CT Sizing

Please provide total specification of transformer.

CT selection for Power transformer.Primary side

1) core 1 for instruments

2) core 2 for Protection for IDMT

3) core 3 for Differential protection.

Secondary side

1) core 1 for instruments

2) core 2 for Protection for IDMT

3) core 3 for Differential protection

The transformer primary may be 110 kV and secondary may be 33 kV

For 20 MVA The full load current on primary side will be (20*1000)/1.732/110=104.97 Amps.Therefore you can select CT ratio of 120/1.Supposing you will be loading only 50% only , you can opt for dual ratio 60/120/1.

Similarly for secondary side the full load current would be 20*1000/1.732/33=349.91 Amps.You can opt for 200/400/1 .

Class of accuracy 5P10 for IDMT relays,PS class for differential Protection, class 0.5 for Instruments.

dear Sir,

Hello ,I am Prasad and I request you to please clarify the following doubt.

We are designing for one project where the Transformer is 2 MVA, 33 kv/ 0.415 kv, Dyn 11.Whether REF protection is required for the transformer or not.If it is required how to calculate the size of neutral CT.I kindly request you to clarify the same .

Thanks & Regards,

K.Prasad

Dear Sir ,

I would like to clarrify one point with,could you please answer my quest?

1)Is it possible to connect 100/1 5P10 2.5VA to 200A load contunueosly?

2)1000KVA ,with FLC 41.88 can able to connect 25/1 CT?If yes any problem will occur to the CT?

Regards

Dennis

For the 132 kV System has been explained. Wheras for 400 kV or 220 kV system there will be 5 core CTs. One for Metering, one for O/C, E/F, one for differential, one for distance preotection, norammly these two requires PS core and called as special protection and one more core will be spare.

Each CT cores must be have size 2 core x 2.5 sq.mm to avoid opening in extraordinary cases or can have a size of 1 core x 4 sq.mm.

As usual the Base MVA needs to be considered and the system fault current needs considered for selecting the ratio of CT.

Regards,

N.Suresh