Why Should The Transformer Be Grounded At One Point? Isn’t Multi-Point Grounding More Reliable?

1. Why should the transformer core be grounded?

In the process of transformer operation, the metal structure and parts of the iron core, fixed iron core and winding are in a strong electric field, and the ground potential is high under the action of the electric field. If the iron core is not grounded, there will be a potential difference between it and the grounded wire clamp and the fuel tank, and intermittent discharge may occur under the effect of the potential difference.

In addition, during the operation of the transformer, there is a strong magnetic field around the winding, the iron core, metal structure, parts are in a non-uniform magnetic field, and the distance between them and the winding is not equal, so the electromotive force generated by the magnetic field induction of each metal structure and parts is not equal, and there is a potential difference between them. Although the potential difference is not large, it can also break through a small insulation gap, and may also cause sustained micro-discharge.

Whether it is the intermittent discharge phenomenon caused by the potential difference, or the continuous micro-discharge phenomenon caused by the breakdown of the small insulation gap, it is not allowed, and it is difficult to check the location of these intermittent discharges.

The effective solution is to ground the metal structure and parts of the iron core, fixed iron core and winding reliably, so that they are at the same grounding potential with the fuel tank. The core ground of the transformer is one point ground and only one point ground. Because the silicon steel sheets of the core are insulated from each other, this is to prevent large vortices. Therefore, all the silicon steel sheet must not be grounded or multi-point grounded, otherwise it will produce a large eddy current, and the core is seriously heated.

The iron core of the transformer is grounded, and usually any silicon steel sheet in the iron core is grounded. Although the silicon steel sheet is insulated, the insulation resistance is very small. The uneven strong electric and magnetic fields can make the high voltage charge induced in the silicon steel sheet flow from the ground point to the earth through the silicon steel sheet, but it can prevent the eddy current from flowing from one piece to another. Therefore, as long as any piece of the silicon steel sheet of the iron core is grounded, the entire iron core is grounded.

2. Why can’t the transformer core be grounded at multiple points?

Because the transformer core laminates can only be grounded at one point, because if there are more than two ground points, a loop may be formed between the ground points. When the main track passes through this closed loop, circulating current is generated in it, causing internal overheating and accidents. The melted local iron core will cause a short circuit fault between the iron filings, increase the iron loss, seriously affect the performance and normal operation of the transformer, and can only be repaired by replacing the core silicon steel sheet. Therefore, the transformer does not allow multi-point grounding, only one point grounding.

3. Multi-point grounding is easy to form circulation and heating.

During the operation of the transformer, metal parts such as the iron core and wire clip are in a strong electric field, because electrostatic induction will generate floating potential on the iron core and metal parts, and this potential will discharge to the ground, which is of course unacceptable. Therefore, components such as the iron core and its clamp need to be properly and reliably grounded (except for through-hole bolts). The core is only allowed to ground at one point. If there are two or more ground points, the core will form a closed loop with the ground point and the earth. When the transformer is running, the magnetic flux will pass through this closed loop, generating the so-called circulating current, causing local overheating of the iron core, and even burning out metal parts and insulation layers.