Running differential communication channels 3

[Mbrooke]

What is the best place to run fiber optics for commutation between line differential relays? Is a different (secondary) form recommended for back up?

 

[Marmite]

In an ideal World it would be better to have diverse comms bearers for main and backup protection, but that isn't always economically viable. If by "line" you mean overhead line, then OPGW is generally a very reliable solution, but there are plenty of others.
Regards
Marmite

 

[Mbrooke]

Correct, the communications cable will be within the over head line grounding means itself. The idea is to have circuit one (1) on tower A with tower A having the primary channel for circuit one (1) and the backup channel for circuit two (2); circuit two (2) will be on tower B with tower B having the primary channel for circuit two (2) and the back up channel for circuit one (1).


What other forums of communications are you aware of and how do they perform? PLC was considered but in so far I am not sure where it stands.

 

[DTR2011]

I have recently placed into service (2) 345kV lines that have had their protection upgraded to the latest standards. In each case, there is SEL-411L with both direct (OPGW) and networked (SEL ICON) fiber connections. The backup protection is SEL-311C with PLC (DCUB) scheme.

End to end tests show that the 411L scheme can be 2-6 ms quicker for the same fault case.

 

[davidbeach]

I can't imagine any circumstances where I'd want that combination. I'd much prefer a pair of 411Ls, but would settle for a pair of 311Ls, or even 311Cs. Why mix the two? That just means twice as many ways to make a settings error.

I've never heard anything about DCUB that doesn't suggest it being a misop waiting to happen; why not POTT instead? Far more secure and essentially just as dependable. Everything thing I've read about DCB and DCUB suggests that POTT is far more secure.

We've done A and B different and rue every instance. Our experience is that misoperations come from setting errors or Tech errors, not equipment errors; with that history every different type of relay is simply added risk for no added gain. But if it works for you...

 

[DTR2011]

@davidbeach-

I knew I could count on lively comments. I'm merely the Commissioning Engineer, not system protection. I believe the 411/311 decision is based on SEL recommendation of different HW platform - single point of failure. These lines are 30-40 miles long.

The previous protection system was (3) sets of relays - REL-352, SEL-421 & SEL 321, with PLC and audiotone communications, and no 85CO switches. That was indeed a problem waiting to happen. Reclosing / BF is handled by a separate SEL-451 relay.

The Comm scheme is again above my pay grade and oddly not implemented in SEL logic, but in the TCF-10B logic.

In some urban areas, 411L/311L are being utilized, where each relay has a single channel via OPGW.

 

[davidbeach]

I know where it comes from, and 20 years ago it may well have been appropriate. The "use different" approach introduces a strong bias toward dependability at a definite cost in security while misoperation shows that we, as an industry, really need to focusing on security. We see that the vast majority of protection system misoperations are unnecessary trips. And of those unnecessary trips, the single most common cause is setting errors.

If I'm setting two different relays I have twice as many settings to work with as I do when I'm setting two of the same relays. If my greatest risk is a bad setting that causes an unnecessary trip I've just doubled my total risk of a misoperation. When the numeric relays were new and unproved, it may have been very prudent to take an approach of "if this one doesn't trip, hopefully that one does". But that's not the problem the industry has today. I've heard told that at one point, particularly when event analysis was far more difficult, that "well, it reclosed, so that's good" was a common attitude. Now we have PRC-004 and the requirement to analyze every operation to ferret out all of the misoperations and correct them. Security failures seem to be very low hanging fruit and, to me, the lowest of that low hanging fruit is the security risk associated with using different relays.

 

[Mbrooke]

@DavidBeach, excellent perspective!

FWIW I deliberately used different relays from different manufactures in hopes if one did not trip for what ever reason the other would. As I get wiser I am gravitating toward identical relays from the same manufacture provided each has its own communication channel. Right now line differential protection is taking the place of nearly all primary relaying (excluding pure radial lines) both because of speed and simplicity. As is most SEL relays can also do back protection in a single package.

 

[Mbrooke]

BTW, @David Beech, I would like to offer my apologies in regards to the fault current thread I was not directing my angst toward you rather the concept at hand as I was having difficulty understanding it at no ones fault but my own. My most sincere apology.

 

[cranky108]

I tend to agree with David, however with primary and secondary relaying, the logic is a little different. That is to say the primary relay may have reclosing, and sync-check, where the secondary does not. Likely not a problem unless one gets too many differences in the logic.

There is a risk with line differential only in that a failed, say bus differential relay and fault of same, can't be remotely cleared with this protection.

 

[davidbeach]

When both set are the same, programmed the same, there's no reason they can't both do reclosing and synch-check. Synch-check isn't much of a problem even if the two are different, but if there's any difference in the reclose timing, you can't have them both doing reclosing.

Our rule is if the reclosing logic, and supporting settings, are identical then both relays can reclose, but if there's any difference between the two then we have to pick one to reclose and block reclosing on the other. That's much more of a problem on distribution than it is on transmission; when we have to block reclosing we also slow down that relay so that relay that's doing reclosing will trip first and the reclosing will happen.

Back in the day, we were using the SEL-311 family on two breaker terminals (ring, 1.5 breaker) and one relay would do synch-check for one breaker and the other relay for the other breaker. Close supervision was then wired external to the relays. That got much simpler when we started using the 421 and now with the 411L.

 

[cranky108]

The sync-check is easy, we just don't wire to the secondary relay. If the reclose does not happen, and it is not required for stability, it's only annoying.

On ring and breaker and a half, we use a different logic then for radial bus, such that on a ring we need use a breaker failure/sync-check for each breaker.

 

[Mbrooke]

There is a risk with line differential only in that a failed, say bus differential relay and fault of same, can't be remotely cleared with this protection.

What do you mean by this?

 

[mgtrp]

I think he means that line differential will only protect the line, and not provide backup protection for faults at the remote bus or beyond--if you provide line differential only.

Of course, you would typically provide either sufficient redundancy at the remote substation and CB-fail/transfer trip protection, or include overreaching distance protection as a backup.

 

[Mbrooke]

Dual (redundant) buss differential relays would fulfill this satisfactory?

 

[DTR2011]

Most modern line differential relays include a step distance back up protection. Zone 2 from the remote end(s) should clear a local bus fault in Zone 2 time.

 

[cranky108]

Yes redundant bus protection would fix this for that one case, but if you use this you need to make sure everything is redundant. At issue is if one thing is not redundant, you could be in for a big outage.

What about your protection of your non-BES elements? Are they redundant? Do your maintenance procedures remove part of your redundant protection, when you might need them?

 

[Mbrooke]

Thanks


Everything is already redundant as is including newer non BES (BPS) facilities). Maintenance on a relay removes the redundant protection however.