Breaker and a Half Bus Configuration

By "breaker failing," it is meant that the breaker fails to operate (trip), not necessarily that the breaker catastrophically fails (althought that is possible). The more common modes of failure include things such as burned out trip coils, loss of spring charge motor power, failed protective relays, and such. A breaker failure scheme is designed so that if a breaker doesn't operate to isolate a fault, the next "layer" of breakers is tripped to do it. Another possible failure mode is that the breaker is grossly damaged in some way, and thus when it tries to trip it doesn't (possibly with the smoke coming out of it!).

If the breaker that "fails" does so because of fault current above its interrupting capability, then you have more than a breaker failure scenario, you have a serious design issue. You should never have installed a breaker with lower interrupting capacity than the system where it is being used. It is not that difficult these days to determine the available fault current, so that the appropriate breakers and other apparatus can be specified.

Alright, so say a center breaker fails to trip (come open). Why would that result in the loss of the two circuits?

I could see if the two busses were "shorted" together (just imagining the failed breaker as a node). But then shouldn't there be some sort of controls instrumentation to automatically open one of the outer breakers to avoid that? And thus the two circuits would be fed off of the opposite bus.

I guess to restate/ask a new question. Why does the failure of a center breaker result in the loss of two circuits? Or is it just saying that if the center breaker fails to operate correctly, the other two breakers will still be doing what they are supposed to do, and so the two circuits on that line would become "unreliable" rather than lost?

Say there's a fault in the left transformer in the bottom bay of your diagram. The two breakers that connect that tfmr to the left and right buses (left and middle breakers) should both trip - your protective relays would be set up to trip both of them.

If for some reason the center breaker fails to trip, the breaker failure scheme should sense that the breaker has not opened. It will then do 2 things. It will resend a trip the left breaker (for good measure - it should already be tripped), and it will send a trip to the right breaker. That isolates the transformer from both of your buses.

Unfortunately, that also leaves the transmission line in the right half of that bay feeding the fault in the transformer (or if it's a 2 line bay, leaves one line feeding the fault on the other). Depending on how the protection scheme is set up, a communications-assisted trip (such as DTT, 'direct transfer trip') may be sent to the other end of the line, or else it will just feed the fault until conditions are sensed from the other end to detect the fault and trip the breaker there. The main function of the breaker-and-a-half scheme is to keep the buses operating, or in the event of a bus fault, to keep all the line / bay connections operating.

I don't know enough of the protection engineering principles to explain in much more detail how the relay schemes work, but at least this will give you an overview of how the breaker-and-a-half setup works.

Alright, I think I've been thinking about this all wrong. And now that I'm typing it out it's starting to make sense... I was imagining the middle breaker operating in a normally open state (which doesn't make sense lol). Because the way I saw it Bus A and Bus B couldn't be "shorted" together (like they would essentially be if all three breakers in that like were closed). So if one of the outter breakers opened, the middle one would then close to connect to the opposite bus.

I just wasn't fully understanding the definition of a "bus", but your example was spot on to make something click in my mind.

But in reference to your third paragraph. When both of the outter breakers trip (assuming the middle one is stuck closed) wouldn't that remove any sort of power to the fault in the transformer? I know that the t line you referenced will still be energized initially, but wont that "drain" relatively quickly unless it is connected to another power source since when that breaker on the right opens it will be disconnected from that bus?

By tripping the outer 2 breakers, you remove any source from your own buses (left & right) that are fed from the transmission lines & transformers on them. However, you don't know what the T-line in the bay with the failed breaker (in your general example) has for a source. Unless it is only a radial line feeding a load, there is energy available to flow into the fault at the transformer. It doesn't just "drain away" into the fault and then stop on its own - the generation at the other end(s) will keep pumping current as much as the fault can pull from it, until something upstream trips and isolates the faulted item (transformer, line, etc.). You have to take into account how the whole transmission system connected to your station is configured, not just the station itself.

Hope this helps.

i am not sure whether your problem is the same as what i have seen. If not, please ignore.

In IEC parlance that i am used to, the outer breakers are called "Incomer" and the centre one is called a "Bus Coupler" or "Bus Tie". At any point of time, two out of the three are ON. Normally the incomers are on with the bus coupler off. In the event of failure of one of the sources, that particular incomer is switched off, and the bus coupler is switched on, restoring supply to the affected loads on the faulty source.

Breaker "Failure", if taken to mean "Failure to trip" would invariably result in an upstream breaker tripping. So, the loss of power would be even more widespread, and that is why a lot of care is given to discrimination when designing distribution schemes. i daresay that "Failure" is more likely to mean "Failure to close" here.

As i said before, i may be addressing a completely different issue, if so, sorry.

