Breakers in series 1

[Mbrooke]

How does one go about controlling two breakers in series? Do I just wire the 52A contacts in series and the trip / close contacts in parallel and program the relaying as with a single breaker or am I overly simplifying it?

 

[davidbeach]

It would help to know why you have two breakers in series and what you hope to accomplish in doing so.

 

[Mbrooke]

Being done in order to mitigate the failure of another breaker. Ie, if one breaker becomes stuck (no trip), the other one will take care of it.

 

[HamburgerHelper]

Is this for stability? You can't wait for breaker failure to time out?

 

[Mbrooke]

Reliability. Breaker-and-a-half where in one bay I'd like to eliminate the simultaneous loss of two circuits for a center breaker contingency (see example pic).

Also common place for series breakers are split bus substation where two bus breakers are used in series so a bus fault does not clear the entire substation via stuck bus tie breaker.

 

[cranky108]

My transmission planners have been asking about this type of arrangement.

That might be one way to set up the tripping. Another might be by using the re-trip to trip the second breaker.

How does the impact of operating three breakers at the same time affect your battery life?

 

[Sn00ze]

@Mbrooke, we take the single pole status of both breakers and then coordinate 1-pole, 3-pole tripping and BF between the two as well as teleprotection for remote breaker with PUTT/DTT

 

[Mbrooke]

What type of relay do you use? Never thought about using the 1 pole feature to cover two breakers... can you elaborate on this?

 

[Sn00ze]

I've only seen single pole tripping for 240kV and higher. We use GE-C60. takes in the 52a (or b) status of your breakers. then it takes the inputs from your other protections (Line, TX, etc). for line protection we use sel-421 and siemenes 7SA522 to trip the C60 and the breaker control module with 3-pole tripping. we also designate another contact from the line protection to do 3-pole tripping independently. this can be done for 79 and BF. Sometimes, for whatever reason they just parallel the 3 contacts from the line protection to send a single signal to the C60 to do 3-pole tripping. I think this depends on how much you want to spend on settings because the C60 can do both. I am not a settings guy though, so my knowledge in that dept is limited.

 

[marks1080]

Mbrooke, I may be making a poor assumption but basically you want a double breaker double bus station?

I don't see why any special considerations need to be made for breaker control outside of a breaker and a half station.

If this is so, the only thing I really see you gaining is cost. Unless there's a very special customer connected I wouldn't go down this road. It seem like one of the consequences you have for trying to minimize the impact of a failed breaker is adding more breakers, which can then fail. In my opinion the decreased security doesn't justify the cost of an incremental increase of reliability.

 

[Mbrooke]

Mbrooke, I may be making a poor assumption but basically you want a double breaker double bus station?

Technically in this cases you would indeed place the element on its own bay and treat it as double breaker double bus, but for an existing station its not always possible to expand the substation, especially if that also involves moving a line outside the substation all the way down to a new bay. Having to go over or under other circuits becomes a tricky mess.


Also, as mentioned by another member here (David Beach) breaker-and-a-half is more reliable than double breaker double bus in that you can not clear an entire station without multiple breaker failures in each bay (technically zone two might also open in that case, but you get the point). Id further add in BAAH you can loose both bus bars and still be able to pass power via the center breakers, which is of major benefit especially in a bulk flow gate application that is just cutting over to supply load. Plus BAAH, even with two series breakers tends to be more compact (less land) than DBDB. So even if you took two series in new construction, there are still benefits over DBDB.

I don't see why any special considerations need to be made for breaker control outside of a breaker and a half station.

If this is so, the only thing I really see you gaining is cost. Unless there's a very special customer connected I wouldn't go down this road. It seem like one of the consequences you have for trying to minimize the impact of a failed breaker is adding more breakers, which can then fail.

Not so much as a special customer- but rather a special set of lines feeding many customers- 345kv 1,500MW rated lines that is.

In my opinion the decreased security doesn't justify the cost of an incremental increase of reliability.

Why would security become decreased? I can't see that- but then again I don't know.

 

[davidbeach]

So, from what I've gathered Double Bus Double Breaker is the most reliable if you never have more than one breaker failure for any one event. I now know of two Double Bus Double Breaker installations that tripped everything for cascading breaker failures that might have been contained in a 1.5 breaker station. Pick your poison, no extra outages for a single breaker failure or a better hope at containing cascading breaker failure. Breaker and a third might be even better at stopping cascading failures.

 

[Mbrooke]

Not only cascading failures, but picture one bus out for maintenance and the other clears (faults). I've read documented cases in India and South America where this has happened, and it was prized as an advantage because generation and lines continued to work via the center breakers preventing a blackout. In many ways I'd argue breaker and a half is the ideal substation when all is factored together. Technically two center breakers could be called breaker and a third, but nothing connects between them in my case.

