LV Earth Fault on HV side

[AusLee]

Hi,

Quick question please, if a transformer has 11kV at primary and 415V at secondary, why is it wrong to say that an earth fault of X amps on the LV side is equivalent to a current of:
Y = X * 415 / 11000?

Why doesn't the earth fault follow the turn ratio?

Thanks.

 

[jghrist]

Assuming a delta-wye transformer, it is because the turns ratio is not 415/11000. There is current in only one primary and one secondary winding. The primary winding is connected between two phases and the current comes in from one phase and out the other.

 

[AusLee]

Hi jghrist,

Thanks yes that's the exact configuration. In this case what is the transformation ratio if not 415/11000?

Thanks.

 

[ScottyUK]

Best way to visualise this is to draw the windings out and mark up the voltages applied to the windings rather than between phases. In this case you have a 11,000V primary winding on the delta side and a 240V secondary winding on the star side. The 415V secondary voltage is by virtue of interconnecting three phase-shifted 240V windings in a star, but the winding itself is still 240V.

A more direct answer is that the ratio is 11,000 : 240 or 45.83 : 1