Ex-3: - Two transformer in parallel with calculations of source impedance for 3-phase fault.

Problem

Consider a S/S having two transformers of (a) 132/33kV 25 MVA having leakage reactance of 9.5% and (b) 132/33kV 50 MVA having leakage reactance of 12.5%. For this substation 132kV bus 3-phase fault level is 1700 MVA. Find out the 3-phase 33kV Bus fault current.

Solution

Consider 100 MVA as base MVA

Convert 25 MVA transformer leakage reactance to 100 MVA base

Hence X_T1-100MVA = X_T1-25MVA*(100/25) = 9.5*4 = 38% ----- (using formula – 2)

Convert 50 MVA transformer leakage reactance to 100 MVA base

X_T2-100MVA = X_T2-50MVA*(100/50) = 12.5*2 = 25% ----- (using formula - 2)

FIND OUT SOURCE % REACTANCE FOR 100 MVA BASE FOR 132kV BUS

BY REVERSE ENGINEERING

1700 =   ----- (using formula – 3)

HENCE

 

Find equivalent reactance between source and fault point

X = Xs + (X_T1||X_T2) = 5.88 + 38*25/(38+25) = 20.96%

Find 3-phase short circuit MVA (Base MVA = 100 MVA)

3-Ph short circuit MVA = 100x100/20.96% = 477 MVA ---- (using formula – 3)

For 33kV voltage 1 MVA = 17.5 Amp

Hence this current will be 17.5*477 = 8347 Amp.

Comments

We can calculate source reactance by reverse engineering if source bus fault level is known. Generally up to 132kV bus fault levels are provided by Corporate Office.