When the KGPS series variable frequency power supply itself or the load malfunctions, the power cabinet will usually shut down automatically and enter a protected state. In this case, the power should be immediately turned off, and the cause of the fault should be investigated and repaired. It should be noted that the failure rate of components carrying high current in the main circuit (such as thyristors and fast-acting fuses) is much higher than that of the control board. These components should be the focus of the inspection.
Based on the fault symptoms, faults can be divided into two main categories: complete failure to start and failure to operate normally after starting. As a general principle, when a fault occurs, a comprehensive inspection of the entire system should be performed with the power off. This includes the following aspects:
1. Input voltage
After disconnecting the main circuit, use a multimeter to measure whether there is voltage after the main circuit switch (contactor) and the control power fuse. This will rule out the possibility of open circuits in these components.
2. Rectifier
It includes two rectifiers, each with 6 thyristors and 6 pulse transformers.
A simple method for measuring thyristors is to use a multimeter's resistance range (200 ohms) to measure the resistance between the anode and cathode and between the gate and cathode. The thyristor does not need to be removed during measurement. Under normal circumstances, the resistance between the anode and cathode should be ∞, and the resistance between the gate and cathode should be between 10 and 50 ohms. Too low or too high a resistance indicates that the gate of the thyristor has failed and cannot be triggered to conduct.
The secondary side of the pulse transformer is connected to the thyristor, and the primary side is connected to the main control board. Use a multimeter to measure the resistance of the primary side; the resistance value should be approximately 50 ohms.
3. Inverter
Includes 16 fast thyristors and 8 pulse transformers, which can be checked using the method described above.
4. Compensation capacitors
The compensation capacitors connected in parallel with the load may be short-circuited. The capacitors are generally installed in groups on the capacitor rack. When checking, first determine the group containing the short-circuited capacitor, disconnect the connection point between the busbar of each group of capacitors and the main busbar, and measure the resistance between the two busbars of each group of capacitors. Under normal conditions, the resistance should be ∞. After identifying the faulty group, disconnect the copper busbars leading from each capacitor to the busbar, and then check each capacitor individually to find the one that has failed. Each capacitor consists of 8 cores; the casing is one terminal, and the other terminal is connected to the end cap through an insulator. Generally, only one core will fail. By disconnecting the lead on this insulator, the capacitor can be used intermittently.
Another fault of the capacitor is oil leakage. A leaking capacitor can continue to be used for a short time, but it should be replaced as soon as possible. The angle steel supporting the capacitor and the capacitor rack are insulated. If the insulation breaks down, it will cause a ground fault in the main circuit. Measuring the resistance between the capacitor casing lead and the capacitor rack can determine the insulation condition.
5. Water-cooled cables
These are part of the load circuit and are subjected to tension and torsion during operation. After a long time, they may break at the flexible connection. Testing the resistance between the two output busbars of the power cabinet can determine whether the load circuit is properly connected; the resistance should be zero under normal conditions.
6. Rectifier trigger pulse
There are working indicator lights on the pulse transformer of the rectifier thyristor. Turn on the power, press the "Medium Frequency Start" button, and then open the cabinet door to observe whether each pulse indicator light is on. If it is not on, it means that there is no trigger pulse in this circuit.
