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Relaying breaker and a half

Relaying breaker and a half

Relaying breaker and a half

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

RE: Relaying breaker and a half

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]

RE: Relaying breaker and a half

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

RE: Relaying breaker and a half

(OP)
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?

RE: Relaying breaker and a half

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.

RE: Relaying breaker and a half

(OP)
"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. :D 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?

RE: Relaying breaker and a half

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.

RE: Relaying breaker and a half

(OP)
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.

RE: Relaying breaker and a half

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.

RE: Relaying breaker and a half

(OP)
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?

RE: Relaying breaker and a half

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.

RE: Relaying breaker and a half

(OP)
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.

RE: Relaying breaker and a half

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.

RE: Relaying breaker and a half

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

Thanks again :D

RE: Relaying breaker and a half

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.

RE: Relaying breaker and a half

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

RE: Relaying breaker and a half

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.

RE: Relaying breaker and a half

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]

RE: Relaying breaker and a half

"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?

RE: Relaying breaker and a half

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! ponder

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]

RE: Relaying breaker and a half

(OP)
IMO, I don't see a risk in opening an air disconnect when de-energized, in some case even energized. Its done all the time automatically in all voltage ring buss applications to restore the original ring structure, circuit switches are often used to remove a failed, but none bolted fault transformer from service. There is an arc interrupting full load but it breaks.

At smaller substations like a 34 to 13.8kv its done all the time. When a supply line fails to reclose (lock out), motorized air breaks open to isolate the normal feed line from the buss, and normally open air breaks from the alternate line close to pick the substation back up on line. In this scenario there is risk for older none communicating (SCADA) situations since the line may have tripped on a buss fault within the station itself. However one solution has been either SCADA with fault logic or operating a split buss design where all energized reclosing is done via a center breaker that can automatically open in case restoring a buss fault.

There is one disadvantage to air breaks which also creates a risk. During an ice storm air breaks can be iced over, so opening them can do harm as well as trying to close one.

But other than that I am not aware of any risk. Unless your company knows something?

RE: Relaying breaker and a half

(OP)
FWIW, There is this operating procedure that might make some of you go bugeyed

In feed through station applications, where no breaker is present at a transmission to distribution substation, if the lines fault on either side of the transmission to distribution substation breakers will open at both supply substations. At the T-to-D substation both circuit switchers open automatically. A reclosing sequence begins on both lines. When the first of either two lines successfully auto reclose, the circuit switcher on that line will be re-closed to pick the T-to-D station load back up. If the second line successfully recloses the second circuit switcher closes to restore the loop, if no successful reclose takes place the second circuit switcher remains open.

Im not sure how common or safe this method is, but it is routinely done.

RE: Relaying breaker and a half

Yep, we call it a sectionalizing station, have a bunch of them on our system. Plus a lot more of the transfer stations that will auto transfer from a tripped source to the available source. Once upon a time that was all done with MODs. We've been replacing the MODs with circuit switchers but still have quite a few MODs remaining.

RE: Relaying breaker and a half

(OP)
Ok good to know that. The comment "At some point there has to be a device used as an isolation point. That's where the danger is." raised the question.

Thanks

RE: Relaying breaker and a half

One danger of remote operation of disconnect switches is false indication of switch position due to mechanism failure. If the auxiliary switch indicates the wrong position to SCADA and status is not verified visually, damage can occur.

One generating station destroyed a turbine generator when the ring bus breakers were opened to take the generator off line and the GSU transformer’s disconnect was opened to isolate the generator prior to reclosing the ring. The switch was stuck closed due to a broken mechanism but the control board indicated an open switch. The operator did not verify switch status due to nasty weather. When the first ring breaker closed, the turbine generator was torn apart by the out of synch inadvertent energization.

In most T&D situations, having a stuck switch with incorrect indication will not cause that much damage. But it is something to consider, especially around generation.


RE: Relaying breaker and a half

(OP)
Forgive me if this should be a new thread.

I have been thinking about this extensively and so far one uncertainty has sparked debate. In breaker and a half, where should supply transformers be terminated? My understanding has been on the busses themselves, however I came across this online design manual that recommends terminating supply transformers in the individual bays:

"The reason 345-kv autotransfomers may not be terminated on the main busses is because of the design problem
imposed by breaker failure contingencies for breaker-and-a-half stations containing 3 or more autotransformers.
With 2 autotransformers at a station (1 on each main bus), installation of the 3rd is problematic because wherever
the 3rd autotransformer is terminated, a breaker failure contingency could result in a tripping of 2 autotransformers.
Terminating the autotransformer in individual bays eliminates this potential operating problem."

RE: Relaying breaker and a half

We never connect the transformers to the bus in breaker-and-a-half, nor in double bus-double breaker. One of the major drivers for this type of configuration is to allow breaker maintenance without removing lines or transformers from service. Single breaker to the bus forces the transformer out for breaker maintenance. Not sure what possible benefits (other than one less breaker) there could be to a single breaker connection; though I'm sure someone will proceed to enlighten me.

RE: Relaying breaker and a half

I wish my utility did not regularly connect the low side of our 500/230 and 230/115 autotranformers directly to the busses [with only an intervening disconnect switch] instead of into breaker-and-a-half bays; unloading a tranformer means opening one end breaker in each bay plus any bus tie breakers where so equipped, opening the transformer low-side [motorized] disconnect, then returning the bus to service.

This configuration becomes especially problematic during contingency situations where there are pre-existing outages already in effect in some of the bays involved...

CR

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

RE: Relaying breaker and a half

(OP)
I would say both of you are correct in seeing that way.

For reasons I cant always explain there are many substations where transformers are connected directly to the busses. Its tempting when the buss ends up directly in front of the transformer, especially when the buss spacing is near equal to transformer spacing. The only advantage is cost (that I know of); using only one secondary breaker or none at all.

If loosing 1 transformer does not have a significant impact I could understand the cost being worth it, but if not, I will admit that can defeat the whole breaker redundancy plan to some degree or another.

Here is another foot note from the guide:

"The elements in the 345-kV grid tend to be extremely critical to the transmission grid reliability, thus the added
reliability of a breaker-and-a-half scheme will be considered good utility practice if 4 or more transmission
connections are forecasted."

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