87T multi-terminal application

[kartracer087]

Hello,

I am wondering if it is possible to connect a circuit like the one I have shown in the attached PDF where the 87T differential current circuit will only compare the current flowing through my TR-1 transformer? Is that what an 1:1 Aux CT would do for me? The issue is that I don't want the relay to think any current flowing to TR-2 is fault current as it is just normal load current flowing out of zone. I only want in-zone fault current to operate the 87T relay and was wondering if using an Aux CT would do this for me to isolate only that part of the tapped circuit for current comparison purposes.

Thanks.

 

[kartracer087]

I looked at this in more detail and I don't think its possible to accurately detect faults pertaining to the transformer or the portion of the tapped line. The only way I think this is possible is putting the current transformers specifically at the tap point so it only sees the TR-1 current in and out of that zone. Otherwise placing the CT at the 13.2kV breaker it will see all out of zone faults including those on the secondary sides of other transformers like TR-2. It needs to only see that segment associated with TR-1. Even with magnetic separation afforded by an auxiliary CT the issue is it will still drive current through the primary side CT circuit any time including for faults outside of my orange zone. I just don't think its possible to effectively use high speed transformer protection relaying unless only that portion of the primary and secondary circuit is compared directly. The relay cannot see any other current involved with the other transformers or after the tap.

 

[sushilksk]

CT location on 13.2kV side should be below tap point for correct operation of differential protection. Further, relay current at full load current on 13.2kV side will be 328A, which is too high to handle and not a good option of CT ratio.

 

[kartracer087]

CT location on 13.2kV side should be below tap point for correct operation of differential protection. Further, relay current at full load current on 13.2kV side will be 328A, which is too high to handle and not a good option of CT ratio.

I agree using a 400:5A CT is more appropriate for this application given the full load current will only be 328A as stated you will lose a lot of resolution and prone to more error using a 1200:5A CT especially under lower loads.

That's how I ended up designing it was I put the CT right at the location of the load interrupter switch just after the fuse. So the primary cable from the load interrupter switch to the transformer and from the transformer to the 2.4kV gear is in the protection zone. My drawing was also incorrect in that I have the secondary 2.4kV CT's on the load side of the main so that the main breaker is also included in the zone of the circuit.

I omitted some additional detail because the load interrupter switch is also a duplex design so it has (2) primary lines coming into it. Which further would have complicated the design. Putting the CT's on the load side of the fuse is proper. Any major transformer fault would trip the 13.2kV bus supply feeder breaker and the secondary 2.4kV breaker on a fault, which is exactly what the design was for. The question is if the user wants to err on the side of caution and have the 87T relay beat the fuse (in essence the fuse would be a backup) or if we want the fuse to try and operate first then if there is still a problem to have the 87T trip out. Can be done by adding a definite time delay to the 87T operation. However single phasing is possible with the fuse operation so again safest way is to have the 87T operate first before the fuse attempts to clear. Only issue is on a dead short the 87T won't beat the fuse because the fuse will clear in a half cycle to 1 cycle (max) which is faster than the breaker can even part contacts. I am not sure coordination is super important at this part of the plant the idea is if the transformer fails everything should shut down including that 13.2kV breaker. If both the breaker and the fuse happen to clear this is OK. This is how operations wants it to be they would rather have the safety of preventing an eventful failure. We also have the sudden pressure relay in the relay tripping circuit as well.

 

[dpc]

I don't think that will work out. The 5 MVA transformer is too close in size to the 7.5 MVA. The differential setting would have to be extremely de-sensitized to avoid tripping on load current. You really need a three terminal configuration to account for the current in both transformers.

 

[jghrist]

You need a three terminal 87T, with the third terminal connected to CTs on the 4.16 kV side of TR-2k.