Neutral Current Transformer.

hi!

Max. un balaned current flowing in neutral ct = 1.732 * Ir

this is being a protection ct, 5p10 class, 15 va , CT ratio - 2(1.732*Ir) / 5 or 1 Amp

depends upon the relay range.

e.g. for 5 mva 33kv/433v transformer, - Ir = 6667 Amp. on secondary star winding.

Max. un balanced current - 11547 Amp

So, CT ratio > 12000/5 Amp,

We select (2 * 11547 = 23094 A) 24000 / 5Amp

Bar primary CT. of class 5P10 , 15 VAburden.

With the above thumb rule, you will find the relay setting on the linear portion of the inverse curve of the relay characteristic.