The magnetizing inrush current generated by the large-capacity transformer no-load closing is one of the main problems causing the protection action and damaging the internal structure of the transformer. By analyzing the hysteresis characteristics of ferromagnetic materials

the mechanism of the residual magnetism of the transformer is clarified. Taking the no-load closing model of the double-winding transformer as an example

the calculation formula of the transient magnetic induction intensity and the magnetizing inrush current after the air-to-space change is derived. The convergence angle is 0° and the maximum magnetizing inrush current is the same as the remanence. The 500 kV transformer simulation calculation model compares the closing angles of 0° and 90°

which proves that in the presence of residual magnetism

the magnetizing inrush current will make the air-to-air transition have a probability of failure

and will protect the action and damage the equipment. Therefore

it is necessary to demagnetize large capacity transformers after shutdown.The DC demagnetization method is often used in the field because of its portability. The simulation results show that after DC demagnetization

the residual magnetic content in the transformer is very small

and the transformer can ensure the successful closing.