An isolated three-phase load is supplied from a 33 kV/11 kV substation via a 5MVA transformer with an impedance of 6% (calculated on transformer rating) and a 38 km long three-phase 11 kV overhead line. The overhead line has the following series parameters:
Series inductive reactance = 0.264 /km/phase
Series resistance = 0.245 /km/phase
Line capacitance and shunt leakage effects may be neglected.
The line supplies a small industrial site fed from a 750 KVA, 11 kV/660 V delta/star transformer with an impedance of 5% (calculated on transformer rating) and negligible resistance. The transformer secondary is solidly earthed.
The site has a load of 650 kVA at a power factor of 0.92 lagging. The voltage at the 33kV bus at the substation (Vs) is maintained at 33.1kV and the 33kV fault level at the substation is 600 MVA.
Section A
Draw a line diagram of the circuit and calculate the voltage at the LV load bus (Vr), the substation power and reactive power flows and the power loss in the 11kV transmission line.
Section B
Calculate the site fault level at the LV side of the transformer.
i. For a phase-to-earth fault at the LV terminal box of the site transformer, calculate (a) the rms value of the fault current, (b) the three phase voltages at the point of fault and (c) the referred line currents on the primary (11kV) side of the transformer.
ii. For a phase-to-phase fault at the LV terminal box of the site transformer, calculate (a) the rms value of the fault current, (b) the three phase voltages at the point of fault and (c) the referred line currents on the primary (11kV) side of the transformer.
Section C
The site 650 kVA load is actually a three-phase, six pulse thyristor rectifier feeding a dc bus.
The site engineer decides that a 95 kVA three-phase power factor correction capacitor bank should be connected across the incoming ac supply to improve the site's power factor and to support the local voltage level.
Assuming that the voltage waveform at the site 11 kV bus is purely sinusoidal, calculate the Total Harmonic Distortion (THD) at the 660 V bus.
Is this level of harmonic distortion acceptable? If not, give a brief view on how might you go about solving the problem?