neutral protection from residual connection OR zigzag ct

[JLuc]

we have a new installation: a 25 kV feeder from a Y-D txfo, with a zigzag txfo. what is the common practice to protect that feeder from phase/ground faults. should I use residual CT connection from the overcurrent protection, or sould I use a CT from the neutral point of the zigzag. the overcurrent protection and the neutral overcurrent protection are done on the same relay -> multilin MIF II.

thanks

JL

 

[dpc]

Normally, I'd use both, if possible. And I'd consider a backup relay if this is a major feeder.

 

[davidbeach]

Phase CTs located between the two transformers won't seen the ground fault current. You would need to locate the phase CTs beyond the grounding transformer or use a CT in the neutral. Either works, which is best depends on the particulars of the installation.

 

[RRaghunath]

Assuming the Zigzag trafo is connected at the transformer secondary terminals (before the incomer breaker at the switchboard), both phase CTs located in the switchboard incomer panel as well as the neutral CT in Z-trafo should see the earth faults downstream.
Thus, you will be able to use either or both of the methods (residual current or neutral CT)for earth fault protection.
As dpc mentioned, it is better if you could have two earth fault protection elements, one connected to the residual input and the other neutral CT input.
However, checkout the relay manual, many relays don't have provision for accepting two inputs.
In such a case, the neutral CT input becomes obvious choice. The residual connection always leaves a possibility of spurious earth fault protection operation, due to differences in the phase CT characteristics.

 

[slavag]

Small aditional remark.
You can use MIFII with residual connection for ground fault protection. MIFII are include only one CT input for 51N function.
CT on neutral side of zigzag xfr. is possible connect to transformer protection.
Of course you need provide some coordination between these functions.
Regards.
Slava