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?
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Relaying breaker and a half
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RE: Relaying breaker and a half
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Relaying breaker and a half
RE: Relaying breaker and a half
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
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
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 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
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
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
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
RE: Relaying breaker and a half
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
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
Thanks again :D
RE: Relaying breaker and a half
RE: Relaying breaker and a half
RE: Relaying breaker and a half
RE: Relaying breaker and a half
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
RE: Relaying breaker and a half
...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]
RE: Relaying breaker and a half
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
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
RE: Relaying breaker and a half
Thanks
RE: Relaying breaker and a half
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
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
RE: Relaying breaker and a half
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
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."