Questions on Three phase Circuits
1. A delta connected load of impedance 600+j300 Ω per limb is connected to a 10kV,
50Hz, 3 phase grid substation via a line of impedance 1+j3 Ω per phase. a.. Derive a per phase equivalent circuit for this system. b. Calculate the current drawn by the load. (25.6A) c. Calculate the line to line voltage at the load. (9.91kV) d. Calculate the power (P) and reactive power (Q) drawn by the load. (392kW,
196kvar) e. Calculate the power factor of the load. (0.89)
2. A 100kVA balanced load of power factor 0.85 lagging is connected to a 50Hz, 3 phase supply via a line of impedance 0.016+j0.032
Ω per phase. The voltage at the load is exactly 400V line to line. a. Derive a per phase equivalent circuit for this system. b. Calculate the power (P) and reactive power (Q) drawn by the load. (85kW,
52.7kvar) c. Calculate the current drawn by the load.(144A) d. Calculate the line to line voltage of the supply.(406V) e. Calculate the power loss in the line. (995W)
3.
A 3 phase 400V, 50Hz supply feeds a delta connected load over a line of impedance 0.1+j0.2Ω per phase. Each limb of the delta connected load has impedance 10+j5Ω. Calculate: a.
The current in the load (59.1A) b.
The volage at the load (381V line to line) c.
P&Q of the load. (34.9kW, 17.5kvars)
4. A three phase 50 Hz substation supplies a load through a three phase feeder that has a per phase impedance of 0.05+j0.2Ω. The load draws 54kW at 0.9PF lagging. The line to line voltage at the load terminals is 400V. Determine: i) The per phase equivalent circuit assuming a star connected load. ii) The magnitude of the line to line voltage at the substation busbar. iii) The total real and reactive power at the substation busbar. iv) The power loss in the line.