PQ-Classes and Causes

POWER SYSTEMS QUALITY Definitions and Classes

What is Power Quality?  Any

Power problem manifested in voltage, current, or frequency deviations that results in failure or misoperation of customer equipment

 Power

Quality = Voltage Quality

1

What is Power Quality?  Any

Power problem manifested in voltage, current, or frequency deviations that results in failure or misoperation of customer equipment

 Power

Quality = Voltage Quality

1

Types Of Power Quality Disturbances (per IEEE 1159) 1.

Transients

2.

Short-Duration Voltage Variations

3.

Long-/Short-Duration Voltage Variations

4.

Voltage Imbalance

5.

Waveform Distortion

6.

Voltage Fluctuation

7.

Power Frequency Variations

DC Offset Harmonics Interharmonics Notching

2

Causes of Power Quality Problems Georgia Power Survey, mid 1990’s 

Customer Perception  Natural

60%

 Utility

17%

 Customer

12%

 Neighbor

8%

 Other

3% 3

Why are we Concerned about Power Quality?  The

ultimate reason is economic value

Economic impacts on utilities Economic impacts on customers Economic impacts on supplier of load equipment

 $$$$$$$$$$$  $$$$$$$$$ $$ 4

1. Transients 

Transient: undesirable event and momentary 1.1 Impulsive Transient 1.2 Oscillatory Transient Possible Causes



Possible Effects



PF cap energization



Data corruption



Lightning



Equipment damage



Loose connection



Data transmission errors



Load or source switching



Intermittent equipment operation



RF burst



Reduced equipment life



Irreproducible problems

5

1. Transients Unipolar  Positive

Bipolar  Notching

Oscillatory

200

100

0

-100

-200

Negative

Multiple Zero Crossings 6

1.1 Impulsive Transients 

Sudden non-power frequency change in the steady-state condition of voltage, current, or both.



Unidirectional in polarity (either +Ve or -Ve).



Characterized by their rise and decay times.

(1.2/50 s Lightning Surge, 250/2500 Switching Surge)

7

Lightning Stroke Impulsive Current

8

1.2 Oscillatory Transients 

Sudden non-power frequency change in the steadystate condition of voltage, current, or both, that

includes both +Ve and -Ve polarity values. 

Frequency Spectral Content Subclasses: High: > 500 kHz (s duration)  Result of system response to an impulsive transient.

Medium: 5 -500 kHz (tens of s duration)  Back-to-Back Capacitor Energization  Cable Switching 9

Medium Frequency Oscillatory Transient Current

10

1.2 Oscillatory Transients  Low: < 5 kHz (0.3 ms to 50 ms duration) 

Frequently encountered on Distribution Systems



Typical peak magnitude between 1.3 - 1.5 pu



Caused by many types of events  Capacitor Bank Energization

(300 and 900 Hz frequency Oscillations)

11

Low Frequency Oscillatory Transient Current

12

Low Frequency Oscillatory Transient Current

Upstream Capacitor Switching 13

2. SHORT DURATION RMS Voltage Variations

 Depending on the fault locations and the system conditions  Voltage

drops (sags)



Voltage rises (swells)



Complete loss of voltage (interruptions)

14

RMS Variations IEEE1159 Characterizations Short

Duration (below 1 minute)

 Instantaneous (0.5 - 30 cycles)  Sag (0.1 - 0.9 pu)  Swell (1.1 - 1.8 pu)  Momentary (30 cycles - 3 sec)  Interruption (< 0.1 pu, 0.5 cycles - 3s)  Sag

 Swell  Temporary (3 sec - 1 minute) Long

Duration (below 1 minute)

Overvoltages Undervoltages Sustained Interruptions

15

RMS Voltage Variations

Sag   Swell   Interruption 100

0

-100

16

RMS Voltage Variations Causes and Effects

Possible Causes Sudden



change in load current

Possible Effects 

Process interruption



Fault on feeder 



Data loss



Fault on parallel feeder 



Data transmission errors



Motor start



PLC or computer misoperation



Undersized distribution



Damaged product



Motor failure

system

17

2.1 Voltage Sag ( Dip) 

A sag is a decrease to between 0.1 pu and 0.9 pu in rms voltage or current at the power frequency for duration from 0.5 cycles to one minute (short duration undervoltage).

 Voltage sags are usually associated with system faults  Energization of heavy loads  Starting of large motors 18

Causes of Voltage Sags 

Voltage Sags are caused by faults on the power system. 

