selecting the ratio of C.T.’s employed in grounding transformer protection schemes

[pawlos]

can i get a guide on how i can select the ratio of C.T.’s employed in grounding transformer protection schemes?the plan is to use this neutral CT for REF protection i want to know how to decide on the ratio for the neutral CT of the earthing transformer.

 

[dpc]

Without any knowledge of your system, I find it odd that you would have REF on a grounding transformer since these are generally used as source of ground fault current for clearing faults downstream.

 

[pawlos]

To give more detail on the issue the Transformer is a 132/15 kv Ynd11 the LV side is earthed through an earthing transformer.the question is how to decide the ratio for the neutral CT of the earthing transformer and main transformer.

 

[RRaghunath]

I suppose you are discussing Yd connected main transformer with a grounding transformer at D-side terminals.
The REF protection will be taking phase CTs from the switchboard and the neutral CT of the grounding transformer. If yes, it is recommended to have neutral CT ratio to be identical to that of phase CTs, for REF protection.
The neutral CT for standby ground fault protection may have a lower primary rating, equal to the rated ground fault current or even lower so as to allow sensitive setting.

 

[pawlos]

@RRaghunath thank you ,your assumption is right.just to be sure when you mean the rated ground fault current are you talking about rating of the grounding transformer?

 

[RRaghunath]

pawlos,
it is the design ground fault current that is computed taking the zero sequence impedance of the grounding transformer and the NGR (if any).

 

[pawlos]

@RRaghunath ,sorry to bother you can you give me more explanation on the design ground fault current or do you have any material you can refer me to?

thank you in advance

 

[RRaghunath]

Pawlos,
Design ground fault current means the earth fault current magnitude fixed during design of the 15kV system. This forms the basis for selecting the grounding transformer impedances and the Neutral Earthing Resistor value.
Your design department should be able to help you further on this.