Numerically computed zero sequence current
Numerically computed zero sequence current
(OP)
Modern protective relays can use either numerically calculated Io or actual neutral current fed via residual connecting of three phase CTs.I not sure if there is any difference between the two? Is one better than other?






RE: Numerically computed zero sequence current
Whereas the Io is >= the primary current of the CTs, the error are negligible and you can use either the calculated or the residual current without big differences.
Differences may arise if the numerical relay has only one A/D converter and the sampling frequence is too low. So the current are sampled at "very" different moments and the numerical vectorial summation may be affected by errors. Otherwise the residual connected CTs are your solution.
RE: Numerically computed zero sequence current
RE: Numerically computed zero sequence current
I think that theoretically, there should be an preference to using the directly measured residual CT current instead of using a calculated value, but there probably is no practical advantage.
RE: Numerically computed zero sequence current
I'd be careful using any canned logic from any relay manufacturer (and I work for one of them). I think the canned logic is a great point of departure and an excellent way of seeing how someone who knows the relay very well would solve a particular protection problem. But was the protection problem they solved exactly the protection problem you need solved? Maybe, maybe not, but in either case how will you answer the question as to why the program was what it was if something goes wrong? Probably best to make the whole program your own.