Short circuit calculation using MVA method (manual)

Utility: 150KV, 1000 MVAsc

Transformer 1: 170 MVA, 150/13.8KV, 15% Z

13.8KV Bus

Generator: 100MVA, X"d = 0.2

Transformer 2: 30 MVA, 13.8/6.6KV, 15% Z

6.6KV Bus

Transformer 3: 2MVA, 6.6KV/400V, 10% Z

Motor 1: 10 MVA (Lumped), 20% Z

400V Bus

Motor 2: 1000 KVA (Lumped), 20% Z

Motor 3: 600 KVA (Lumped), 10% Z

In the event of a short circuit, the sources of short circuit current are

1. Utility

2. Generators

3. Motors

 Static loads such as heaters and lighting do not contribute to short circuit.

"EquivalentMVA" are:

Transformers and Motors

 Generators

 Cables and Reactors

So, here are the results of MVAsc:

Utility: MVAsc = 1000MVA

Transformer 1: MVAsc = 170 / 0.15 = 1133.33 MVA

13.8KV Bus

Generator: MVAsc = 100 / 0.2 = 500 MVA

Transnformer 2: MVAsc = 30 / 0.15 = 200 MVA

6.6KV Bus

Transformer 3: MVAsc = 2 / 0.1 = 20 MVA

Motor 1: MVAsc = 10 / 0.2 = 50 MVA

400V Bus

Motor 2: MVAsc = 1 / 0.2 = 5 MVA

Motor 3: MVAsc = 0.6 / 0.1 = 6 MVA

Now we calculate the upstream contribution :

At Transformer 1:

MVAsc @ 150KV = 1000 MVA

MVAsc @ 13.8KV = 1/ (1 / 1000 + 1 /1133.33) = 531.25 MVA

At Transformer 2:

MVAsc @ 13.8KV = 531.25 + 500 = 1031.25 MVA

MVAsc @ 6.6KV = 1/ (1 / 1031.25 + 1 / 200) = 167.51 MVA

At Transformer 3:

MVAsc @ 6.6KV = 167.51 + 50 = 217.51 MVA

MVAsc @ 400V = 1/ (1 / 217.51 + 1 / 20) = 18.31 MVA

At 400V Motors

Motor 3: MVAsc = 18.31 x 5 / ( 5 + 6 ) = 8.3 MVA

Motor 4: MVAsc = 18.31 x 6 / ( 5 + 6 ) = 9.98 MVA

The fault MVAsc @bus 400V = 18.31 + 5 + 6 = 29.31MVAsc

The three phase  I_f = 29.31/(1.732*(0.4)) = 42.3 kA.

Now we come to fault single phase to ground :

For single phase faults, positive sequence, negative sequence and zero sequence

impedances need to be calculated.

I_f = 3 (I1 + I2 + I0)

Examining the circuit in above, at the 400V Bus, on Transformer 3 contributes to the

zero sequence current.

For transformers, the negative sequence and zero sequence impedance are equal to the positive sequence impedance.

Z1 = Z2 = Z0 or

MVA1 = MVA2 = MVA0

@bus 400V;

1 / MVAsc =1/3 (1 / MVAsc1 + 1 / MVAsc2 + 1 / MVAsc0)

1 / MVAsc = 1/3 (1 / 29.31 + 1 / 29.31 + 1 / 20 )

1/ MVAsc = 

MVAsc = 1/   = 25.4 MVAsc

If= 25.4 / (1.732 x 0.4) = 36.6 kA