves, especially during low wind speeds.

The control system development for the MPPT is less

complicated with the passive converters. The cost and

complexity associated with the gate drivers also decreases as the semiconductor switches decrease from 6 to 1 [171].

To increase power handling capacity, more number of

boost converters and 2L-VSCs can be connected in parallel

similar to the BTB converters (refer to Figs. 13 and 14).

Due to the interleaving operation of boost converters, the

inductor current ripple becomes lower and thus the size of

dc choke becomes smaller. This configuration has been

implemented with WRSG in Enercon E82 turbines.

3) Diode Rectifier + 3L-Boost Converter + 3L-VSC: The diode-rectifiers can also be used in MV WECS. As shown in

Fig. 20, the generator-side NPC rectifier can be replaced

with a diode-rectifier and 3L-boost converter. With this

topology, the advantages of generator-side passive con-

verters and grid-side multilevel converters are combined

[178], [179]. The voltage rating for the IGBT/IGCT switches and diodes is half of the net dc-bus voltage. The

3L boost converter offers many benefits compared to the

standard boost converter: reduced switching and reverse

recovery losses, and balancing of capacitor voltages [180].

Fig. 20. Type 4 WECS with diode rectifier, 3L boost converter and 3L-VSI. (Promising power converter configuration).

Fig. 19. Type 4 WECS with diode rectifier, 2L boost converter and 2L-VSI. (Commercial power converter configuration).

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Compared to the BTB NPC converters, the number of active switches in generator-side converter decreases from

12 to 2, and it leads to a cost-effective solution for MV

turbines. This configuration leads to enhanced power

quality and better grid code compliance compared to the

BTB-NPC converters [85], [142].

Similar to the 3L boost converter, a 4L boost converter

can be used in conjunction with the 4L diode-clamped

converter [112], [164]. As mentioned earlier, the balancing of the dc-link capacitor voltages with the 4L-DCC is very

complicated compared to the NPC converter [181], [182].

But the intermediate 4L-boost converter ensures balancing

of the capacitor voltages during all the operating conditions.

Even with the higher number of clamping diodes in the grid-

side converter, the cost of this configuration becomes

approximately equal to the BTB-NPC converters [112], [164].

4) Diode Rectifier + Buck Converter + CSC: The current source converters can also be designed with passive

generator-side converters. The configuration of WECS

with diode rectifier, 2L buck converter and PWM CSI is

shown in Fig. 21 [183]. This configuration is simple and

less complicated compared to the previous configurations

[184], [185]. In VSC’s the dc-link voltage is boosted, while

in CSCs the dc-link current is boosted. Like others, this configuration also suffers from the high torque ripple.

The summary of passive generator-side converters

along with technical merits and demerits is provided in

Table 8 in the Appendix. The present and future wind

turbine projects are also listed in Table 8 in the Appendix.

V I I . C O N V E R T E R S F O R M U L T I P H A S E G E N E R A T O R S

As discussed in the previous two sections, the parallel

power converters increase the power handling capacity.

The wind energy industry also practiced multiphase gene-

rators and distributed converters as an alternative approach

to increase the power handling capacity. A generator

having two-sets of three-phase windings is denoted as a

six-phase generator. The generators which have more than six-phases are expressed as open-winding generators. In

this section, the distributed and cascaded converter config-

urations for six-phase and open-winding generators are

presented.

A. Converters for Six-Phase Generators The six-phase machines, both induction and synchro-

nous, are widely used in the electric drives industry. The wind energy industry also adopted the use of six-phase

generators in Type 4 turbines. With the two sets of wind-

ings, the insulation level for each winding decreases. The

power handling capacity also increases by two-times as

each set of winding carries half the rated current [186].

1) Distributed Converters: A six-phase generator based WECS with BTB 2L-VSCs is shown in Fig. 22 [67], [131]. The stator windings are separated by 30�, and thus the stator voltages are phase shifted by 30�. The phase-shift causes cancelation of 5th and 7th harmonics in the stator currents

with which the torque ripples are minimized [172]. This

feature leads to lower size or even elimination of generator-

side harmonic filters. Since the two sets of windings are

separated, no circulating current flows through the gener-

ator-side converters, and thus the power density (high-power per ampere) of system increases. The Envision E128-3.6 MW

wind turbines use such a six-phase configuration with

DD-PMSG. The present industry is also investigating the use

of BTB-NPC converters in place of BTB 2L-VSCs for MV

operation [144]. For the details of few other six-phase con-

figurations, refer to Tables 9 to 11 in the Appendix.

A similar six-phase configuration with passive gener-

ator-side converters is shown in Fig. 23. The disadvantage of high torque ripples with the passive front-end can be

mitigated by employing six-phase configuration with 30�

phase shift. A six-phase configuration employed by the

Vensys V70/77 and Goldwind GW70/77 wind turbines is

shown in Fig. 24 [187]. The power converter is realized by

six-phase diode rectifier, three-channel boost converter

and two-channel VSCs. The dc-link is configured as a

Fig. 21. Type 4 WECS with diode rectifier, 2L buck converter and CSI. (Promising power converter configuration).

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common element for VSCs. The boost converters and VSCs

operate in interleaving mode to decrease the inductor and

grid current ripple, respectively.

2) Cascaded Converters: The MV operation of wind tur- bines is cost effective and a promising solution for mega-

watt wind turbines. In the standard approach, the output

of MV generator is connected to the MV grid through a

MV power converter [131], [143], [145]. But, one of the

shortcomings in the current wind energy industry is the

lack of availability of the MV generators. The use of LV converters at the generator-side and MV converters at

the grid side represents a very promising approach. The

off-the-shelf LV generators and MV converters can be re-

arranged to increase the efficiency of wind energy con-

version while reducing the overall cost [188].

Fig. 22. Type 4 WECS with six-phase generator and parallel 2L VSC modules. (Commercial power converter configuration).

Fig. 23. Type 4 WECS with six-phase generator and parallel boost converter + 2L-VSC modules. (Promising power converter configuration).

Fig. 24. Type 4 WECS with six-phase generator and three-channel boost converter + parallel 2L-VSC. (Commercial power converter configuration).

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A six-phase configuration with two series-connected LV converters on the generator-side, and an NPC converter on

the grid-side is shown in Fig. 25 [189]. The generator-side

2L converter outputs are connected in a series to achieve

higher dc-link voltage, and also to provide midpoint for the

grid-side NPC. The power output of the generator is dis-

tributed among the two converters and thus the ampere-

per-phase ratio decreases. The midpoint voltage control is a

challenging issue with this particular configuration. This configuration may not work during the whole wind speed

range due to the fact that the dc-link voltage becomes

insufficient during low wind speed conditions.

As shown in Fig. 26, passive generator-side converters

can also be used to decrease the cost of the turbine and to

operate during the whole wind speed range [190], [191].

