Grounding Essentials and Composition Status of Transformer Cores

Transformer cores are primarily composed of silicon steel sheets during manufacturing. To reduce eddy currents to a certain extent, there is a specific insulation resistance between the sheets. However, due to the extremely large inter-sheet capacitance, the sheets can be considered electrically connected in an alternating electric field. Therefore, grounding the core at a single point is sufficient to clamp the potential of the entire stacked laminations to ground potential.

If the core or its metal components are grounded at two or more points (multi-point grounding), a closed loop will be formed between the grounding points. This loop links part of the magnetic flux, inducing an electromotive force (EMF) and creating a circulating current. This can lead to localized overheating and even burn out the core.

Key Points for Transformer Core Grounding

1. Install a separate grounding lead for the yoke clamps. If the core accidentally contacts the upper clamp and causes a multi-point grounding fault, the fault current will only circulate within the core and clamp assembly. Without a separate lead, no current will flow through the main core grounding lead, potentially leading maintenance personnel to mistakenly believe that no fault exists. By installing a separate lead, any contact between the core and the clamp will form a loop through the two external grounding leads, allowing the grounding current to be accurately detected externally.

2. Position the core grounding shunt at the center of the yoke's cross-section. With this placement, regardless of the position of the insulating bolts on the yoke tie bands or the location of the fault grounding point, the induced voltage in the loop will be only 1/4 of the turn voltage. Consequently, the grounding current will be limited to just a few amperes, which is significantly lower than when the grounding shunt is placed in other positions.