Who blocks zone 2 / zone 3 reclosing? 3

[Mbrooke]

I seem to get answers in the extremes, either you should block (being a really good idea) or not block (very bad idea). I guess there are pros and cons to both. I want to ask the members here: In practice do you block zone 2 or zone 3 from re-closing? If so, why so?

For me I block zone 3 because this would mean that protection elsewhere has failed to clear a fault, and reclosing would seem to be re-energizing an already consequential problem.

 

[cranky108]

We block reclosing on all time delayed operations. Which would be zone 2 time operations (not for POTT operations), and zone 4 operations.
We don't trip for zone 3.
We also don't reclose for three phase operations.

 

[Mbrooke]

Thanks

Any specific reason for you guys or just how its always been done?

 

[marks1080]

Block on zone 2 timed. The idea being is that it's a 'back-up' protection. If you see zone 2 go on timed for a POTT scheme it's safe to assume 'something' went wrong and you don't know what it is. Therefore, don't reclose. Zone 2 instant -> reclose.

 

[cranky108]

We really don't need reclosing for stability, but more for operator to not have to close it manually.

Timed zone 2 usually will not hold anyway.

 

[davidbeach]

We reclose on all line trips (overhead anyway). We've had a number of interesting cases where the trip was explainable (and settings changed afterward), but unnecessary; the reclose kept things from getting worse.

One big reason to reclose on all trips is that coordination begins to fall apart under N-1, N-2, N-1-1, and other related contingencies. I know that historically settings didn't (maybe still don't) account for all of the impactive N-1 contingencies. Beyond that it begins to approach impossible. If I consider N-2 contingencies at the left end of the line and then consider N-2 contingencies at the right end of the line I'm very likely to find that what helps with one makes the other far more problematic.

I've got at least one location where N-0 is all we can assure proper operation for and that for a bunch of different N-1 contingencies we'll get over tripping as the reach at a particular location goes way up. The first time I stumbled onto that one I spent hours trying to fix it and couldn't come up with anything that didn't make system normal performance worse. So if it has to trip, at least it will reclose. Some day I'll be rid of those EM relays and can implement ground distance and the in-feed problem diminishes but doesn't go away.

Another consideration is understanding the protection dynamics - having it all modeled helps. An element at one terminal that appears to reach an appropriate distance down each remote line may then rear its ugly head two or three zones away when parallel paths have reconverged. If it's parallel paths from a lower voltage to a higher voltage the transformer effect magnifies the reach and the scope of the possible over tripping. In a lot of cases it's multiple zones tripping simultaneously, and if the reclosing intervals aren't all identical the system condition will be different on reclose.

 

[cranky108]

David, it sounds like you have some issues with your transmission planners.

The difference might be the load density, in that where load density is higher, you generally have more, and shorter, lines. So in this case you likely can survive without a line in service. In areas where load is not dense, there are fewer lines, and stability likey can suffer.

So the answer is 'it depends'.

Although I suspect most of us don't adjust our reclosing because of the changing topography. We try to keep things the same, when maybe we should not.

 

[marks1080]

Anyone doing double shot on HV lines?

 

[bacon4life]

We do a second shot in locations with where motor operated disconnects will isolate a portion of a line after the first reclose attempt.

 

[marks1080]

What kind of time delay do you use on that? 30 seconds?

 

[Mbrooke]

One big reason to reclose on all trips is that coordination begins to fall apart under N-1, N-2, N-1-1, and other related contingencies.

I'm really curious why that is. Are you using anything besides MHO for ground and phase? I never use anything beside MHO unless there is a problem like LOP. Reason being that not only N-1 greater changes fault current but generation dispatch varies so much fault current varies greatly.


Perhaps the day (solution) will come when everything is 87L and 21P/G is only activated for loss fiber communication.

 

[davidbeach]

Where the relays are capable, we use mho phase and quad ground; mho phase and directional ground OC for POTT (where implemented); and non-directional OC for LOP conditions, but do not attempt to provide beyond the line backup protection under LOP conditions. The step distance and the LOP are always implemented regardless of transfer trip capabilities except that when the line has 87L the LOP tripping also requires loss of the 87L channel.

