Earthing or Grounding Transformer - Construction, Working And Advantages

Construction and Working: 

Neutral is not available when the system is connected in delta or in many systems the neutral is available but it is not possible to connect it.

In such circumstances arrangement of the artificial neutral has to be made. Earthing transformer is used for this.

This is called the neutral earthing compensator also.

There is a three-limbed core in earthing transformer. Two windings are provided on each limb. 

One set of winding is connected in star and the neutral lead is taken out of this. This winding is connected to the winding of another set such that the currents of the two windings kept on each limb are in the opposite directions. 

So in the normal conditions less exciting current flows.

The impedance of the earthing transformer is less so the magnitude of the fault current is high. So to reduce the value of the fault current, the current limiting resistor is connected either in the neutral circuit or in the winding of the earthing transformer.

The figure shows the connections of the earthing transformer connected to the transmission line fed from a star delta-connected transformer.

Current limiting resistor R' is connected in the neutral circuit.

Application of Earthing Transformer:

1. Earthing transformer provides a low impedance path to the ground. so earthing transformer constantly maintain system neutral at or near to ground potential.

2. Earthing transformer limits the magnitude of transient overvoltages when restriking ground faults occur.

3. Earthing Transformer provides a source of ground-fault current during line-to-ground faults.

4. Earthing Transformer permits the connection of phase-to-neutral loads when desired.