What Are the Different Types of Circuit Protection?

Circuit protection is mainly to protect components in electronic circuits from damage under overvoltage, overcurrent, surge, and electromagnetic interference. With the development of science and technology, power / electronic products have become increasingly diverse and complicated. The circuit protection components used are no longer simple glass tube fuses of the past. Usually the protection devices include varistor, TVS, gas discharge tube, etc. Has developed into a wide range of emerging electronic component fields.

In various types of electronic products, the trend of installing overcurrent protection and overvoltage protection components is increasing. The reason for this is mainly summarized in the following aspects:

(1) With the development of electronic products,

Circuit protection

Causes of overvoltage

1 Operating overvoltage: Overvoltage caused by electromagnetic processes during frequent operations such as closing, closing, and fast DC switch off.

2 Surge overvoltage: Overvoltage that enters the converter from the power grid due to accidental reasons such as lightning strikes.

3 Power electronic device shutdown overvoltage: Overvoltage generated when the power electronic 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 ^[3] feedback braking is also called the pumping voltage.

The basic principle of overvoltage protection is to add different additional circuits according to the different parts of overvoltage generated in the circuit. When a fixed overvoltage value is reached, the additional circuit is automatically turned on to allow the overvoltage to form a path through the additional circuit and consume the overvoltage. Stored electromagnetic energy, so that over-voltage energy will not be added to the main switching device, protecting power electronic devices.

Circuit protection

Causes of overcurrent

When a device in a power electronic converter breaks down or short-circuits, the trigger circuit or control circuit fails, overload occurs, the DC side is short-circuited, the reversible drive system generates circulating current or inverter failure, and the AC power voltage is too high or too low Equal, can cause the current of the components in the converter to exceed the normal operating current, that is, overcurrent occurs. Because the current overload capability of power electronic devices is much worse than that of general electrical equipment, the converter must be properly protected against overcurrent. The converter's overcurrent is generally divided into two categories: overload and short circuit.

Over current protection method

(1) AC line reactor or ^[4] rectifier transformer with large leakage reactance, which uses reactance to limit short-circuit current. However, there is a large AC voltage drop during normal operation.

(2) Current detection device. A signal is issued when overcurrent occurs. On the one hand, the overcurrent signal can block the trigger circuit and quickly reduce the fault current of the converter to zero, thereby effectively suppressing the current. On the other hand, the over-current relay is controlled to make the AC contactor contacts open and cut off the power. However, it takes a certain time (100 ~ 200ms) for the overcurrent relay and AC contactor to operate, so this protection can only work if the current is not large.

(3) DC fast switch. For large and medium capacity converters, fast fuses are expensive and inconvenient to replace. In order to prevent the fast fuse from being blown during overcurrent, a DC fast switch with an operating time of only 2ms is used, which can protect power electronic devices before the fast fuse operates.

(4) Fast-acting fuse. The fast fuse is the last line of defense to prevent the converter from overcurrent damage. In thyristor converters, the fast fuse is the most common overcurrent protection measure and can be used in the AC side, DC side and the main circuit of the device. Among them, the AC side fast fuse can protect the thyristor element short circuit and the DC side short circuit. However, during normal operation, the fast fuse current rating must be greater than the thyristor current rating. This protects the short circuit fault of the component. difference.