Description
An equivalent source at 69 kV has a three-phase short circuit capacity of 1180 MVA with X/R = 10. It feeds a dedicated feeder through a 30 MVA, 69 kV/12.47 kV transformer with a leakage impedance of 5%. A group of nonlinear loads consisting of computers and network server is fed by a 1.5 MVA, 12.47 kV/480 V service transformer. The leakage impedance of the transformer is 4%. A wye-grounded capacitor bank rated at 3.287 MVAr, 12.47 kV is installed at the secondary of the 69 kV/12.47 kV transformer. The nonlinear load has the fundamental current of 450 A and it produces 11th order harmonic current of 5.3 % of the fundamental. The system is shown in Figure 5.
1). Draw an equivalent circuit at the fundamental frequency of the 12.47 kV system. The equivalent circuit must show values of impedances and current magnitude at the 12.47 kV feeder.
2) Determine the system parallel resonant frequency at the 12.47 kV system and repeat question (1) at the resonant frequency.
3) Mitigate the harmonic problem by shifting the parallel resonant frequency to a non-characteristic harmonic frequency by modifying the size of the capacitor. What is the appropriate size of the capacitor bank while keeping the steady state voltage rise less than 2.0%?
4). Mitigate the harmonic problem by blocking the harmonic current from entering the 12.47 kV systems. Install the filter for the 11th harmonic at the 480-V bus. The filter will also improve the power factor from 0.75 lagging to 0.96 lagging. Determine the appropriate capacitor and reactor size for the filter.
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Explanation & Answer
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Question 1.
Equivalent Circuit at the fundamental Frequency of 12.47kV
Regulator
Feeder
Source
12.47kV
Nonlinear Load 1...