Distribution system and power quality

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ECE 5750 Distribution System & Power Quality

Part 6: Power quality concepts, phenomena, & sources of transient overvoltages

Instructor: Dr. Ha Le Department of Electrical and Computer Engineering

California State Polytechnic University, Pomona

What will be presented? 1) Basic power quality (PQ) concepts

2) IEC and IEEE classifications of PQ disturbances

3) PQ definitions: Transients, short- and long-duration voltage variations, voltage unbalance, waveform distortion, voltage fluctuation, power frequency variations.

4) Sources of transient overvoltages

Reading: PQ textbook, Chapter 1, 2, 5

Power quality concepts

What is power quality ? IEC 61000-1-1 (1992)

Define PQ in terms of EMC (electromagnetic compatibility) between equipment and supply system serving the equipment.

 EMC: The ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment.

 Emission: Electromagnetic pollution produced by a device.

 Immunity: Device’s ability to withstand the electromagnetic pollution.

Developer: International Electrotechnical Commission (IEC)

Common definitions of power quality IEEE Std 1100-2005 and IEEE Std 1159-2009

“The concept of powering and grounding electronic equipment in a manner that is suitable to the operation of the equipment and compatible with the premise wiring system and other connected equipment”

IEC 61000-1-1 “The ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment”

From a customer standpoint, power quality is any power problem manifested in voltage, current, or frequency deviations that results in failure or misoperation of the customer equipment.

PQ in distribution systems

PQ disturbances generally take place in distribution systems rather than in transmission systems.

Reasons:

 More loads are served at the distribution level.

 Distribution systems are generally radial and remote from generation sources making them electrically weaker.

Common locations of PQ disturbances

Central generating station

18 - 24 kV

230 - 765 kV

115 kV

Tie-line to other power

systems

Large industrial loads

13.8 kV

Transmission system

Subtransmission system

Distribution system

Characteristics of T & D

 Each level feeds one below it.

 Each level has more pieces of equipment in it than the one above.

A power system serving 300,000 households:

 50 transmission lines

 100 substations (e.g. 138/24 kV, 69/13.8 kV)

 600 feeders (e.g. 24, 13.8, 7.2, 4.16 kV)

 60,000 service TRF (7.2kV/120V/240V)

Importance of current and impedance

 Equipment failures or misoperations are caused by significant deviations in the voltage waveshape.

 Voltage is influenced by the current flowing through the system impedance.

 When the current waveshape passing through the system impedance deviates significantly, the quality of the voltage will be impacted.

 System impedance in a PS comes from overhead lines and transformers, thus utilities have control over impedance.

 End-users have control over the currents since their equipment draws current from the system.

Root causes of PQ disturbances

Load Electrical

Requirements

Utility Supply Electrical

Characteristics

Mismatch or Incompatible

Demand a near perfect sinusoidal voltage

waveform

Root cause: Current & Impedance (1)

perfect voltage waveshape

voltage drops

Customer BusUtility

)sin()( 1111   tZItV s

)sin()( 11 tIti 

)sin()( 111   tZIVtV ssload

Root cause: Current & Impedance (2)

perfect voltage waveshape

voltage drops

Customer Bus

Utility

))(sin(

)sin(

1 ,...7,5,3

1111

jnXRtnIV

tZIV

n n





 





 



 ,...7,5,3

111 )sin()sin()( n

n tnItIti 

 

  

  



 ,...7,5,3 1111 ))(sin()sin(

n nnssload jnXRtnItZIVV 

Control of current & impedance Who has control over currents:

 Harmonics: End-users have control over the currents since their equipment draw currents from the system.

 Short-circuit currents: Faults in the system causes voltage sags or interruptions.

 Lightning: Currents cause high-impulse voltage, leading to flashover.

Who has control over impedances:

 Utilities: Overhead lines, underground lines, transformers.

Absolute PQ solutions?

Got holistic solutions to all PQ problems ?

Technologies to get rid of offending currents

Technologies to get rid of system impedances

Got ideal solutions to all PQ problems ?

Get rid of offending currents and the system impedances

Cost of PQ disturbances (1) Based on finding of Lawrence Berkeley National Lab, 2004

Cost per year

Source: Understanding the Cost of Power Interruptions to U.S. Electricity Consumers, Ernest Orlando Lawrence Berkeley National Laboratory, 9/2004

Momentary: 30 cycles - 3 seconds

Cost of PQ disturbances (2) Based on finding of EPRI / CEIDS, 2001

Power outages (momentary and sustained interruptions):

$104 billion to $164 billion a year

Power quality phenomena: (transients, sags, harmonics)

$15 billion to $24 billion

EPRI is Electric Power Research Institute

Power quality phenomena

Classification of PQ disturbances

10-7 10-6 10-5 10-4 10-3 10-2 10-1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 10 7

Lightning

System Switching

Subsynchronous Resonance

Transient Stability

Long-term Dynamics

Tie-line R

egulation

Load following

1 usec

1 cycle of 60 H

1 second

1 m inute

1 hour

1 day

Electromagnetic Transient

Transient Stability

Economic Operations

 The term PQ is applied to a wide variety of electromagnetic (EM) transient phenomena.

