Main-Tie-Main differential protection
Main-Tie-Main differential protection
(OP)
Can anyone provide any guidance/recommendations for providing differential protection on a double ended sub with Main, and tie breakers? Both main breakes have CT's and tie breaker has CT's on both sides of breaker. Does anyone know of a good reference for this?
I am also interested in providing 67N directional ground fault protection with main and tie breakers. Can anyone provide a refence for setting up this protection?
I am also interested in providing 67N directional ground fault protection with main and tie breakers. Can anyone provide a refence for setting up this protection?






RE: Main-Tie-Main differential protection
RE: Main-Tie-Main differential protection
Explain what you want to do with the 67N.
RE: Main-Tie-Main differential protection
For directional overcurrent, you generally will need voltage transformers to polarize the relay to respond to current in the desired direction.
RE: Main-Tie-Main differential protection
In my case the tie-breaker is normally closed and the facility runs with one main breaker closed or the other operating off of the one utility source.
So looking at the drawing Lets assume Main breaker 1 is open and the Tie and Main breaker 2 are closed. For faults on any of the feeder breaekrs the feeder breakers should trip and and the tie or main should not trip. If there is a fault on Bus A then should the tie breaker trip? And if there is a fault on Bus B then the main should trip?
From what I looked at it seems like this scheme combines a main breaker overcurrent settings and tie breaker overcurrent setting into one curve/setting? Normally I am used to having main settings and curves and then a tie breaker setting and curve. Does this scheme allow for only two curves instead of 3 with the tie and mains rolled into one curve?
RE: Main-Tie-Main differential protection
Thanks
RE: Main-Tie-Main differential protection
If you had a fault on the other bus, the current in the main and tie breakers would subtract and you would not get a trip from the partial differential. The directional relays on the other bus would trip the tie breaker. This would allow you to set the partial differential and directional overcurrent relays with the same time-current characteristics and avoid having to coordinate between tie breaker and main breaker overcurrent relays.
It's a little difficult to figure out the scheme because of the way the one-line is drawn. Normally, you would have solid lines representing the ac voltages and currents and dashed lines for control. Your one-line looks like there are CT circuits going into lockout relays. There is a directional and a power element shown with no voltage input. It's all very confusing.
RE: Main-Tie-Main differential protection
So with the relaying schemes shown it sounds like there is not any traditional overcurrent relay setting for the main and tie breakers individually it sounds like the main and tie are set as combination (eg buss diff and directional) is this correct?
Can you explain how the directional overcurrent relay on the "other bus" would tirp the tie breaker? I'm not sure I follow this.
Also there is a relay function shown for a 67N however I dont see any ground CT's at the tie breaker as there is on the mains. Does this mean that ground fault cannot be used in a partial differential or directional function or is it possible that you have to use the residual connection of the phase CT's for ground functions?
RE: Main-Tie-Main differential protection
The 67N could use the CT residual current.
RE: Main-Tie-Main differential protection
Actually, I'm not see any reasons for use 67 and 67N protection.
Only in few special cases, for example as back-up protection of 87L or/and back-up protection of 87B, some kind of directional zone selectivity.