How to diagnose transformer core faults

During normal transformer operation, an electric field exists between the energized windings and the oil tank, with the core and other metal components situated within this field. Due to uneven capacitance distribution and varying field strengths, if the core is not properly connected, charging and discharging phenomena will occur. This damages the dielectric strength of both the solid insulation and the oil. Therefore, the core must have one reliable grounding point.

The core is composed of silicon steel sheets. To reduce eddy currents, there is a certain insulation resistance between the sheets. However, the inter-lamination capacitance is extremely large, which can be considered a conductive path in an alternating electric field. Grounding the core at just one point is sufficient to clamp the electrical potential of the entire stack of laminations.

If the core or its metal components have two or more grounding points (multi-point grounding), a closed loop will form between the grounding points. This loop links a portion of the magnetic flux, inducing an electromotive force (EMF) and creating a circulating current. This results in localized overheating and can burn out the core.

For a transformer core, single-point grounding is the only correct and normal method. In other words, the core must be grounded, and it must be grounded at exactly one point. Core faults are mainly caused by two factors: poor construction workmanship leading to short circuits, and accessories or external factors causing multi-point grounding.

Once monitoring detects a multi-point grounding fault in the transformer, the transformer should be shut down promptly. After being taken out of service, the multi-point grounding fault must be completely eliminated. To resolve such faults, corresponding maintenance measures should be adopted based on the type and cause of the multi-point grounding.