Lehman Engineering Power Quiz
"Overcurrent Protection"

Consider the following distribution line.
A bolted ground fault occurs from CØ to ground at F1:

  • G = generator, grounded Y connected (Ignore its impedance in your calculation)
  • TX = transformer, 3Ø, Grounded Y- Delta , 13.2 kv / 480 volts,
    1000 KVA, 3% impedance (ZTX)
  • ZXL = system line reactance, ZXL = 2% on 1000 KVA base
  • F1 = bolted ground fault from CØ to ground (Assume no fault impedance; ZF1 = 0)
  • M = 10 HP, 3Ø, 480 volt, Power Factor = 0.8, Transient Reactance (x'd) = 5%
  • (All impedances are on 1000 KVA base and 480 base volts)


Question #1:
Calculate the short circuit current for the ground fault at F1.


Question #2:
What current will flow in the unfaulted phases (A & B) during the fault?


Hint: Remember that the transformer secondary is Delta connected.


Solution