Relaying breaker and a half

[Mbrooke]

How does Relaying breaker and a half compare to double breaker double buss? Is it worth the advantage?

 

[crshears]

If I understand your question correctly...the "advantage" of breaker-and-a-half is in the savings of using one less breaker per "diameter," not in any relaying savings, since the CT overlap required to provide complete zone protection doesn't change all that much.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[davidbeach]

The relaying would be exactly the same except that you get two positions per bay instead of only one position per bay.

 

[Mbrooke]

Ok, thanks!

I guess the only advantage comes from increased reliability since a stuck breaker could also clear a neighboring line as apposed to double breaker double buss?

 

[davidbeach]

Double bus-double breaker has 33% more breakers than breaker-and-a-half. A single stuck breaker in double bus-double breaker only takes out one position and one bus, leaving all other positions in service on a single bus. A single stuck breaker in breaker-and-a-half has a 50-50 chance of resulting in the same thing as double bus-double breaker or of taking out one more position.

There's a perception in the industry that double bus-double breaker solves the stuck (failed) breaker problem. A utility that I know of (not mine) managed to suffer a simultaneous failure of both breakers associated with one position, resulting in a complete clearing of the entire station. Impossible to clear an entire breaker-and-a-half station with out at least one failed breaker per bay.

 

[Mbrooke]

"There's a perception in the industry that double bus-double breaker solves the stuck (failed) breaker problem. A utility that I know of (not mine) managed to suffer a simultaneous failure of both breakers associated with one position, resulting in a complete clearing of the entire station. Impossible to clear an entire breaker-and-a-half station with out at least one failed breaker per bay."

I never thought about it that way before! Excellent point. The irony is that its the 50-50 chance that drives the double breaker double buss preference. The fear is that if the center breaker failed, that could take out a line supplying bulk generation or a leg from a 650mva transformer (though the latter should really be on the buss and not on a position... but trust me I see everything)

The best way to explain the relaying for DB-DB is that its identical to single breaker single buss just a times 2 in everything. However, with the addition of a midpoint breaker, something must be different to sequence it?

 

[davidbeach]

For me, using SEL-411L, SEL-421, and SEL-487E relays for all two breaker applications (ring, breaker-and-a-half, double bus-double breaker, main and transfer), there a two nearly trivial differences between what we'd do for breaker-and-a-half and what we'd do for double bus-double breaker.

For each position (line or transformer), the position relays do their own breaker failure protection and will do breaker failure protection for the bus side breaker when the bus relay trips to bus lockout. Thus for breaker-and-a-half we need that input on one side and not the other, but on double bus-double breaker we would need that input on both sides.

For line reclosing on breaker-and-a-half we lead with the bus breaker and follow with the shared breaker. For double bus-double breaker there'd have to be a determination as to which bus breaker to lead with.

Otherwise, I can't think of any other differences.

 

[Mbrooke]

That's what I was looking for. DB-DB does require 2 inputs, but 1 for Breaker and a half. The thing I like about SEL relays is that they come with preconfigured setups along with detailed instructions. I think I am starting to like them more than GE multilin.


One more question. In DB-DB, when a line faults/locks out, both breakers are tripped while the motorized air breaks are left to open manually or through manual TSCADA; they are left closed after a fault.

However, in breaker and a half, would any down side exist to opening the motorized air breaks/circuit switchers after a line has locked out (isolating it from position) and reclosing the 2 breakers? Theoretically, by doing so, if any buss was cleared for what ever reason, the opposite healthy lines would still have access to the other buss if such a contingency took place.

 

[davidbeach]

We don't use motorized disconnects so I can't really say for sure. On a ring we generally have a line disconnect so we can close up the ring with the line out but typically have enough bays in breaker-and-a-half that leaving one bay open isn't considered high risk. To date the breaker-and-a-half are all air insulted pipe bus installations; our first GIS breaker-and-a-half will be installed in a couple of years. It may have line disconnects where we don't presently have any.

One problem with automatically opening a line disconnect and reclosing the breakers is that the fault might exist between the breakers and the line disconnect. Where I've seen the use of automatic opening of the MODs is in response to breaker failure; trip the surrounding breakers, then open the disconnects on each side of the failed breaker, and then reclose the remaining breakers. That way the failure of a bus side breaker on breaker-and-a-half doesn't leave the whole bus out. This is typically applied at 500kV.

 

[Mbrooke]

All the substations in question are open air. Each line position is equipped with either manual or automatic line isolation. Ring buss designs by all means employ this type of behavior since multiple line failures (unlikely during a blue sky day, but very realistic during ice storms or other serve weather) will break the ring into segments resulting in unintended circuit configurations (load flows).

