Why is the core of a power transformer grounded?

Transformer is a static piece of equipment and it has primary and secondary winding. The low voltage winding has few turns of wire wound around the core. The high voltage winding has many turns of wire wound around the low voltage winding. This scheme of winding reduces the electric stress between the core and the winding.

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transformer core grounding

Transformer Core

The transformer core is formed with thin sheets of laminated CRGO core. The laminated sheets provide a conductive path to the magnetic field, but on the other hand, it provides electrical insulation between the sheets.  The laminated sheets have very low reluctance, and because of this, most of the flux pass through the core without leakage. The eddy current sets up in the transformer core because of the induced voltage in the core. The transformer is earthed to have a minimum resistance for fault current to pass to the ground.

How does Core grounding Protects the equipment?

In case of insulation failure of the winding, a high voltage is induced into the core. The induced high voltage causes partial discharge and, the transformer may fail. Thus, it is absolutely necessary to provide a low resistance path to the fault current so that the upstream breaker can be tripped to isolate the faulty equipment.

For equipment safety, we connect the transformer core to the ground from a single point. The fault current finds its path from core to the ground in case the core attains high potential caused by insulation failure. There are two methods of transformer core grounding.

  •  Transformer core connection to transformer grounded tank 
  •  Separate bushing for core grounding connection and connecting bushing to separate earth point. 

How to ground the core of a large power transformer?

In the case of a large rating transformer, the core of the transformer has multiple sections. The cooling ducts insulate the core sections. The core jumpers bond the core sections together and the final jumpers connection connects core sections to single point earth.

The heavy magnetic field develops at the time of charging, as a result, a high voltage develops because of capacitive coupling between winding and core. The voltage developed in the core discharge through the earth point of the core. The protection device operates in the event of a winding to core insulation failure. The core of the power transformer is grounded at a single earth point in order to avoid circulating current with multiple earthing of the core.

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