Oh no, anyone with experience actually working with this typ of configuration would have a better idea than me. I was just handed a book and told understanding the breaker and a half configuration might be benificial. In my last post in response to PeterT, I stated that what I imagined as happening was that two are on while one is off type situation (just in a less straightforward manner).

It makes sense, because if any two are on, then power will be being delivered to both circuits on that line (in your image I believe my "circuits" are represented by your "loads").

But the problem with referring to a failure as "failure to close", is that if the coupler fails to close, one of the two circuits/loads should still be operating just fine. Say Bus #1 has a fault and M1 is forced open (the far left breaker), and the coupler remains open, you only lost load 1, not load two.

MY interpretation (at the moment), is that like you said originally, the incomers are normally closed and the coupler is normally open. A failure of an incomer would be failure to close, but a failure of the coupler would be failure to open. Because then if there was a fault in Bus #1, far left breaker would open, coupler would "fail to open", and so the far right incomer would open, and both circuits/loads would be lost.

If just the far left incomer failed to close, and the coupler was operating normally (and was in an open state because of that) only one load would be lost.

Sorry for the nit-pickyness, its just that EVERYWHERE has it stated that "the failure of an incomer results in the loss of the adjacent 'circuit', and the failure of a coupler results in the loss of both 'circuits' on that line".

.... its just that EVERYWHERE has it stated that "the failure of an incomer results in the loss of the adjacent 'circuit', and the failure of a coupler results in the loss of both 'circuits' on that line".

Yes, i did note this statement, and was somewhat perplexed. Distribution systems are usually designed for maximum continuity of supply to the maximum number of circuits/loads.

If an incomer fails, the bus-coupler would switch on, and supply would be resumed to the circuits/loads on that failed incomer. This is the very purpose of the system....continuity of supply.

Failure (to trip) of a bus-coupler in the event of a short-circuit would cause both supplies to supply the short-circuited circuit/load. Consequently, both incomers would trip to protect their sources, thereby rendering all circuits power-less. The engineer who chose and passed the bus-coupler, and maybe some maintenance guys, would be looking for new jobs

Seriously though, it is expected that a periodic testing and maitenance schedule is the norm in most places.

I appreciate your explinations, definitely helped to clear up many questions I had.

After talking with PeterT another question has arose though. What would happen if all three breakers were closed? I am picturing the two busses as parallel connections to the same source (see drawing). If that is the case for the two busses, then I don't see there being much of an issue if all three breakers were closed. But if they two busses came from two seperate sources, I could see a problem occuring.

Sorry to be getting so far off the original topic, but if you feel like you want to keep answering my question please do haha.

Thanks!

Wow. This is a good thread you started i think.... some lateral thinking is bound to result from this....

Questions :

1. It appears from your schematic that the primary and secondary of the two transformers are both shorted. Why ? i don't think this is ever done.... (a) it is never necessary and (b) it is certainly going to give rise to circulating currents, since there are never going to be two absolutely identical transformers !

2. What does the middle breaker feed? It seems to be just shorting the two incomers....what is its purpose?

As i said, i am familiar with TWO sources feeding one interlinked system. Not one source feeding two circuits/loads.

i need to really understand your system. Do you have some weblinks to help me read some more stuff ?

Here's a good, free reference on substation design that has a discussion of different bus configurations and their relative merits & costs. See section 4.6 on typical bus configurations. I think that dcpppf may have drawn his original diagram from this, or at least a source that used the same diagram.

http://www.usda.gov/rus/electric/pubs/1724e300/1724e300.pdf

Ah, it appears there is some basic misunderstanding of the buses! The main buses (vertical lines on the left and right) are just that - physical buses. The sources that energize them are the transmission lines or transformers that come into the bays. All the buses do is distribute the current between the bays, so your diagram of the "source" connected between the buses is incorrect.

A transmission system is generally run with the phasing of the transmission lines fundamentally "in phase" (other than tweaking that is done at sources feeding into the lines such as the generating plants, usually done by VAR control,and usually to manage power flows). Thus, all the lines coming into your bkr-and-a-half station can be assumed to be, for discussion purposes, in-phase. It is not unusual (and I would venture to say, usually the case) that the breaker-and-a-half station is operated with all the breakers closed unless there is a need to isolate some specific line, bus, or connected equipment or to control flow by separating certain lines. This allows power to be "shared" all around the station.

Thus, the normal protection for a fault on a connected line or transformer is to trip the 2 breakers at its connection to the station to isolate it, and the rest of the station continues to operate as it was. It's only in the case of a breaker failure that you start to lose more of the system.

Does that make sense?

Whew, you all are quick with the answers haha. Before I forget, pwr2thepeople, your link happens to be to the same address as the one in my quote on my initial post. Just thought it was convenient haha.