 

[davidbeach]

Agreed, I wouldn't call yours as breaker and a third since there isn't a third position in there. Breaker and a half doesn't require bus protection in the bays, nor would breaker and a third; but you'd have to have dedicated bus protection in each bay where you have two breakers with nothing between them.

To the original question, I think I'd trip both of them simultaneously but hope that I never have to turn that from a theoretical answer to a practical answer. To date I've not had to deal with critical clearing times that would cause one to entertain such outlandish schemes; but I've heard of conditions that would make something of the sort a reasonable reaction. I think, though, if I ever got there I'd try to have the extra breaker on the position. The bay, from bus to bus, would still have only three breakers for two positions, but then I'd add a single breaker (possibly with a bypass switch) on the line or transformer. Then I'd have a 4 or 5 cycle breaker failure trip of that breaker followed by the 10 cycle (our standard, others might be 7 or 8 cycles) breaker failure trip of the adjacent bus and the far end of the bay. I'd like to keep that as a pencil and paper exercise.

 

[Mbrooke]

Agreed, I wouldn't call yours as breaker and a third since there isn't a third position in there. Breaker and a half doesn't require bus protection in the bays, nor would breaker and a third; but you'd have to have dedicated bus protection in each bay where you have two breakers with nothing between them.

Correct and agreed.

To the original question, I think I'd trip both of them simultaneously but hope that I never have to turn that from a theoretical answer to a practical answer.

Hate to say it, but my bets are you will one day encounter such a setup. To be honest I'm surprised you have not seen this at least once in the wild- but then again every utility has vastly different standards and conditions that it must work with.

To date I've not had to deal with critical clearing times that would cause one to entertain such outlandish schemes;

In this case its not critical clearing time forcing it, but rather the removal of the line directly across from it should the center breaker fail to clear; otherwise the breaker on the other side connecting to the bus would need to be duplicated as well if that was the case.

The system has a rough critical clearing time of about 12 cycles for a 3 phase fault and about 20 for a single phase fault. When a fault occurs the line protection relay will send a trip signal to each line breaker consisting of 3 single pole breakers which should clear in about 2 to 4 cycles, after which the 3 phase fault is down graded to a single phase fault if one of the breakers stick (it is assumed that only one of the 3 single pole breakers will fail to open, and 3 single pole breakers are deliberately chosen at higher voltages over 3 pole units to increase the critical clearing time, ie a single phase fault can be tolerated much longer than a 3 phase fault). If current is still present on any breaker CTs 6 to 8 cycles latter, a 3 phase trip is sent to all breakers attached to that bus for a side breaker failure which should clear in about 2-4 cycles. For a stuck middle breaker its about the same, after 6-8 cycles a DTT is sent to the remote terminal plus tripping of the other side breaker; in 5 cycles both the remote end and the side breaker should clear. In total in about 16 cycles a fault is removed involving a stuck breaker, about 5 cycles with working breakers- both below the critical clearing time.

However, once two 345kv lines are removed (possibly an auto transformer if the remote end has one attached directly to that line), the remaining 345kv lines can overload during peak periods with reduced local generation, hence the need make sure a failed breaker does not remove other elements.

but I've heard of conditions that would make something of the sort a reasonable reaction. I think, though, if I ever got there I'd try to have the extra breaker on the position. The bay, from bus to bus, would still have only three breakers for two positions, but then I'd add a single breaker (possibly with a bypass switch) on the line or transformer.

If critical clearing time was the factor (ie fault could not persist for more than 8 cycles) I would certainly do that. FWIW there have been cases involving very large generators (over 1,500MW) where faults in front of it had greatly reduced clearing times due to the lower inertia as compared to smaller paralleled generators. I've also heard of increased renawbles increasing the CCT.

Then I'd have a 4 or 5 cycle breaker failure trip of that breaker followed by the 10 cycle (our standard, others might be 7 or 8 cycles) breaker failure trip of the adjacent bus and the far end of the bay. I'd like to keep that as a pencil and paper exercise.

At this point since I have CCT coverage, I am debating tripping both breakers, or just tripping one and then the other 10 cycles latter.

 

[Mbrooke]

But- to keep the overly complicated simple- view it like this. Lets say you had a straight bus and had two line breakers in series. How would you control those two breakers?

 

[marks1080]

Mbrooke: Security is reduced because you've added 'stuff.' One of the balancing acts between reliability and security.

So I agree with you when you say the double bus double breaker is more reliable. But my point is that the very small amount of reliability you gain doesn't justify the cost and decreased security. Security is decreased just by adding more elements which could represent a failure point. Generally, "A station with more breakers than another station will experience more breaker fails." is a true statement.

