Generally, at every voltage level, the neutral point is available from the generator or transformer i.e., when the star connected. But when there is a delta connection or when the neutral is desired on the busbars etc., then the neutral point will not be available. In such cases, the most common method used for neutral grounding is through earthing transformer.
The earthing transformer, also known as the grounding transformer provides a neutral point where it is not available. The earthing or grounding transformer provides a low impedance path to the neutral and overvoltages due to arcing grounds, static induced charges, and lightning surges are eliminated by discharging to earth. By using earthing transformer a neutral point is created in the following way,
- Using a zig-zag earthing transformer, and
- Using star-delta earthing transformer.
Zig-Zag Earthing Transformer :
The system neutral grounded through a zig-zag transformer is shown below. It is a three-phase, dry-type, air-cooled autotransformer.
There is no secondary winding in a zig-zag transformer. The schematic arrangement of winding in an earthing transformer is shown in the below figure.
In a zig-zag transformer, there are three limbs and each limb is provided with two identical windings. These windings are wound such that under normal conditions the two fluxes on each limb oppose each other in such a manner that the net flux on each limb is negligibly small. Hence, during normal conditions, the transformer draws very little magnetizing current.
During an earth fault on the circuit, the zero-sequence current will flow in the transformer winding through the earthing. The earth fault current finds little impedance. Earthing transformers are of short time rating (10 seconds to 1 minute) which makes them small in size when compared to the power transformer of the same rating.
Star-Delta Earthing Transformer :
In case the zig-zag transformer is not available, then a star-delta transformer can be used without loading the delta side as shown below.
In a star-delta transformer, the primary winding of the transformer is in star connection with neutral connected to the ground. The terminals of the primary winding are connected to the three-phase ungrounded system. The secondary of the transformer is in delta connection and is not connected to any external circuit i.e., it operates unloaded.
The operation of this transformer is similar to the zig-zag transformer, which offers high impedance to the current under normal conditions, and on the occurrence of a ground fault, a low impedance path is provided for the ground fault currents.
It should be ensured that not to operate more than one earthing transformer connected to the same supply system at the same time and also short-circuit protection should be provided in each line of the transformer.
