Study of transformer recovery inrush after clearance of external faults

The transient of transformer voltage recovery after clearance of external faults has been considered to be the same with that of transformer switching-on without load. In fact, the transient during transformer voltage recovery has its own characteristic. Taking into account the constraint that a breaker opens at the fault zero-crossing current, this paper analyzes the transformer magnetic-electric transient during the entire period of external faults. The fault transferring residual flux is firstly presented and four variables influencing the transformer recovery inrush current are studied. Furthermore, The digital simulation results and dynamic simulation data indicate that transformer recovery inrush itself cannot result in mal-operation of differential protection.