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

While transformer operating separately LV side protection can be provided by Non-Directional relay. When alarm and load trimming scheme is not used; overcurrent PSM will be corresponding to its LV side full load current. In case of adoption of alarm and load trimming scheme; overcurrent PSM would be 105-110%. 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 fault current considering fault on LV bus. 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 with time delay of 350 ms. This is due to two reasons. 

1.)  As we know that fault level at any bus is calculated considering all elements of the power system and all generators are in service. Actual fault level is always less than this depending on outages on element and generators and fault impedance. If we select HS current setting as 100% of LV bus fault level chances of it’s operation will be very remote owing to above fact. Hence we have to select HS current setting less than this calculated fault current value to ensure its operation. It may be less by any quantity typically ranging from 70% to 90%. To ensure maximum chances for operation of HS element we have chosen 70%. Typical operating characteristics for LV side and HV side relays plotted against common LV side fault current on X axis is as shown here. 

2.)  Figure also shows that with use of HS by this way LV operating characteristics gets a shape where time discrimination between HV side relay and LV side relay goes on increasing as fault current decreases (which is true for most of the cases) from maximum possible fault current.