Relay

The neutral current to setting 51G relay must be calculated base on zero frequency current at lv side of transformer. it depend on configuration of upstream system. we can not calculated only by false level at HV and LV side of transformer.

Basicaly, for setting a relay we need:

1. Pick up current - calculated base on fail poind and current transformer ratio

2. Time dial setting (or time multial setting) - base on co-cordination between delay time relay (Nolmaly 51G is not neceesary for cordination) and charistoristic of relay's curve

you can refer to some book or standard (ex. IEEE sd 242) for more information.

The fault levels indicated by you are the rated fault withstanding currents (short time ratings ) of the primary & secondary side of the Transformer.This has nothing to do with the actual fault current that is likely to flow through the transformer in case of fault on the LV side.For this, you will need the rated % impedance of your transformer and the actual fault level of the 33 kV Bus to which the transformer is connected.

Based on these you can calculate the value of maximum 3 phase S.C.fault current.Approximately 86% of this value can be considered as the maximum Earth Fault current which may flow through the Neutral.The E/F relay (51G) has to be set below this value.If the LV side is feeding single phase loads, then certain unbalance current will flow through the Neutral under normal conditions.Hence, the relay has to be set above this value.The maximum unbalance current in the Neutral can be upto 50% of the line current depending upon the No. of single phase loads.Uou may fine tune the setting during testing to ensure reliability and sensitivity of the protection.

Hope,this will help you.

stand by earth fault relay connected in the transformer neutral bushing needs to be co-ordinated with the down stream relay. Earth fault current setting has to be set above the down stream earth fault current with respect to time.

Obviously.However,it may be kept in mind that the down stream E/F protection relays/releases will be as many as the No. of O/G feeders connected to the LV Bus.

51G relay is called sensitive earth fault or stand by earth fault relay. whe n there is an earth fault occur the path of this current is through neutral and the main earth fault relay tripped but if there is no magnitude for main relay trrping then this relay tripped.