Examples of circuit protection methods

Overvoltage protection
Causes of overvoltage
1 Operating overvoltage: overvoltage caused by electromagnetic processes in regular operation such as pulling, closing, and cutting off fast DC switches.
2 Surge overvoltage: Overvoltage caused by accidental causes such as lightning strikes and entering the converter from the grid.
3 Power electronics shutdown overvoltage: The overvoltage generated when the power electronics device is turned off.
4 In the power electronic converter-motor speed regulation system, the overvoltage caused by the excessive DC voltage on the DC side due to the motor  feedback braking, also known as the pump boost voltage.
The basic principle of overvoltage protection is: according to the different parts of the overvoltage in the circuit, add different additional circuits, when the fixed overvoltage value is reached, the additional circuit is automatically opened, so that the overvoltage forms a path through the additional circuit, and the electromagnetic energy stored by the overvoltage is consumed, so that the overvoltage energy will not be added to the main switching device, protecting the power electronic device.

Overcurrent protection
The cause of overcurrent
When a device inside the power electronic converter breaks down or short circuits, the trigger circuit or control circuit fails, overload, DC side short circuit, reversible transmission system circulating current or inverter failure, and AC power supply voltage is too high or too low, phase loss, etc., can cause the current of the components in the converter to exceed the normal working current, that is, overcurrent. Since the current overload capability of power electronics is much worse than that of ordinary electrical equipment, the converter must be properly protected against overcurrent. The overcurrent of converters is generally divided into two main categories: overload overcurrent and short-circuit overcurrent.

Methods for overcurrent protection
(1) AC inlet reactor, or rectifier transformer with large leakage reactance, using reactance to limit short-circuit current. However, there is a large AC voltage drop during normal operation.
(2) Current detection device. The signal is sent when the overcurrent is overcurrent, and the overcurrent signal can block the trigger circuit on the one hand, so that the fault current of the converter quickly drops to zero, thereby effectively suppressing the current. On the other hand, the overcurrent relay is controlled to make the AC contactor contact jump open and cut off the power supply. However, the action of the overcurrent relay and the AC contactor takes a certain amount of time (100 ~ 200ms), so this protection can only be effective when the current is not large.
(3) DC fast switch. For large and medium capacity converters, the price of fast fuses is high and it is inconvenient to replace. In order to avoid blowing the fast fuse during overcurrent, a DC fast switch with an action time of only 2ms is adopted, which can protect the power electronics before the fast fuse action.
(4) Fast fuse. Fast fuses are the last line of defense against overcurrent damage in converters, and in thyristor converters, fast fuses are the most commonly used overcurrent protection measures, which can be used on the AC side, DC side and the main circuit of the device. Among them, the AC side fast fuse can protect the short circuit of the thyristor element and the short circuit of the DC side, but it is required to work normally, the current quota of the fast fuse is greater than the current quota of the thyristor, which protects the short circuit fault of the component poorly.