Applied Harmonics Control of Harmonics

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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 other customers I L customer under study utility system PCC I L customer under study other customers 3

• • • 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) 4

Total harmonic distortion of voltage is normalized with the system nominal rms voltage : T HD V n  h   1 V n V h 2  100 % Total demand distortion for current (as before) : TDD  h   1 I h 2 I L  100 % 5

Bus Voltage at PCC, Vn [kV,LL] Vn < 69 69

Table 6.2 Harmonic current limits

Vn < 69 kV Isc/IL 0-20 h<11 4.0% 111000 7.0% 10.0% 12.0% 15.0% 3.5% 4.5% 5.5% 7.0% 2.5% 4.0% 5.0% 6.0% 1.0% 1.5% 2.0% 2.5% 0.5% 0.7% 1.0% 1.4% 8.0% 12.0% 15.0% 20.0% 691000 161 kV50 h<11 2.0% 3.5% 5.0% 6.0% 7.5% h<11 2.0% 3.0% 11

• I h is rms magnitude of individual harmonic current • I sc is the short-circuit current at PCC • I L 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 8

• 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 I L  kWD kWD pf   3  kV rated average billed demand in kW pf  average billed power factor kV rated  nominal LL voltage in kV 2 .

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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 10

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 11

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 12

C A B C B A N Zig-zag transformer N 13

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 14

Placement of ZZ transformer

a b c I 3 n 3I 3 Unloads the neutral conductor upstream 15

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 17

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 18

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 19

Filters

• In-line reactors (or chokes) for ASD • Isolation transformers can help • Isolation transformers can be reconnected with different phase shifts X s X t reactor 0-5% on ASD kVA ASD M 20

Isolation transformers X~5% 480 V bus ASD M M ASD 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 I h = 0 for h = 5, 7 21

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 22