Common Abnormalities
1、Unbalanced three-phase voltage indication: One phase drops (may be zero) while the other two are normal, line voltage is abnormal, or accompanied by audio and visual signals. This may be due to the melting of the high-voltage or low-voltage fuse of the transformer.
2、Unbalanced three-phase voltage indication in an uneffectively grounded neutral system: One phase drops (may be zero), and the other two rise (up to line voltage) or the pointer fluctuates. This may be a single-phase grounding fault or fundamental frequency resonance. If all three-phase voltages rise simultaneously and exceed the line voltage (the pointer may swing to the maximum), it may be subharmonic or high-frequency resonance.
3、Repeated melting of the high-voltage fuse may be caused by severe internal insulation damage, such as interlayer or turn-to-turn short circuit of the winding.
4、In an effectively grounded neutral system, during bus switching operations, phase voltage rises with low-frequency fluctuation, which is generally a series resonance phenomenon. If phase voltage suddenly rises or drops abnormally without any operation, it may be due to internal insulation damage of the transformer, such as insulation bracket winding, interlayer or turn-to-turn short circuit of the winding.
5、In an effectively grounded neutral system, unstable voltmeter indication when the voltage transformer is put into operation may be due to poor contact of the N (X) terminal grounding of the high-voltage winding.
6、Handling of open circuit in the voltage transformer circuit.
Handling Methods
1、In accordance with the relevant provisions of relay protection and automatic devices, withdraw the relevant protection to prevent misoperation.
2、Check whether the high-voltage and low-voltage fuses and automatic air switches are normal. If the fuse is melted, find out the cause and replace it immediately. Exercise caution if it melts again.
3、Inspect all joints in the voltage circuit for looseness or disconnection, and check the switching circuit for poor contact.
