understanding power quality

8/4/2019 understanding Power Quality

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Power Quality

Impact of Poor Power Quality

Elements of Power Quality Harmonics

Power Quality Improvement

Harmonic Mitigation


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Power quality is the measurement of howperfect an electrical voltage/current is

at any given point or time The Measurement is in terms of:

Continuity

Magnitude

Frequency

Waveform


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Losses on account of poor power quality

Tangible Losses

Production losses due to power outages Equipment Failure

Intangible Losses

Productivity losses Troubleshooting

Reduced equipment life


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A study was performed by EuropeanCopper Institute in 2001, covering 1400 sitesin 8 countries

Any given site in Europe has 5-20%probability of suffering from one or more ofthe problem listed.

Computer Lockups Flickering screens

Flickering lights.

Overheating of transformers at moderate load


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The Annual losses that Industries in EUface on account of poor power quality is

to the tune of 10 Billion Euros The hourly losses experienced by a few

businesses in the USA

Airline reservation centers: $67,000 -$112,000

ATM network and service fees: $12,000- $17,000

Brokerage (retail): $5.6$7.3 million

Credit card sales authorizations: $2.2$3.1 million

Telephone ticket sales: $56,000$82,000

Catalog sales centers (large retailers):$60,000 - $120,000.


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Utility

Supply quality

Disturbances due to other connected loads Network

Effect of Network impedance

Load Non Linearity


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Voltage Surge

Voltage Swell

Overvoltage Blackout

Voltage Sag

Brownout

Flicker

Harmonics


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Voltage Surge

- Over voltage thatcan reachthousands of volts

- Lasts less than onecycle

- Origin Lightning surges

Switching surges


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Direct Lightning stroke

Indirect lightning stroke

Arrives on the service drop

Nearby stroke

Ground potential rise

Radiation of electromagnetic fields


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Peak transient voltage magnitude = 1.2 to 1.8times normal voltage

Transient oscillating frequency = 300 to 1000000Hz

Surge protecting devices will not operate

Capacitorswitching

surges


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Voltage Swell

Over-voltage that lasts for a few cycles to a fewseconds

20% to 30% increase in line voltage


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Abnormality that remains for relativelylonger duration

Causes

Open neutral

Abrupt reduction of load

Overcompensation by Capacitors

Surge suppressors may not operate, asmagnitude will be less than 2 times


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Voltage sag

Line voltage less than 80% of normal voltage Equipment malfunction or shutdown of control

circuits

Source: faults in the feeders


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Fault at F3

Dip to 0% at Load 3

Dip to 64% at Load 2

Dip to 98% at Load 1

Fault at F1

Dip to 0% at Load 1

50% dip for otherloads


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Sustained under-voltage

Utilities deliberately reduce the voltage to reduce powerdrawn

Brownouts result in

Over heating of compressor motors Loss of microprocessor memory

Reduced motor torque, increased stalling

Overheating of motors (insulation failure)

Tripping of protective devices


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Zero-voltage condition that lastsfor more than two cycles


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Cyclic variation of intensity oflamps

Repetitive sags/fluctuation ofsupply voltage results inflickering

Causes Loads with higher rate of

change of power likewelding, rolling mills, cranes,etc.

Inter-harmonics

Effects

Psychological effects likefatigue and reducedconcentration levels

Disruption in productionprocesses


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Harmonics aresinusoidal componentsof a wave, whosefrequencies are

integral multiples ofthe fundamentalfrequency

For the fundamentalfrequency 50 Hzintegral multiples are2, 3, 4,etc. So, 3rdHarmonic is 3 X 50 Hz =150 Hz


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Current Harmonics through Connectedloads Non linear loads

AC/ DC Drives

CFLs

SMPS

Electronic Dimmers

Voltage harmonics through the Utility Non linear loads connected in nearby

industries


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SMPS

CFL 12 P Drive

6 P Drive


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Total HarmonicDistortion (as % of

fundamental)

Total HarmonicDistortion (as % of

total RMS)


