A new scheme of protective system for three-phase transformers

Three-phase transformers are key equipment in power systems and power plants. Security and stability of three-phase transformers are both important and necessary for system operation. When a three-phase transformer is energized, the inrush current due to flux saturation in the core results in a malfunction of the differential relay and also causes a stress impact to the transformer. These have a bad effect on the security and stability of the power system. In this paper, an optimum nonsynchronous switching-on suppressing method of three-phase transformers inrush current is proposed. The proposed method is examined by experiment. It has excellent performance for suppressing three-phase inrush current. It is applied to design the protective system of a three-phase transformer. For the optimum nonsynchronous switching-on suppressing method, a protective system based on a single-chip controller is implemented. The system can distinguish fault current from inrush current. Therefore stress impact and overvoltage resulting from three-phase inrush current can be avoided. The active protection of three-phase transformer is obtained to improve the protective relaying system. Now, problems caused by three-phase inrush current can be detected and solved effectively.