Low SF6 action 8

[Mbrooke]

What are the typical choices of action when dealing with low SF6 and an SF6 cutout? Is an automatic relaying action initiated or is the decision left to operators via remote SCADA?

 

[marks1080]

Ya I'm in Canada as well. I've worked with 735kV and 500kV air blast breakers with a couple different utilities. Always have your fingers crossed during those cold winter storms. Hell, you keep your fingers crossed on sunny calm cold days.

I also find the talent pool of people who are really good at working with air blast breakers (either at the maintenance or engineering level) is dwindling. The company I work for now is actively replacing all air blast breakers. Probably will take another 5 to 10 years to swap them all out.

SF6 breakers are beginning to get a bad name now do to the environmental impact. I wouldn't be surprised to see something else on the market in the next decade.

For your own information, my ABB 145PM63-20B breaker control cct has the 63-2 switch pick up a 63X1 relay for trip coil 1 and 63Z2 and 63Z1 relays in trip coil 2 (not sure why the add two relays here, maybe just spare contacts?). 63X1 will pick up trip coil 1 and 63Z2 picks up trip coil 2. 63X1 and 63Z2 are also used to supervise the close coil (block close). I'd upload a schematic, but honestly I'm not sure if it's proprietary or not. Either way you can probably imagine what I'm describing.

 

[Mbrooke]

Hi Mbrooke,

I know that these breakers are generally rated to open with low or no SF6 for load currents. However, I personally think it would be a huge mistake to use this fact in operating decisions. If a breaker is known to have low-low SF6 it should be taken out of service and isolated immediately. Look at it from the point of view of how many possible contingencies away from catastrophic failure you are. If you don't isolate the breaker when you know its unhealthy how will it perform if exposed to some other system fault or condition? Treat the breaker as being 'failed' before it actually blows up. Initiate breaker fail on low-low gas and let the breaker fail protection clear the zone. Don't clear the zone if it's not necessary to do so. Do not modify the manufactures lock-out and trip logic. I'm fairly amazed those discussions are even taking place at your company.

Use the 'SF6 Low Gas Trip' contacts to initiate breaker fail. For these scenarios you need to rely on the breaker tripping itself. Sending signals via SCADA is an operational practice, nothing to do with protections. If you know your breaker is unhealthy (low-low SF6 for example) the fastest way to trip it (faster than BF or any other protection) is to let it trip itself. You are only a single contact operation away from picking up the trip coil then, so you trip speed is the speed of your motorized SF6 pressure switches plus one contact. If you trip via any other system you will add extra time to the trip action. You initiate breaker fail to take care of clearing the zone in the even that the breaker can't break the current, or for some other concurrent failure. Remember, when things go bad in primary equipment you can reasonably expect the chances for concurrent failures to go up.

For stage 1 low-SF6 you should alarm and dispatch. No need to take the breaker out of service immediately, although the dispatched workers who inspect the breaker would probably appreciate an outage. This can all be done via operator control, and no protection operation necessary.

OK, am confused. I am not sure where you got the notion the breaker will be tripping with no SF6.

Second, the breaker will trip as soon as the densimeter hits low-low:

If you know your breaker is unhealthy (low-low SF6 for example) the fastest way to trip it (faster than BF or any other protection) is to let it trip itself. You are only a single contact operation away from picking up the trip coil then, so you trip speed is the speed of your motorized SF6 pressure switches plus one contact.

You bring up also initiating breaker fail, and that sounds like a good idea but need more info on that.

One last point on Breaker Fail: I'm not sure but I got the impression from your above response that BF opens said breaker. This is not true. Breaker Fail does not try to trip it's own breaker, only the breakers in the adjacent zone. There's actually a really good reason not to have a breaker fail protection send a trip into it's own breaker, and it has to do with making sure you don't leave your 125/250V DC signal on the trip coil, eventually cooking it to destruction.

I did not, BF is when the breaker does not open and uses remaining breakers (and transfer trip is applicable) to deenergize the stuck breaker.

 

[Mbrooke]

Are you suggesting that on top of having the breaker trip due to low gas, that the breaker fail protection clear the affected zone? That could get ugly pretty quickly.

I am curious, what to you know about this? Id imagine clearing the breaker via healthy breakers would be a good thing in any case.

