Why Are Voltage Transformers Burning Out? Find the Real Causes
In power circuits, voltage transformers (VTs) are often damaged or burned out. If the root cause is not identified and only the transformer is replaced, the new unit may quickly fail again, disrupting power supply to users. Therefore, the following checks should be performed to determine the cause of VT failure:
If the voltage transformer has ruptured and oil residue is found on the silicon steel laminations, the damage was likely caused by ferroresonance. This occurs when unbalanced voltages or harmonic sources in the circuit cause voltage fluctuations that form an oscillating circuit with system inductance. This resonance severely damages the VT's core laminations and typically results in the failure of one or two phases.
If there is a strong burnt odor from the VT, or blackening and burn marks on the secondary terminals and wiring, this indicates a secondary-side ground fault, which causes the primary-side phase-to-phase voltage to rise. Inspect the secondary wiring for insulation damage, excessively stripped wire ends, or exposed copper strands that may contact grounded parts. Also check if the secondary fuse or connected components have failed due to insulation breakdown causing grounding.
If the primary terminal is blackened due to overheating and the mounting bolts are deformed, the cause is often excessive discharge current—particularly when the VT is used as a discharge coil for capacitor banks. Check whether the primary fuse element is oversized or incorrectly installed. The primary fuse rating for a VT is typically 0.5 A, and for low-voltage VTs, it generally does not exceed 1 A.
If no obvious external damage is found after the VT fails, inspect external components and wiring for abnormalities. If nothing is found, interview the on-duty personnel to determine if there were "cracking" or "popping" sounds before failure. Such noises indicate internal inter-turn discharge in the transformer winding, usually due to poor manufacturing quality of the voltage transformer.