87L is nice, but not universally the be-all and end-all of line protection. Comm asymmetry can sink an 87L scheme that relies on ping-pong timing to determine the one-way time delay; been there done that. Tapped distribution loads can sink an 87L scheme that has the negative sequence differential set too sensitively during distribution faults; got that T-shirt. On lines with the full package, I generally see zone 1 clearing as clearly the fastest way to a trip. If the line has POTT it's likely that both ends are keying before either zone 1 trips, if one end would be zone 2 without the POTT it generally trips very shortly after the zone 1 trip at the fast end. The 87L trip generally lags by 0.25 to 0.75 cycles. POTT takes one bit of data, 87L takes lots and lots of bits of data. I'd really like to get to the point I have new relays and POTT capable transfer trip pretty much everywhere; I can get there far sooner than being able to turn on 87L everywhere. Lines with automatic sectionalizing stations in the line can probably never make effective use of 87L. But every line terminal being installed is capable of 87L once everything else needed is in place.

The contingency problem is due to the infeed affect. Say I'm at a bus with relatively weak sources behind it and looking at a bus with a couple of particularly strong sources. All that infeed means that my second and third forward zones see an apparent impedance much higher than the physical impedance between the relay and the fault. Now, take out one or both of those strong sources and the apparent impedance reduces relative to the actual physical impedance. More "distance" for the same fault and the same reach, more opportunity for the second zone to trip where the third zone was intended to operate or for the third zone to reach beyond a bus that it wouldn't see under normal conditions.

Now consider a line with relatively balanced sources at both ends but one or two dominant sources at each end. Our setting process considers all single contingencies. But two sources out behind the relay produces a very different set of reaches than two sources out at the far bus. It may be difficult/impossible to reach far enough with local sources out while also not reaching too far with them in and the remote sources out.

Fortunately most of our system is far enough away from most of the generation that generation dispatch patterns have less impact than they might if the generation was closer. If the generation was closer that would be more contingency cases to worry about.

In short, most of the time the tripping of a backup zone is likely to result in it locking out so not reclosing would just shorten the process. But sometimes, when things are trying to fall apart, the system configuration may result in over tripping that is recoverable, if it auto recloses. They're all rare events, but we've decided to go for the second chance to hold as much of the system together as we can rather than only tripping once for apparent out of zone faults.

Someday we may have an event that causes us to reconsider, but so far, so good.

 

[Mbrooke]

David, best case I've heard thus far for re-closing zone 2 and zone 3. Honestly, if your substations have breaker failure and redundant bus bar protection (main 1 and main 2 from different batteries) I see absolutely no problem in re-closing. I think it could just be that blocking zone trips originated from the old EM days when backup zones were often just that: a real backup. And often still is in other countries.


87L- I agree and thank you for sharing this. However I beg to hold 87L in regard for power swing tolerance and on Out-of-step blocking

 

[davidbeach]

Another reason to always reclose is so that when the third forward zone trips for a slow clearing feeder fault on a tapped distribution station you don't wind up with one end of the line open and the other remaining closed.

 

[Mbrooke]

Good point- thank you

 

[davidbeach]

Yet another reason to always reclose is that the typical setting development process ignores load, other than as a setting for load encroachment. But in actual use the relays are looking at a combination of load current and fault current. Under heavy loading conditions the relays at the sending end of the line become more sensitive, the apparent impedance of a fault beyond the remote (receiving) end goes down. At the same time the relays at the receiving end of the line become desensitized for faults beyond the remote (sending) bus.

The event that prompted the previous comment occurred on a line sufficiently loaded that only the sending end responded initially. It was only after the sending end tripped that what had been the receiving end picked up. Fortunately the feeder finally tripped shortly after the first end of the line tripped.

My experience leads me to ask - If you don't reclose, why are you deliberately handicapping yourself?

 

[marks1080]

There's a hyperbolic argument that someone (not me) might make saying the entire reason we have a relay building with a DC system is purely for reclosing... Delete the relay building; delete the DC system; and delete the secondary system.... after a fault replace the fuses.

 

[davidbeach]

During a hard fault there may not be enough AC to get the breaker open. You might be able to use a capacitive trip device, but there has to be something. Or you just go back to days of somebody watching a meter for faults and overloads and opening the knife switch when needed; then waving his bowler back and forth to blow out the arc.