The torque ripples can be minimized because of the phase-

shifted windings, and meanwhile the LV-MV operation can be accomplished. This configuration is more promising

compared to the active generator-side converters because

during low wind speed operation, the boost converters can

operate with a higher duty cycle to maintain sufficient dc-

link voltage [190]. Similar to the VSC converters, the LV to

MV conversion can also be achieved by current source converters. The PWM-CSI configuration is much simpler

and there are no issues related to the balancing, unlike

those in the NPC converters presented in Fig. 26. This

configuration has already been introduced in the MV

drives industry [192], and it can be adopted for high power

wind turbines.

B. Converters for Open-Winding Generators The wind generators with multiple windings (also

named open-winding structure) are also used in the present

wind turbines. The generator is equipped with more than

two sets of windings, and offers new possibilities to connect

the converters in different configurations [52], [57], [67],

[193], [194].

1) Distributed Converters: The distributed converters are similar to the one shown in Fig. 22, except that the number of phases or converter channels are more than

two. One of the practical wind turbines, which use six sets

of three-phase windings, is shown in Fig. 27 [195], [196].

The Gamesa G10x 4.5 MW wind turbines use this concept

Fig. 25. Type 4 WECS with six-phase generator, series connected 2L-VSRs, and 3L-VSI. (Promising power converter configuration).

Fig. 26. Type 4 WECS with six-phase generator, series connected 2L-boost converters, and 3L-VSI. (Promising power converter configuration).

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with 6 BTB 2L-VSCs modules in parallel. Each module is

rated for 690 V with its own harmonic filters, circuit

breaker, measurement and control boards. The design also

includes the step-up transformer in the nacelle to decrease

the cable cost and losses [195]. The works in [131], [144],

[197] also investigated the use of different number of con-

verter channels with the open-winding generators. In

comparison to the parallel VSC modules discussed earlier in Section V, the circulating currents in the generator-side

converters are eliminated. As a result, the power rating of

the overall system increases, in proportion to the number

of channels employed. The distributed windings also pro-

vide insulation between the converters. These generators

are specially designed at a higher cost. To minimize the

circulating current in grid-side converters, open-windings

transformers can also be used.

2) Cascaded Converters: A high-power wind turbine using cascaded converters is shown in Fig. 28. This

converter configuration is similar to the cascaded H-bridge

converter (CHB) used in the electric drives industry. The

converter requires isolated dc sources which are generated

by the open winding generator. The generator contains

multiple sets of two-phase windings with a phase displace- ment of 90�. When more number of converter cells is connected in series, the system voltage and power rating

increases. Each module can be realized using the power

converters introduced in Fig. 29 [57], [193], [194]. The

transformerless grid connection is also possible with this

configuration as system voltages of 10–35 kV can be

reached by connecting more modules in series. Moreover,

the LV switching devices can be used in the power con-

verter to achieve MV operation levels. The modularity,

redundancy and fault-tolerant operation are other added advantages of this configuration [146].

A three-level H-bridge converter is shown in Fig. 29(a).

It is a simple structure and its output contains three levels.

This converter can be realized by mass produced two-level

VSCs [171]. With three cells in series as shown in Fig. 28,

and 3L H-bridge converter, the output voltage contains

7 levels and there by the dv=dt and harmonic filter size decreases significantly. To achieve the higher system volt- age operation with a lower number of modules, NPC

H-bridge modules can be used as shown in Fig. 29(b) to

form a 5L-HB converter. In this case, the generator wind-

ings should also be designed for medium voltage operation.

The cost associated with the generator-side active

converters can be decreased by using single-phase diode-

rectifier and boost converter similar to the topology pre-

sented in Fig. 19 [193], [198]. Another possibility is to use medium frequency transformers (MFTs) in the BTB

H-bridge modules (Fig. 29(c)). This concept has been

Fig. 27. Type 4 WECS with multiphase generator, multiple 2L VSC modules and open-winding transformer. (Commercial power converter configuration).

Fig. 28. Type 4 WECS with multiphase generator and cascaded 2L VSC modules. (Promising power converter configuration.)

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proposed in [199] for UNIFLEX-PM project. The dc/dc converters provide galvanic insulation and thus the grid-

side transformer can be eliminated by connecting more

modules in series as shown in Fig. 28. The MFT operates at

several kHz and there by reduces overall size of the

converter significantly [57]. The large number of compo-

nents decreases the overall reliability of this configuration.

The pros and cons for various converter configurations

discussed in this section are summarized in Table 8 in the Appendix. The market penetration of these converters is

also highlighted.

V I I I . P O W E R C O N V E R T E R S W I T H O U T D C - L I N K

The matrix converters (MC) provide direct ac-ac conver-

sion without any intermediate dc-link, leading to more

silicon based conversion with low cost and small foot print [120], [200], [201]. The cycloconverters also perform di-

rect ac-ac conversion, but their output voltage/frequency is

just a fraction of input voltage/frequency [121]. Compared

to the cycloconverters, the MCs can produce wide ranges

for output voltage/frequency. The MC is able to convert

variable voltage/frequency to fixed voltage/frequency to

connect the wind turbine to grid [59], [119]. Due to the

elimination of dc-link capacitors, it offers reliable solution for wind turbines, especially when they are employed in

offshore. The summary of comparison between the voltage

source converters and matrix converters is given in Table 7

for MV operation of wind turbines [202]–[204]. Since

there is no intermediate dc-link, the variations in the

generator-side variables strongly affect the grid-side va-

riables and vice versa [205]. The development of proper

control system is crucial to ensure the correct operation of MC when employed in wind turbines.

A. LV Matrix Converters The direct matrix converter (DMC) based LV wind

turbine is shown in Fig. 30, where the generator and grid

are rated for LV operation. It employs bidirectional

switches (with common emitter or collector) and CL filter on the grid-side similar to the current source converters

discussed before. The power flow is bidirectional and thus

this converter can be employed with SCIG, DFIG, WRSG,

and PMSG turbines [119], [206]–[208]. The low voltage

gain is a major challenge for DMCs compared to the VSCs which lead to poor semiconductor device utilization. The

large number of semiconductor devices and gate drivers is

another drawback of the matrix converter functional. Cur-

rently, the DMCs are used in low power and LV industrial

applications, but not practiced in WECS yet. The use of

indirect matrix converters is also reported in literature for

wind energy application [209].

B. MV Matrix Converters Another possibility of using MCs for the MV turbines is

to arrange them in a modular way. A MV wind turbine

with nine modules is shown in Fig. 31. The basic power

cell is realized with 3-phase to 1-phase matrix converter (also called single-phase matrix converter) as shown in

Fig. 32 [210]. To increase the MV operation further more

modules can be connected in series [211]–[213]. This

configuration is called multi-modular matrix converter

(MMMC) or cascaded matrix converter (CMC), and it

offers sinusoidal input and output currents. In addition

to the step-up transformer, this configuration requires

Fig. 29. Sub-module configurations for the cascaded power conversion system (refer to Fig. 28).

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phase-shifting transformer to ensure series connection of

matrix converter modules. The phase-shifting transformer

cancels the lower order harmonics there by the grid cur-

rent quality is improved. The phase-shifting transformers

are more expensive and bulky compared to the standard transformers.

