Auckland University Power Quality Systems Electrical Engineering Questions
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An equivalent source at 69 kV has a three-phase short circuit capacity of 1180 MVAwith X/R = 10. It feeds a dedicated feeder through a 30 MVA, 69 kV/12.47 kVtransformer with a leakage impedance of 5%. A group of nonlinear loads consisting ofcomputers and network server is fed by a 1.5 MVA, 12.47 kV/480 V servicetransformer. The leakage impedance of the transformer is 4%. A wye-groundedcapacitor bank rated at 3.287 MVAr, 12.47 kV is installed at the secondary of the 69kV/12.47 kV transformer. The nonlinear load has the fundamental current of 450 Aand it produces 11th order harmonic current of 5.3 % of the fundamental. The systemis shown in Figure 5.
1). Draw an equivalent circuit at the fundamental frequency of the 12.47 kVsystem. The equivalent circuit must show values of impedances and currentmagnitude at the 12.47 kV feeder.
2) Determine the system parallel resonant frequency at the 12.47 kV system andrepeat question (1) at the resonant frequency.
3) Mitigate the harmonic problem by shifting the parallel resonant frequency to anon-characteristic harmonic frequency by modifying the size of the capacitor.What is the appropriate size of the capacitor bank while keeping the steadystate voltage rise less than 2.0%?
4). Mitigate the harmonic problem by blocking the harmonic current fromentering the 12.47 kV systems. Install the filter for the 11th harmonic at the480-V bus. The filter will also improve the power factor from 0.75 lagging to0.96 lagging. Determine the appropriate capacitor and reactor size for thefilter.
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