Applied Harmonics Control of Harmonics
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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
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