The semiconductor device voltage rating is fraction of

the system operating voltage, but the component count is

significantly higher. For example, the 9-module MMMC

requires 108 switches in contrast to 72 switches used in

9-module cascaded voltage source converters shown in

Fig. 28. The output waveform contains 7 levels similar to the

one obtained by Fig. 28, therefore the output voltage wave-

form is close to sinusoidal. Due to large number of switches,

the reliability of system becomes lower. This configuration

also requires complex modulation scheme and control sys- tem. This technology is not being applied in the wind tur-

bines yet, but a 9 module MMMC is available in market for

wind turbine applications (Yaskawa Enewin-MX1) [119].

The advantages and disadvantages of matrix converters

for wind energy conversion application are given in Table 8

in the Appendix.

Table 7 Comparison of Voltage-Source and Matrix Converters for MV-WECS

Fig. 30. Type 3 and 4 WECS with three-phase to three-phase direct matrix converter. (Promising power converter configuration.)

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I X . W I N D F A R M C O N F I G U R A T I O N S

As a contrary to the conventional power plants, a group of

wind turbines form the wind power generation units, often

called wind farms. The early wind farms have been on land to take the advantage of low initial and maintenance costs.

Nowadays, the offshore wind farms are gaining more

attention because the power production can be increased

and stabilized with the help of stronger and steady winds,

the impact on land use and landscapes can be reduced,

audible noise and visual impacts can be mitigated and less

opposition by the ‘‘Not In My Back Yard (NIMBY)’’ move-

ment [14], [17], [31]. The most of the offshore projects (nearly three quarters)

are located in the European countries. The United Kingdom

(U.K.), Denmark, Belgium, Netherlands, Germany, Sweden,

Finland and Ireland are the key players for offshore wind in

Europe [4]. The offshore wind capacity account only 2% of

installed wind power capacity by 2012. The offshore project

proposals and the present trends indicate that by 2020, the

offshore wind power capacity would reach 40 GW [214].

The largest offshore wind farm to date is London Array with 630 MW installed capacity. The future offshore wind

farms are proposed in the range of 1200–2500 MW. Few

such projects are Blekinge Offshore, Sweden (2500 MW);

Korea Offshore, South Korea (2500 MW); and Moray Firth,

United Kingdom (1300 MW) [13].

A. Overview of Wind Farm Interconnection Methods The optimal interconnection of wind turbines play a

crucial role in decreasing the cost, and increasing

Fig. 31. Type 4 WECS with multimodular (9 module version) matrix converter. (Promising power converter configuration).

Fig. 32. Configuration of 3 � 1 matrix converter. (Sub-module of Fig. 31.)

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efficiency, reliability and performance of wind farm [31]. Many configurations have been proposed in literature, but

only few of them have made their way to the practical

implementation. In this section, we will study the practical

and most promising wind farm configurations. In Fig. 33,

four different configurations are shown. These configura-

tions exhibit three distinct features as defined here [15], [18], [52], [54]:

• Series or parallel connection of WT output terminals, • Coupling of WT output ac or dc terminals, and • Connection of wind farm to utility grid by ac or dc

transmission lines.

Fig. 33. Onshore and offshore wind farm configurations.

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All the wind farm configurations which have been re- searched and commissioned till date combine these three

discrete classes in different manner. The parallel connec-

tion of wind turbines increases the current and power

capacity, while series connection increases the voltage and

power rating. The parallel connection is most widely in the

present wind farms, while some works propose that the

series connection could increase the power density and

decrease the transmission losses. However, this configu- ration causes high power losses in the converters [54].

The ac coupling enables use of all the five types of wind

turbines depicted in Fig. 5 because the WT step-up trans-

former is a common element to all these. This is the most

commonly used configuration with both ac and dc trans-

mission lines. The dc coupling is possible with the Type 4

wind turbines only.

The power to be delivered and distance of wind farm to the nearby utility grid are two important factors that play a

crucial role in deciding between ac and dc transmission

systems. The high voltage ac (HVAC) systems are favorable

for low power wind farms which are located close to the

utility grid. For power ratings and distances greater than

400 MW and 60 km, the HVDC transmission is the most

preferable choice [38], [215]. The Fig. 33 shows only one

collector point, however in the practical wind farms more number of collector systems are used to increase the power

handling capacity and also reliability [31].

B. Parallel AC Configuration + HVAC Transmission The parallel-ac configuration of wind farm with high

voltage ac (HVAC) transmission is shown in Fig. 33(a) [31].

As the name implies, it uses parallel connection, ac cou-

pling and HVAC transmission from the aforementioned classification. This configuration offers low initial cost for

the wind farm substation. For the present wind turbines,

the most standard output voltages are 690 or 3000 V. The

WT output voltages are converted to medium voltage level

(33 or 34.5 kV) by the wind turbine step-up transformer.

The three-phase output terminals from the step-up trans-

formers are connected in parallel to form a medium voltage

ac (MVAC) collection system. The MVAC is then stepped- up to HVAC in the range of 60–245 kV by the wind farm

substation [38], [215]. The HVAC transmission lines con-

nect the wind farm substation to the national electric grid.

A step-down transformer may be employed at the receiving

end to connect to the distribution lines.

It should be noted that though the Fig. 33(a) shows only

Type 4 wind turbine, all other wind turbines depicted in

Fig. 5 can be used to form the wind farm because the step- up transformers are common elements in all these con-

figurations. The earlier offshore wind farms were located

close to the shore and they used same technology as on-

shore farms shown in Fig. 33(a) along with submarine

three-phase ac cables. An example offshore wind farm em-

ploying this interconnection approach is Horns Rev,

Denmark with a capacity of 160 MW. This interconnection

approach is associated with few disadvantages. To improve the transmission efficiency, additional reactive power com-

pensators such as static compensator (STATCOM) or static

VAr compensator (SVC) should be connected at both-sides

of HVAC transmission lines [16], [22]. The faults on the

HVAC lines adversely affect the wind farm and vice versa.

C. Parallel AC Configuration + HVDC Transmission The parallel-ac configuration of wind farm with HVDC

transmission is shown in Fig. 33(b) [216]. This is the most

promising interconnection approach for far located

offshore wind farms with larger power capacity [15]. The

MVAC of WT collection system is first converted to HVAC

by a step-up transformer and then the HVAC is converted

to the HVDC by an ac/dc power converter. The step-up

transformer and ac/dc converter are housed in the offshore

platform. The receiving-end onshore substation contains dc/ac converter and a step-down transformer. The offshore

substation is connected to the onshore substation through

submarine HVDC cables [20]. As discussed earlier, all the

five types of WTs (with STATCOM) can also be used in

this configuration because WT step-up transformers are

common to all of them.

The current source and voltage source converters can be

employed for the HVDC applications. The classic HVDC systems for high power applications use current source

converters with line-commutated Thyristor valves (also

known as LCC converters) and external reactive power

compensation equipment [53]. The voltage source con-

verters based HVDC systems feature compact size, flexible

layout of substation, independent control of active and re-

active power, black-start capability and fast dynamic re-

sponse [19]. The traditional VSC-HVDC systems have been built using two-level VSC or three-level NPC converters

with hundreds of switching devices in series [21].

