When 2 circuit breakers are in series on a Main Incomer - WHY would they need to be COORDINATED ?

[bdn2004]

We've got a 12.47kV Utility feeder to a switchgear that has a main circuit breaker. And there are 5 feeder breakers. There is bus differential protection on the switchgear bus.

In order to properly coordinate any of the feeder breakers with the Main Breaker there's supposed to be at minimum 0.15 seconds between the relay settings (Main Breaker vs. feeders) at the fault current they both see on a fault. That's the way I understand it.

Upstream of the main is the Utility's circuit breaker. I understand there needs to be coordination with the feeder breakers and the Main Breaker. Any feeder needs to trip before the Main Breaker does. But I don't see the need to delay - as in coordinate the Utility with the Main Breaker. The only time these two would be in the circuit alone and a fault - is if there were a bus fault, and the bus differential will react and trip the Main Breaker. If the bus differential didn't work, the Main Breaker and the Utility's breaker would back it up.

What I'm suggesting is to set the Main on the switchgear to the exact same settings as the Utility.
Is this the way this is generally done? If not why?

 

[Palletjack]

No. They need to be coordinated. Say you have a fault and your main is set the same as the utility breaker, which one opens first on a fault on the load side of your main? It’s basically a race between the two. When the utility trips on 50-51, and your main didn’t trip, where is the fault zone? Forget the 87 differential, it’s not coming into play here.
It becomes a troubleshooting nightmare unless it’s as obvious as blown to bits parts laying everywhere.

 

[bdn2004]

I know it's a race between the two. But it's going to be a major problem - and our Plant will be down no matter what. So it's just to tell you who faulted? Seems like you are trading that convenience for slowing down the protection.

There's also differential protection on the incomer. I don't understand why that and the bus differential can't be considered as part of the overall protection scheme. "on the load side of the main" are the feeder breakers and the bus, that's it.

 

[waross]

You have control of your own breaker.
In the event that your breaker clears first you may re-energize when you have determined that it is safe to do so.
In the event that the utility breaker clears first, you must wait for the utility to investigate and determine when it is safe to re-energize.
That may take a lot longer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[bdn2004]

Those are good reasons...

We've got a situation where the Utility relay settings are where they are. And trying to get the required time separation for a fault close to our bus, with the highest fault current, is proving about impossible to do without the Utility changing their settings. That's a bridge I didn't want to have to cross.

 

[Palletjack]

The differential will only trip for just that, a differential fault. The utility breaker also has overcurrent and/or instantaneous settings. How far apart are these breakers? Utility, main, and feeder?
Like it was stated, if you trip the utility out, no matter what you say, they are going to do their own investigation. May even pull an event file on site. I’m not saying this will take hours, but it could take longer than you want to wait, especially if you have the problem isolated. With the proper coordination, you could go back to work and not even let the utility know.
Have you asked them to change a curve or other setting? All they can do is say no. We would investigate and see if we could bump our setting up to help you out.

 

[bdn2004]

I’m looking at the sequence of events data. For a fault at our first load, which is 1000’ from the sub - the system coordinates. Feeder instantaneous, main ahout 10x longer, Utility .2 sec past that.

But if it faults right at the load side of the feeder breaker inside the switchgear, the fault current maximum, the separation time drops to like .1 sec between the main and the Utility and the coordination flunks.

A bus differential fault is a fault at that same point.

The Utilty has changed their settings before. I had to get into this because the load on the feeder was doubled. I just wanted to see if we can get this done without them having to change anything.

 

[davidbeach]

As pointed out, the scope of the outage is the same, but the restoration efforts won't be.

In general, the utility doesn't really want to trip for faults on your system but will do so in a manner which limits the impact to the rest of the customers on the feeder.

How long are your feeders and where do they go? If you've really got a thousand plus feet your ought to be able to set an instantaneous at the feeder breaker that covers a ways out. If they each run to a single location you can add line differential on the feeders and trip instantaneously for any fault on any of the feeders.

You might even be able to pay for better protection at the utility breaker, say to add line diff or a POTT scheme between the utility breaker and your main breaker so that they can trip instantaneously for anything between the two and then might be willing to go a bit slower for faults beyond your main breaker. Might be willing.

On the other hand, if they clear for a fault in your gear, how bad would that be, really?

 

[bdn2004]

The feeders feed multiple daisy chained unit substations. The lengths are between 1000 - - 2000’. The instantaneous is set on the feeders but it’s still set pretty high for all of the inrush current on the transformers it’s gotta get past.

And the feeders are 3/c armored cables in cable tray 40’up in pipe racks and in the building girders to feed the unit subs from below. The chances of disturbance is like none.

The Utility’s feeder does have line differential. Plus (2) overcurrent relays set at exactly the same.

This system seems it’s already protected well with multiple layers. Does it really matter that much that our main is not perfectly coordinated with them at every point where the chance of a fault is very slim or is already protected by a redundant scheme?

 

[Palletjack]

Not if it doesn’t bother you that their breaker will trip and you wait on them to get it back on... if it happens a couple of times due to miscoordination they may balk then...