Your example makes perfect sense, very well explained. But what if you turn your definition of the center breaker "failing" to mean that it fails to close (after it has been opened for whatever reason). Say breaker breaker H2 has been opened for some purpose and has remained open. If a fault in Bus A occurs, breakers A1, A2, and A3 will open and breakers H1, H2, and H3 will close (if not already closed, but for the sake of this argument assume they were open). If H2 "fails" to close, then it will only result in the loss of one load/circuit, not two (load 2).

If this is a situation that will most likely never occur just let me know, but with the setup of a breaker and a half configuration it seems quite possible to me. Or maybe to avoid a long drawn out definition, they just say loss of two circuits as a "majority" of the time occurrence and/or worst case scenario.

I'm sorry for the confusion associated with my schematic kvsridhar. I simply included it so that some people could see what was being pictured in my head, which thankfully sparked PeterT's interest.

PeterT, yes that makes sense. It seems like the system would be more passive the way you describe it with all three breakers closed. Because the way I was originally thinking of it, if one breaker opened, there would need to be some sort of instrumentation that told another to close. Where as with your description two will "auto" open, and that's that, time to go fix the problem since it is now isolated.

But yes, after your third post I thought I may have been misunderstanding what exactly may energize busses. When power is delivered to a substation I understand that there are multiple different sizes of step-down transformers, just say 220V/400V/34.5kV. Now for just the schematics we are looking at there are TWO busses, in phase, and the same voltage (I'm assuming the same voltage since they should be interchangeable to their loads). So to get two separate busses of the "exact" same voltage (still in phase), where does that split occur? If the busses were 220V, would there be two 220V step down transformers? Or in my second schematic could that "HV Source" be replaced by a "220V Step Down Transformer" and that split would occur after the voltage is stepped down?

Thanks again for joining the discussion, I am definitely learning some things.

Your analysis of a failure to close on any of the "H" breakers seems to make sense, but is abnormal for this type of configuration. The center breakers are not set up to close automatically. There's no need since they are already closed in normal configuration. The breaker and a half design is pretty expensive, so it is usually only used at major utility nodes (generating stations and large substations with 3 or more main transmission lines) with very high available fault current. It is specifically intended for a network configuration where all of the various ins and outs are closed and operating in parallel. Because of the possibility that sources may become disconnected and out of synchronization from one another, each bus and line is equipped with a potential transformer which connects to a synchronizing system. The sync system ensures that the voltages and frequencies are matched before closing the breaker.

We use this design at our largest generating station. Referencing my earlier drawing, Source 1 and Source 2 at my substation both connect to the regional transmission grid. T1 and T2 are generator step-up transformers. Load 1 and Load 2 supply radial distribution substations. The primary concern is the ability of the generators to get out to the larger network. Loss of significant generation will cause the entire network to collapse as occurred in the northeast US and Canada in 2003. As long as all equipment in the breaker and a half system works properly, loss of any single source or load connection will not prevent both generators from sending power to the grid.

I get the impression you're thinking of a "self-healing" scheme where a normally open switch or breaker separates two circuits. A fault on one of the circuits would result in an automatic sequence of events which opens the nearest switches on either side of the fault, then closes the normally open switches to restore power to as much of the circuit as possible. That design works wonderfully in radial distribution systems but not so well in an interconnected grid network. The preferred scenario on a transmission grid is for the system operator to verify that conditions are appropriate, then take positive physical action (enter a command into a computer, turn a switch, push a button, etc.) to close the center breaker.

After reading the sequence of events of the northeastern blackout I can see why a "self-healing" scheme wouldn't be ideal for an interconnected grid network. I haven't been working with substations very long so I just have a very basic understanding of most things. I knew some substations need to be more reliable and certain bus configurations can add to the reliability of those substations. I appreciate all of the theoretical examples as well as the link to the blackout. Really made the purpose of this configuration as well as it merits stand out to me.

dcpppf, I wish I could show you the station I'm working on right now. It would illustrate your example well. If you took your example station and made it 7 bays instead of 3, that would just be the 115kV portion. There are also 8 bays of 345kV. Consider the two transformers you show as 345kV - 115kV step-downs, so the two portions of the station are linked by them.

The 115kV lines feed the local transmission system, including some limited generation. The 345kV lines come from some major generation plants both local and cross-state. This station is basically a big "cross-roads" through which the power flows.

So in the big picture, overall power flow comes in and through the 345kV portion, and is also stepped down to 115kV, which has flow out and through it. Hope that's clear enough.