I will have to consider the possibility of a fault between the motorized air breaks and the breakers, though to be honest I don't recall very many (if any) situations, but none the less a realistic concern.

Installing the air-breaks or circuit switchers between the buss and breaker might be an even better option for such. Good food for thought.

From looking at other utilities around the globe as a whole, breaker and a half seems to be becoming the dominant design replacing the old single breaker double buss topology (once common in Europe)and ring buss very common to the US and Canada.

In your opinion, is there ever a time to consider double breaker double buss? I ask because for me it is the dominant design even at lower (less reliability concern area) voltage levels?

 

[davidbeach]

I'm just a Protection Engineer - it's the planners that determine bus configuration. There are utilities that go double bus-double breaker at 115kV and higher. Most of the double bus-double breaker bays we have at this point are incomplete breaker-and-a-half bays. Our planners like ring for up to 6 positions and breaker-and-a-half beyond that. We don't have significant stability issues, so maybe we can focus on the economics of breaker-and-a-half vs. double bus-double breaker.

 

[Mbrooke]

In your case, how do the planers choose between buss configurations, if you know? Im guessing economics out weigh the small risk, if any. Thinking about it
where the system is strong, meshed, with transmission networks in between generation having sufficient reserve capacity and access to bulk networks at multiple points; losing a neighboring line under a center breaker failure contingency would not cause any adverse effects.

Generally the driver here and for myself are a set of reliability standards that force certain configurations rather than cost (not always smart). For example, any contingencies or multiple set of contingencies can not cause any transmission outages or at the distribution level can only effect a very small amount of customers. Good practice, but, there are times where cost makes me re-think a contingency that may not be a concern to start with.

 

[davidbeach]

I guess our system is robust enough that loss of two lines connecting end-to-end doesn't violate the reliability standards. At our largest breaker-and-a-half station it would be one line and one distribution transformer for a center breaker failure. Most of our newly constructed stations are intended to serve load rather than increase the robustness of the transmission system. Where that load is highly critical, there is typically a tie between the low-sides of transformers in different bays, so we can get the load back quickly should there be a breaker failure.

If you have motor operated disconnects on that middle breaker, you can respond to a middle breaker failure by opening the disconnects as soon as both bus breakers in the bay open and then reclose the bus breaker that was opened in response to the breaker failure. In that regard you're set for a quicker response than we might be.

 

[Mbrooke]

Makes sense. You've given me a lot to consider, and a new way to approach this.

Thanks again

 

[marks1080]

The utility I work for would never remotely operate a motorized disconnect switch without having personal on the ground to verify correct operation. HV Disconnects commonly fail to operate as expected for many many reasons. In my opinion it would be a mistake to have an operating procedure rely on this. Bad things will eventually happen.

 

[Mbrooke]

Would the same risk take place for de-energized operation?

 

[marks1080]

Yes - if you eventually plan to energize something. What's the point in operating dead equipment? At some point there has to be a device used as an isolation point. That's where the danger is.

 

[crshears]

Neither would my utility return the equipment adjacent to a failed breaker to service without a visual confirmation that the motorized and remotely controllable breaker disconnects were open. Where we have two open points in series but without either verified, we might in some instances [such as when firm load remains interrupted otherwise] take the risk of energizing the next diameter out.

Absent a stuck or failed breaker, however, we do routinely rely on remotely operated motorized disconnects to operate correctly without having someone there to visually confirm their operation.

In the past, before major restructuring, there was one procedure we undertook which has since been abandoned, namely:

With a breaker-and-a-half scheme switchyard consisting of multiple diameters, on a day with no adverse weather about, when a breaker wouldn't open from either remote control, local control, or the manual trip plunger, the operators would confirm a loop of at least one parallel path existed from one side of the stuck breaker to the other, then manually open the disconnects on either side of the breaker to isolate it from the system.

I was the switch operator on one of these occasions; the first switch drew a yellowish arc for maybe 5-10 cm before it broke; the second drew the expected arc associated with removing the stuck breaker from potential.

It worked like a thing of beauty; I can't understand why we've lost the confidence to do this nowadays...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[davidbeach]

"Diameter" Not a term I've encountered in that context; are you referring to what I'd call a bay - a string between the main buses with three breakers and two positions?

 

[crshears]

Yes; your bay is our diameter. Even though our construction forces tend to use the term "bay," whereas Operations uses "diameter," neither department calls the other wrong; potayto, potahto, I suppose...

...Now you've gone and made me think again!

Upon further consideration, I must admit that our use of the term "diameter" is somewhat elastic; in differentiation from the context just described, we also use the term to denote all of the breakers that operate to isolate a failed circuit breaker from the system, since these devices draw an electrical circle around the failed component.

Hmmm...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]