Solved Example for 1-Ph Short-Circuit Calculations

In the power system under consideration 315 MVA ICT and 200 MVA ICT needs to be replaced by respective three terminal network. As there is no source connected to ICT tertiary it need not be connected to +Ve reference bus and –Ve reference bus in respective network but as tertiary is Delta connected it needs to be connected to zero reference bus. Accordingly calculations and sequence networks for the power system would be as below.

1)    Generator

Xg1(new) = Xg1(given)*(Base MVA/Rated MVA)

            Xg1(new) = 0.15*100/60 = 0.25 or 25%

            -Ve and Zero Sequence reactance for generator are same as +Ve sequence reactance

2)    Generator transformer

Calculate values for X_P, X_S and X_T for 100 MVA ICT from name plate values of Xps, Xpt and Xst are Xps = 14%, Xpt = 40%, Xst = 28% thus

X_P = (Xps + Xpt – Xst) = 13%

X_S = (Xps + Xst – Xpt) = 1%

X_T = (Xpt + Xst – Xps) = 27%

For Transformers and ICT -Ve and Zero Sequence reactance are same as +Ve sequence reactance

3)    132kV Transmission lines

Ohmic value of line reactance = 0.4 * 35 = 14 Ω

Positive Sequence Pu value XL₁₁ = XL₂₁ = 14*100/(132)² = 0.0803 or 8.03%

For line zero sequence reactance will be 3 times more (or as equal to actual value available), thus

XL₁₀ = XL₂₀ = 24.09%

4)    Similarly for 220kV Lines

XLa1 = XLb1 = 4.54% and XLa0 = XLb0 = 13.62%

5)    For 200 MVA and 100 MVA ICTs

For 200 MVA ICT from name plate values of Xps, Xpt and Xst are Xps = 13%, Xpt = 37%, Xst = 26% thus

X_P1 = (Xps + Xpt – Xst) = 12%

X_S1 = (Xps + Xst – Xpt) = 1%

X_T1 = (Xpt + Xst – Xps) = 25%

For the base of 100 MVA these values will be

X_P1 = 6.0%

X_S1 = 0.5% and

X_T1 = 12.5%.

For Transformers and ICT -Ve and Zero Sequence reactance are same as +Ve sequence reactance.

For 100 MVA ICT tertiary delta is open hence

XT21 = 0.127*100/100 = 0.127 (or 12.7%) again For Transformers and ICT -Ve and Zero Sequence reactance are same as +Ve sequence reactance.

1)    Power Grid

Using formula

X_s1% = 100x100/1200 = 8.33%

Now to find out X_s0 use formula

Thus

2*8.33 + X_s0% = 3*100*100 / 1000

2*8.33 + X_s0% = 30

Hence X_s0% = 30 – 16.66%

X_s0 = 13.34%

From these values we can draw the +Ve, -Ve and Zero sequence network and considering SLG fault at 132kV Bus we can interconnect them as shown below.

Reducing +Ve or –Ve sequence network to single reactance between respective reference bus and fault point is straight forward. However to reduce zero sequence network to a single reactance requires conversion of star to delta in respect of 220/132kV 200 MVA ICT equivalent circuit.