Instrument Transformer and Power Management (P1) Course
Chapter (6) : Current Transformers
6.5 Earthing of current transformer
6.5.1 Secondary winding :
All current transformers should have their secondary windings grounded in service because a high voltage may develop in the secondary winding due to capacitor coupling between the windings.
The grounded secondary winding also protects personnel if there is a breakdown of insulation between primary and secondary windings.
6.5.2 Frame :
All current transformers must have their frames grounded, an ungrounded current transformer could develop a charged condition in the same manner as a capacitor. The capacity of the charged state would depend on the potential supplying the transformer and the physical size of the transformer.
6.5.3 Grounding effect in protection circuit :
The secondary winding grounding is very important to the protection scheme in the substation, especially for distance relay.
It gives a very important looking to the current direction at normal and fault condition," The polarity has no effect on the direction of current in or out the relay".
There are two types of earthing :
1- Earthing in direction of feeder .( feeder earthing)
2- Earthing in direction of Bus bar.( bus bar earthing)
If the relay require current inject coil , then the CT must be feeder earthing, but if the relay require current rejection, then the CT must be bus bar earthing.
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