 Classification of PQ disturbances is based on EM transient phenomena.

IEC classification of PQ disturbances (1)  Electromagnetic phenomena causing power quality disturbances can

reach equipment via conductive and radiative coupling pathways.

 Conductive coupling occurs when a physical pathway exists

 Radiative coupling occurs without a physical pathway, instead propagating through electric and magnetic fields

 Electromagnetic phenomena occurs over a range of frequencies, can be broken down into low and high frequency ranges.

IEC classification of PQ disturbances (2)  IEC classifies EM phenomena causing PQ disturbances into six general

categories based on the coupling pathway and frequency range. Steady- state and/or transient disturbance phenomena may exist in each category.

IEC classification of PQ disturbances (3) IEEE Std.1159-2009 provides a general classification of power quality phenomena based on: 1) principal spectral content, 2) duration, 3) magnitude of the disturbance.

IEC classification of PQ disturbances (3)

Note: The terms and categories in the IEC table apply to PQ measurement and are not to be confused with similar terms in IEEE Std. 1366 2003 [B27] and other reliability-related standards, recommended practices, and guides.

Transients  Electrical transients are the results of a sudden and rapid

change in the electrical system

 Generally undesirable and short lived

 IEEE Std. 100-1992

 A transient is “that part of the change in a variable that disappears during transition from one steady-state operating condition to another”

 Transients category consists of two sub-categories:

 Impulsive transient

 Oscillatory transient

Impulsive transients (1)  Sudden, non-power

frequency change in steady-state condition of voltage, current, or both, that is unidirectional in polarity.

 Most commonly associated with lightning strokes

 Figure 2.1 illustrates a typical current impulsive transient caused by lightning

Impulsive transients (2)  The electrical

characteristics of impulsive transients are defined by the peak magnitude of the crest, the time- to-crest value tr , and the time-to-half value th

 An impulse wave is expressed as tr / th

   ttc eeVtv    02.1 Vc is the crest value; α and β are 1.43 x 104 and 4.4 x 106 respectively

The standard impulse wave can be approximated as

Oscillatory transients (1)  Oscillatory transient is a sudden, non-power frequency

change in the steady-state condition of voltage, current, or both.

 The instantaneous value of an oscillatory transient changes polarity rapidly, fluctuating between positive and negative.

 Characterized by its spectral content, duration of oscillation, and the maximum absolute value during the oscillation

 Can be further divided into low, medium, and high frequency oscillatory transient characteristics.

Oscillatory transients (2)  Oscillatory transients with a principal frequency of less than 5 kHz are

considered as low-frequency transients.  Duration: 0.3 - 50 ms  Cause: Capacitor switching, TRF energizing, line energizing etc.

Oscillatory transients (3)  Voltage and current waveforms during line energizing

operations

Oscillatory transients (4) The most common oscillatory transient in PS is that of capacitor energizing:

 Frequencies: 300 Hz - 900 Hz  Overvoltage magnitude: Typically between 1.1 and 1.3 pu; can reach

2.0 pu in theory.  Usually damp out in less than 0.5 cycles.

Oscillatory transients (5)

Oscillatory transients (6)  Oscillatory transients with

a principal frequency between 5 kHz and 500 kHz are considered medium frequency transients.

 Typical durations are much less than those of low frequency transients, i.e., 20 μs.

 A back-to-back energizing transient can produce transients in this frequency range.

Fig. 2.7 The above back to back capacitor switching transient has two principal frequency components – 18kHz and 350Hz. 18-kHz component = medium- while 350-Hz component = low-frequency oscillatory transients.

Oscillatory transients (7)  High-frequency oscillatory transients range between

500 kHz and 5 MHz with a typical duration of 5μs.

 Absolute maximum overvoltage during the transient is generally up to 4 pu.

 High-frequency oscillatory transients are common in gas insulated substations (GIS).

 High-frequency oscillatory transients in GIS are called very fast transients.

 When a sudden change in voltage occurs due to the opening or closing of a disconnect switch, a ground switch or a circuit breaker, or due to a short-circuit condition, very high oscillatory transients can result. Overvoltage of up to 2.5 pu can result.

