Talking about the method of thyristor overcurrent protection

In order to prevent damage to the thyristor caused by the failure of the AC circuit inversion of the thyristor, the traditional method is to use a fast fuse for overcurrent protection. The advantage is that the line is simple, but the reliability is poor. The reason is that this method does not monitor whether the thyristor is in the inverter failure state, but monitors the magnitude of the current through the device for protection.

It may happen that when the main circuit is just in the inverter failure state, the thyristor has been out of control, and just started to be in the "long-term" conduction in the inverter failure state.

However, due to the leakage inductance and stray inductance of the transformer, there is a transition process in the rise of the main loop current, and the current will not immediately reach the temperature that causes the fuse to blow. At this time, if the main circuit can be cut off in time, the thyristor will be protected. However, if only fuses are used for over-current protection, because the fuse blowing speed is related to the rationality of fuse selection, fuse structure, ambient temperature and other factors, it is necessary to ensure that the thyristor is monitored in the first one or two cycles just out of control It is technically difficult to reach and cut off the main circuit.

Therefore, the simple use of this protection method is often when the fuse is blown, the circuit has been in inverter failure for some time, and the thyristor may have been damaged because the PN junction temperature rises too high.