A Zone 2 fault is an 'out of zone' fault, so ideally you want the fault cleared by the protection for the zone it's in. I think what you are referring to as DTT would imply Z2 tripping without permission - or an overtrip. To be honest, I'm not sure that is what you mean by DTT or not, so sorry if this is way off base.
Lets say you have a 6 mile two terminal line, the west side is protected by relay "A" and the east side on the other end 6 miles away relay "B". A fault occurs 1 mile away from relay A. Relay A trips on zone 1, and in non communicating system relay B will wait 20 cycles to open on zone 2. Now, if the same occurs, why can't relay "A" send a DTT to relay "B" bypassing the 20 cycle timer? If terminal A opens, that means terminal B must also open.
Why not just disable the DTT during testing? If the DTT is being initiated via 125 volts DC from an output on the back of the line protection relay, why not route it through an operable test switch?
It's not really an either/or type of question. If we do either, we almost always do both (the exceptions are all old). We also have a switch to disable all transfer trip during testing or other work when tripping the remote end would be bad.
All our newer transfer trip runs on SEL's mirrored bits; bit 1 is DUTT/DT, bit 2 is POTT, and bit 3 remote drive to lockout.
The DUTT part of bit 1 is the conventional under-reaching zone 1 trip; the DT part is anything else that makes one end of the line want the other end to open for, such as breaker failure. Well understood, and easy to implement.
The POTT scheme is really more than traditional POTT since it can take advantage of the reverse block and echo capabilities in the relay logic.
The remote drive to lockout allows one end of the line to keep the other from reclosing. This can be for breaker failure, it can also allow one end of the line to reclose and if it trips it can then block the remote end from closing.
The POTT scheme does a pretty good job for nearly everything; the DUTT/DT occasionally gets there first, but the POTT scheme can't force the remote end open. If the remote end is already open, the echo logic on the POTT scheme provides high-speed coverage of the whole line, where the in-service terminal would never get a DUTT/DT signal from open end for a fault near the open end and would trip zone 2.
POTT is overreaching so it will handle high impedance faults better.
DTT can be hard to set on short lines.
With POTT sides have to see the fault for the comm. scheme to operate. One bad instrument, tech, or relay won't trip it out.
We have a short 500kV line (350') protected by redundant differential relays (SEL 411L and GE L90). Separate fiber routes are used,for redundancy: overhead OPGW and UG conduit. If communications fail, backup distance protection is enabled but the lowest available Zone 1 setting over-reaches from both directions, so both Zone 1 & Zone 2 have to be delayed to coordinate.
When I first read this thread, I thought I should use POTT as backup. But if communications are down, there is no way to send a permissive trip signal and we are back to a time delayed Zone 1.
Am I missing something that might make that backup faster when communication fails?
Just trip on total loss of comms. Both A and B comms will ever only be out simultaneously once. After that time they'll be scrambling to get the failed comms back in service before the other one goes out.
Properly set, sure... On the other hand, stuff happens, events have to be investigated, and reports have to be written. Happens with any scheme, though...
I'd written a whole lot more, it sounded really bleak, and I deleted it. I tried again. So, let's just say that no scheme is perfect. POTT is pretty darn good, but when it fails you get good stories if you dig down and get to the bottom of why it failed. I'll leave it there...
Mr. Beach, I don't think I have read anything bleak from you thus far. And in general anything posted here by members no matter how mundane sounding has been of value.
But for now- being a long night I will trust your experience and take your word for it.
I was sitting there thinking about all the ways I've seen POTT schemes fail to be secure over the years. Some of them make pretty good stories, some were just embarrassing screw ups. What I was writing kept running off into the weeds, sounding like trying to get it right could be rather hopeless. It's not hopeless by any means, just that memories of the failures are much clearer than than all the times they've worked right.
But, all in all, I do like being able to have POTT on a line. It really does have a decent degree of security, but not perfect. It's not a bad way to protect a line at all.
POTT gets tricky when your primary configuration gets more complicated. What do you do when your Zone 2 POTT scheme sees through the low side of a tapped station? What do you do when apparent impedance seems to imply there's no way you can set your zone 2? These issues are more common than not in my experience (based on the system I've worked on). We often deploy DCB schemes because we decide it's more secure in some scenarios than a POTT setting could be, so we accept the risk of overtrip, rather than be exposed to a scenario when we don't trip in zone.
Now you've gone and done it, brought that DCB stuff into the conversation... ;-)
That seems to be a eastern interconnect vs. western interconnect thing. DCB seems common back east but is uncommon in the west while POTT is much more common in the west than in the east. Most of the Protection Engineers I know of throughout the west see DCB as a misop just waiting to happen; as we look at the quarterly misop reports it is clear that the vast majority are unnecessary trips. It seems to me that as a whole, the protection profession seems to have our thumbs firmly planted on the dependability side of the dependability-security scale. I wonder if we can ease up just a bit there and reduce the over tripping without creating a rise in the number of under trips.
I'm having a hard time trying to picture how a reasonably set zone 2 would reach through to the low side of a tapped station. When we deliberately try to reach through the necessary setting is much larger than any zone 2. On the other hand, a negative sequence line differential can respond to low side faults at tapped stations oh so easily.
Where a POTT scheme will completely fall apart is when applied on a three terminal line with outfeed. That's were for a fault on one branch, fault current would flow out of one of the other branches, through the surrounding system, and back in on the faulted branch. In that case you've got to wait for the faulted branch to trip on distance and the outfeed turn into infeed. Or just apply line differential.
DCB absolutely is a misop waiting to happen. But that's better than a non-trip waiting to happen
There are plenty of line configurations in our system where you would choose DCB over POTT. There are even some configurations where you would choose DCB over an 87 relay, usually due to equipment limitations.
DCB is also the best choice to use whenever you have a PLC communications system.
David, the POTT zone 2 can easily see into the LV network on a tapped station when you have a long, three ended line with a weak in-feed terminal. Actually, I've come to expect this in some of our more 'congested' areas of our network. Now we even have our LV protections equipped with a 'HV line blocking' signal to supervise (via comms) the Zone 2 terminal protections.
Our system is small, but we solve the three terminal problem by not having any three terminal lines.
Our system planners are aware that more than two terminals is likely to increase protection times, and decrease reliability for customers.
It's like the old joke, "Doctor it hurts when I do this. Well then don't do that".
If you are using two fibers on a line for line protection, and you lose both, there is a different problem that you need to solve. So tripping the line seems to be a good call.
