Applied Harmonics Control of Harmonics

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).

utility system customer under study PCC IL other customers utility system customer under study PCC IL other customers

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)

Table 6.1 Harmonic voltage distortion limits in % of nominal fundamental- frequency voltage

Table 6.2 Harmonic current limits

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

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

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

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

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

Zig-zag transformer

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

Placement of ZZ transformer b c I3 n 3I3 Unloads the neutral conductor upstream

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).

Passive Filters Shunt passive filters are high-pass notch filter notch filters, tuned to a specific harmonic frequency, or high-pass filters high-pass filter notch filter

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

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

Filters In-line reactors (or chokes) for ASD Isolation transformers can help Isolation transformers can be reconnected with different phase shifts ASD M Xs Xt reactor 0-5% on ASD kVA

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

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