1 Locating Harmonic Sources System Impedance Impacts of Harmonics K-factor Interharmonics
2 System Effects Harmonic currents in a radial distribution system will generally flow back to the source (fig 5.23) unless they are diverted to a nearby capacitor bank (fig 5.24). System impedance is generally inductive reactance X L = L, so remember that for harmonic h, the reactance is X h = h L = h X 1
3 Capacitor banks Capacitor impedance Z c = -j X c where reactance X c = 1/( C) = 1/(2 f C) Since capacitor banks are rated in terms of voltage and reactive power: X c = (kV) 2 /MVAr = 1000(kV) 2 /kVAr Again remember that X c is proportional to 1/f, so at harmonic h: X ch = X c1 /h
4 Effects of resistance Resistance does not “damp out harmonics” Resistance will tend to damp out resonance, which may alleviate (to some extent) effects of harmonics Power factor correction capacitors are likely to cause some low-order harmonic resonance problems even with resistance in the circuit
5 Effects of harmonics Impact on Capacitors Impact on Transformers Impact on Motors Impact on Telecommunications Impact on Energy and Demand Metering
6 Effects of Capacitors Capacitors have some tolerances in ratings according to IEEE Standard : 135% of nameplate kVAr 110% of rated rms voltage (including fundamental and harmonics) 180% of rated rms current (including fundamental and harmonics) 120% of peak voltage
7 Effects on Transformers Harmonics affect transformers by –increasing the rms current which increases the transformer copper loss (winding resistance loss) –increasing the eddy-current losses, which usually increase as the square of the frequency
8 If voltage distortion is being caused by harmonics in the load current:
9 TypeMVAP EC-R Dry0<S<13-8% 1<S< % S> % Oil filled0<S<2.51% 2.5<S<51-5% S>59-15%
10 K indicates how much extra loss is incurred due to harmonic currents increasing eddy-current losses.
11 K factor Harmonics increase losses in transformers K factor is used to derate transformers
12 Interharmonics and Losses