Investigation on the Operation Analysis And Setting Calculation of 110kV Main Transformer Backup Protection

In the substation with two parallel operating three-winding transformers,if the short-circuit fault between the 10kV(or 35kV) protection CT of the main transformer to the same side bus occurs,the switch on 10kV(or 35kV) trips and fault still keeps,or the switch refuses to trip and the fault can not be isolated,which will cause the whole substation to loss power.This paper puts forward to add the compound voltage lockout directional overcurrent protection in backup protection of main transformer 10kV(or 35kV) side and the direction pointing from the same side bus to the main transformer,and introduces the setting calculation principles for the overcurrent protection,which ensures that losing power scope is the smallest.