Ct saturation

[Madka]

I have a question about ct saturation calculation
I know that when the current transformer is saturated, the secondary current is disturbed and drops
In some documents and manuals instead of the maximum short-circuit current, relay setting current considered for saturation calculations.
The maximum short-circuit current is definitely higher than the relay setting current, and the current transformer may become saturated in this current and the relay may not recognize it correctly.

 

[che12345]

"...The maximum short-circuit current is definitely higher than the relay setting current, and the current transformer may become saturated in this current and the relay may not recognize it correctly."
I have the following opinion for your consideration.
1. In the IEC world refer to IEC 60044-1 for CT selection. For "protection", the standard Classes are 5P and 10P. The members identify the maximum permissible total error with rated accuracy limit current. Note: the letter P stands for "protection". The " Accuracy limit factors" : 5, 10, 15, 20, 30. E.g. 5P5, 5P30, ...
2. Suggestion: In case a 5P30 is under-rated for the application, use a 1A instead of 5A secondary current, with higher (I[sub]p[/sub]/I[sub]s[/sub]) current ratio and with higher rated (VA) burden.
Che Kuan Yau (Singapore)

 

[DiscoP]

I'd suggest reading the manuals very carefully, and consulting with the product application support experts of the relay manufacturer.

It is true that sometimes a value less than the maximum fault level will be OK and some protection schemes can tolerate some saturation, but there is normally a multiplying factor applied (often with some very specific limitiations and conditions applied).

If these are not understood correctly, then there is a chance the scheme will not perform as desired.

The other issue to bear in mind is if the relay fails and you need to use something else then a low CT rating may limit future options.