Short-duration voltage variations Characterized by the duration and voltage magnitude of the disturbance

 Typically have durations ranging between 0.5 cycles and 1 minute

 Typically have voltage magnitude of zero to 1.2 pu

Based on the duration and voltage magnitude characteristics, the category short-duration variation are grouped as:

Duration:

 instantaneous (0.5-30 cycles),  momentary (30 cycles – 3 seconds), and  temporary (3 seconds – 1 minute)

Magnitude:

 interruption (< 0.1 pu),  sag or dip (0.1 – 0.9 pu),  swell (1.1 – 1.4 pu)

Interruptions  Occurs when the supply voltage or load current decreases to less than 0.1

pu for a period of time not exceeding 1 minute.  Can result from PS faults, equipment failures, and other disasters.  Many instantaneous interruptions are due to the operation of overcurrent

protective devices.

Voltage sags (1)  A voltage sag is a decrease in the rms AC voltage to between 0.1and 0.9

pu of the nominal voltage at the power frequency for duration no longer than 1 minute.

 Root causes of voltage sags are faults on the power system and the starting of large loads.

Voltage sags (2)  The associated RMS profile of the voltage sag waveform and its

current waveform.

Voltage sags (3)  Starting large loads such as motors can also cause voltage sag

Voltage swells A voltage swell is defined as an increase in rms voltage or current at the power frequency for duration from 0.5 cycles to 1 minute.

 Typical magnitudes between 1.1 and 1.8 pu

 Voltage swells are comparatively much less common than voltage sags.

Occur in both solidly grounded and ungrounded systems

 In a grounded system, voltage swells occur during a single-line to ground fault where the equivalent zero- sequence impedance at the fault location is larger than the equivalent positive-sequence impedance

 In an ungrounded system, the line-to-ground voltages of the unfaulted phases can rise to 1.73 times the nominal line-to-ground voltages during a single-line to ground fault condition.

Long-duration voltage variations (1)  Long duration variations encompass RMS deviations at

power system frequencies for > 1 minute

 Long duration variations are characterized by the voltage magnitude. They can be:

a) Interruption

b) Undervoltage

c) Overvoltage

 Long-duration overvoltages and undervoltages are caused by load variations on the system and system switching operations.

 Sustained interruptions are caused by faults on the system.

Long-duration voltage variations (2) Overvoltage: Increase in the rms voltage > 1.1 pu for a duration > 1 minute

 Usually the result of load switching off or energizing a capacitor bank.

Undervoltage: Decrease in the rms voltage to < 0.9 pu for a duration > 1 minute

 A load switching on or a capacitor bank switching off can cause an undervoltage until voltage regulation equipment on the system can bring the voltage back to within tolerances.

Sustained interruptions: Supply voltage < 0.1 pu for a duration > 1 minute

 Sustained interruptions are often permanent and require human intervention to repair the system for restoration.

Voltage unbalance (1)  Voltage unbalance is a condition in a polyphase system in which

the RMS values of the line voltages or the phase angles between consecutive lines are not equal.

Voltage unbalance (2)

 IEEE 1159-2009 and IEC 61000-3-4 quantify the degree of the imbalance in terms of ratios of the negative and zero sequence components to the positive sequence voltage.

 Voltage unbalance for a one-week trend of imbalance on a 12.47 kV feeder is summarized in Figure 2.13 and 2.14.

Voltage unbalance (3)  Voltage unbalance as defined by ANSI C84.1 is the ratio of the

maximum deviation from average of the 3-phase voltages to the average voltage of the 3-phase voltages.

Waveform distortion (1)  Waveform distortion is defined as a steady state

deviation from an ideal sine wave at power frequency, principally characterized by the spectral content of the deviation.

 There are 5 primary types of waveform distortion 1) DC offset

2) Harmonics

3) Interharmonics

4) Notching

5) Noise

Waveform distortion (2) Interharmonics: Voltages or currents having frequency components that are not integer multiples of the fundamental frequency.

 Generally the result of frequency conversion and is often not constant, varying with the load.

Notching: Periodic voltage disturbance caused by the normal operation of power electronic devices when current is commutated from one phase to another.

Noise: Unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed on the power system voltage or current.

 Basically consists of any unwanted distortion of the power signal that cannot be classified as harmonic distortion or transients.

Waveform distortion (3)  An example of voltage notching from a 3-phase converter that

produces continuous DC current.

Voltage fluctuation (1)  Voltage fluctuations are systematic variations of the voltage

envelope or a series of random voltage changes.

 An example of a 10-second RMS voltage trend demonstrating voltage fluctuation caused by arc furnace operation.

Voltage fluctuation (2)  IEEE voltage flicker working group has adopted a standard to describe the

potential for visible light flicker through voltage measurements

a) Simulates the lamp/eye/brain transfer function and produces a fundamental metric called the Pst short term flicker sensation

b) Another measure called the Plt long term flicker sensation

A trend of Pst measurements taken at a bus serving an arc furnace load

Power frequency variations (1)  Defined as the deviation of the power system fundamental

frequency from its specified nominal value.  Slight variations in frequency result as the dynamic balance

between load and generation changes.

