CT wiring for switch gear

[NCTHAI]

Can experts share their experience pertaiing to CT Wiring? Here is the conditions for substation under advance stage of energization:

1) CBCT (Core Balance Current X'mer) is used for earth fault protection in motor feeders and distribution trasnformer feeders (outgoing). CBCT Ratio 150/1 A, Max fault current is limited to 400 A - Limited by use of NGR installed in
- CBCT class is "Class 1"

Any issue with this? Is it normal? What are the implications? Is it not required to have Px or XPXX class CT?


2) For incomer circuits on PCC (power Control Center) differential relay is provided. Also there is separate CT for earth fault potection - From Neutral of trasnformer. This CT is 5P10 class. Differential CT provided is Class Px.

Is there issue with this? The problem is we are mixing CT for differentil and earth fault. So the questuin is is it aceotable to use different class CT for protectin,


Thanks in advane for you advice.

 

[jose0007]

Differential protection involves 2 nos. CT for the protected section. Please check the CTs at other end (trafo phase end) and they will be of the same type, i.e. Px

In your case, the 5P10 class CT is for earth fault protection (most likely SEF)

 

[NCTHAI]

Jose0007

Yes it is correct. All differential CTs - on each side of X'mer are PX class. 5P10 CT is installed on neutral and is in REF circuit.

 

[jose0007]

Can you tell me which relay are you using here?

I checked up some of our old projects, and we have used 5P20 neutral CT in case of Siemens 7UT61 relay for REF protection.