By hvhipot 
(1) Unbalanced three-phase voltage indication: One phase decreases (can be zero), the other two phases are normal, the line voltage is abnormal, or accompanied by sound or light signals, which may be due to the melting of the high or low voltage fuse of the transformer;
(2) Neutral point non effective grounding system, three-phase voltage indication imbalance: one phase decreases (can be zero), the other two phases increase (can reach line voltage) or pointer swings, which may be a single-phase grounding fault or fundamental frequency resonance. If the three-phase voltages rise simultaneously and exceed the line voltage (pointer can swing to the end), it may be a frequency division or high-frequency resonance;
(3) High voltage fuses have blown multiple times, which may be due to severe damage to the internal insulation, such as short circuits between winding layers or turns;
(4) When the neutral point is effectively grounded in the system, the phase voltage rises and oscillates at low frequencies during the busbar switching operation, which is generally a series resonance phenomenon; If there is no operation and the phase voltage suddenly rises or falls abnormally, it may be due to internal insulation damage of the transformer, such as insulation support winding, winding layer or turn short circuit fault;
(5) The neutral point effective grounding system may have unstable voltmeter readings when the voltage transformer is put into operation, which may be due to poor grounding contact at the N (X) end of the high-voltage winding.