How to calculate the short circuit level of a electrical power system?

Question

In this case I am considering a transformer of rating 20 MVA and voltage level of 33/11 KV

Answer 1

Short circuit calculations are basically an application of Ohm's Law using the line voltage and the impedance of all the upstream components.

Many real-world engineering transformer fault current calcuations assume an infinte bus -- zero impedance of all devices upstream of the transformer. Only the impedance of the transformer itself.

The piece of data you did not furnish is the impedance of the transformer -- generally expressed as %Z. This is a parameter stamped on the transformer's nameplate.

Once you have the %Z, the formula for the secondary fault current is:

(100%÷ Z%) x Total VA
---------------------------------
(secondary volts) √3

If we assume a transformer impedance of 8%, the resulting fault current is:

I = ((100% ÷ Z%) x 20,000,000) ÷ ((11,000)√3)
I = ((100% ÷ 8%) x 20,000,000) ÷ ((11,000)√3)

I = ((12.5) x 20,000,000) ÷ 19,052 = 13,121 amps

You will need to obtain the actual impedance from your transformer and repeat the calcuations.

Answer 2

With a few assumptions, we can get close to the right value for short circuit current with the information you have given.

1. The transformer impedance (i.e. leakage reactance) is the most important parameter. We will assume 7.5% (0.075 per unit) since that is in the typical range of impedance values for this size transformer.

2. We will only attempt to calculate 3-phase short circuit, since you give no information about winding vector groups.

3. We will assume that the source on the 30 kV side of the transformer has no source impedance. This is understood to be a simplifying, conservative, assumption. There may be quite a lot of impedance in the 30 kV source.

4. I sc = 20 MVA / (sqrt(3) * 11 kV * 0.075 pu ) = 13,996 A

This is 14 kA for your purposes.

Also, this is close enough to standard electrical equipment short circuit withstand ratings of 12.5 kA that you should look very closely at the additional details: transformer impedance, source (30 kV) impedance, etc.

Note that we used 1.0 pu source voltage, while the IEC standards specify use of 1.05 pu source voltage for equipment rating studies.

If you need a more accurate answer, consult an electrical engineer and be sure to collect the necessary information for a proper short circuit study.

Answer 3

thank you