Engineering Essay

EXAMINATION OF NEW CURRENT CONTROL METHODS FOR MODERN PWM CONTROLLED AC ELECTRIC

LOCOMOTIVES

G.G. *, , I. Schmidt*, P. Kiss*

*Budapest University of Technology and Economics, Department of Electric Power Engineering, Budapest, Hungary, email: [email protected], tel: +36-1-463-3609

DiFilTON-ARC Ltd., Budapest, Hungary Keywords: Electric locomotive, current control, harmonic distortion, measurements

Abstract A railway electrification system supplies electrical energy to railway locomotives and multiple units. There are several different electrification systems in use throughout the world. The single-phase AC network systems are widespread (25 kV 50 Hz or 15 kV 16 2/3 Hz). The Hungarian system is 25kV 50 Hz AC. This article is just dealing with the AC network supplied locomotives. Nowadays in our country the series wound DC traction motor driven locomotives are still widely used. These vehicles are equipped with diode or thyristor rectifier circuits that inject harmonics into the AC line and distort the line voltage. In our work we examined and compared current control methods that can be achieved by

- We worked out a new current control strategy that possesses several advantages. The modern locomotives endeavour to consume sinusoidal current from the AC network, in phase with the network voltage fundamental. In generator mode these endeavour to supply back to the grid sinusoidal current in antiphase to the voltage fundamental. We compared current control methods

. One of them can reduce the consumed root mean square (RMS) or fundamental current of a distorted line connected modern locomotive in motor mode. Other one can increase the generated RMS and fundamental current in generator mode. With these strategies the harmonic currents can be used for active power. Moreover it turned out that the harmonic content of the network can be reduced by

. For the study, we built a test system. We can model the line- side converter of a modern locomotive DC-link frequency converter with the system. A common solution in locomotives is when several line-side converters feed two DC-links. In the test system we modelled these with one converter, while the motor-side voltage source inverters and the electric traction motors were taken into account as a controllable current source DC-link.

1 Introduction Hungary has a great tradition in locomotive manufacturing. One of the first electrified rail line in the world was

established by a Hungarian company called Ganz in 1902 in Valtellina (Italy). The company designed and built the whole electrification system and produced special electric locomotives for Valtellina. From then to 1980s several types of locomotives were produced by Ganz. In Hungary still large numbers of electric locomotives are running that were constructed by the company in the 1960s and 1970s. The most commonly used electric locomotives are the V43 and the V63. These are driven by series wound DC traction motors. The V43 contains diode rectifier circuit and two motors (total power: 2200 kW). The V63 is driven by six motors and contains half controlled thyristor rectifiers (total power: 3600 kW). [8] Both locomotives generate large amount of harmonic currents into the grid. The following figure represents a possible harmonic current spectrum of a V43 and V63.

Figure 1: Harmonic current spectrum of a V43 and V63

The high harmonic-content current of the locomotives cause that the RMS (root mean square) current load of the AC network substantially grows. Extra losses reduce the electric energy transmission capacity of the network. The locomotive current is full of harmonics, so the originally sinusoidal network voltage waveform become distorted. Eventually, qualitative parameters of the energy supply are affected: potential difficulties include various kinds of economic and technological problems, breakdowns, switching surges, overheating and electromagnetic disturbances. Locomotives equipped with diode or thyristor rectifiers can cause stark voltage distortion on the rail grid, which total

harmonic distorion (THDU) can reach 50% (e.g. Fig. 2.,

Figure 2: Rail grid THDU, measured data

Growing numbers of PWM controlled modern locomotives connect to the rail grid distorted by DC traction motor driven locomotives. These locomotives are equipped with DC link frequency converter (Fig.3.). [4,8] In our study we are dealing with the control of its line-side converter. These are able to generate the breaking energy; therefore these kind of line-side converters are called four quadrant power supplies (4QS). [4,6] These have two operating mode: a.) Rectifier mode: When the traction motors of the locomotives are in motor mode, the 4QS operating as a rectifier: (power flow: AC rail grid 4QS DC link). b.) Inverter mode: When the traction motors of the locomotives are in breaking mode, the 4QS operating as an inverter: (power flow: AC rail grid 4QS DC link). In our work we compared current control strategies can be achieved by PWM controlled locomotives.