The modular multilevel converter (MMC) has become

an important milestone in VSC-HVDC systems. This con-

verter features low size, modular structure, transformerless

operation, enhanced reliability, less derating of semicon-

ductor switches, fault-tolerant operation, near sinusoidal

output waveforms and high efficiency [217], [218]. The MMC-VSC-HVDC systems are being offered by Siemens,

ABB, Alstom and EPRI for the interconnection of offshore

wind farms. The discussion of MMCs is beyond the scope of

this paper. The readers are suggested to refer to [19], [217]–

[223], and references therein for more details about

converter configuration, modulation, control and their ap-

plications to offshore wind energy. The huge initial and

maintenance costs are the main drawbacks of this inter- connection approach. The list of HVDC-based offshore wind

farm projects can be found in [13], [21], [48], [214], [217].

D. Parallel DC Configuration + HVDC Transmission The parallel-dc configuration of wind farm with HVDC

transmission is shown in Fig. 33(c) [224]–[226]. As men-

tioned earlier, this configuration is only applicable for the

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770 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

Type 4 wind turbines. An intermediate dc/dc converter increases the dc output voltage (1200/5000 V dc corre-

sponding to the 690/3000 V ac) of ac/dc converters to

medium voltage dc (MVDC). The output dc voltages of dc/

dc converters are then connected in parallel to form a

MVDC collection system (30–50 kV). The MVDC is then

stepped-up to HVDC using a central wind farm dc/dc

converter. The configuration of HVDC transmission and

receiving-end substation are similar to the previous configuration.

The size and weight of central wind farm dc/dc con-

verter is smaller compared to the step-up transformer and

ac/dc converter employed in Fig. 33(b). The dc/dc con-

verters use internally dc/ac conversion stage, medium or

high frequency transformer, and then ac/dc conversion

stage [227], [228]. A higher insulation level is needed for

WT generator and ac/dc converter which is the main challenge with this interconnection approach. Though this

configuration is highly researched, it has not been imple-

mented yet.

E. Series DC Configuration + HVDC Transmission The Fig. 33(d) shows the series-dc configuration of

wind farm with HVDC transmission [229], [230]. The dc

output terminals of the WT ac/dc converter are connected in series to reach HVDC level. For example, 250 LV wind

turbines with 1200 V dc output can be connected to reach

a transmission dc voltage level of 300 kV. If MV wind

turbines are employed as in the case of Nordsee Ost off-

shore wind farm (Germany), only 60 ac/dc converters

needs to be connected in series to reach 300 kV dc voltage

level. The works [229], [231], [232] analyzed the feasibility

of this interconnection approach with both voltage and currents source converters.

This configuration leads to huge cost savings as dc/ac

converters in each wind turbine, and central wind farm

substation is not needed. Due to less number of power

converters, the losses are lower and overall efficiency is

higher. A bypass switch is usually connected across the dc-

link of each ac/dc converter to ensure that the wind tur-

bine will be isolated from the dc network during mainte- nance or down times. If large numbers of wind turbines are

bypassed due to unexpected problems, the complete wind

farm may have to be shutdown because the transmission dc

voltage falls below the threshold limit.

X . F A U L T R I D E - T H R O U G H C O M P L I A N C E I N M W - W E C S

The fault-ride through (FRT) operation is a major concern

for the wind turbine and power converter manufacturers.

In this section, the FRT compliance in Type 3 and 4 tur-

bines will be discussed. The Type 1, 2 and 5 turbines are

omitted in the analysis, because they are outdated or not

being popular in the current wind energy industry. To

understand the FRT compliance in WECS, firstly it is

important to know the grid code requirements, and the effect of grid faults on the electric generator and/or power

conversion systems. Having the clear idea about these two

topics, it would be very easy to understand the broad range

of practical and promising FRT compliance methods.

A. Requirement for Reactive Current During Grid Faults

The grid codes imply that the wind turbine (WT) sys-

tem must provide reactive current during grid faults such

that the grid voltage recovers back to the normal value. As

an example, according to the E.ON regulation [27], the FRT function should start when the grid voltage falls below

90% of its nominal value. For the grid voltages in the range

of 50%–90%, the generation system should provide 2%

reactive current for each 1% voltage dip, and when the grid

voltage falls below 50%, the system should provide 100%

reactive current. In other words, the WT must provide

100% capacitive reactive power and 0% active power out-

put. Moreover, a smooth transition from normal operation and FRT operation must be accomplished within 20 ms in

case the grid voltage falls below the 90% value.

B. Effect of Grid Faults on Electric Generators and Power Converters

As aforementioned, the Type 4 turbines completely

decouple the generator from the utility grid, and thus the

transmission faults are invisible to the wind generator,

drive train and gearbox, if any. With the initiation of elec-

trical control system, the active power output from the

grid-side converter can be decreased to zero within milli-

seconds. However, the mechanical system response is much slower than the electrical response, and thus the

wind generator injects active power to the power con-

verter. According to the power balance theorem, the dif-

ference between the generated power and power injected

to the grid must be handled by the WT system [70]. In

literature it is called ‘‘surplus active power,’’ but in this

paper, we denote it as ‘‘surplus energy’’ as it is more

realistic in nature. Due to surplus energy, the net dc-bus voltage increases exponentially with respect to the grid

fault duration, and this will destroy the complete power

conversion system.

The FRT compliance with the Type 3 turbines is little

complicated compared to the Type 4 turbines, because the

DFIG stator terminal is directly connected to three-phase

grid. Because of this, the stator of DFIG experiences

uncontrolled dynamics during grid faults [138]. Since the rotor is electromagnetically coupled to the stator, the rotor

voltages rise to very high values. The FRT transients also

cause severe torque transients leading to high-stress on

drive-train and gearbox failure [233]. The controllability of

DFIG WECS during grid faults is poorer compared to the

Type 4 WECS, and for this reason hardware protection is

mandatory.

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C. Solutions for FRT Compliance The methodologies to comply with the fault ride-

through (FRT) in Type 3 and 4 wind turbines and wind

farms are shown in Fig. 34. To solve the issue of surplus

energy during grid faults, many methods are proposed in

literature such as [89]–[91], [234]–[236]: (i) regulation of active power generation using a pitch control system;

(ii) storage of surplus energy in the dc-link, external ener-

gy storage systems, and turbine-generator rotor inertia;

(iii) dissipation of surplus energy in the dc-link chopper, ac

crowbar, and braking resistor; (iv) use of compensation

devices; and (v) use of control systems at wind farm, wind

turbine and power converter level. In this section, these

methods are discussed in detail. As shown in Fig. 34, the grid variables such as voltage,

current and frequency are measured on the low-voltage

side of the wind turbine step-up transformer by a grid in-

tegration supervisory system (also called as grid manage-

ment system). The measured information is continuously

transmitted to the master control system. This enables the

wind turbine system including pitch controller and power

electronic converters to react to the changes in grid variables.

