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Why Should a Transformer Core be Grounded?

During the operation of a transformer, the core and the metallic structures and parts that secure the core and windings are subjected to a strong electric field. This induces a high potential difference between these components and the ground. If the core is not grounded, a potential difference between the core and grounded components such as clamps and the tank can cause intermittent discharge. Additionally, the magnetic field around the windings induces varying electromotive forces (EMF) in different metallic components due to their differing distances from the windings. These small potential differences can still cause continuous minor discharges through tiny insulation gaps, which are unacceptable and difficult to locate.

The effective solution is to reliably ground the core and all associated metal structures, ensuring they are at the same potential as the tank. The transformer core grounding must be done at a single point only. The core laminations are insulated from each other to prevent significant eddy currents, which would otherwise cause excessive heating. Hence, multiple grounding points for the core are prohibited, as they would create unwanted eddy currents leading to severe core overheating.

Why a Transformer Core Should Not Be Grounded at Multiple Points:

If the core laminations are grounded at more than one point, a closed loop may form between the grounding points. When the main magnetic flux passes through this loop, it induces circulating currents within it, causing internal overheating and potentially severe damage. Such damage can include local core burning and short circuits between core laminations, increasing core losses and affecting transformer performance. In severe cases, this can lead to transformer failure and necessitate core replacement or repair.

Risks of Multiple Grounding Points:

During transformer operation, the core and metal parts within the strong electric field will have induced voltages, causing them to discharge to the ground if not grounded properly. Single-point grounding prevents circulating currents (ring currents) that can occur if the core is grounded at multiple points. These circulating currents cause localized overheating and can damage the metal components and insulation, risking the operation of the transformer.

Thus, grounding at one point is essential for safe and efficient transformer operation.

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