Class of Neutral CT

Not necessarily, but it will surely make your life easier. Test the CTs and compare the magnetising curves. If different, ensure your relay settings suit the configuration.

"should the neutral CT be of the same class PS?"

Answer: Yes

" how do i ensure that the parameters like knee point voltage and saturation levels are same for phase CTs and neutral CTs?"

Answer: You need to specify the parameters of Neutral CT while tendering. Even after the award of purchase order you can provide these details to the transformer vendor.

For NCT you need to specify, CT ratio, class of accuracy, Knee Point voltage and Magnetising current Im.

For this particular case, specify 2500/1 A , PS class CT.

knee point voltage vk should be greater than or equal to 2 If(Rct + 2RL)

If is fault current reflected on CT secondary side. For 1600 kVA, the impedance is 6.25 %. With this we get fault current of 34.13 kA. Hence If is (1/2500)*34.13kA = 13.65 A

Here Rct CT secondary resistance. Get this from CT supplier. Usually transformer manufacturer outsources this CT from some vendors.

RL -- lead resistance of cable from CT to relay(For REF ANSI no.64). This you can calculate by finding the distance between HT Panel and Transformer. Use 4 Sq.mm or 2.5 sq.mm wire Copper.

Magnetising current should be less than 30 milli Amp @ Vk/2.

Prashanth

thank you sir.

i presume that the knee point voltage for the CTs in phase busbars shall be similar to the value computed by the formula for NCT and the same should be specified in specifications being given to panel manufacturer while ordering.