1) Power Limitation by Pitch Control System: A simple approach to dealing with the surplus energy in Type 3 and

4 turbines during grid faults is to lower the power

captured from the wind. The pitch angle variation of 5–

10�/s is employed during normal operating conditions. However, during the emergency conditions such as grid

faults, the pitch angle can be varied rapidly, i.e., 10–20�/s [36]. The turbine-generator rotor possesses large moment

of inertia and thus the response time of pitch control

system is very slow (usually in seconds) compared to the

time imposed (usually in milliseconds) by the grid codes.

For this reason the pitch control method alone cannot

guarantee compliance to the grid codes and safe operation

of power converters. However, this method can be used

with other FRT enhancement methods to achieve better results.

2) Storage of Surplus Energy: During the grid faults, the surplus energy can be stored in the energy storage systems

which can be broadly represented as electrical (capacitor,

superconducting magnetic energy storage), mechanical

(flywheel, compressed air energy storage, pumped-storage

hydroelectricity), and electrochemical (batteries, super capacitors) systems or combination of them based on eco-

nomical, technical and operational advantages [52], [237].

A small scale battery energy storage system is shown in

Fig. 34 as a possible solution proposed by some research

works [238]. The charging and discharging of the battery

banks is assisted by a bidirectional converter. The power

stored in the battery banks can be transferred back to the

utility grid when the WECS recovers back to normal ope- ration. Since the power is processed by the power elec-

tronic converters, the response time is much faster

compared to the pitch control system discussed earlier.

The capital cost of energy storage systems is very high,

Fig. 34. Fault ride-through compliance methods for Type 3 and 4 WECS.

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772 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

especially at MW power level, and thus they must not be restricted to FRT enhancement only. During normal ope-

ration, storage elements can be used to make the WECS

output power smoother, even though the wind speed

changes irregularly [53], [54]. The sizing of storage sys-

tems is very crucial at the MW-level, and an optimization

is required to make a compromise between the initial cost

and improvement in the system performance.

For compliance with grid code requirements, the rather old concept of storing energy in the inertia of rotating

masses can be applied to the wind energy system, and this

approach reduces the cost and complexity associated with

the additional hardware [239]. The works in [85], [90]

verified this method with the voltage source converters,

and [89], [240] analyzed the system with current source

converters. This FRT compliance strategy is not practiced

in the wind energy industry yet, but it is a promising solution especially for Type 4 direct-drive wind turbines.

3) Dissipation of Surplus Energy in Type 3 WECS: In Type 3 WECS, with the ac crowbar protection system, the rotor

windings of the DFIG are short circuited through resistors.

This is most widely used method in the present MW Type 3

wind turbines [40]. This arrangement makes the DFIG to

operate similar to the WRIG in Type 2 WECS (refer to Fig. 5). The surplus power in the rotor circuit is dissipated

through the ac crowbar resistors [16], [138]. This scheme

disconnects the RSC and thus protects it from the rotor-

side over voltages.

The resistor value is usually chosen large enough to

decrease the rotor currents to nominal value in short period

of time. The switch which is shown in series with the

resistor can be implemented by Thyristor or IGBT such that the rotor short circuit period can be controlled dynamically

[23], [241]. The control scheme for ac crowbar is coordi-

nated with the advanced wind turbine control schemes so

that the WT contributes to the reactive power injection.

The dc chopper dissipates the surplus energy in the dc-link.

It can also be used in conjunction with the ac crowbar to

dissipate the surplus energy. Another possible application

is to use resistors in series with the three-phase stator and/ or rotor terminals to limit the fault current magnitude

[242], [243].

4) Dissipation of Surplus Energy in Type 4 WECS: In Type 4 WECS, the surplus energy causes exponential rise in the

dc-link voltage. By employing a chopper in the dc-link, the

surplus energy can be easily dissipated. This is the most

widely used method in the present Type 4 turbines due to its fast response time and relatively low initial cost. This

idea has been originated from the ‘‘electromagnetic brak-

ing’’ concept, which is a commonly used in the electric

drives industry to surpass the torque variations [236].

The braking resistor is connected across the dc-link

capacitor through a power electronic switch. The voltage

rating for the dc chopper switches is the same as the active

switches employed in the power converter. The braking resistor value is designed according to the rated power of

WECS. The control scheme for the dc chopper must

consider transients of the system, and the heat dissipation

capability of braking resistors and crowbar switches [234].

Though it is a simple method, it is less efficient because

this approach deals with the dissipation of energy.

5) Use of Compensation Devices: A different perspective to comply with the FRT requirements is to make the wind

generator and power converter ‘‘invisible’’ to the grid faults.

During grid faults, a dynamic voltage restorer (DVR) can

inject three-phase voltages such that the equivalent volt-

ages seen by the wind generator and power converter

correspond to the normal operating condition [37], [244],

[245]. The DVR 1 and 2 compensates the stator and rotor

voltage dips in Type 3 WECS, respectively. By theory, the DVR 2 can also be used with Type 4 wind turbines, but it is

not necessary to employ them. The DVR 3 compensates the

grid voltages at wind farm level [246], and thus it can be

used for Type 1 to 5 turbines. The DVR arrangement re-

quires additional power converter of very large power rat-

ing and injection transformer, and thus it increases the

overall cost of the system.

X I . T R E N D S I N H I G H - P O W E R W E C S

Wind energy is an interdisciplinary subject consisting of

various branches of engineering and science. In this

section, the past, present and emerging future trends in

high-power WECS are highlighted along with technical

challenges to motivate further works in this field. The

current largest wind turbine is 8 MW (Vestas V164) and some manufacturers have announced 10–15 MW wind

turbines as their future projects. At the moment there is no

set standard to anticipate how large will be the wind

turbines by 2020 and 2030.

As shown in Fig. 35, from our perspective, the trends

in WECS can be broadly classified into four categories:

(i) mechanical technologies; (ii) electrical technologies;

(iii) integration to power systems; and (iv) control theory. The cost, weight, reliability, modularity, redundancy,

fault-tolerance, efficiency and grid code compliance are

the major influential factors that govern the commercial-

ization of any renewable energy technology. The afore-

mentioned WECS trends will be discussed in the following

subsections considering these influential factors.

A. Trends in Mechanical Technologies The mechanical technologies play a crucial role in

practical development of multi-megawatt wind turbines.

In this section, the major trends in the mechanical tech-

nologies are discussed in brief.

1) Gearbox and Drive-Train Technologies: The mechanical components such as blades, gearbox and tower will be

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main restriction for power levels beyond 15–20 MW.

To increase the wind turbine power ratings beyond

15–20 MW’s, the wind industry must develop light weight

mechanical components. The direct drive technology will

help to reduce the maintenance requirements in offshore wind turbines [66]. Currently, the direct drive wind tur-

bines contribute to 28.1% of the global market share [5],

and they continue to dominate the future WECS market as

highlighted in Table 11. The multiple drive train technol-

ogy will also help to reduce the mechanical torque on each

generator shaft. As an extension to the quad drive-train

developed by the Clipper, the work in [174] shows the

feasibility of applying octo drive-train. The innovations in the novel drive train technology will help boosting the

wind turbine power ratings further.

