Starpoint of current transformers

[Blackberry77]

I am having a problem understanding the reason for the starpoint connected in a certain way. For example, I know by convention that in differential protection, the starpoint of both CT's covering the zone of protection is always pointed towards the protected object. Why is this the case, is it only because of a standard or is it because of how the current will flow? Also I assume that it doesn't matter if X1 or X2 is oriented towards the protected object as long as that end has been starpointed.....correct me if I am wrong. Also does anyone make current transformers that have H1 with respect to X2, and H2 with respect to X1 (polarity wise). Thanks

 

[dpc]

The only thing that really matters is that the CT secondary currents going into the relay have the relative polarities expected by the relay.



David Castor

http://www.cvoes.com

 

[Ohmly]

Not sure what standard uses H1,X1 etc, but I assume it is similar to IEC60044 where P1 has the same polarity to S1.

This is something you cannot change (ie you cannot have P1 and S2 with the same polarity).

However it doesn't matter which way P1 or P2 points.

Also I assume that it doesn't matter if X1 or X2 is oriented towards the protected object as long as that end has been starpointed.....

This statement is normally correct

Star point towards the zone is used by our organisation to keep a simple convention.

You can also have both star points pointing away from the zone and it will work correctly too. I know of other organisations that have different conventions again (eg star point towards the load flow), it makes it complicated but you can still get it to work by changing connections at the relay.

You can work with any convention as long as you know that when current is flowing in P1, it flows out S1 (and vice versa).