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?

 

[Mbrooke]

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.

Sure, that is true if you put straight bus directly in the place of where BAAH, DBDB and ring bus is used today. You can have a very, very reliable system with straight bus that takes breaker failure and bus faults into account, provided you design the system for N12 and up. This is certainly doable, but you need more lines, more stations, and possibly higher voltages. The magnitude of which costs many, many times more than if you were to have the most expensive substation, say triple breaker triple bus or ring bus with a breaker on each line as well.

An example would be places like France, single breaker double bus is used almost exclusively at the 380kv levels, and there lies the rise of clearing the whole station under the right conditions. As a result a 380kv ring is built around Paris. That means more capacity/lines and more sub-stations for the same level of reliability that breaker redundant substations give, ie DB,DB.

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.

Correct, these which I have in mind having breakers in series.

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.

But you would be if transmission capacity was starting to dwindle beyond N-1 and you wanted to keep reliability, correct?

 

[marks1080]

Mbrooke: To your very last statement - There are many many many reasons why brown field work is done in various ways. A lot of these reasons are way beyond the power system. If the economy was roaring and the province I'm in wasn't in massive debt, perhaps we would bulldoze the site I'm working at and start a new. Although the connected generators may have an issue with being down long term. Or maybe they won't because our government will just pay them anyway (hence the debt lol) Or perhaps not. Either way I'm just replacing EOL equipment and I'm ok with not re configuring the entire site. So I really don't know how to answer that question practically.

My only comment towards your initial question is that I think the cost doesn't justify changing station configuration to double bus double breaker. I don't think the extra reliability will justify the decreased security. This is a subjective area, and my thoughts may be different than others. If your company is ok with funding a double bus double breaker rebuild and you have the real-estate and clearances to do so, along with a healthy purse, than go for it. But it IS a mistake to think that the reliability gained doesn't come at a cost to security. I will confidently argue that the reliability gained in your case doesn't justify the security lost. I would be happy to back that up statistically if I had time and access to analyze the stats. Other (better) people have already done this. I've read their books and am ok accepting breaker and a half as the best solution in most situations. I am aware there are one off's where I would prefer a different solution, but in general I wouldn't.

When you tally your costs don't just look at the cost of an extra breaker and hardware. There's an increase in administration costs as well as routine maintenance (which also impacts work force).

As an anecdote: Do you know when you can possibly have a power system with reliability and security ratings that are both a 1 (or 100%)? This is only partly a trick question....

Answer: When the system is offline. A dead power system is both 100% reliable and 100% secure. Please don't pitch this idea to your management, because in my experience they don't always know when you're joking or not.

 

[Mbrooke]

Mbrooke: To your very last statement - There are many many many reasons why brown field work is done in various ways. A lot of these reasons are way beyond the power system. If the economy was roaring and the province I'm in wasn't in massive debt, perhaps we would bulldoze the site I'm working at and start a new. Although the connected generators may have an issue with being down long term. Or maybe they won't because our government will just pay them anyway (hence the debt lol) Or perhaps not. Either way I'm just replacing EOL equipment and I'm ok with not re configuring the entire site. So I really don't know how to answer that question practically.

Its ok- but question. What is Brown Field work?

My only comment towards your initial question is that I think the cost doesn't justify changing station configuration to double bus double breaker. I don't think the extra reliability will justify the decreased security. This is a subjective area, and my thoughts may be different than others. If your company is ok with funding a double bus double breaker rebuild and you have the real-estate and clearances to do so, along with a healthy purse, than go for it. But it IS a mistake to think that the reliability gained doesn't come at a cost to security. I will confidently argue that the reliability gained in your case doesn't justify the security lost. I would be happy to back that up statistically if I had time and access to analyze the stats. Other (better) people have already done this. I've read their books and am ok accepting breaker and a half as the best solution in most situations. I am aware there are one off's where I would prefer a different solution, but in general I wouldn't.

When you tally your costs don't just look at the cost of an extra breaker and hardware. There's an increase in administration costs as well as routine maintenance (which also impacts work force).

As an anecdote: Do you know when you can possibly have a power system with reliability and security ratings that are both a 1 (or 100%)? This is only partly a trick question....

Ok, I guess I have to apologize. I mis read. I thought you were advocating for not having a series breaker and choosing other options instead. My mistake on that.

Answer: When the system is offline. A dead power system is both 100% reliable and 100% secure. Please don't pitch this idea to your management, because in my experience they don't always know when you're joking or not.

Or when there are no clowns working on it LOL (joking)

 

[marks1080]

No apologies necessary.

Green field refers to work on a brand new site. Brown field refers to work at an existing site.

I'm not sure how common that terminology is around the world, but pretty common in North America.

I believe your project would be a brown field. Brown field is typically more difficult in this industry as sites tend to get really old before major upgrade work. Usually that means at best you're working with poor drawings. Worst case, there are no drawings. Most large utilities today can probably put up a brand new green field station with 90% of the engineering already done on templates.... at least I think that's the case.