2) Turbine Blades and Offshore Foundations: The wind turbine blades are the most distinctive, visible and crucial

components. Over the past three decades, the wind turbine

blade technology has evolved rapidly in terms of aerody-

namic design and materials. The most commonly used materials in the modern wind turbines are aluminium, fib-

reglass, polyester resin, balsa wood or carbon-fiber com-

posites [72]. The three-blade design is most standard for the

current large wind turbines and it gives symmetrical load-

ing [62]. The blades requirements such as lighting protec-

tion, audible noise reduction, optimum shape, higher

power-to-area ratio, easy manufacturing impose a great

challenge to the turbine manufacturers. The recent devel- opments in the blade sandwich technology can lower the

manufacturing and transportation costs. The wind turbine

blades have a maximum aerodynamic efficiency of 59%

according to the Betz limit [42]. It is prime importance for

mechanical engineers to improve the aerodynamic effi-

ciency, the highest achieved to date is 50% (in Enercon,

Shandong SwissElectric and Wikov blades) [247].

The wind turbine foundations directly support the tower, nacelle and rotor blades. The foundations for the

onshore turbines are simple and it includes slab, monopile

and multipile types. Because of different water depths and

soil types, and harsh weather conditions, the design of

foundations for offshore wind turbines is quite challenging

task. This issue has triggered many research and develop-

ment activities, and currently many offshore foundation

concepts are available such as gravity, monopile, tripod, tripile, jacket and floating. The gravity and monpile foun-

dations are applicable for shallow waters with depth less

than 30 m [48]. The other foundations especially floating

Fig. 35. Representation of trends in high-power WECS.

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774 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

foundations which have already been used in offshore oil and gas industry are promising for future deep water

(> 40 m) projects [7], [248].

B. Trends in Electrical Technologies As mentioned earlier, by 2020, wind turbines in the

range of 15–20 MW will be operational. The electrical

technologies have been used successfully up to tens of

MW’s in electric drives industry. But due to limited space in WT nacelle and tower, high power density electrical

components must be developed. In this section, the trends

in electrical technologies are discussed in detail.

1) Electric Generators: Though the Type 3 wind turbines dominate the current wind market, it is expected that in

future the Type 4 turbines will take over the market. The

Type 4 wind turbines with squirrel cage induction gene- rator or synchronous generator will dominate the future

wind energy market as they increase the wind energy

conversion efficiency and also meet the various grid codes

without needing additional hardware [68]. The permanent

magnet (PM) materials have shown significant variations

in their price. The wind turbine manufacturers are looking

for alternative generator configurations with high power

density. An attempt by wind industry to increase the power

density of electrical generators is to use high temperature

super conducting (HTS) synchronous generator in place of

PMSG/WRSG [249], [250]. The HTS is more favorable for

wind turbines rated above 6 MW. The HTS generators

eliminate the need rare earth materials. The size and

weight of HTS generator is significantly lower compared to

the standard WRSG/PMSG. This technology has been ap- plied in drives industry up to 33 MW [63]. The AMSC and

ABB have announced to use HTS generators in their future

10 and 15 MW wind turbines, respectively.

2) Semiconductor Switching Devices: The semiconductor switching devices are the key elements in any power con-

verter. The evolution and technological improvements in

semiconductor devices directly affect the power converter technology and their key operating characteristics such as

cost, reliability, efficiency, power density, operating volt-

age levels, and power limits [61], [195]. The losses in the

power electronic devices can be categorized as conduction

and switching losses. The first one is produced due to the

switching device type and the second one is caused by

the modulation/control scheme of the power converter. In

the current commercial high power converters, these losses are less than 3% [122]. The emerging switching

devices and advanced modulation and control schemes can

improve the power conversion efficiency further.

The asymmetric IGCTs are popular for the multilevel

voltage source converters, while symmetric GCTs are used

in the current source converters [122]. In wind energy, the

insulated gate bipolar transistor (IGBT) is dominantly used

for both LV and MV power converters. The gate driver requirement is simple for IGBT switching devices. They

can operate at higher switching frequencies compared to

the IGCTs, and increase the power quality and power

density [53], [57].

The next-generation power semiconductor devices are

based on wide band-gap materials. These devices have sub-

stantial performance improvements over the silicon based

counterparts. These devices are able to operate at higher temperatures, power densities, voltages and switching fre-

quencies. The two wide band-gap materials for power ap-

plications are Gallium Nitride (GaN) and Silicon Carbide

(SiC) [251]–[255]. The SiC devices have exhibited success

at higher voltages (1200 V or higher) and they are smaller

than the silicon based IGBTs. The GaN devices are cost

effective at lower voltages compared to both silicon and

SiC based switching devices. The GaN and SiC devices will have a positive impact on the next generation high-power

and high-voltage wind energy power converters [253],

[256], [257].

3) Power Converters: The increasing power level in wind turbines is driving the power electronics technology to-

wards the medium-voltage operation. The MV power

converters will be dominant for the next generation multi-megawatt wind turbines as they offer cost effective

and compact design. Research works on novel MV power

converters and advanced control schemes will play a crucial

role in the development of high-power wind turbines. The

COE can be reduced at least 2.5% by moving from LV to

MV technology in high-power WECS [45]. The MV tech-

nology is a low-risk choice because it is a well-proven

technology in high-power electric drives industry. The perceived technical risks associated with the MV operation

of wind turbines are thus mitigated.

The maintenance costs are very high for offshore wind

farms and they account for 30% of COE [63]. The power

electronics technology is reported to be one of the major

source for the failure or downtime of wind turbines [258].

The reliability improvement of power converters and as

well fault tolerance operation will eventually decreases the turbine down times and increase annual energy produc-

tion. Due to limited space in nacelle, high power density

converters are need by the wind turbine manufacturers

[157]. A customized and efficient design along with proper

cooling system can decrease the overall size of power con-

verters. The Gamesa G10x 4.5 MW wind turbines were

reported to use liquid cooling and optimized design lead-

ing to 30% smaller foot print compared to the competitor products, and higher power density of 0.58 MW/m3 [195].

Apart from the classical multilevel converter topologies

(NPC, FC and CHB), many new converters have been

proposed in literature with new structures or as a hybrid

combination of classical multilevel converters. As an ex-

ample, the nested neutral-point clamped (NNPC) con-

verter combines the best features of the classical NPC and

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FC converters and produces four-level output waveform [259]. The scholarly works in [260], [261] demonstrate

that the modular multilevel converters (MMC) which have

been used in HVDC application are also promising for

wind energy systems. These converters offer modular

structure, higher operational voltage levels leading to

transformerless grid integration and improved grid current

quality. It is anticipated that the research and development

activities in the MV power converters area would become more rigorous in the coming years.

C. Trends in Integration to Power Systems The conventional power plants show excellent perfor-

mance indices such as active/reactive power control,

voltage/frequency regulation, short-circuit capability, and

black start capability. The black start capability indicates

the ability of power generation unit to restore to normal operation from shutdown mode without using any external

power network. The electric utilities expect the high-

power WECS to behave similar to the convention power

generation units. This subsection briefly summarizes the

trends related to the WECS integration to power systems.

