Equipment electrical maintenance knowledge

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

Time: 2019-08-15

 Fault analysis and processing: This is a fault phenomenon that is difficult to repair. The price of thyristors is relatively high, and the burnout of thyristors is unpredictable, so care must be taken when repairing such faults. Burning the thyristor for the following reasons.

    a. When the thyristor is reversely correlated, the instantaneous glitch voltage subjected to the reverse voltage is too high. In the main circuit of the intermediate frequency power supply, the instantaneous reverse glitch voltage is absorbed by the RC absorption circuit. If the resistor or capacitor is open in the snubber circuit, the reverse glitch voltage will be too high to burn out the thyristor. In the case of power failure, use a multimeter to measure the resistance of the absorption resistor and the capacity of the absorption capacitor to determine whether the resistance absorption circuit is faulty. Loose connections can also create high voltages.

    b. The load is reduced to ground insulation. The insulation of the load circuit is reduced, causing the load to ignite between the ground, disturbing the triggering time of the pulse, or forming a high voltage across the thyristor to burn out the thyristor components.

    c. The pulse triggers a loop failure. If the trigger pulse is suddenly lost during the operation of the device, the inverter thyristor will be opened, and the output of the intermediate frequency power supply will generate high voltage, which will burn out the thyristor components. This type of fault is generally a circuit fault formed by an inverter pulse. It can be inspected by an oscilloscope, or it may be a poor contact of the inverter pulse lead. The wire joint can be shaken by hand to find the fault location.

    d. The device is loaded with an open circuit during operation. When the device is running at high power, if the load is suddenly open, a high voltage burnout component will be formed at the output.

    e. The device is shorted during operation. When the device is running at high power, if the load is suddenly short-circuited. There will be a large short-circuit current surge on the thyristor. If the overcurrent protection action is too late to protect, the thyristor component will be burned out.

    f. Protection system failure (protection failure). Whether the thyristor can be safe is mainly guaranteed by the protection system. If the protection system fails, the equipment will work slightly abnormally, and the thyristor will be safe. Therefore, inspection of the protection system is essential when the thyristor burns out.

    g. The thyristor cooling system is faulty. Thyristors generate a lot of heat during operation and need to be cooled to ensure normal operation. Generally, there are two ways to cool the thyristors: one is water cooling and the other is air cooling. Water cooling is widely used, and air cooling is generally only used for power equipment below 100 kW. The medium-frequency equipment that usually uses water-cooled mode is equipped with a water pressure protection circuit, but basically it is the protection of the total influent water. If there is water blockage on a certain road, it cannot be protected.

    h. Reactor failure. The internal ignition of the reactor will cause the current on the inverter side to be interrupted, and a high voltage burnout thyristor will also be generated on the inverter input side. In addition, if the reactor is replaced during maintenance, and the inductance and core area of the reactor are less than the required value, the reactor will burn out the thyristor due to the loss of current limitation due to magnetic saturation during high current operation.

    i. The commutation inductance has water seepage and the inter-turn insulation is reduced to cause current instability.

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