Hazards of Multi-point Grounding Faults in Transformer Cores

Hazards of Multi-point Grounding Faults in Transformer Cores

During normal transformer operation, multi-point grounding of the core is strictly prohibited. This is because, under normal operating conditions, alternating magnetic fields exist around the windings. Due to electromagnetic induction, parasitic capacitances are formed between the high-voltage and low-voltage windings, between the low-voltage windings and the core, and between the core and the tank.

Through the coupling effect of these parasitic capacitances, the energized windings induce a floating potential on the core relative to the ground. Since the distances between the core (and other metal components) and the windings are unequal, potential differences exist between the various components. When the potential difference between two points is high enough to break down the insulation between them, spark discharge occurs. This type of discharge is intermittent, and over the long term, it adversely affects both the transformer oil and the solid insulation.

To eliminate this phenomenon, the core is reliably connected to the tank, bringing it to the same electrical potential as the tank. However, if the core or other metal components have two or more grounding points, a closed loop will form between these points. This creates a circulating current, leading to localized overheating and oil decomposition, which degrades the insulation performance. In severe cases, it can burn out the core's silicon steel sheets, resulting in a major power transformer accident. Therefore, the main transformer core must be grounded at exactly one point.