pq-classes and causes

7/24/2019 PQ-Classes and Causes

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POWER SYSTEMS QUALITYPOWER SYSTEMS QUALITY

Definitions and ClassesDefinitions and Classes


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What is Power Quality?What is Power Quality?

Any Power problem manifested in

voltage, current, or frequency deviationsthat results in failure or misoperation of

customer equipment

Power Quality = Voltage Quality


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Types Of Power Quality Disturbances

(per IEEE 1159)

1. Transients2. Short-Duration Voltage Variations

3. Long-/Short-Duration Voltage Variations4. Voltage Imbalance

5. Waveform Distortion6. Voltage Fluctuation

7. Power Frequency Variations

DC Offset

HarmonicsInterharmonics

Notching


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Why are we Concerned about PowerWhy are we Concerned about Power

Quality?Quality?

The ultimate reason is economic value

Economic impacts on utilities

Economic impacts on customers

Economic impacts on supplier of load equipment

$$$$$$$$$$$

4


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1.1. TransientsTransients

Transient: undesirable event and momentary

1.1 Impulsive Transient

1.2 Oscillatory Transient

Possible Causes

PF cap energization

Lightning

Loose connection

Load or source switching

RF burst

Possible Effects

Data corruption

Equipment damage

Data transmission errors

Intermittent equipment operation

Reduced equipment life Irreproducible problems


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1. Transients

-200

-100

0

100

200

Unipolar

Positive

Negative

NotchingOscillatory

Multiple Zero Crossings

Bipolar


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1.11.1 Impulsive TransientsImpulsive 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)


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Lightning Stroke Impulsive Current


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1.21.2 Oscillatory TransientsOscillatory Transients

Sudden non-power frequency change in the steady-

state 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


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Medium Frequency

Oscillatory Transient Current


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1.21.2 Oscillatory TransientsOscillatory 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)


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Low Frequency Oscillatory Transient Current


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Upstream Capacitor Switching

Low Frequency Oscillatory Transient Current


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2.2. SHORT DURATIONSHORT DURATION

RMS Voltage Variations

Depending on the fault locations and the system conditions

Voltage drops (sags)

Voltage rises (swells)

Complete loss of voltage (interruptions)


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RMS Variations

IEEE1159 Characterizations

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)

Short Duration (below 1 minute)

Long Duration (below 1 minute)Overvoltages

Undervoltages

Sustained

Interruptions


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0

Sag Swell Interruption

100

-100


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Possible Causes

Sudden change in load current

Fault on feeder

Fault on parallel feeder

Motor start

Undersized distribution

system

Possible Effects

Process interruption

Data loss

Data transmission errors

PLC or computer misoperation

Damaged product

Motor failure


Causes and Effects


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2.1 Voltage Sag (Dip)

Asag is a decrease to between 0.1 pu and 0.9 pu

in rms voltage or current at the powerfrequency 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


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Causes of Voltage SagsCauses 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.


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2.12.1 Voltage Sag

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


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Temporary Voltage Sag (3 sec - 1 minute)


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Motor Starting Timeplot Chart

09/13/96 09:49:00.50 - 09/13/96 09:49:04.00

Min Max Median

CHA Vrms 206.11 222.25 219.19

CHA Irms 1.40 847.71 207.16

CHA Vrms CHA Irms

09:49:00.5 09:49:01.0 09:49:01.5 09:49:02.0 09:49:02.5 09:49:03.0 09:49:03.5 09:49:04.0

Volts

205.0

207.5

210.0

212.5

215.0

217.5

220.0

222.5

Amps

0

100

200

300

400

500

600

700

800

900

Temporary Voltage Sag


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2.22.2 Voltage SwellVoltage Swell

Aswell is an increase to between 1.1 pu and1.8 pu in rms voltage or current at the powerfrequency for duration from 0.5 cycles to oneminute (short duration, momentary,

overvoltage).

Voltage swell are usually associated with system

faults Switching off a large load

Energizing a large capacitor bank


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2.22.2 Voltage Swell

Voltage rise on the unfaulted phases during a SLG fault


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2.32.3 Voltage InterruptionsVoltage Interruptions

An interruption occurs when the supply voltage

or load current decreases to less than 0.1 pu fora period of time not exceeding 1 minute

Voltage Interruptions can be the result of:

Power System Faults

Equipment Failures

Control Malfunctions


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2.32.3 Voltage InterruptionsVoltage 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


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Momentary Interruption (30 cycles - 3 sec)


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3.23.2 UndervoltagesUndervoltages

An undervoltage is a decrease in the rms ac voltageto 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


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3.33.3 Sustained InterruptionsSustained 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.