Figure 3: Schematic circuit diagram of a modern PWM controlled locomotive

2 Current control strategies

The following three strategies can be used in the current control of a locomotive line-side converter. In the brief description of the three methods the effect of current pulsation caused by the PWM is not considered. The distorted grid voltage at the pantograph of the locomotive can be written as follows:

2 1 )sin(sin)( tUtUtu (1)

( U : harmonic voltage amplitudes, : phase angles of harmonic components, 1, f1: fundamental frequency) 1. According to the first strategy, the converter enforces sinusoidal current (isin) in the AC line, both in rectifier and in inverter mode. This can be considered as the common method because it is widely used. [2,3,4] In this case, the converter AC side current is proportional to the fundamental of the distorted line voltage waveform. a.) Rectifier mode:

tUktIti sinsin)( 1sinsin_1sin (2.a) b.) Inverter mode:

tUktIti sinsin)( 1sinsin_1sin (2.b) (I1_sin: current amplitude, ksin [A/V]: gain)

Figure 4: Waveforms of the first strategy (left: rectifier mode, right: inverter mode)

2. If the second strategy is used, the converter consumes or generates current (iohm) proportional to the line voltage waveform. In rectifier mode this strategy consumes the harmonic currents injected by non-linear loads, therefore

- locomotive can be reduced. It means that the converter behaves like a resistive load. This strategy is known, but it is not applied in locomotives. [5,7] a.) Rectifier mode:

2 __1 )sin(sin)( tItIti ohmohmohm

2 1 )sin(sin tUtUk ohm

(3.a)

b.) Inverter mode:

2 __1 )sin(sin)( tItIti ohmohmohm

2 1 )sin(sin tUtUk ohm (3.b)

(Because of the ohmic behavior ohm subscript is used. I1_ohm: fundamental current amplitude of iohm, I : harmonic currents amplitudes of iohm, kohm [A/V]: gain)

Figure 5: Waveforms of the second strategy (left: rectifier mode, right: inverter mode)

3. We worked out a new control strategy. The main advantage of this appears in inverter mode. According to the strategy, at inverter mode the generated fundamental current of the converter (irev) is in antiphase with the line voltage fundamental while the harmonics of the generated current are in phase with the harmonics of the line voltage and are proportional with them. Therefore, it operates as a fundamental current generator and harmonic current consumer. At a given DC side power, the converter generates higher fundamental current into the line than in the former two strategies and operates as a harmonic compensator. a.) Rectifier mode:

2 __1 )sin(sin)( tItIti revrevrev

2 1 )sin(sin tUtUk rev (4.a)

b.) Inverter mode:

2 __1 )sin(sin)( tItIti revrevrev

2 1 )sin(sin tUtUk rev (4.b)

(The fundamental current is reversed from the harmonic currents; so rev subscript is used. I1_rev: fundamental current amplitude of irev, I : harmonic currents amplitudes of irev, krev [A/V]: gain)

Figure 6: Waveforms of the third strategy (left: rectifier mode, right: inverter mode)

3 Test system This chapter represents the structure of our test system. It consists of two main parts: the power electronic circuit and the control system. We can model the line-side converter of a modern locomotive DC-link frequency converter with this test system. A common solution in locomotives is when several line-side converters feed one or two DC-links. In this study we modelled these with one converter, while the motor- side voltage source inverters and the electric traction motors were taken into account as a controllable current source DC- link.

3.1 Power electronic circuit

In our test system we used a bridge-type four-quadrant electric power converter (Fig.7.). The AC side was connected to the single-phase line, while the DC side was connected to the same line - through load resistors - with a diode rectifier. We connected buffer capacitors into the DC link. To set the proper voltage levels toroid transformers were put into the circuit (T1-3, T5, T8), therefore the AC line voltage and the

For galvanic isolation we connected transformers (T4, T6, T7).

Figure 7: Schematic circuit diagram of the test system

The converter contains IGBT switching elements. The system enables bidirectional power flow, therefore it can operate in rectifier mode (AC/DC) or in inverter mode (DC/AC). To examine different current control methods, we built a controller for the power converter. The AC and the DC side voltages and currents of the converter could be measured by current and voltage meters. For taking into account the effect of the line distortion, we connected a non-linear load into the AC line circuit that contains a diode rectifier circuit and causes strong distortion on the line voltage. The current of the non-linear load could be varied. Fig.7. presents the schematic circuit diagram of our test system.