1) Policies and Grid Code Requirements: The integration of wind turbines and wind farms to the power system will require more attention in future. The wind farms, and

their performance and operation in terms of supply secu-

rity, reliability and power quality would be of great interest

to the power system operators. As a result, the grid code

requirements in terms of frequency support, active and

reactive power control, short-circuit power level, voltage

variations, flicker, harmonics and stability would become

more vigorous in the coming years compared to what we see nowadays [23]. It is anticipated that the ride-through

requirements continue to evolve and would have signifi-

cant impact on the design and operation of modern high-

power wind turbines. The asymmetrical grid fault condi-

tions are the new challenges for the wind generation sys-

tems [33], [37]. A more rigorous research is needed in this

area to study the performance of WECS with different

generator-converter configurations and wind farm inter- connection methods. The full-scale voltage source

converters meet the FRT requirements straightforward,

however current source converters and partial scale voltage

source converters need some extra care in the design and

operation.

2) Interconnection of Onshore and Offshore Wind Farms: As discussed in Section IX, four possible configurations are available to connect the wind turbines as wind farms and

integrate them to the grid. The present industry uses pa-

rallel ac configuration with HVAC transmission that needs

bulky low frequency (50/60 Hz) transformer. The future

offshore wind farms have been planned in gigawatt (GW)

range and in deep sea. The cost, size, reliability and effi-

ciency of interconnection approaches thus become very

crucial. Due to the advantages of HVDC systems over HVAC systems, the HVDC transmission systems will take

over the current HVAC transmission to achieve econom-

ical and technical benefits and remove the bulky transfor-

mers. Though the dc collections systems are in research

stage only, they will see future implementations to have

potential benefits especially for offshore projects. It is

foreseen that the power converters for the HVDC appli-

cations will witness enormous research and development activities.

D. Trends in Control Theory The control theory has evolved as an important discip-

line in the modern wind turbines and wind farms. The

control schemes enforce the WECS to achieve desired

operation, increase the wind energy conversion efficiency,

reduce cost of energy, increase life time of wind turbine components, decrease structural loading, reduce turbine

down times, and provide superior dynamic and steady-

state performance [7], [30]. In this section, the control of

mechanical and electrical power conversion units will be

discussed with more focus on the latter one.

1) Control of Power Converters and Wind Generators: The power converters and electric generator and their accurate control lead to higher energy conversion. The generator-

side converters in WECS are usually controlled by vector

control or field oriented control (FOC) (rotor field

oriented in SCIG, stator field oriented in DFIG, maximum

torque per ampere in PMSG), direct torque control (DTC)

or direct power control (DPC) [58], [69], [262], [263]. The

main objective for these control schemes is to regulate

independently: (i) dq axis generator currents in FOC scheme; (ii) generator torque and flux in DTC; and (iii)

generator active and reactive powers in DPC.

The reference control variables such as generator speed,

torque or power for these schemes are usually provided by a

MPPT algorithm. The MPPT is an important issue in va-

riable speed WECS to enhance the wind energy extraction.

The broad range of advanced and sophisticated MPPT

methods include optimal tip-speed-ratio control, power signal feedback control, hill climbing searching control,

optimal torque control, and optimal power control [16],

[114], [115], [264]. The power converter switching actions

are adjusted such that the WECS variables are maintained

at their reference value. The grid-side converters in Type 3

and 4 WECS are controlled by decoupled voltage oriented

control (VOC) or DPC [265]. In VOC, through the regula-

tion of dq axis grid currents, the net dc-bus voltage (active power) and reactive power are controlled, respectively. In

DPC scheme, similar to the generator-side control scheme,

the grid active and reactive powers are controlled

independently.

Recently, finite control-set model predictive control

(FCS-MPC) strategy appeared as a simple and promising

alternative to control the power converters and wind

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776 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

Table 8 Main Features of Power Converters for Megawatt Wind Turbines

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Table 9 List of Commercial Type 3 and Type 4 Megawatt Wind TurbinesVPart-I

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778 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

Table 10 List of Commercial Type 3 and Type 4 Megawatt Wind TurbinesVPart-II

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Table 11 List of Variable-Speed Megawatt Wind Turbine Projects in the Development Stage

Yaramasu et al.: High-Power Wind Energy Conversion Systems

780 Proceedings of the IEEE | Vol. 103, No. 5, May 2015

energy systems, and this method eliminates the need for linear regulators and modulators [70], [266]. The cost

which is paid to use predictive control is higher computa-

tional burden, but the modern digital signal processors can

perform large amount of calculations at low cost. There is

lot more room to develop advanced control schemes for

power converters and electric generators. The research

and advancements in the model predictive control [70],

H1 control [34], sliding mode control [267], artificial in- telligence based control [41] and other control schemes

[30], [33], [37] for power conversion units will enable

wind energy further.

2) Control of Wind Turbines and Wind Farms: The turbine output power can be controlled aerodynamically by passive

stall, active stall or pitch control [16]. The modern variable-

speed wind turbines use pitch mechanism to change the rotation of blades in their longitudinal-axis. Three individ-

ual electric pitch drives are more commonly used in the

present wind turbines. The aerodynamic interaction of

wind turbines could lead to negative impact on the wind

farm. To solve such issue, many wind farms control tech-

niques have been proposed. The readers are suggested to

refer to [7], [268]–[270] and references there in for more

details regarding the wind farm control schemes.

X I I . C O N C L U S I O N

In this paper, a comprehensive review of high-power wind

energy systems is presented with main focus on electrical

technologies. The generator-converter configurations em-

ployed with SCIG, DFIG, PMSG and WRSG WECS are

investigated and analyzed. The most common configura- tions for the state-of-the-art and emerging wind turbines

are discussed. Various technical issues related to these

configurations are discussed, including generator types,

power converter topologies, active power control, energy conversion efficiency, and grid-side reactive power com-

pensation. The back-to-back connected converters, passive

generator-side converters, converters for six-phase and

open winding generators, and converters without interme-

diate dc-link are presented, and their features and draw-

backs are analyzed. The wind farm interconnection

technologies, grid code compliance methods are analyzed

with respect to the practical and promising approaches. The trends in various sectors of wind energy including mechan-

ical technologies, electrical technologies, integration to

power systems and control theory are presented in detail.

The future will see enormous growth of wind energy

systems in the world. Technological development of wind

energy conversion systems will play a major role in making

wind power systems highly efficient. Switching to wind

power will ensure energy independence and pollution free environment. The oil is expensive, but wind is free–now

and forever, and for every nation. h

A P P E N D I X The main features of the power converters discussed in

Sections V to VIII are listed in Table 8. The advantages,

disadvantages, market penetration and technology status are highlighted. The summary of commercial Type 3 and 4

wind turbines which use DFIG/SCIG/PMSG/WRSG and

power converters is given in Tables 9 and 10. For each

manufacturer, only those wind turbine models which have

distinct feature from their preceding model are presented.

The wind turbines with power rating greater than 1.0 MW

are considered and listed here. The future wind turbine

projects which are in the development stage are summa- rized in Table 11. These details are gathered from the re-

spective WT product brochures, company press news

releases, and online portals [12], [13], [28], [29].