 

[protoslash]

it gets me pissed off just reading this. Imagine every fault at your bus trips the utility breaker, unnecessarily creating outage to everyone on the same feeder. At this point, why bother having your own protection at all, just let the utility clear everything for you.

you can try talking to utility, but remember their breaker also coordinate with whatever their upstream device, so there might not be room to slow down the speed. At the end of the day, you gotta work with what you have. If the utility's reliability suffers because you screwed up protection, be prepared for some angry folks knocking.

 

[davidbeach]

The last post points out that perhaps we don’t have enough information to answer the question. My responses have been based on the assumption the the utility “breaker” was an interrupting device immediately ahead of the customer and would only interrupt that customer. If the breaker is the feeder breaker at the substation then it imperative that coordination be provided across all possible faults.

 

[bdn2004]

No need to get pissed off. I’m just asking a question to you that have much more experience than I do. I’ve already been provided some excellent reasons not to do this. And I’ve worked to make it correct and prove it on paper.

Incidentally this plant has operated for 30 years and there is no record - at least in the software model we inherited that anyone ever attempted to coordinate with the Utility before in the existing system. And with the existing settings on that I don’t see how it’s possible.

There are two main feeders from the Utilty’s substation that only feeds this Plant. They go to a Main - Tie - Main 12.47kV switchgear arrangement.

Also I’ve watched a number of training videos On coordination and a valid strategy is to set two breakers in series to the same settings when it doesn’t really matter which one trips first - you just want the thing turned off as fast as possible. And this allows for decreasing the time for the upstream devices that do need to coordinate.

 

[ppedUK]

@bdn

The main point that the others have made, and which you seem to be overlooking, is that if the Utility breaker trips first, you will be significantly inconvenienced waiting for the Utility to investigate the fault and re-instate your supply. You say there are no other consumers on the feeder(s), so no problem for any other users, but it is you that will have the hassle with the Utility to get your supplies back on.

If you owned and operated both breakers that are in series, then the info in your training videos might be an acceptable solution, but you don't

There are many solutions to this problem, some of which others have suggested. You could for example lower you incomer settings to grade with the utility, by installing a simple blocking scheme from your outgoing feeders to the incomer breaker(s). BUT if you can't engineer this problem out, then you have to accept the consequences of having a race between the breakers. The first approach would be to ask the Utility if they can increase their settings, even if only a little. Remember though that text book co-ordination is very rarely achieved in this situation without implementing other measures.

 

[jghrist]

The only time both breakers will trip is if (1) the bus differential fails for a bus fault or (2) the feeder fails to trip for a feeder fault. I'd say the probability of either is low and I'd rather risk that inconvenience than compromise coordination of the feeders by trying to squeeze another coordination layer in.

 

[bdn2004]

They actually had a feeder fault last year on the existing system during construction due to a ground fault at the high voltage termination at one of the transformers. It did the worst of all worlds - it tripped the feeder breaker, and it tripped the Utility - which shut off half the Plant. Everybody went home for 2 days. And in the end the Utility got blamed. They never accepted blame - but that was the consensus from the Plant.

It certainly wasn't cut and dried that it was the Utility's fault - and we even knew exactly what happened and where the fault occurred. That investigation took some time and lots of people got involved.

The main-tie-main arrangement allows the tie to be closed and the entire plant run on either Utility feed.

Unfortunately I'm not involved with the design of these systems. These engineering solutions are good ideas but they all cost $$ and corporate approvals and all that stuff. I'm just trying to work with what we got.

 

[jghrist]

Do you have an NGR to limit the ground fault current? Coordination with the utility for ground faults can be tricky in that situation. We have a client that has a 4 kV substation with an NGR and the client's plant ground grid is isolated from the utility substation ground grid. The NGR is grounded to the plant ground grid, but if there were a fault on the utility ground grid the return current would have to flow through earth to get to the plant ground grid and the NGR before it returns to the utility transformer neutral.

 

[bdn2004]

jghrist,

No there is not an NGR. BTW your last post is saying what I'm originally trying to say. The probability is very unlikely and there is bus differential protection at the one point it doesn't perfectly coordinate. That seems a valid reason to not worry about getting the Utility involved, outages planned, and all the other stuff you have to do to change something like that.

 

[Palletjack]

You stated earlier that if the bus differential didn’t work, the main would pick it up.
It’s getting confusing...need a flow chart...

Questions..
Does your main have overcurrent, (instantaneous and/or time overcurrent)? I realize it has differential.
How far is it from the edge of the differential zone, through the mains, to the feeder breakers. Is there a chance something could get into the wires, bus, etc there.

And most importantly.

Is there anything else, any other customers rather than you, after the utility breaker?

 

[bdn2004]

Palletjack,

The main has time overcurrent but the instantaneous is not set. The bus differential zone is around the line side of the main incomer and on the load side of each of the 5 feeder breakers. There is also line differential around the Utility's incomer. That CT is in the incoming section as well.

This is new switchgear in a new building. If you are asking can snakes or rodents or other creeping things get in it, again highly unlikely.
And it has an insulated bus.

The Utility built a new substation to feed this switchgear(s) (there are two feeds, two switchgears exactly the same). No other customers are on these feeders. No other customers are on the Utility's substation. This new substation was built to shed load in the existing substation that has increased over the years. The old one still exists - but it will now feed half the load, and this new one the other half.