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Amplitude

Frequency

Phase Crest Factor

Ratio of the peak Value to the RMS value


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R Y B

Fundamental +120o 0o -120o

Second harmonic+240o 0o -240o

-120o 0o +120o

Third harmonic+360o 0o -360o

0o 0o 0o

Positive sequence

Negative sequence

Zero sequence

Y

R

BR

B

YR

Y

B

Positive sequence Negative sequence Zero sequence


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Ratio

Iscc / Iload

Harmonicodd

numbers(35)

ITHD

< 20 4.0 % 0.3 % 5.0 %

20 - 50 7.0 % 0.5 % 8.0 %

50 - 100 10.0 % 0.7 % 12.0 %

>1000 15.0 % 1.4 % 20.0 %

IEEE 519-1992 Standard:


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Triple-N harmonics are additive

Neutral current may rise up to 150% to 210% of the phasecurrent

Heating of neutralconductors

At high frequency eddy current will be high

Increased neutral current because of triple-N harmonics

In delta circulating currents will be more (3N harmonics)

Overheatingtransformers

Heating because of eddy current, skin effect

Uneven torque results in mechanical damage of shaftOn motors

Because of increased voltage drop along the lineLow voltage at

end loads


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Harmonic current results in harmonicvoltage drop, which distorts the supplyvoltage

Distorted voltage

Higher harmonic frequencies may interferewith the data and communication line thatare laid near

Communicationproblems

Capacitors offer low impedance for higherfrequencies

So heavy harmonic current results in capacitordamage

Over-stressing of PFC

capacitors

Most portable do not measure true RMS value andcan underestimate non-sinusoidal current by 40%

Frequent tripping of relays and circuit breakers

Currentmeasurement

problems


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DC current flow Low frequency

AC current flow

High frequency

AC current flow

The effective area of the conductor, available for current flow,reduces as the frequency of the AC current increases. Hence, the

resistance of the conductor increases, as the frequency increases


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Trip level set lower than the fundamental value.

The relay should trip as the fundamental value

is higher than the trip level. But the presence of

harmonics has reduced the peak value. Hence

the protective relay will not trip.

Mal-operation

Triplevel set higher than the fundamental value.

The relay should not trip as the fundamental

value is lower than the trip level. But the

presence of harmonics has increased the peak

value. Hence the protective relay will trip.

Nuisance tripping


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Capacitive Reactance is inverselyproportional to the supply frequency

Offers a low impedance path to theharmonic currents resulting in harmonicamplification

Can be observed through the change inTHD levels before and after switching oncapacitor banks


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Resonanceoccurs when

XL=X

C

Maximumcurrent flow

Impedancewill be

minimum


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Resonanceoccurs when

XL=X

C

Minimumcurrent flow

Impedancewill be

Maximum


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Surgesuppressor

Voltage surges

VoltageregulatorsVoltage swellVoltage sag

Brownout

Flicker

UPS Voltage swellVoltage sag

BlackoutBrownout

Generators Blackout

Brownout


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Disturbingequipment

Sensitiveequipment

Disturbingequipment

Sensitiveequipment

Disturbingequipment

Sensitiveequipment

Not recommended

Transformer

Better Excellent


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Harmonicfilter

Passiveharmonic

filter

Detunedfilter

7% 14%

Tunedfilter

Activeharmonic

filterHybrid filter


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Harmonic filter comprises of a Reactor (L) in series with aCapacitor (C)

Such a filter has a unique resonance frequency FR at whichinductive reactance of reactor equals capacitivereactance of capacitor. FR = 1/(2LC)

Below FRthe filter is capacitive

Above FRthe filter is inductive


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Resonance Frequency FR< 90% of lowest dominantHarmonic frequency

Detuned or Harmonic Suppression Filters

Resonance frequency FR within 10% of thefrequency of the Harmonic to be absorbed

Tuned or Harmonic Absorption Filters


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Standard detuned filters have a fixed percentage tuningfactor p

Percentage tuning factor %P= (XL /XC )*100

Standard detuned filters are available for 7% tuning factor

The resonant frequency of the filter fR is related to tuningfactor p by fR = fS/ (p/100) = 189 Hz for 7% filter


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Linearity

One of the most important factor thataffects the performance of a filter

Linearity: Change in inductance/Change incurrent

Q factor

Ratio of the Tuning frequency to theBandwidth


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understanding power quality

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