You want to consider real life scenarios where you gain anything by having a double bus double breaker. It's easy to imagine those scenarios. Now try to find real life situations where they happened and how double bus double breaker would have helped. If you can actually find these scenarios ask how adding the extra breaker would have helped. After finding that out ask yourself if it would have been worth the cost to get that extra reliability. I think you'll find the answer to that is 'No' because there simply haven't been enough of those operations to justify catering for above a breaker and a half configuration. Add with the security you lose by adding the extra breakers, the answer should most definitely be "No". This is why you see breaker and a half used so much. Like I said originally, unless you have a very special customer connected, where a loss of power equals a large law suit, or it's some large generating station critical to maintaining system stability, generally the best solution is breaker and a half.

This discussion actually reminds me of a book I read called "Confessions of an Economic Hitman." At least I think that's what it's called. It's about an ex-CIA operative who worked with American utilities who went over seas to build power systems for third world countries. One of his CIA duties was to get these countries to over build their power systems to a point where it was just indebting them to American companies for no good reason. This guy would have built plenty of double bus double breaker stations I'm sure.

 

[Mbrooke]

Mbrooke: Security is reduced because you've added 'stuff.' One of the balancing acts between reliability and security.

So I agree with you when you say the double bus double breaker is more reliable. But my point is that the very small amount of reliability you gain doesn't justify the cost and decreased security. Security is decreased just by adding more elements which could represent a failure point. Generally, "A station with more breakers than another station will experience more breaker fails." is a true statement.

I agree with what you are saying in general, but ask yourself this question: why do we choose breaker and a half over straight bus or single breaker double bus? Adding all those extra breakers only increases the odds of a stuck breaker, makes relaying far more complicated (yes easier with modern relays but settings get more complex and thus error), adds a lot of cost, and there is more equipment to maintain. Bus faults are rare, and in single breaker double bus you can do maintenance on any bus without shutting down the station.

You want to consider real life scenarios where you gain anything by having a double bus double breaker. It's easy to imagine those scenarios. Now try to find real life situations where they happened and how double bus double breaker would have helped.

In this case double breaker double bus would be the "standard" solution, but said station would need to be expanded, and said line would need to be moved to a new bay.

If you can actually find these scenarios ask how adding the extra breaker would have helped. After finding that out ask yourself if it would have been worth the cost to get that extra reliability. I think you'll find the answer to that is 'No' because there simply haven't been enough of those operations to justify catering for above a breaker and a half configuration.

What if I told you I can list as least 20 substations in the US the have the above configuration, often in more than one bay? I think this is more of not being seen in the wild than actually being a poor idea. There are two other solutions to this problem, one costs 5x as much the other +45x as much.

Add with the security you lose by adding the extra breakers, the answer should most definitely be "No". This is why you see breaker and a half used so much. Like I said originally, unless you have a very special customer connected, where a loss of power equals a large law suit, or it's some large generating station critical to maintaining system stability, generally the best solution is breaker and a half.

Am I more likely to inadvertently loose a single element? Sure. But I am also more likely not to loose a second element when another one fails.

This discussion actually reminds me of a book I read called "Confessions of an Economic Hitman." At least I think that's what it's called. It's about an ex-CIA operative who worked with American utilities who went over seas to build power systems for third world countries. One of his CIA duties was to get these countries to over build their power systems to a point where it was just indebting them to American companies for no good reason. This guy would have built plenty of double bus double breaker stations I'm sure.

Or rather plenty of straight bus... I'm not kidding.

 

[marks1080]

Hi Mbrooke... I think you need to go back and review reliability vs secruity philosophy. Obviously straight bus is very secure, but it's reliability is just one notch above having a black-out. Therefore the balance between security and reliability hasn't been properly achieved. Considering how large the US is I absolutely believe there are at least 20 stations in the whole country that don't use breaker and a half. I bet you will find a lot more than 20 if you look hard enough. I am personally working on a job now rebuilding a double ring bus station. Am I upgrading it to breaker and a half? Hell no. Way too expensive. But if I was building a green-field site you can bet it would be breaker and a half.

Basically your question boils down to this: What station design gives you the best balance of Security and Reliability at the best cost. The answer, in general, is breaker and a half. The industry has known this for some time now... but not always, as you can see from your old stations that have weird configurations. And before everyone replies with their one off situations that go against this I am speaking generally. In general, breaker and a half is the best choice. If you can get the funds approved to over build, by all means, overbuild

 

[marks1080]

Sorry Mbrooke: I also should have added my two cents on how to handle a double breaker double bus system. If I were going to implement that I wouldn't trip both of the middle breakers simultaneously. I would still zone out my station using breakers and I would still only trip adjacent breakers from any zone protection. Keep in my you will have to either overlap breaker CTs or you will have to find a way to protect the chunk of bus in between them. There's nothing complicated about that, just that you need to be aware of it. You don't want to have a chunk of bus between two breakers that isn't protected.