Why Transformers Only Allow Single-Point Grounding? Iron Core Grounding Principles

Transformers are common equipment in power systems used to change the magnitude of AC voltage.  The grounding connection is not only crucial for their normal operation but also directly affects personal safety and power system stability. Many people wonder: why can transformers only be grounded at one point? Wouldn't multi-point grounding be more reliable? Below, we will explain this in detail from two aspects: "Why does the iron core need grounding?" and "Why can't it be grounded at multiple points?"

Why Does the Transformer Iron Core Need Grounding?

During transformer operation, the iron core and the metal structures, parts, and components that fix the iron core and windings are all in a strong electric field and a non-uniform strong magnetic field. This will generate a high potential to ground due to electric field induction, and a potential difference between different components due to magnetic field induction—

Problems caused by the electric field: If the iron core is not grounded, there will be a potential difference between it and the grounded clamps, oil tank, etc., which may cause intermittent discharge.

Problems caused by the magnetic field: Different metal components have different induced electromotive forces due to their different distances from the windings. Even if the potential difference is not large, it may break down tiny insulation gaps, leading to continuous micro-discharges.

These discharge phenomena are not only unacceptable, but also extremely difficult to locate. The effective solution is to reliably ground the iron core and related metal components, so that they are at the same ground potential as the oil tank, avoiding discharges caused by potential differences.

It should be noted that the silicon steel sheets of the iron core are insulated from each other (the purpose is to prevent the generation of large eddy currents). Therefore, it is not necessary to ground all the silicon steel sheets—only one silicon steel sheet needs to be grounded. Through weak conduction between the silicon steel sheets (under strong electric and magnetic fields, the induced high-voltage charge can flow to the ground through the silicon steel sheets), the "entire iron core is effectively grounded," while avoiding eddy currents flowing across the sheets.

Why can't the Transformer Iron Core be Grounded at Multiple Points

Multi-point grounding is one of the common faults of transformers. The core risk lies in the formation of a closed loop and the generation of circulating current, leading to local overheating. The specific principle is as follows:

Generation of loops and circulating currents: If the iron core has two or more grounding points, a closed loop will be formed between the grounding points. When the transformer is in operation, the main magnetic flux passes through the closed circuit, inducing a circulating current (eddy current).

The dangers caused by eddy currents: Eddy currents can cause local overheating of the iron core, and in severe cases, may melt parts of the iron core, causing short circuits between the iron core laminations, significantly increasing iron core losses, and ultimately affecting the transformer's performance or even rendering it inoperable. Such faults usually require replacing the iron core silicon steel sheets to repair.

Conclusion

the core purpose of grounding the iron core is to "eliminate potential differences and prevent discharge," and "single-point grounding" is a key design to achieve this purpose while preventing the formation of eddy currents and avoiding overheating risks. Therefore, the transformer iron core must and can only be grounded at one point; two-point or multi-point grounding is strictly prohibited.