Grounding Transformers

Grounding Transformers are used in Ungrounded systems to provide a earth point.  Grounding Transformers are classified into two types

1) Zig Zag Transformers and

2) The Star-Delta Grounding Transformer with secondary unloaded.

We had looked at the Zig-zag Transformer in an earlier Post (Click here). 

Let us now look at the Star-Delta Grounding Transformer

The Star-delta grounding Transformer has a primary which is star connected and a delta secondary.  The phases of the star primary are connected to the busbar while the neutral is grounded.  The secondary of the transformer which is delta connected is usually left unloaded, though it can also be used to supply power.  The delta serves to provide a return flux path for unbalanced loads.  

During an earth fault, the zero sequence currents can flow through the grounded neutral of the transformer.  If the current is to be limited, a resistor can be added in series to the neutral of the transformer primary.