A New Principle Of Transformer Protection

Power transformers as very important elements in power system require very reliable protection. Differential protection has been adopted as main protection against inner fault of transformer for a long time, since it has high reliability and high sensitivity, which are most important for transformer protection. But there are some problems troubling differential protection: identification of inrush current from short circuit current; unbalance current in differential circuit when outside short circuit happens; make sure differential protection device won't mal-operate in process of voltage recovering after cutting off outside short circuit; make sure that the saturation of CT won't influence the operation of differential protection. Identification of inrush current has been a key problem to be resolved. Many researchers have done a lot of work on this aspect. Because the inrush current of transformer is influenced by many factors such as residual flux of iron core, saturation flux density, system impedance, mode of connection, iron core structure, closing angle and so on, but all of the proposed methods can't make sure 100% reliability. Identification of inrush current is the most critical factor that limits differential protection operation.This paper proposes a new criterion for differential protection. Compared with the existed criterions, this criterion has great improvement on reliability and sensitivity. To solve the problem that differential protection may mal-operate when a faulted transformer closing onto system or after cutting off out side fault, an impedance relay is added in this criterion. Identification of inrush current is replaced by impedance relay, so some wrong operation caused by wrong result of detection of inrush current is eliminated. This principle has great sensitivity to inner fault and great brake ability to outside fault. It doesn't reflect to inrush current but can reflect to fault when closing onto system.In this paper, some simulations of transformer are done when no-load closing under different conditions by using MATLAB. Results of the simulations also proved the priority of this principle.