Main-Tie-Main Arrangement with Tie Breaker Normally Closed 3

[NickParker]

1. What protections would be required in a Main - Tie - Main arrangement with the Tie in the "closed" position in normal conditions?
2. How should the coordination between the Main and Tie Breakers be handled?

 

[davidbeach]

It depends. Heck, you didn't even specify a voltage. What might be done at medium voltage might be vastly more difficult at low voltage. If process/load requirements dictate the closed tie, is there a protection budget commiserate with that need or are you looking for the cheapest out? Detail are needed.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[RRaghunath]

The main disadvantage with 'Bus Tie breaker closed' operation is that there is an additional protection grading step added between outgoing feeders and the incomer. This means an additional 300ms (typical) in operation of incomer protection operation, be it as a backup to the feeder protection or be it as a main protection for busbar faults (assuming there is no dedicated busbar protection).
Adding two CTs one on either side of bus tie breaker and paralleling the CT signal with that of the incomer will help avoid this additional time. The arrangement sums up the current from the incomer as well as the tie that is coming in to the subject bus section while timing the Overcurrent protection. The protection trips the subject incomer as well as the bus tie breakers.
You can find more details of the scheme in IEEE 242 (Buff book)

 

[davidbeach]

The additional time delay is a fact of life, if all you're using is inverse time overcurrent. But, put in transformer diff, bus diff, and line diff on the outgoing lines and there's no time penalty at all. As I said, it depends. Partial diff, as describe above helps, full diff helps even more. Zone Selective Interlocking can eliminate, or nearly so, the time penalty on low voltage gear. What voltage, what type of breakers, etc., etc.?

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[unclebob]

Flat short time delay on the tie, while I2t delays on both mains (LV LSIG Breakers). May be a 90% pick up on the tie's long delay, if coordination is that critical. Lastly, consider removing the instantaneous element on both mains while using it on the tie. That's it, almost full coordination. But, be aware that incident energy will be very high, if no other mean of protection is to be used.

 

[dpc]

Coordination between the main breakers and tie breaker is often not worth the increase in clearing time and arc flash energy. It is only a factor for bus faults. With digital relays it is fairly simple to create a fast bus tripping scheme with a blocking scheme on a fast definite time relay. In a bare bones system, the tie breaker probably doesn't really need any protection at all, since the main breakers should clear any faults not cleared by the feeders. As David said, it all depends on what you are willing to live with.

 

[davidbeach]

Some folks assume low voltage, other assume medium voltage. I've seen enough posts here over the years that I can't fully rule out high voltage. We're all spinning our wheels until Nick buddy provides a bit more info. Solutions at low voltage are very different from, and a lot more difficult than, solutions where relays can be reasonably assumed. All unit protection, when relays are a given is a much easier reach than unit protection when the breakers have built in trip units. Lots of answers to an unanswerable question.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[NickParker]

The question specifically asked about medium voltage, but I'm curious how the scheme differs for low voltage switchgears.

 

[davidbeach]

Low voltage breakers with built in trip units, and without separate CTs, make it more difficult to much of anything beyond time overcurrent. On the other hand, the low voltage gear is likely to have provisions for zone selective interlocking which becomes more involved when using separate relays.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[bacon4life]

If this is being fed by transformers with LTCs, running with a closed tie requires the LTC work in parallel.

In a bare bones system, the tie breaker probably doesn't really need any protection at all, since the main breakers should clear any faults not cleared by the feeders.

I only recently learned of the bare bones method. One our older stations cleared the entire low side bus via both mains when a feeder breaker failed to open. The lack of protection zones ended up requiring inspection of both sections of bus before any restoration could occur.