Ex-1: - Single transformer with negligible source impedance.

Problem

Consider a S/S having one transformers of 132/33kV 25 MVA having leakage reactance of 9.5%. Let the S/S source impedance be negligible as compared to transformer leakage reactance. Find out the 3-phase and 1-phase fault current for 33kV Bus fault.

Solution

3-Ph short circuit MVA = 25x100/9.5 = 263 MVA  ---- ( using formula – 3)

1-Ph short circuit MVA = 3x25x100/(9.5+9.5+9.5) = 263 MVA  - ( using formula – 4)

(As for transformer X₁ = X₂ = X₀)

For 33kV voltage 1 MVA = 17.5 Amp

Hence this current will be 17.5*263 = 4602 Amp.

Comments

In actual practice source impedance is not negligible hence 3-Ph fault MVA will be less than this. It shall be a good approximation to consider it as 80% of calculated value. Hence 3-phase fault current shall be 0.8*4602 ≈ 3500 Amp. (and 1-phase fault current too)