1. Concept of overvoltage:

When the transformer in the box type substation is running, if the voltage of the relay protection calibrator exceeds its maximum allowable working voltage, it is called the over-voltage of the transformer.

2. Classification of overvoltage:

Overvoltage often causes great harm to the insulation of transformer, and even causes insulation breakdown.

Overvoltage can be divided into atmospheric overvoltage and operating overvoltage:

(1) Atmospheric over-voltage: when the transmission line is directly struck by lightning or discharged by lightning cloud, the over-voltage caused by the drastic change of electromagnetic field is called atmospheric over-voltage;

(2) Switching overvoltage: When the transformer in the box type substation or the switch on the line is switched on or off, the overvoltage caused by electromagnetic energy oscillation and accumulation in the system is called switching overvoltage.

These two kinds of overvoltage of transformer are transient processes with short action time. The operating over-voltage of relay protection calibrator is generally 3.0-4.5 times of the rated voltage, while the atmospheric over-voltage value is very high, reaching 8-12 times of the rated voltage. The voltage distribution in the winding is extremely uneven, and the voltage on some wire turns at the inlet terminal is very high. Therefore, necessary measures must be taken to prevent overvoltage and provide effective protection.