Lightning



Cable failures



Cable dig-ins



Equipment failures



Animals



Trees



Insulator contamination



etc. 19

2.1 Voltage Sag

Voltage sag as a result of a SLG fault on another feeder 20

Temporary Voltage Sag

(3 sec - 1 minute)

21

Temporary Voltage Sag Motor Starting Timeplot Chart

 Volts

 Amps

222.5

900 CHA Vrms CHA Irms

Min 206.11 1.40

Max Median 222.25 219.19 847.71 207.16

800 220.0

700 217.5 600

215.0

500

400

212.5

300 210.0 200

207.5 100

205.0 09:49:00.5

09:49:01.0

09:49:01.5

09:49:02.0 CHA Vrms

09:49:02.5 CHA Irms

09:49:03.0

09:49:03.5

0 09:49:04.0

22

2.2 Voltage Swell 

A swell is an increase to between 1.1 pu and 1.8 pu in rms voltage or current at the power frequency for duration from 0.5 cycles to one minute (short duration, momentary, overvoltage).

Voltage swell are usually associated with system

faults  Switching off a large load  Energizing a large capacitor bank  23

2.2 Voltage Swell

Voltage rise on the unfaulted phases during a SLG fault 24

2.2 Voltage Swell  Severity of voltage swell during a fault condition is function of:  Fault Location System impedance  System Grounding  VLG = 1.73 pu on ungrounded system

 No voltage rise on grounded system  A 15% swell due to faults on 4-wire, multi-

grounded feeders 25

2.3 Voltage Interruptions 

An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 minute



Voltage Interruptions can be the result of:  Power System Faults  Equipment Failures  Control Malfunctions

26

2.3 Voltage Interruptions 

|Voltage| interruption < than 10% of nominal



The duration of an interruption due to a fault is determined by the operating time of protective devices 

Instantaneous reclosing limit interruption to < 30 cycles



Momentary (30 cycles - 3 sec) caused by delayed reclosing of the protective device





Temporary interruption (3 sec - 1 minute)

Equipment malfunctions or loose connections can produce irregular duration of an interruption 27

Momentary Interruption

(30 cycles - 3 sec)

28

3. LONG DURATION VOLTAGE VARIATIONS



Variations encompass rms deviations at power frequencies for longer than one minute.



Forms of long duration variations:  Overvoltages

}

 Undervoltages  Sustained

 Load variation on the system  System switching operations

Interruptions 29

3.1 Overvoltages 

An overvoltage is an increase in the rms ac voltage greater than 110% at the power frequency for a duration longer than 1 minute.



Causes of overvoltages: 

Switching off a large load



Energizing a capacitor bank 



System is too weak for the desired voltage regulation



Voltage controls are inadequate



Incorrect tap settings on transformers 30

3.2 Undervoltages 

An undervoltage is a decrease in the rms ac voltage to less than 90% at the power frequency for a duration longer than 1 minute.



Causes of undervoltages: 

Switching on a load



Switching off a capacitor bank 



Overloaded circuits

31

3.3 Sustained Interruptions 

Supply voltage has been zero for a period of time in excess of 1 minute (or permanent), and requires human intervention to repair the system for restoration.

32

4. VOLTAGE IMBALANCE 

Voltage imbalance is defined as the maximum deviation from the average of the three phase voltages or currents, divided by the average of the three phase voltages or currents, expressed in %.

Voltage Unbalance Factor (%) 



 Maximum Voltage  Deviation From  Average Voltage  Average Voltage

V 

 LL max

 V  LLavg

V  LLavg



 100

33

 100

Phase Voltage/Current Unbalance Unbalance Factor Example For example, if a 480-V application shows voltages  V   AB = 473 V V   BC = 478 V V CA = 486 V,  V   avg = (473 + 478 + 486)/3= 479 V, The maximum deviation from that average is (486 - 479 = 7 V) Voltage unbalance Factor

Voltage Unbalance Factor (%) 

486  479

479  1.46%

 100 

7 479

 100

The most apparent effects of voltage unbalance are decreased motor efficiency and performance.. 34 34

4. VOLTAGE IMBALANCE 

Alternatively, imbalance is defined as the ratio of either the -Ve or 0-sequence component to the +Ve sequence.

Voltage Unbalance Factor  

Voltage Unbalance Factor  

V  V 





V 0 V 







 Negative Sequence Component  Positive Sequence Componen

 Negative Sequence Component  Positive Sequence Componen

35

Imbalance Trend for a Residential Feeder

36

4. VOLTAGE IMBALANCE 

Causes of voltage imbalances: 

Single-phase loads on a three-phase circuit



Unequal loading



Blown fuses in one phase of a 3-phase capacitor bank 



Unequal source voltage



Unbalanced fault

37

5. WAVEFORM DISTORTION 

Waveform distortion is a steady state deviation from an ideal sine wave of power frequency.



