multi-ratio CT calculation questions 1

[metsfan1395]

Hoping someone can show me the proper method of how to calculate the following. Not sure how to calculate these and would like some guidance as to how it is done, not the actual voltage itself....

a) What voltage can a multi-ratio 600:5 C400 class CT drive while maintaining accuracy within 10% on the 600 amp tap?

b) What voltage can a multi-ratio 600:5 C800 class relay drive while maintaining accuracy within 10% on the 400 amp tap?

c) What voltage can a multi-ratio 1200:5 C200 class relay drive while maintaining accuracy within 10% on the 200 amp tap?

 

[scottf]

a) 400V

b) 533V (400/600 * 800V)

c) 33V

The above assumes that the secondary winding is equally distributed, which is not a given. Most manufacturers try to do it, but it's not required under IEEEC57.13.

For a multi-ratio CT with an equally distributed winding, the relay accuracy performance is proportional to the winding ratio, i.e. 1200:5 C800 would meet C400 at the 600:5A tap.

 

[metsfan1395]

Thanks for the reply Scott. So, it is that simple? You just take the voltage rating as indicated by the C class and multiply it by the ratio of the tap to the full primary current (assuming the secondary winding is equally distributed)?

 

[scottf]

Yeah...pretty much.