3.2 Control circuit

To fulfil the requirements of network-friendly locomotive, the converter needs to have an adequate control circuit. We implemented cascade control into our controller. The primary control loop is a DC-voltage-control and the secondary loop is line current control. This structure guarantees a high power factor and an adequate line current shape. Three different current control methods can be examined with the circuit. Figure 8 shows the block diagram of the control circuit. If it is observed left-to-right, the following parts can be distinguished. The DC voltage (UDC) is measured to get the feedback signal for the primary control loop. The DC voltage reference signal ( DCU ) can be set, but we kept in the same level during each measurement. A proportional integral controller (PI) was applied for the DC voltage control. The output of the controller is limited. For the adequate operation of the control circuit, we inserted an active Wien-Robinson notch filter to eliminate the 100 Hz pulsation of the DC voltage loop [9]. The DC voltage loop generates the proper gain (ksin, kohm, krev) of the line current reference signal ( ACi ).

A multiplier produces ( ACi ). The network current is measured to define the feedback signal. A PI current controller is built in the circuit. It produces the control signal for the PWM generator that drives the switching elements of the converter. In the control circuit the measuring parts can be considered as proportional terms. These can be seen on the figure.

The three control method can be selected by the control strategy switch, for each strategy the AC voltage is measured ( ): 1. The line voltage fundamental need to be defined for the first strategy. We used two series-connected Chebyshev low- pass filters to generate the line voltage fundamental. The cut- off frequency of the filters was tuned to 80 Hz and the suppression is: 80dB/decade. [9] The phase offset of the filters had to be compensated; therefore we built a phase compensator into the circuit.

tUutu sin)( '1 ' 1sin (5)

2. The second strategy directly uses .

2

'' 1 )sin(sin')( tUtUutuohm (6)

3. For the third method is subtracted from the double of the fundamental.

2

'' 1

' 1

' 1 )sin(sin)sin(2'2)( tUtUtUuutu rev

2

'' 1 )sin(sin tUtU (7)

(Labeled values represent the measured values; this makes differences between equations 5-7 and equations 2-4.)

Figure 9: Test system in DiFiLTON laboratory

Figure 8: Control circuit block diagram

4 Measured results With measurements we examined the AC side RMS current of the converter (IAC) and the THDU of the line voltage with each strategy in rectifier and in inverter mode. In our test system we measured the and currents and the DC side power. The waveforms of the

current were observed by oscilloscope (Fig.4-6). The oscilloscope shows the THDU. During each measurement we kept the DC voltage (350 V), the absolute value of the DC current (1.2 A), and the RMS value of the AC voltage (200 V) at the same level. Several measurements were performed Table 1 presents one of them. We received approximately the same results in each measurement. Mode IAC IAC [A] THDU

1.(sin) Rectifier 47.1 2.355 11.20% Inverter 40.8 2.04 10.10%

2.(ohm) Rectifier 46.9 2.345 10.30% Inverter 40 2 10.50%

3.(rev) Rectifier 48.1 2.4025 12.20%

Inverter 42 2.1 9.19%

Table 1: Measured results (IAC: read values, IAC[A]: real values /IAC multiplied by the transfer of the ammeter/)

Based on the measurements it turned out that in rectifier mode the third strategy increases the THDU and at same DC side power it needs more RMS current than the first, sinusoidal method. Therefore it generates harmonics in rectifier mode. In inverter mode the second strategy increases the THDU and at same DC side power it generates less RMS current than the first, sinusoidal method. Nevertheless the main results of our measurements appear if the second strategy is observed in rectifier mode and the third is examined in inverter mode. In rectifier mode the second strategy needs less current than the sinusoidal strategy at same DC side power, and it reduces the THDU of the network voltage, the relative difference is 8.74%. In inverter mode the third strategy generates more current than the sinusoidal strategy at same DC side power, and it decreases the THDU of the network voltage, the relative difference is 9.90%. We defined coefficients, which depend only on the total harmonic distortion of the network voltage. With these coefficients, the differences of the consumed/generated fundamental and RMS currents of the strategies can be exactly determined. We received the same theoretical and measurement results. This will be a topic of a forthcoming paper.

5 Conclusions In this study, we worked out a new current control method for a PWM controlled locomotive line-side converter that possesses several advantages in inverter mode. We showed that it is worth applying a non-used strategy the second one

in rectifier mode.

We built a test system to examine the former three control strategies with measurements. Based on our research, we can conclude that in single-phase line connected AC electric locomotives it is better to apply the second strategy in rectifier (motor) mode and the third strategy in inverter (generator) mode than the common first sinusoidal strategy. The presented principles can also be applied in three-phase line connected converters, but the focus of the current paper is the application in single-phase systems.

Acknowledgements The authors are indebted to DiFiLTON ARC Ltd. for the

kind support in establishing the measurements. We are grateful to invo- Co. for helping us in

obtaining adequate data. (Fig.2.)

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