 

[Mbrooke]

@Marks1080: More than that. Coordinating fuses or anything that uses classical over current on a transmission system is very difficult if not impossible due to the varying generation dispatch and looped nature of the system. I mean, it is technically possible if the system was very, very meshed like a Con Edison secondary 120/208 network- every transmission line was replicated by 5-8x (if not more) and impedance was about equal between substations with load and generation evenly injected into the network, so that any short circuit on any one line resulted in the faulted line fuse seeing full fault current and all others 5-8 less fault current- so only that particular fuse would blow first and not the others. But toss in multiple contingencies and real world scenarios and watch how one could have more than one set of fuses blowing for a fault. And that is just a line. What about a buss fault at a major substation? That would be (say) 24 lines faulting at once- there will by wide spread fuse blowing on a state level- followed by the potential collapse of the system if many other lines and generators get taken out. Also think of a typical substation fed by two 3 phase lines from another substation. How do you coordinate so that a fault on any one 115kv supply line only blows the fuses associated for only that line? How do you get the 300E source fuses to blow and the 200E load station fuse to blow on any faulted line when the other line's 300E and 200E fuses see about the same current? At minimum such a setup would either require either directional over current (at the load station, distance at the supply) or ditching the 200Es at the load station and running the bus normally open.


In short selective coordination would be hit or miss. Plus the "stacking" of time current curves as you head out radially and adjusting for preloading/ambient/tolerances (further widening the gap between curves) would greatly increase the clearing time of a fault- which may exceed the critical clearing time of the system.


Second would come the nature (limitation) of changing fuses- 138kv fuses are already big enough and S&C even has a manual showing a wheel pulley to lift the fuse in place, page 5:

https://www.sandc.com/globalassets/...--all-documents/instruction-sheet-212-501.pdf

Mind you that is for a 250 amp 20ka fuse. The idea is that SMD fuses will protect small 0.5-30MVA transformers out on rural lines that will most likely never blow their fuse. In reality you would need 40, 63, 80 and 90ka fuses, at 2000-5000amps, though if your system is meshed for the reasons I gave above I guess you could get away with lower rated fuses. The higher interrupting would add both cost and weight if not also size. Now think of a 345, 500 or 765kv fuse... I'd imagine you would need multiple fuses rated in series for those levels because the size of one would be simply to great. Mounted high if they exhaust when clearing. A single lighting stroke would cost 10s of thousands in fuses and then a several hours of labor on top of that. Multiple strokes like those seen on Con Ed's 345kv system during some summers and you would be changing out a dozen or more bays.

Also you would still need a device to interrupt full load current- either a circuit switcher or adding SF6 interrupters to your gang disconnects. MODs to make the isolators automatic- though for remedial action, load shedding ect they may not be quick enough- so a circuit switcher would be better. But in the end a circuit switcher designed to simply interrupt load and its transients is already half way to being a live tank circuit breaker. Also in theory your fuses would still need isolators (visible gaps) on both sides when reinstalling your fuses. So nothing is saved in that regard- MODs still need a hut for power and remote control.


Finally there are 2 nightmare scenario: A major storm that results in hundreds or thousands of line trips (ie Hurricane Sandy), or even worse, a system collapse scenario. Both would result in hundreds if not tens of thousands of fuses needing to be changed. A single storm or major blackout could bankrupt a utility many times over. Also switching to save a system or isolate a collapsing one may need to interrupt current levels near that of short circuit (ie lines feeding power from a strong stable system into one that is on the verge of complete collapse or severe power swings)- requiring basically breakers.



But this isn't me coming down on you. Only thinking aloud from my understanding of why fuses aren't used more in transmission. Your idea is actually a good one- FWIW I am currently evaluating several substation designs where 34.5kv or 69kv is stepped down to 13.8kv with everything protected by load break switches and fuses. The idea is to make it mostly maintenance free and cut cost on batteries, equipment and control hut, ect. Its corny but very elegant. Reclosing is not needed since the feeds are underground. And one of these subs might even be feeding an old 4.8kv delta overhead system that had no reclosing

 

[Mbrooke]

Here is a fuse and switch substation in real life:

https://www.sandc.com/globalassets/...ments---all-documents/photo-sheet-761-751.pdf

Very elegant and this is one of the few cases where I can actually see breakers and a control hut being a poor idea. If there were more than 2 secondary cables, fuses could be added to the 4.4kv side of that substation coordianted with the primary trafo fuses.


Also from a German EE book, fused substation 10kv-0.4kv. Switches with Xs are breaker, switches with balls are load breaks, plane switches are just basic isolators.

https://files.engineering.com/getfi...ae-4fe8-9049-c399c55ba4fa&file=10kv-0.4kv.jpg