 

[cranky108]

If your building 5 or 6 or more greenfield substations a year then a template is the way to go. But if your building one every few years, the technology is changing too fast to maintain the templates.

On brownfield substations the issue is people in the past usually did not leave room for new equipment.

 

[Mbrooke]

Thanks and thanks everyone on the terminology And yes, old stations not only lack new addtion space, but also working space LOL.


In so far I came up with something more simple and straight forward: Have the left side SEL411L control only one center breaker while having the right SEL411L controlling the other center breaker. CTs overlap to form an overlapping zone of protection. If the normally called to trip center breaker sticks, BF then opens the other center breaker after 8 cycles. In my eyes there is no programming logic change, and the relays for both lines simply believe this is a classic breaker-and-a-half station.

 

[davidbeach]

That will do just fine until that very rare fault between the two center breakers; instead of just opening those two breakers to clear the bus fault you'll open all four and interrupt both lines.

 

[Mbrooke]

Very true and good point. I may add a differential zone between the two breakers since I have no bus protection to connect the remaining CTs to- but will see. Its rare "enough" so I will weigh my options.

 

[bacon4life]

I may have missed it, but why would you delay the second breaker? Tripping both center breakers at the same time seems much more dependable.

Since faults at the terminals of a transmission breaker are the hardest to interrupt, I have wondered how much series breakers actually reduce the likelihood of a complete bus outage. There are a number of instances of series breakers as bus ties in the northwestern USA.

 

[HamburgerHelper]

Are fuses more reliable than breakers? Do people ever use fuses at transmission level in series with a breaker?

 

[cranky108]

Fuses are a little slow for transmission work.

 

[crshears]

Of insufficient current rupturing capability as well, no?

CR

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

 

[marks1080]

Tripping series breakers 'at the same time' might be a recipe for false breaker fail.

 

[Mbrooke]

I may have missed it, but why would you delay the second breaker? Tripping both center breakers at the same time seems much more dependable.

Since faults at the terminals of a transmission breaker are the hardest to interrupt, I have wondered how much series breakers actually reduce the likelihood of a complete bus outage. There are a number of instances of series breakers as bus ties in the northwestern USA.

The blunt truth is I have no idea how to get both breakers to trip other than tripping both of them and having the 52A contact in series. I have nothing to go by- never seen it done before on paper. Those breakers you have up North, do you have any idea how they are wired or if any special wiring or systems take place to account for the two breakers in series?

 

[Mbrooke]

Are fuses more reliable than breakers? Do people ever use fuses at transmission level in series with a breaker?

Fuses are much more reliable than breakers since they have few if any moving parts. The only place I have seen fuses in series with breakers have been smaller transformers tapping off a bus with lines connected to it (technically in series). In this case its 345kv lines, and they don't make 345kv fuses, and even if they did I doubt they would have a 2000 or 3000amp rating like these breakers.

 

[bacon4life]

Marks1080-what do you mean by false breaker fail?

We used to have a number of 115/12 kV transformers protected with fuses, but the max interrupting rating we could find was 10,000 amps. As fault levels increased over time we had to replace the fuses with circuit breakers/switchers.

 

[davidbeach]

Fuses fail too. Too many other things they can't do, like differential and reclosing.

 

[Mbrooke]

Or step distance as in the case of transmission lines. But I have to admit when dealing with a tap substation and small transformers (12MVA and under) fuses can be very attractive from cost and maintance standpoint, especially when your 12.47kv breakers are just reclosers and the pole mounted boxes that go along with them. Rural substations tend to be very capitol efficent.

 

[davidbeach]

That seems to be the common wisdom of a few decades ago. Can't wait to be rid of the last transformer fuse. We had the misfortune of loosing two transformers last winter. One had fuses and we had oil to clean up all over the place following an event that was finally, fully, cleared by transmission tripping. A few weeks later when the second one went it had a circuit switcher and a differential relay and cleared without the companion overcurrent relay hardly noticing. No oil outside the tank, no fuss, no muss. Told the Tech that went to test the transformer what it was and he said it was the quickest transformer test ever; straight to the tests that wouldn't pass without taking time to perform tests that fault wouldn't cause to fail. No more fuses, thanks.

 

[Mbrooke]

I can see that happening unfortunately. Contrary to what might be portrayed, most transformer faults are not hard bolted faults, but rather turn-to-turn faults or low level internal faults. Fuses if anything are just meant to prevent failures from taking out the main line, and a last resort backup to an uncleared secondary fault. The dirty secret in fused protection is one hopes the transformers itself never fails internally, and with proper care it usually does not over its 40-60 years in service- hence why sometimes the risk is taken.

FWIW in defense of fuses there is one major advantage: if you loose the station DC supply and there is no SCADA to catch that, should a fault occur you minimize the risk of catastrophic transformer failure. Rumor was that happened here with a fault occurring latter on:

https://www.youtube.com/watch?v=fzbQjd_Oo4Q