Settings of Transformer LV Side Overcurrent and Earthfault Relay While Transformer operating in Parallel.

Irrespective of transformer LV side protection scheme selected; the methodology for deciding overcurrent and earthfault relay settings remains same.

For Non-Directional (Reverse Direction i.e. towards LV bus) setting methodology for PSM, TMS and HS is as below.  

When alarm and load trimming scheme is not used; overcurrent PSM will be corresponding to LV side full load current of respective transformer. In case of adoption of alarm and load trimming scheme; overcurrent PSM would be 105-110% of LV side full load current of respective transformer. Earthfault PSM shall be corresponding to 20% of transformer full load current as explained in previous section.

LV side overcurrent and earthfault relay time of operation shall be 350 ms for 70% of fault current contribution by respective transformer considering fault on LV bus. Here 70% of fault current selected to shape the characteristics properly when we are using it along with HS element. Also it allows for fault resistance and 33kV feeder impedance; when fault is remote from LV bus. In case of non-availability of HS element one may prefer to select this value as 100%.This will allow sufficient time for clearing the fault by LV feeder breaker.

We may select Highset setting corresponding to 70% of LV bus fault current contribution by respective transformer. Time delay for HS shall be 350 ms.

For Forward Direction (Towards Transformer) setting methodology for PSM, TMS and HS is as below.  

Overcurrent PSM will be corresponding to LV side full load current of respective transformer. Earthfault PSM shall be corresponding to 20% of transformer full load current as explained in previous section.

LV side forward direction overcurrent and earthfault relay (or relay element) works as backup to Differential Protection; time of operation shall be minimum. Hence TMS shall be selected as 0.05.

We may select HighSet setting corresponding to 70% of LV LA fault current contribution by respective transformer. Time delay for HS shall be 50 ms.

You may have better understanding of the methodology of deciding overcurrent and earthfault relay setting in case of transformers after going through next two examples. First example demonstrates setting methodology in case of transformers operating separately, while second example explains setting methodology in case of transformer operating in parallel.