 

[Mbrooke]

I can say that all the dead-tank SF6 breakers I've worked with (mitsubishi, siemens, abb) at the 115kV or higher level all trip themselves out on low-low gas. Avoiding this would require modifying the manufacturers wiring from what I've seen. It's kind like messing with an old oil breaker anti-pump or anti-slam scheme. You just wouldn't do it. Well not if you really understand the consequences of doing so.

Isn't it the opposite? You have to ask the manufacture to wire it during production so it trips on low-low instead of lockout?

 

[Mbrooke]

For your own information, my ABB 145PM63-20B breaker control cct has the 63-2 switch pick up a 63X1 relay for trip coil 1 and 63Z2 and 63Z1 relays in trip coil 2 (not sure why the add two relays here, maybe just spare contacts?). 63X1 will pick up trip coil 1 and 63Z2 picks up trip coil 2. 63X1 and 63Z2 are also used to supervise the close coil (block close). I'd upload a schematic, but honestly I'm not sure if it's proprietary or not. Either way you can probably imagine what I'm describing.

I would be more than happy to see the schematic or setup you guys have in place. I could be wrong, but 2 relays might be used to ensure that no single failure point exists (ie one bad relay does not render intended operation inoperative)

 

[FrozenE-]

marks1080:

Absolutely, the old air blast experts are in short supply, but there's still some around; we're also on the track to remove all ours, one would HOPE within the same timeframe, but you know how that goes....

Mbrooke:

Tripping via healthy breakers is absolutely necessary in the case of a locked out breaker. The distinction I was trying to make was is the lockout causing the BF to "initiate", therefor clearing the zone immediately, or does the BF scheme wait for a trip to be called for, see that a breaker (the locked out one) does not initiate, then clears the zone. Our systems typically work via the latter. Having the BF clear the zone immediately could result in a rather substantial outage, depending of course on where in the system this occurred, and how quickly the afflicted breaker could be isolated from the system.

 

[FrozenE-]

marks1080, I'm looking at a schem for an older 145PM40-20 here, but I have the 63-1, -2, -3 pressure switches along with a 33 (spring charge limit switch)all paralleled. These pick up a 63AX which opens a normally closed contact in the CC, TC-1, and TC-2 circuits (lockout on low gas or low spring charge). Alarms are done separately. I have seen newer schematics that had the dual 63 auxiliaries as well.

 

[marks1080]

Mbrooke,

If you have a breaker go into low-low gas why would you want to trip any other breakers if you don't have to? Let the breaker trip itself out. Initiate breaker fail. If the breaker fail conditions are true than the zone will clear via breaker fail. If the breaker fail condition isn't true, than your worst case scenario is you lost a single breaker, and very likely no primary elements are out of service because of it.

 

[Mbrooke]

Mark, thank you, it makes perfect sense now. I got confused when you said initiating breaker fail, but I understand that is only if said breaker does not open when called upon and thus other/remote breakers are tripped to cover that.

 

[marks1080]

Don't confuse a 'stuck' breaker with a failed breaker. Sure, a stuck breaker can be failed... but breakers can fail in many other ways, some much more spectacular. One of the most destructive types of failed breaker is one that does actually open (is not stuck) but fails to break the electrical cct, ie: breaker contacts are open and still arcing. If I had to pick and choose between which type of failed breaker I would prefer to have, it would be the stuck breaker.

In my opinion, for whatever that's worth, any trip signal to a breaker should also have a combined, or separate (depending on the implementation between IEDs and electro-mech relays) breaker fail initiate signal. If you send your SF6 gas trip signal into an IED you can marshal the breaker fail initiate logic through the IED logic. If you're using electro-mechanical relaying you're going to want to bring a SF6 gas trip contact from the breaker mech box into your breaker fail initiate cct. It's always nice to include a blocking switch when doing this so the signal can be isolated during maintenance. Keep in mind, in industry, breaker failure trips (extended zone trips) are more commonly executed inadvertently, rather than a legitimately failed breaker during a fault. I'm not sure if it is common industry practice or not, but all of our breaker fail systems have something we call Early Trip. It's basically a safety net for workers where there is potential to accidentally pick up the breaker fail trip bus during maintenance. The early trip will trip the breaker immediately, which should cancel all breaker fail timers based on 52/a pallet logic in the breaker fail timers.

 

[Mbrooke]

But internal arcing is not a risk at low-low provided you trip as soon as you hit low-low?


