Research on Parallel Access of Transformer Differential ProtectionDevice at the Secondary Side of CT

Differential protection is the primary protection, and it should remove the fault accurately and rapidly without malfunction. In this paper, as to the transformers in the way of internal bridge connection, the action of differential protection in fault protection region is analyzed when protection devices are parallel accessed into the secondary side of differential CT, which will detect fault outside of protection region and malfunction of differential protection with ratio braking caused by large unbalance current because of small braking current and serious saturation of CT after no-load closing of transformer at near side. It can be found from the simulation that the current waveform of transformer with load deteriorates rapidly when transformer at near side closes with no load, which will make differential current increase suddenly and the content of second harmonics is too low, therefore cause the maloperation of differential protection. In the end, the corresponding measures are proposed.