Power frequency variations (2) Frequency variations that go outside of accepted limits for normal steady- state operation of the PS can be caused by faults on the bulk power transmission system, a large block of load being disconnected, or a large source of generation going off-line.

Sources of transient overvoltages

TRANSIENT OVERVOLTAGES

Internal Sources

Switching surges Power Frequency overvoltages Harmonic Resonance

External Sources e.g. Lightning

E.g. Poor voltage regulation,

ferroresonance etc.

E.g. Utility system is lightly damped

E.g. Switching of capacitor,

transformer etc.

Capacitor switching transients (1)  Shunt capacitors are commonly used for power factor correction

 They can either be always energized or can be switched depending on need.

 Capacitor banks are usually switched at the same time every day.

 Capacitor switching causes malfunction of electronic equipment and adjustable speed drives to trip.

Capacitor switching transients (2) 1) Capacitor switch contacts close at a

point near the system voltage peak. The voltage across the capacitor at this instant is zero.

2) Since the capacitor voltage cannot change instantaneously, the voltage at capacitor location falls to zero and then builds up as capacitor charges to system voltage.

3) As power system source is inductive, the capacitor voltage overshoots and rings at the natural frequency

4) Utility capacitor switching transients are commonly between 1.3 pu to 1.4 pu.

5) The energizing transient is accompanied by inrush current which may be several times more than the normal current peak value

Steady-state voltage rise  If the voltage is low, capacitor provides an

increase to bring the voltage back to tolerable limits.

 If the capacitor is left energized when the load is turned off, the voltage can rise too high, resulting in sustained overvoltage

 In the diagram, applying KVL, the voltage rise is given by:

withoutCswithCs VVV ,, 

sc cap

MVA Q

X XV

3



scMVA 3 is 3-P short circuit capacity at the capacitor bus in kVA or MVA

Issue related to power factor correction  Capacitor banks are used to provide

reactive power in an inductive system. This helps in improving the power factor

 The required compensation from a capacitor bank is given by:

i corresponds to initial power factor

D corresponds to desired power factor

 It must be checked that

a) Voltage will not rise above 110% when load is minimum

b) Harmonic resonance does not occur

)tan(tan diLPQ  

Lightning transient phenomena (1)  An electric discharge occuring in

the atmosphere, possessing the characteristics of high current, short duration and a path length of several miles is known as lightning

 There are four types of cloud-to- ground lightning:

a) Downward negative lightning

b) Downward positive lightning

c) Upward positive lightning

d) Upward negative lightning

 Negative cloud-to-ground lightning is most frequent. A typical flash is made of three or four strokes.

Lightning transient phenomena (2)

Lightning transient phenomena (3) 1) Due to lower region (N) being populated by negative charges, the

region below it (P) gets positively charge. The first discharge is the preliminary discharge within the cloud between N and P regions.

2) This leads to the formation of stepped leader that proceeds to ground in discrete steps as a series of partial discharges.

3) As the leader approaches the ground, sharp objects or uneven portions on the ground itself become sites where the voltage gradient is high enough for breakdown of air to occur.

4) The positive charge moves upwards rapidly along the path of ionization to neutralize partially the negative charge in the cloud. This discharge is known as the first return stroke. Duration of return stroke is about 100µs and the usual peak current value is 30kA.

5) The flash may end after the return stroke current stops. Once the dart leader reaches the ground, a second stroke may take place.

Lightning activity  Keraunic data is the data pertaining to the frequency of

the occurrence of thunderstorms.

 An isokeraunic region is assumed to have roughly the same likelihood of a lightning event occuring at any point within it.

 A more detailed approach of estimating the likelihood of lightning events is through the use of ground flash density (GFD) maps. GFD is the average number of lightning strokes per unit area at a particular location.

 The GFD is useful in determining the exposure of a system to lightning.

References 1. S. Santoso, Fundamentals of Electric Power Quality, 2012.

2. R. C. Dugan, M. F. McGranaghan, S. Santoso, W. Beaty, Electrical Power Systems Quality, McGraw Hill 2012.

3. T. A. Short, Electric Power Distribution Handbook, 2003.

4. J. D. Glover, M. S. Sarma, T. J Overbye, Power System Analysis and Design, 5th Ed., CENGAGE Learning, 2012.

5. T. Gonen, Electric Power Distribution Engineering, 3rd ed., 2014, CRC Press, ISBN 9781482207002.

6. W. H. Kersting, Distribution System Modeling and Analysis, 3rd ed., CRC Press, 2012.

7. IEEE Std. 519-1992.

8. Other sources