Transcript Harmonics (continued)

Applied Harmonics
Control of Harmonics
1
Harmonic Distortion Evaluations
IEEE Standard 519-1992
• Limit harmonic current injections from end users so
that harmonic voltage distortion is tolerable.
• Limit harmonic voltage (responsibility of utility).
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utility system
PCC
IL
customer
under
study
other customers
utility system
customer
PCC
under
IL study
other customers
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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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Total harmonic distortion of voltage is normalized
with the systemnominal rms voltage :
THD Vn 
 Vh
h 1
Vn
2
 100%
Total demand distortion for current (as before):
TDD 
 Ih
h 1
IL
2
 100%
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Individual
Bus Voltage harmonic
at PCC, Vn
voltage
[kV,LL]
distortion
Vn < 69
3.00%
69<Vn<161
1.50%
161<Vn
1.00%
Total
voltage
distortion
THD_Vn
5.00%
2.50%
1.50%
Table 6.1 Harmonic voltage distortion
limits in % of nominal fundamentalfrequency voltage
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Table 6.2 Harmonic current limits
Vn < 69 kV
Isc/IL
0-20
20-50
50-100
100-1000
>1000
69<Vn <161 kV
Isc/IL
0-20 *
20-50
50-100
100-1000
>1000
161 kV<Vn
Isc/IL
0-50
>50
* line applies to all
h<11
4.0%
7.0%
10.0%
12.0%
15.0%
11<h<17 17<h<23 23<h<35
2.0%
1.5%
0.6%
3.5%
2.5%
1.0%
4.5%
4.0%
1.5%
5.5%
5.0%
2.0%
7.0%
6.0%
2.5%
35<h
0.3%
0.5%
0.7%
1.0%
1.4%
TDD
5.0%
8.0%
12.0%
15.0%
20.0%
h<11
2.0%
3.5%
5.0%
6.0%
7.5%
11<h<17 17<h<23 23<h<35
1.0%
0.75%
0.3%
1.75%
1.25%
0.5%
2.25%
2.0%
0.75%
2.75%
2.5%
1.00%
3.50%
3.0%
1.25%
35<h
0.15%
0.25%
0.35%
0.50%
0.70%
TDD
2.5%
4.0%
6.0%
7.5%
10.0%
h<11
11<h<17 17<h<23 23<h<35
35<h
2.0%
1.0%
0.75%
0.3%
0.15%
3.0%
1.50%
1.15%
0.5%
0.22%
power generation equipment regardless of Isc/IL ratio
TDD
2.5%
3.8%
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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 q / 6
For example, if q  12, then multiply by 2.
IL 
kWD
pf  3  kVrated
kWD  averagebilled demand in kW
pf  averagebilled power factor
kVrated  nominal LL voltage in kV
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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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A
B
C
C
B
A
N
Zig-zag transformer
N
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Zig zag transformer
• Place the transformer to supply balanced
triplen harmonics (and any other zerosequence 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
a
b
c
I3
n
3I3
Unloads the neutral conductor upstream
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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-pass
filter
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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
ASD
Xs
M
Xt
reactor 0-5%
on ASD kVA
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Isolation transformers X~5%
480 V bus
M
ASD
M
ASD
Approximates a 12-pulse converter with 6pulse converters by putting half on DeltaDelta transformers and the half on Delta-Y.
12-pulse has Ih = 0 for h = 5, 7
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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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