Short Circuit Calculation

[mako]

Good Day.

If the utility has an infinite Short Circuit MVA Value and downstream has a transformer, what will be the short circuit value or result at the secondary of the transformer? Say 11 KV Primary and 3.3 KV Secondary with a transformer Impedance of 6%, 3 Phase, 8 MVA Power Transformer.

Thanks.

 

[ron]

Calculate the full load current on the secondary then divide that by the impedance of the transformer.
For example, if the full load current on the secondary is 1400A and the impedance is 6%, then the let through current out of the secondary, can be as high as 23,355A

 

[jtester]

It is common to use an actual utility impedance value in calculations with transformers as big as 8 MVA. It can change the fault currents values calculated, sometimes significantly.

I try to determine a safe, but realistic, system impedance when using transformers 2.5 mva or so and larger. Deciding on an infinite high side is conservative, but sometimes costly.

Jim T

 

[mako]

Short Circuit Calculation

Pardon but still I can't get it.It means that if the Ultility SCMVA is infinity, then it will not be included in the calculation at the secondary of Transformer,i.e. only the SCMVA of the Transformer is the short circuit considered. Am I right?

 

[ron]

If the theoretical case of infinite fault current availablitiy on the transformer primary, then the only limitation to fault current provided out of the secondary of the transformer is the transformer impedance itself.

 

[dsteves]

Pardon but still I can't get it.It means that if the Ultility SCMVA is infinity, then it will not be included in the calculation at the secondary of Transformer,i.e. only the SCMVA of the Transformer is the short circuit considered. Am I right?

Not really. The infinite source is the given utility model. Your model of the circuit will have 8 MVA / .06 = 133.3 MVA short circuit. Divide that by (secondary rated L-L voltage * sqrt(3)) and you'll get 23.3 kA.

The MVA model of calculation takes the MVA fault levels each segment can supply or pass and performs calculation of the series circuit by using what looks on paper like a parallel resistance model. With the infinite source you do

(1/infinity) + (1/133 MVA) = 1/(Available at xfmr sec) which becomes
0 + 1/133 = 1/133. Take the reciprocal of the RHS and you have 133 MVA SC level (23.3 kA).

If the utility told you that they had 1000MVA SC available at the transformer primary, you'd do this.

1/1000 + 1/133 = 1/(available at xfmr sec)

.001 + .0075 = .0085

1/.0085 = 117.4 MVA available at the transformer secondary.

This reduces to 20.54 kA Isc. Not a lot of difference compared to the infinite source model (11.7% reduction)

If the utility had 100 MVA available at the transformer primary,

1/100 + 1/133 = 1/available at sec

.01 + .0075 = .0175

1/.0175 = 57.1 MVA (9.99 kA) at the transformer secondary. This is a lot less than the infinite source model (57% reduction)

As the utility SC MVA drops, so does yours. If they have a really stiff system, they may be able to deliver a boatload of MVA to your transformer primary during a fault. jtester is absolutely right. Try to get worst case Isc or MVAsc at the transformer primary from the utility. If you instead use the infinite source model, you'll end up specifying overrated equipment, which could be significantly overrated. But you can live on knowing you'll never poof the gear

Caveat emptor : if the utility comes back later and stiffens up the system, you could be in trouble until you tweak the service by reducing the peak fault current after the transformer secondary. The infinite source model would have protected you from this scenario.

Hope this helps.

edited to recalc 3.3 kV, not 3.0 kV . D'oh!

Dan

 

[mako]

Short Circuit calculation

Short Circuit calculation

Thanks to everybody's comments/explanations on this topic.At last, at least it has been already clear to me what Infinite Utility SCMVA means.It was really a great help as it was well explained.

Thank you very much guys.

Expect more queries the next time.