Grounding / Earthing Transformer

Grounding Transformer is often called Earthing Transformer. The purpose of Grounding Transformer is to artificially create neutral point. In Delta Connected Transformer or Generator where neutral point is unavailable or in case where neutral point is inaccessible, Grounding Transformer is used to create neutral point.

Constructional Design of Grounding Transformer

Grounding Transformer is so designed to only provide neutral point. Its VA is very low. Such transformer do not have any secondary winding rather only primary winding. The primary winding is split into two parts and each part is having the same number of turns. These parts of winding are connected in series on limb of core in such a manner that current in one part is opposite in direction to the current in second part of winding. This means mmf produced by one part of winding is equal but opposite to the mmf produced due to second part. Thus the resultant mmf in any of the core limb is zero under normal condition. Hence the inductance of windings of Grounding Transformer is ideally zero under normal operating condition. 

During line to ground fault, fault current flows through the grounded transformer and the fault kVA is supplied by the source (Delta connected Generator / Transformer). Therefore it is very important to ground the neutral point of Grounding Transformer by high resistance to limit the current. 

Grounding Transformers are short time rated in the range of 10 sec to 1 minute. Therefore the size reduces when compared with a power transformer of same rating. It becomes important in high voltage circuit to have neutral point for protection purposes. Therefore with the help of such transformer, neutral point is created.