Question

QUESTION 3. (a) The positive-, negative- and zero-sequence reactances of a 20-MVA, 13.2-LV synchronous generator are 0.3 pu, 0.2 pu and 0.1 pu, respectively. The generator is solidly grounded and is not loaded. A line-to-ground fault occurs on phase a. Neglecting all the resistances, determine the fault current. Assume V, = 120° pu. (b) Explain why a bolted single-line-to-ground fault at the sending end of a transmission line might be higher than the bolted three-phase short circuit on the same side? How does this situation compare with what happens at the receiving end of the line? [10+10 Marks

Answer 1

(a) Given positive, Negative and zero sequence reactance of a synchronary generator are 0.3, 0.2 and oil pu respectively. X=0.3, X2 = 0.2 and Xo = 0.1 for L-G fault fault current If= 3ERy X+Xztxo 3.110 0.3 +0.2701 If 5 Pu (MVA) 1b = IND 2/3) (18-25) น 0.87477/KA Per unit Actual value Base value > Actual fault current perunit fault current & Base value 5 x 0.87427 KA If = 4.3738 KA when line to ground fault occurs, the fault current is if 4.3738 KA

By fault current 3ERI for LG Fault ar x + x2 + xo for 3 phase fault If = ERI * for synchronous generator, Bero sequenance reactans ce to is very low compared to positive reactance. to is very low compared At sending and side At sending and side, if a fault occurs, It involves only synchronous generator reactance. to positive sequenug reactance. [...x, W highest Xit*,+ xo < 3X 3ERY → If 3Eri Xi+Xqtxo <3X1 for L-G fault If > ERI Х. But Er is fault current for 3-0 X fault current fault wrrent for L-G fault > for 3-4 for sending and side.

is is more We know that, Zero sequence reactance of transmission lines are more compared to positive sequence reactance due to ground effect Xe 8 (25-3-5) XI At the receiving and means it include transmission line As xo is high. reactance also. X,+Xztxo > 3% 3 ER If for L-G If X +x Xo BERT > 3X1 If ERR L-G fault But If= Er is fault current for 3-0 X1 L Le L-G fault If 3-Ø fault for Receiving end side.