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A B O U T T H E A U T H O R S

Venkata Yaramasu (Member, IEEE) was born in

Karumanchi, Guntur, India. He received the B.Tech

degree in electrical and electronics engineering

from Jawaharlal Nehru Technological University,

Hyderabad, India, in 2005, the M.E. degree in elec-

trical engineering with specialization in power

electronics from S. G. S. Institute of Technology

and Science, Indore, India, in 2008, and the Ph.D.

degree in electrical engineering from Ryerson

University, Toronto, Canada, in 2014.

He is currently a Postdoctoral Research Fellow at the Laboratory for

Electric Drive Applications and Research (LEDAR) and Center for Urban

Energy (CUE), Ryerson University. His research interests include renewable

energy, high power converters, electric vehicles, and predictive control.

Dr. Yaramasu received six Best Student Paper Awards and two first

prizes in National Level Technical Quiz Competitions during his under-

graduate studies in India. During his Ph.D. studies at Ryerson University,

he received the Best Poster Award at the Natural Sciences and Engineer-

ing Research Council of Canada (NSERC)VWind Energy Strategic Network

(WESNet) Annual Meeting-2010, Best Poster Awards from the Electrical

and Computer Engineering (ECE) Department and Faculty of Engineering

and Architectural Science (FEAS) in 2013, Student Research Awards from

the Toronto Hydro, Hydro One and Connect Canada in 2010, 2012, and

2013, Research Excellence Awards from the ECE Department in 2012,

2013, and 2014, and Best Teaching Assistant Award from the FEAS in 2010.

Bin Wu (Fellow, IEEE) received the M.A.Sc. and

Ph.D. degrees in electrical and computer engi-

neering from the University of Toronto, Canada in

1989 and 1993, respectively

In 1993, he joined Ryerson University, where he

is currently a Professor and Senior NSERC/Rock-

well Automation Industrial Research Chair in

Power Electronics and Electric Drives. He has

published more than 300 technical papers,

authored/coauthored two Wiley-IEEE Press

books, and holds more than 25 granted/pending US/European patents

in the area of power conversion, medium voltage drives, and renewable

energy systems.

Dr. Wu received the Gold Medal of the Governor General of Canada in

1993, Premier’s Research Excellence Award in 2001, NSERC Synergy

Award for Innovation in 2002, Ryerson Distinguished Scholar Award in

2003, YSGS Outstanding Contribution to Graduate Education Award, and

Professional Engineers Ontario (PEO) Engineering Excellence Medal in

2014. He is a fellow of Engineering Institute of Canada (EIC) and Canadian

Academy of Engineering (CAE). He currently is an Associate Editor of the

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS and IEEE CANADIAN REVIEW.

Yaramasu et al.: High-Power Wind Energy Conversion Systems

Vol. 103, No. 5, May 2015 | Proceedings of the IEEE 787

Paresh C. Sen (Life Fellow, IEEE) was born in

Chittagong, Bangladesh. He received the B.Sc.

(with honors in physics) and M.Sc. (Tech.) degrees

in applied physics from the University of Calcutta,

West Bengal, India, in 1958 and 1962, respectively,

and the M.A.Sc. and Ph.D. degrees in electrical

engineering from the University of Toronto,

Toronto, ON, Canada, in 1965 and 1967,

respectively.

Dr. Sen is currently Emeritus Professor of Elec-

trical and Computer Engineering at Queen’s University, Canada. He has

worked for industries in India and Canada and was a consultant to

electrical industries in Canada. He has authored more than 215 technical

papers in the general area of electric motor drives and power electronics.

He is the author of two internationally acclaimed textbooks: Principles of

Electric Machines and Power Electronics (Hoboken, NJ: Wiley 1989, 1997,

2013) and Thyristor DC Drives (Hoboken, NJ: Wiley 1981). He has taught

electric machines, power electronics and electric drive systems for over

45 years. His fields of interest include power electronics, electric drive

systems, switching power supplies, wind energy systems, digital control,

and modern control techniques for power electronics and motor drive

systems.

Dr. Sen has served IEEE in various capacities: as an Associate Eeditor,

Distinguished Lecturer, Chairman of the Technical Committees on power

electronics and energy systems, session organizer, session chairperson,

and paper reviewer. He served as an Natural Science and Engineering

Research Council of Canada (NSERC) Scientific Liaison Officer evaluating

university-industry coordinated projects. He is globally recognized as an

authority in power electronics and motor drive systems. He received the

IEEE-IAS (Industry Application Society) Outstanding Achievement Award

in 2008, and the IEEE-Canada Outstanding Engineering Educator Award

in 2006 for his outstanding contributions over four decades as a re-

searcher, supervisor, teacher, author and consultant. He received the

IAS-IDC Prize Paper Award in 1986. He is a fellow of EIC. As an Emeritus

Professor, he continues to be active in research, supervision of graduate

students, and in several IEEE societies.

Samir Kouro (Member, IEEE) received the M.Sc.

and Ph.D. degrees in electronics engineering from

the Universidad Técnica Federico Santa Marı́a

(UTFSM), Valparaı́so, Chile, in 2004 and 2008,

respectively.

In 2004, he joined the Electronics Engineering

Department, UTFSM, where he currently Associate

Professor. From 2009 to 2011 he was a Postdoc-

toral Fellow in the Department of Electrical and

Computer Engineering, Ryerson University,

Toronto, Canada. His research interests include power converters,

variable-speed drives, and renewable energy power conversion systems

(wind and photovoltaic).

Dr. Kouro is a Principal Investigator in the Solar Energy Research

Center SERC-Chile and Titular Researcher in the Advanced Center for

Electrical and Electronic Engineering (AC3E), both research centers of

excellence in Chile. He has coauthored one book, four book chapters and

over 80 refereed journal and conference papers.

Dr. Kouro served as Guest Editor of a Special Section in the IEEE

TRANSACTIONS ON INDUSTRIAL ELECTRONICS and one in IEEE TRANSACTIONS ON

POWER ELECTRONICS. He received the IEEE Power Electronics Society

Richard M. Bass Outstanding Young Power Electronics Engineer Award in

2012, the IEEE Industry Applications Magazine Best Paper Award of 2012,

the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS Best Paper Award of

2011, the IEEE Industrial Electronics Magazine Best Paper Award of 2008.

Mehdi Narimani (Member, IEEE) received the

B.Sc. and M.Sc. degrees from Isfahan University of

Technology (IUT), Isfahan, Iran, in 1999 and 2002,

respectively, and the Ph.D. degree from the Univ-

ersity of Western Ontario, Ontario, Canada, all in

electrical engineering.

He is currently a Postdoctoral Research

Associate with the Department of Electrical and

Computer Engineering, Ryerson University and

Rockwell Automation Canada. From 2002 to

2009, he worked as a faculty member of Isfahan University of Technology

where he was involved in design and implementation of several industrial

projects; he was the leader of a research team for several research pro-

jects. He is the author/coauthor of more than 45 journal and conference

proceeding papers and three patents (pending review). His current

research interests include high power converters, motor drives, control of

power electronics, and renewable energy systems.

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788 Proceedings of the IEEE | Vol. 103, No. 5, May 2015