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4.4. VOLTAGE IMBALANCEVOLTAGE 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 %.

100

100(%)

max

LLavg

LLavgLL

V

VV

VoltageAverage

VoltageAverageFromDeviationVoltageMaximumFactorUnbalanceVoltage


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Phase Voltage/Current Unbalance

%46.1

100479

7100

479

479486(%)

FactorUnbalanceVoltage

Unbalance Factor Example

For example, if a 480-V application shows voltages

VAB = 473 V

VBC = 478 V

VCA = 486 V,

Vavg = (473 + 478 + 486)/3= 479 V,

The maximum deviation from that average is (486 - 479 = 7 V)

Voltage unbalance Factor

The most apparent effects of voltage unbalance are decreased motor

efficiency and performance..34


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4.4. VOLTAGE IMBALANCEVOLTAGE IMBALANCE

Alternatively, imbalance is defined as the ratio of

either the -Ve or 0-sequence component to the +Ve

sequence.

ComponenSequencePositive

ComponentSequenceNegative

V

VFactorUnbalanceVoltage

ComponenSequencePositive

ComponentSequenceNegative

V

VFactorUnbalanceVoltage

0


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Imbalance Trend for a Residential Feeder


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5.25.2 HarmonicsHarmonics

Harmonics are sinusoidal voltages or currents havingfrequencies that are integer multiples of the

fundamental frequency; 50 or 60 Hz.

Distorted waveforms can be decomposed into a sum ofthe 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 eachindividual harmonic component

Total Harmonic Distortion (THD), is used as a measure of

the effective value of harmonic distortion


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Harmonic Decomposition ofDistorted Waveform

Waveforms

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

CHA Amps

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

Amps

-200

-150

-100

-50

0

50

100

150

200


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Linear Voltage / Current

voltage

current voltage

current

No Harmonic Content

Harmonic Content

Effects of Harmonics:

increasing line losses

overloading the neutral conductor

Lower the system PF


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Current Waveform andHarmonic Spectrum of ASD


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5.35.3 InterharmonicsInterharmonics

Interharmonics are sinusoidal voltages or currents

having frequencies that are not integer multiples of

thefundamental frequency.

Interharmonics can appear as discrete frequencies

or as a wide-band spectrum

Interharmonics can be found in networks of all

voltage classes


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5.35.3 InterharmonicsInterharmonics

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


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5.45.4 NotchingNotching

Notching is a periodic voltage disturbancecaused by the normal operation of power

electronics devices when current is commutatedfrom one phase to another (momentary shortcircuit ).

The frequency components associated with notchingcan be quite high and may not be readily

characterized with measurement equipmentnormally used for harmonic analysis


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Voltage Notching


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5.55.5 NoiseNoise

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.


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5.55.5 NoiseNoise

Effects of Noise:

Noise disturbs microcomputers and PLCs

Mitigation of Noise:

Filters

Isolation transformers

Line conditioners

66 VOLTAGE FLUCTUATIONSVOLTAGE FLUCTUATIONS


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6.6. VOLTAGE FLUCTUATIONSVOLTAGE FLUCTUATIONS

Voltage fluctuations (flicker) are systematic variations

of the voltage envelope or a series of random or

continuous voltage changes, the magnitude of whichdoes not normally exceed the voltage ranges of 0.9 pu

to 1.1 pu.

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

Time (s)

v

(t)(V)

Voltage flicker envelopeVoltage flicker signal

ESPRIT estimated Voltage flicker signal


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6.6. VOLTAGE FLUCTUATIONSVOLTAGE 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 canexhibit continuous, rapid variations in the load current

magnitude (arc furnaces, welding machines, etc)


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Voltage Flicker


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7.7. POWER FREQUENCY VARIATIONPOWER 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.

H PQ bl b d d?H PQ bl b d t t d?


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The following symptoms are indicators of PQ problems:

A piece of equipment misoperate at the same time ofday

Circuit breakers trip without being overloaded

Equipment fails during a thunderstorm

Automated systems stop for no apparent reason

Electronic system fail or fail to operate a frequentbasis

Lights dim or blink and electronic system misoperate

Overheating of transformers and motors

Blowing of feeders capacitors fuses

Failures of capacitors

How can PQ problems be detected?How can PQ problems be detected?

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