Approximate or Quick Calculations of Overcurrent and Earthfault Relay Settings for Transformer

In case of non-availability of HV bus fault levels or other resources required for detail calculations; one may choose for quick calculations. Here for calculations of relay time of operation with TMS = 1; either scientific calculator or document for Standard Inverse characteristic curve plotted on double log graph will require.

In such case LV bus fault current shall be considered as 70% of calculated fault current neglecting source impedance. As +Ve sequence and zero sequence leakage reactance of the transformer is same calculated 3-Ph short circuit current and 1-Ph short circuit current for LV bus will be same. In other words it is just sufficient to calculate 3-Ph short circuit current for LV bus and to consider same for 1-Ph short circuit current.

If transformer operates in parallel and when there is a through fault (fault on LV Bus); current shared by them will be in proportion to its capacity. Also while transformer operates separately it’s through fault current is proportional to capacity.

Thus when source impedance is neglected; it is not needed to adopt different procedures for calculations of short circuit MVA for transformer through fault; when transformers operating separately or operating in parallel. We can directly calculate T/F short circuit MVA using same formula for both cases.

For 33kV Bus

Also O/C PSM shall be decided considering LV full load current as per name plate details and HV full load current equal to HV current on normal tap.

To understand the methodology; readers are requested to go through the solved example. Though the example solved using Excel sheet it is encouraged to solve it by hand calculations.

Limitations of Approximate Calculations: Though the decision of approximate calculations may lead to unselective tripping of the transformer if actual fault current is very much higher than considered fault current (70%). In such case by observing the DR one may revise his decision. However for remote sub-station fault current may be even less than considered for quick calculation. Under such circumstances transformer may get subjected to risk. Hence such decision shall be taken cautiously.

Actually for 132/33 kV 2X25 MVA transformer with 132kV Bus as strong source; approximation considering 70% of fault level neglecting source impedance may work. But for other cases following table will help.