What is the reason for burning thyristors frequently?

May 21, 2022

Fault phenomenon: frequently burn out the thyristor components, and burn out immediately after replacing the new thyristor.

Fault analysis and treatment: This is a fault phenomenon that is difficult to maintain. The price of thyristor is relatively expensive, but burning thyristor is impossible to prevent, so we should be extra careful when repairing such faults. There are the following reasons for burning out the thyristor.

a. The instantaneous burr voltage of the thyristor bearing the reverse voltage is too high when the reverse correlation is broken. In the main circuit of intermediate frequency power supply, the instantaneous inverse burr voltage is absorbed by resistance capacitance absorption circuit. If the resistance and capacitance in the absorption circuit are open circuit, the reverse burr voltage will be too high and the thyristor will be burned out. In case of power failure, measure the absorption resistance and absorption capacitance with a multimeter to judge whether the resistance capacitance absorption circuit is faulty. Loose connecting wires will also produce high voltage.

b. The insulation of load to ground is reduced. The insulation of the load circuit is reduced, which causes the load to strike fire between the ground, interferes with the trigger time of the pulse, or forms a high voltage at both ends of the thyristor and burns out the thyristor elements.

c. Pulse trigger circuit failure. If the trigger pulse is suddenly lost during the operation of the equipment, it will cause the open circuit of the inverter thyristor, high voltage will be generated at the output end of the intermediate frequency power supply, and the thyristor components will be burned out. This kind of fault is generally the circuit fault formed by the inverter pulse, which can be checked by oscilloscope. It may also be the poor contact of the inverter pulse lead. Shake the wire connector by hand to find out the fault location.

d. The load is open when the equipment is running. When the equipment is running at high power, if the load is suddenly in an open circuit state, a high-voltage burned element will be formed at the output end.

e. When the equipment is running, the load is short circuited. When the equipment is operating at high power, if the load is suddenly in a short circuit state. There will be a great short-circuit current impact on the thyristor. If the overcurrent protection action is too late for protection, the thyristor components will be burned out.

f. Protection system failure (protection failure). The safety of the thyristor is mainly guaranteed by the protection system. If the protection system fails and the equipment is slightly abnormal, it will endanger the safety of the thyristor. Therefore, when the thyristor is burnt out, the inspection of the protection system is essential.

g. Thyristor cooling system failure. There are two ways to cool the thyristor: one is to cool the thyristor, and the other is to ensure its normal operation. Water cooling is widely used, and air cooling is generally only used for power supply equipment below 100kW. Generally, the medium frequency equipment with water cooling mode is equipped with water pressure protection circuit, but it is basically the protection of total water inflow. If there is water blockage in one way, it cannot be protected.

h. Reactor failure. Ignition inside the reactor will cause intermittent current at the inverter side and high voltage at the inverter input side to burn out the thyristor. In addition, if the reactor is replaced during maintenance, and the inductance and iron core area of the reactor are less than the required value, the reactor will lose the current limiting effect due to magnetic saturation and burn out the thyristor during high current operation.

i. The commutation inductance has water seepage and the turn to turn insulation is reduced, resulting in current instability.