Short Circuit Calculation

Hi,

It depends on the position of the short circuit.

Is it between phases or one phase going to ground on the "wye" star side or on the delta side?

I touched on single phase "wye" star side short circuit calculations in my electrical course some 24 years ago!! Showing my age now. :-(

It takes into account the "earth fault loop impedance" to calculate the fault current and thus the disconnection time based on the protective device used and the characteristics of the same, be it a fuse or a circuit breaker or an earth-leakage ( r.c.d ) device.

Regards,

Dr M.

Dear Dr. Marten

Thank you for your answer.

The problem is as follows:

in a 230-13.8KV substation, I need to set the relay downstream to a power transformer.

This transformer is delta(230) wye(13.8) connected and the impedance is 8%.

In order to calculate the short circuit, I have two possibilities

-consider z t = j 0,08 + 0 and neglect transformer phase shifting(30 degrees)

-or consider the phase shift and will have a different value of the current (zt = 0,08 L(90-30)

My question is: is it valid to neglect the phase shift, for calculation purposes?

The difference will be that I will have a resistive and a reactive current.

Kind regards

Dear Dr

A single phase to earth fault is being examined.

Kind regards

Emmanuel

How does the fault current calculated? Please provide a useful formulas to find the exact value of current required if it is really a fault current or not.

You can sure do components, but why go to that trouble? It's much easier to calc the fault current from the XFMR MVA, Z% and secondary Voltage. If you must do compenents, then perform a delta - wye transform then reduce every thing to per unit per phase (pu pp).

Dear dr

The transformation into symetrical components does not take into consideration the phase displacement between prymary and secondary, i.e., the current reflected to the secondary will be in phase with the secondary current.

Therefore, the question remais open.

Dear folks

I finally discovered how to cope with this matter.

One just need to convert the delta impedances into star impedances.

Then use the impedance obtained in this way and shift it by the delta wye angle between the two windings of the transformer.

After this, the system may be dealt with as a regular symmetrical components circuit.

Thank you again for your efforts in helping me.

Regards

Emmanuel