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Applied Harmonics

Control of Harmonics

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IEEE Standard 519-1992

• Limit harmonic current injections from end users so that harmonic voltage distortion is tolerable.

• Limit harmonic voltage (responsibility of utility).

Harmonic Distortion Evaluations

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other customers

customerunderstudy

utility systemPCC

other customers

customerunderstudy

utility systemPCC

IL

IL

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• End users are limited at PCC in terms of – individual harmonic components and– total demand distortion

• Utility is mainly responsible for limiting voltage distortion at PCC

• Evaluations:– Measurement of currents injected by load

(over one week period)– Calculation of frequency response of system

impedances (using harmonic calculation software)

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%100I

ITDD

:before) (ascurrent for distortion demand Total

%100V

VHDT

: voltagerms nominal system the with

normalized is voltageof distortion harmonic Total

L

1h

2h

n

1h

2h

Vn

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Bus Voltage at PCC, Vn

[kV,LL]

Individual harmonic voltage

distortion

Total voltage

distortion THD_Vn

Vn < 69 3.00% 5.00%69<Vn<161 1.50% 2.50%

161<Vn 1.00% 1.50%

Table 6.1 Harmonic voltage distortionlimits in % of nominal fundamental-frequency voltage

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Vn < 69 kVIsc/IL h<11 11<h<17 17<h<23 23<h<35 35<h TDD0-20 4.0% 2.0% 1.5% 0.6% 0.3% 5.0%

20-50 7.0% 3.5% 2.5% 1.0% 0.5% 8.0%50-100 10.0% 4.5% 4.0% 1.5% 0.7% 12.0%

100-1000 12.0% 5.5% 5.0% 2.0% 1.0% 15.0%>1000 15.0% 7.0% 6.0% 2.5% 1.4% 20.0%

69<Vn <161 kVIsc/IL h<11 11<h<17 17<h<23 23<h<35 35<h TDD0-20 * 2.0% 1.0% 0.75% 0.3% 0.15% 2.5%20-50 3.5% 1.75% 1.25% 0.5% 0.25% 4.0%50-100 5.0% 2.25% 2.0% 0.75% 0.35% 6.0%

100-1000 6.0% 2.75% 2.5% 1.00% 0.50% 7.5%>1000 7.5% 3.50% 3.0% 1.25% 0.70% 10.0%

161 kV<VnIsc/IL h<11 11<h<17 17<h<23 23<h<35 35<h TDD0-50 2.0% 1.0% 0.75% 0.3% 0.15% 2.5%>50 3.0% 1.50% 1.15% 0.5% 0.22% 3.8%

* line applies to all power generation equipment regardless of Isc/IL ratio

Table 6.2 Harmonic current limits

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• Ih is rms magnitude of individual harmonic current

• Isc is the short-circuit current at PCC

• IL is the fundamental component of the maximum demand current (average max demand over 12 months)

• Individual limits apply to odd-order harmonics, even order limits are at 25% of indicated value

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• For power converters with more than 6 pulses, where q = pulse number, multiply limits in table 6.2 by

kV in LL voltage nominalkV

factorpower billed average pf

kW in demand billed averagekWD

kV3pf

kWDI

.2by multiply then 12, q if example,For

rated

rated

L

6/q

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Controlling harmonics

• Control only when harmonics create a problem. Types of problems:– load harmonic currents are too large– path for harmonic currents is too long

electrically (too much impedance) producing voltage distortion or communication-line interference

– response of system magnifies one or more harmonics

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Options for control• Reduce magnitude of harmonic currents

from load• Add filters to do one or more of these:

– short out (siphon off) the harmonic– block harmonic currents from entering part of

the system– supply the harmonics locally

• Modify the frequency response of the system by filters or other means

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Reducing load harmonic current

• Sometimes transformer connections can be changed, for example:– phase shift on some transformers supplying

6-pulse converters– delta windings block triplen currents– zig-zag transformers can supply triplens

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N

AB

C

N

A B C

Zig-zag transformer

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Zig zag transformer

• Place the transformer to supply balanced triplen harmonics (and any other zero-sequence currents) to load

• This will unload zero sequence currents on circuits upstream of the ZZ transformer, with little or no effect downstream

• Fault study results may be affected

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Placement of ZZ transformer

I3

Unloads the neutral conductor upstream

3I3n

ab

c

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Filtering• Shunt passive filter: short-circuit harmonic

currents close to their source

• Series passive filter: block harmonic currents from power delivery system (may cause large load voltage distortion)

• Active shunt filter: electronically supply low-order harmonics to a nonlinear load (used with simple passive filters for higher frequency components).

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Passive Filters• Shunt passive filters are

– notch filters, tuned to a specific harmonic frequency, or

– high-pass filters

notch filter high-passfilter

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Modifying system frequency response

• Add a shunt filter to the system

• Add a reactor (e.g., in series with a pf correction capacitor) to de-tune system

• Change pf correction capacitor size or placement, or remove capacitor bank entirely

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Where to control harmonics

• On utility system or end-user facility– utility system is more difficult to filter unless

we can move or change the size of or reconnect a capacitor

– end-user system may be easier to filter if we can access the feeder(s) where the harmonic currents are being produced

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Filters• In-line reactors (or chokes) for ASD

• Isolation transformers can help

• Isolation transformers can be reconnected with different phase shifts

XsXt

ASDM

reactor 0-5%on ASD kVA

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M480 V bus

M

Approximates a 12-pulse converter with 6-pulse converters by putting half on Delta-Delta transformers and the half on Delta-Y. 12-pulse has Ih = 0 for h = 5, 7

Isolation transformers X~5%

ASD

ASD

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Harmonic studies

• Perform harmonic studies when– a problem occurs, to find a solution– planning large capacitor bank installation on

either utility or industrial system– planning installation of large nonlinear load

such as adjustable speed motor drives (ASD)– designing a harmonic filter or converting a

capacitor to a harmonic filter