power quality harmonics

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POWER QUALITY HARMONICS NOVIE AYUB WINDARKO DAN YAHYA CHUSNA A

13-Nov-2014, Grand Mirama Hotel, Surabaya

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PLAN OF PRESENTATION

1. DEFINITION2. HARMONICS PROBLEMS3. HARMONIC SOURCES4. HARMONIC STANDARDS5. HARMONIC MITIGATION6. CONCLUSIONS

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Definition of Harmonics

Harmonics are defined as currents or voltages with frequencies that are integer multiples of the fundamental power frequency

SIMPLY PUT - Harmonics are used to mathematically describe the shape of a curve that is not sinusoidal.

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What is Harmonic Distortion?

Harmonic Distortion is a mathematical way of describing how non-sinusoidal a wave shape appears

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What is Harmonic Distortion? Fourier Analysis - Sum of the Squares

1

cos)(h

hho thVVtV

% υTHD

100V

V

1

2h

2h

Percentage of the Total Harmonic Distortion of a nonsinusoidal voltage waveform

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What is Harmonic Distortion? Fourier Analysis of the

waveforms found in a three phase diode rectifier shows low order harmonics including the 5th, 7th, 11th , 13th, etc.

Calculation of true power factor considers the energies contained on these additional frequencies. Bottom figure shows the resulting harmonic spectrum based on Fourier analysis of the current waveform shown in top figure.

Three phase diode rectifier, line voltage/ current

-700-600-500-400-300-200-100

0100200300400500600700

Voltage Current

100.00%

30.38%

5.55% 7.16%4.83% 4.32% 3.59%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Magnit

ude (as

% o

f Fundam

enta

l

Harmonic Order

Normalized Harmonic Spectrum

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Harmonics Problems

Increased Transformer Heatingrecommended K-Factor of 4 to 13 on new installations

Increased Conductor Heatinglarger gauge wirerun two wires in parallel

Electromagnetic EquipmentPLCs - more sensitive to Voltage Notching

System resonance - Power Factor Correction utilize input reactors to reduce likelihood of

resonance Lower Power Factor for System PF PF Power Power Power PowerTrue Total al al act Harmonics Re Re Re ./

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Harmonics and transformers

Load

Many transformers are rated by “K factor” which simply describes

their ability to withstand harmonics.Transformers may also be

deratedto compensate for the additionalheating caused by harmonics.Improved transformer

designs have also been developed, with oversized neutral busses, special cores, and specially designed coils.

Load

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Harmonics and transformers K-Factor

 In is the amplitude of the nth harmonic current normalized

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Harmonics and power-correction capacitors

Power correction capacitors can cause series and parallel resonance effects on a power system.

If a harmonic is generated which excitesa system resonance, amplification of thatharmonic may occur.

Calculation of the harmonic resonance frequency for thesystem can give an indication of potential resonance problems.

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Harmonics and power-correction capacitors

EXAMPLE:

Assume a 1500 kVA supply xfmr, with a 5.75% impedance.

Also assume 600 kVA of powercorrection capacitors on the system

1500 kVA5.75%

600 kVAC

The harmonic resonance frequency is defined by:

= hr =

kVAsc

kVAC

1500 / 0.0575 = 6.6

600

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How Harmonics Lower Efficiency

Consider estimating power factor at the terminals of an AC Drive in a system with low source impedance (high available short circuit current) with no input line reactor or DC bus choke.

-500

-400

-300

-200

-100

0

100

200

300

400

500

pf = kW/kVA

I THD = 92.8%

pf = 1/Sqrt(12+.9282)

pf = 73.3%

Power Factor Considering 92.8% I THD

True factor is improved, when current distortion is limited by system impedance. (Including reactors, or bus chokes.)

-500

-400

-300

-200

-100

0

100

200

300

400

500

pf = kW/kVA

I THD = 32.6%

pf = 1/Sqrt(12+.3262)

pf = 95.08!

Power Factor Considering 32.6% I THD

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Power Factor When Harmonics Exist

Apparent Power

Distortion Factor

Power Factor

True Power Factor Representation - Expanded

Appa

rent

Pow

er (k

VA)*

P Real Power (kW)

Q R

eact

ive

Pow

er

X (

kVA

r)

D Distortio

n

Figure 13.1

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a) Current Source nonlinear load

Diode rectifier for ac drives, electronic equipment, etc

HARMONIC SOURCES

Thyristor rectifier for dc drives, heater drives, etc.

Per-phase equivalent circuit of thyristor rectifier

b) Voltage source nonlinear load

Per-phase equivalent circuit of diode rectifier

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INPUT CURRENT OF DIFFERENT NONLINEAR LOADS

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Harmonics Standard

• IEEE 519-1992 United States Standards on harmonic limits

- IEEE limits service entrance harmonics.- The IEEE standard 519-1992 limits the level of harmonics at the

customer service entrance or Point of Common Coupling (PCC).- With this approach the costumer’s current distortion is limited based on

relative size of the load and the power supplier’s voltage distortion based on the voltage level

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The IEEE standard 519-1992

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Harmonics Mitigations

Inductive Reactance

Method: Add a line reactor or isolation transformerto attenuate harmonics.

Benefits: Low cost.

Technically simple.

Concerns: Tends to offer reductions in only higherorder harmonics. Has little effect on the 5th and 7th harmonics.

Because of the associated voltage drop, there are limits to the amount of reactance that may be added.

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Harmonics Mitigations

Passive Filters

Method: Provide a low-impedance path to groundfor the harmonic frequencies.

Benefits: May be tuned to afrequency between two prevalent harmonicsso as to help attenuate both.

Concerns: Tuning the filters may be a labor-intensive process.

Filters are difficult to size, because they offera path for harmonics from any source.

Quite sensitive to any future system changes.

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Harmonics Mitigations

Passive Filters

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Harmonics Mitigations

Active Filters

Method: Inject equal and opposite harmonics onto thepower system to cancel those generated by other equipment.

Benefits: Have proven very effective in reducingharmonics well below required levels.

Concerns: The high performance inverter required for the harmonic injection is costly.

Power transistors are exposed to conditions of the line, so reliability may be a problem.

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Active Filters

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Harmonics Mitigations

12-pulse Rectifiers

Method: Two separate rectifier bridges supply a singleDC bus. The two bridges are fed from phase-shifted supplies.

Benefits: Very effective in the elimination of 5th and 7th

harmonics.Stops harmonics at the source.Insensitive to future system changes.

Concerns: May not meet the IEEE standards in everycase.Does little to attenuate the 11th and 13th harmonics.

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Harmonics Mitigations

18-pulse Rectifier

Method: An integral phase-shift transformer and rectifier Input which draws an almost purely sinusoidalwaveform from the source.

Benefits: Meets the IEEE standards in every case!

Attenuates all harmonics up to the 35th.

Stops harmonics at the source.

Insensitive to future system changes.

Concerns: Can be expensive at smaller HP’s

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Comparison of waveforms

6-pulse converter

12-pulse converter

18-pulse converter

note the level of distortionand steep current rise.

the waveform appears moresinusoidal, but still not very smooth.

virtually indistinguishablefrom the source currentwaveform.

Terimakasih