Types of waveform distortion: 

DC Offset



Harmonics



Interharmonics



Notching



Noise 38

5.1 DC Offset  DC

offset is the presence of a dc voltage or current

in an ac power system. 

Causes of dc offset: Effect



of half-wave rectification

Effects: 

Biasing transformer cores (saturation)



Heating and loss of transformer life

39

5.2 Harmonics   Harmonics

are sinusoidal voltages or currents having frequencies that are integer multiples of the  fundamental frequency; 50 or 60 Hz.



Distorted waveforms can be decomposed into a sum of the fundamental frequency and the harmonics



Harmonic distortion originates in the nonlinear loads and power electronics devices on the power system



Harmonic distortion levels are described by the harmonic spectrum with magnitudes and phase angles of each individual harmonic component



Total Harmonic Distortion (THD), is used as a measure of the effective value of harmonic distortion 40

Harmonic Decomposition of Distorted Waveform

Waveforms  Amps 200

150

100

50

0

-50

-100

-150

-200 14:34:42.20

14:34:42.21

14:34:42.22

14:34:42.23

14:34:42.24

14:34:42.25

14:34:42.26

14:34:42.27

CHA Amps

Waveform event at 03/22/1999 14:34:42.480 File: C:\DranView\Q_DEMO Cycle by cycle harmo nics.dnv

41

Linear Voltage / Current No Harmonic Content

voltage current

Harmonic Content

voltage

current

• Effects of Harmonics: increasing line losses  overloading the neutral conductor  Lower the system PF 

42

Current Waveform and Harmonic Spectrum of ASD

43

5.3 Interharmonics   Interharmonics

are sinusoidal voltages or currents having frequencies that are not integer multiples of the fundamental frequency. 

Interharmonics can appear as discrete frequencies or as a wide-band spectrum



Interharmonics can be found in networks of all voltage classes

44

5.3 Interharmonics 



Sources of interharmonic waveform distortion: 

Static frequency converters



Cyclo-converters



Induction motors



Arcing devices



Power line carrier signals

Effects of Interharmonics:  

Affect power line carrier signaling Induce visual flicker in display devices such as CRTs 45

5.4 Notching  Notching

is a periodic voltage disturbance caused by the normal operation of power electronics devices when current is commutated from one phase to another (momentary short circuit ).



The frequency components associated with notching can be quite high and may not be readily characterized with measurement equipment normally used for harmonic analysis

46

Voltage Notching

47

5.5 Noise  Noise

is defined as unwanted electrical signals

with broadband spectral content lower than 200 kHz superimposed upon the power system voltage or current in phase conductors, or found on neutral conductors or signal lines.

48

5.5 Noise 

Sources of Noise: 

Electronic devices



Control circuits



Arcing equipment



Loads with solid-state rectifiers



Switching power supplies



Improper grounding 49

5.5 Noise 

Effects of Noise: 



Noise disturbs microcomputers and PLCs

Mitigation of Noise: 

Filters



Isolation transformers



Line conditioners

50

6. VOLTAGE FLUCTUATIONS 

Voltage fluctuations (flicker) are systematic variations of the voltage envelope or a series of random or continuous voltage changes, the magnitude of which does not normally exceed the voltage ranges of 0.9 pu 2.5

to 1.1 pu.

2

Voltage flicker signal

Voltage flicker envelope

1.5 1 0.5    )    V    (    )    t    (   v

0 -0.5 -1 -1.5

ESPRIT estimated Voltage flicker signal

-2 -2.5

0

0.02

0.04

0.06

0.08

0.1 0.12 Time (s)

0.14

0.16

0.18

0.2

51

6. VOLTAGE FLUCTUATIONS 

The voltage flicker signal is defined by its rms magnitude expressed as a percent of the fundamental (Vrms/V1).



Magnitudes as low as 0.5% can result in perceptible lamp flicker if the frequencies are between 6-8 Hz



Voltage fluctuations are caused by loads which can exhibit continuous, rapid variations in the load current magnitude (arc furnaces, welding machines, etc) 52

Voltage Flicker

53

7. POWER FREQUENCY VARIATION 

 Power Frequency Variations are defined as the deviation of the power system fundamental frequency from it specified nominal value (e.g. 50 Hz or 60 Hz).



The power system frequency is directly related to the rotational speed of the generators supplying the system.



There are slight variations in frequency as the dynamic balance between load and generation changes.



Frequency variations that go outside of accepted limits for normal steady state operation of the power system 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. 54