My idea is this: When the breaker does trip on low-low, it sends 125 volt DC power to IN105 (or what ever)into the the back of the SEL relay which is programmed for breaker fail. If the relay does not see subsided current on any pole for X time, it does a master trip (tripping of all breakers feeding power into and out of said breaker.)



I will have to review this, but it sounds the most stable instead of directly imitating the BF logic described above.

 

[davidbeach]

Just remember, if you decide to trip on low (or low-low) gas you are calling on the breaker to trip and interrupt current. Failure to interrupt that current creates a fault and the need for breaker failure tripping to clear the surrounding zone. You do have the possibility of breaker opening with no additional impact.

On the other hand, if you automatically block trip until someone gets there post haste to replenish the SF6 supply you may experience nothing except the need to switch out that breaker to provide the isolation needed to service the breaker. It requires a fault before the issue of ability to operate becomes important.

If your ability to get someone on site quickly is good and faults are infrequent you'll probably never need to rely on breaker failure protection to clear a fault if you leave the breaker closed. On the other hand, if getting people to the breaker is difficult and faults are common you will be better off tripping on gas pressure and risking the need for additional clearing.

 

[bacon4life]

We have purchased several different brands of SF6 breaker, all of which provide detailed diagrams as to how the user should wire their preference. We block trip at SCADA monitored stations and immediately open at unmonitored stations.

If immediately initiating breaker fail logic, be sure that the supervising overcurrent elements are set above maximum load current.

A breaker with SF6 at atmospheric pressure is much different than a breaker filled with air. I assume air would mix with the SF6 for large leaks such as a bullet hole or a broken pipe.

Example wiring schematic:

https://res.cloudinary.com/engineer..._low_gas_trip_and-or_block_diagram_dgv5hr.png

 

[Mbrooke]

So by the looks of it there is a possibility that the breaker may arc and fail internally when tripping on low-low. Is this because the small leak may introduce water vapor into the mix? And I take it high speed buss protection will not stop the breaker tank from grenading?


Large leaks -such as bullet holes -I hope I never, ever have to think about that. But in such a case lockout would be ideal so I can see why such a preference would be preferred. In all honesty I did consider it, and if the risk was real the opposite would have been chosen solely on that concern.

Breaker fail pickup current, thanks and will make note. Would this also be a good time to toss breaker position into the mix in the event load current is below BF current?

 

[marks1080]

Mbrooke,

When trying to predict failure modes to determine protection philosophy you should always think of worst case scenario. A breaker going into low-low slowly is not worst case scenario. Hell, a bullet hole through the tank of the breaker isn't worst case scenario. Worst case scenarios happen when you have coincident events happening at the same time. For example, breaker goes into low-low (at whatever rate) and there's a fault on the bus. Now you have a real problem and without proper protection philosophies in mind you are in place where you can start seeing things blow up.

Also your breaker fail pick-up current should be pretty low. I imagine the settings are already below load current. It only cares if there is still current moving through the breaker when the breaker should be open.

 

[Mbrooke]

But how would a fault on the bus with the breaker tripping on low-low create an issue? At low-low this is the last point at which the breaker still has its 40KAIC rating.

BF- makes sense in the event the breaker does not open as anticipated while leaking SF6.


And thanks for clarifying, I no longer count breaker position on BF protection as once can still have "open" while a pole is hung up. Thus a low value of around 20 amps per phase is considered for BF.

 

[FrozenE-]

Happened across this document which seems relevant - some discussion about breaker electrical characteristics as well as different control/mitigation philosophies at reduced SF6 pressures.

Link

 

[HamburgerHelper]

FrozenE, are you from Minnesota? I don't know how much longer that SF6 liquefaction problem will exist. My brother is a wine maker in minnesota and he follows the climate zone maps closely to determine the different varieties he can grow. He gets excited about the zones moving further north. He is or will soon be able to grow varieties that are not as cold hardy. The weather is different though. The last 6times I went home for christmas, only once was there enough snow to ski.

 

[FrozenE-]

HamburgerHelper - No, I'm up in Canada - toured the great lakes this fall and heard some discussions at a few wineries along the way, interesting stuff.

We run primarily mixed gas breakers here (almost all SF6/CF4, with a few /N2's kicking around still).

Manitoba's take on it while I'm posting links

 

[marks1080]

Thanks for teh link FrozenE. I didn't realize that anyone mixed gasses in cold environments. Just gave the doc a read through and some great ideas that I will pass on to some folks here for our northern stations.

Thanks!