During my reply to pwr2thepeople, I was re-reading your description of my error in how the busses were connected, and I'm not sure what it was (maybe a good nights rest), but it finally clicked. The busses are simply connections to link the bays. See attached drawing to clarify.

Sorry to steal your image pwr2thepeople, but I don't have much to work with besides paint and yours can make things a LOT more clear :). Here your say that in the 115kV side (the left one), the sources/loads would be replaced by local transmission grid or some limited generations, as well there would be 7 bays rather than three? And on the 345 kV side, there would be 8 bays and these sources/loads would be replaced by a regional transmission grid (just kind of passing the 345kV along to other substations far away so that they can do with what the 115kV side is doing)?

I believe I get the connections and the properties of a breaker and a half configuration now, but I am a little behind on the what exactly is fed by them. Firstly are there local and regional transmission grids?

You're almost there with your diagram, as I described our station. Eliminate the "common" connection of that big source at the bottom. The transformer that is connected between breakers A3 and H3 of the 115kV is a step-down from the 345kV connection between its A3 and H3 breakers. The tfmr at B2-H2 on the 115kV is a step-down from the B2-H2 bay on the 345kV. They are only "common" as much as the whole station then is common as the bay breakers are all closed.

You seem to keep thinking of a single source for the station or transmission. Actually there are generating plants all over. They may have dedicated lines coming out of them feeding into a transmission switching station such as this, or may just tap off a line along with other loads and generators along its length (I won't go into details here). Each of the transmission lines connecting to the example bkr-and-a-half station(s) could be either feeding power in or carrying it away, depending on many conditions including time of year, time of day, who's generating at the time, etc.

As for local vs. regional transmission, yes, that is often the case. The higher the voltage gets you are usually trying to move bulk levels of power across longer distances. Locally you might have lines (some radial to loads, some between transmission stations) that can be fairly short. You might then tap distribution stations off these, to transform your 115kV down to, say, 13.2kV or 23kV. Those stations would normally just be loads on the transmission system, since they feed local residential, commercial, and industrial customers, and wouldn't usually have significant generation on them. The whole thing can look like quite a spiderweb. Here are a few links to transmission maps (CA seems to have a lot of them online). These are primarily on the bulk power system, although the second one gets into more of the local transmission:

http://www.energy.ca.gov/maps/infrastructure/transmission_lines.html

The caiso map was really neat, getting to see all the different voltages at which it is being transferred, looks like around 500 kV seems to be the most common for longer distances and under 230 kV for the shorter. Which matches everything I've seen and heard, longer distances generally leads to higher voltages.

On the new dwg, it sounds like that is how you explained the step-down transformers to be connected? I just threw the source back in connected as is to help aid with my next question :). If that happenes to be connected the way you were trying to get me to see it, then great haha.

But I'm not sure about the 345kV sources. The way it has been sounding I feel like the two breaker and a half "stations" you have described are, in a sense, connected in series? And there would be four busses, two at 345 for the left "station" (sorry to have reversed them from the last drawing) and two at 115, after the step downs, at the right "station".

But the way the breaker and a half configurations are set up, it doesn't look like it matters where the power comes in. I have been kind of close minded about this since most of the schematics I've seen, if not all of them, have had arrows on the sources/loads going outward, which in my mind told me that they were only sending power away at those locations and not allowed to take any in, but as you said, they "could be either feeding power in or carrying it away".

If that setup for the two 345kV "sources" is acceptable, I guess my next question would be how many external (345kV in this case) sources are generally connected in a breaker and a half configuration. I'm assuming two at a very minimum, but there could easily be more for extra "protection".

I think you've got it! The transformer connections are correct this way. And remember, transmission lines are just wires - they may have a "normal" power flow that tends to go mostly in a certain direction, but especially at the EHV (extra-high voltage), the flows can go either way depending on what the loads are on different parts of the system.

Any of the line connections to a bkr-and-a-half station can potentially be a source or load (or both). As I said, sometimes a generating plant will have a dedicated line or lines into a station like this, just as your sketch showed the two 345kV sources. But lines connecting to other parts of the larger grid can be sources too, depending on how the power flows. That sort of thing is the province of the electrical planning engineers, who examine the load flows and line capabilities, and work with the system operators to get the power where it needs to go. I just build them, I don't run them.

Anyway, this has been quite a good discussion!

Good deal. I can finally try to explain to others, with confidence, how this configuration works. It's nice to get a more broad perspective on what is going on in a substation since my background is relatively slim, as it stands I am still yet an intern haha.

Thanks for your time!

Hi, I'm new to this forum. I wish I had found this a year ago. I am presently working on (3) breaker and a half substations. This thread has been very interesting and has answered many of my questions. I knew I wasn't alone in my questions, because our trip table is constantly being "tweaked". Thanks for all the great information!