Determine the ohmic value of the current limiting reactor per phase external to a 40 MVA, 15 kV, 50 Hz, three phase synchronous generator which can limit the current on short circuit of 6 times the full load current. The reactance of the synchronous generator is 0.06 pu.

• A. 0.8 Ω
• B. 0.2 Ω
• C. 0.4 Ω
• D. 0.6 Ω

Answer: (D)

The ratio of full load current to short circuit current = 1/6
Xsc = j/(1/6)
External reactance required = j*((1/6) - 0.06))
= j*0.106 pu
Full load current = (40*1000)/(√3*15)
= 1539.6 A
Per unit reactance = j*0.106 = (I*Xr)/V
j*0.15 = (1539.6*Xb)/((15/√3)*1000))
= 0.60 ohm