Step distance relaying under double circuit faults
Step distance relaying under double circuit faults
(OP)
Are their any relaying concerns or set backs in regards to the simultaneous faulting of two circuits sharing the same tower? Or with other types of relaying?






RE: Step distance relaying under double circuit faults
Analysis of Faulted Systems by Paul Anderson has a section dedicated the analysis of simultaneous fault if you want to dig into it.
RE: Step distance relaying under double circuit faults
Thank you for the reference btw :)
RE: Step distance relaying under double circuit faults
The new NERC TPL-001-4 requires including common tower outages in power flow studies if the length is more than 1 mile.
RE: Step distance relaying under double circuit faults
If the phasing is swapped on the other circuit, does it help?
RE: Step distance relaying under double circuit faults
Reminds me of the old expression: your current operating practice is based on your last bad experience!
RE: Step distance relaying under double circuit faults
FWIW I came across this:
https://www.pacw.org/fileadmin/doc/SummerIssue08/l...
RE: Step distance relaying under double circuit faults
But in double-circuit line applications, faults near one end of the line may result in a sequential trip operation. This sequential trip happens when the instantaneous relay elements trip the breaker nearest to the fault location (this trip is independent from the communications tripping scheme). The breaker farthest from the fault must wait for a permissive signal. The major problem with this sequential fault current clearance is that it creates a current reversal in the healthy parallel line. If the protection for the healthy line is not equipped to address this reversal, one terminal of the healthy (non-faulted) line may trip incorrectly.
RE: Step distance relaying under double circuit faults
Edit: I guess this would only take care of one bay, but not the other.
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
https://selinc.cachefly.net/assets/Literature/Publ...
RE: Step distance relaying under double circuit faults
You also need modeling software that knows what to do with the mutuals; I'd expect any decent transmission fault analysis program would. I wouldn't want to try it by hand though.
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
Question:
Can the reversal problem be cured in modern none communicating relays or must their be a communication assisted tripping scheme in place to mitigate the problem?
RE: Step distance relaying under double circuit faults
Modelling mutual coupling is not that
hard, and it can be done by hand - it
is just rather tedious to check all the
different cases.
The mutual impedance has units of ohms,
or "volts per amp". In the case of zm0,
the mutual impedance tells you how many
zero sequence volts induced in feeder 1
per amp on feeder 2.
You could model this by inserting an appropriate transformer in your
equivalent circuit model.
From what I've read and seen, power system
engineers from the fifties, sixties and
seventies seemed to have a much better
grasp of fundamental theory than the current
generation.
Having said that, one could probably get
away with a lot by making (conservative)
educated guesses. Mutual coupling always
makes the zone 1 reach setting shorter and
the zone 2 reach setting longer.
Alan
RE: Step distance relaying under double circuit faults
There are also temporary faults for which no evidence was observable when patrolling the whole line from the ground after a reclose. Now, with relay distance-to-fault information, a more detailed inspection of just a few of towers may result in actually identifying very minor damage that gives good evidence for what kind of fault occurred.
RE: Step distance relaying under double circuit faults
Thank you, this is a good info. wye-wye equivalent transformer?
Oh yes! Not bashing anyone here, but computer programs have bypassed all reasoning, critical thinking and theoretical understanding. I myself have long been guilty of that (and still am) relying on an answer where I know nothing about how it came to be. I am blown away by what engineers were able to do 60 years ago with next to nothing in computing or the "dumb" equipment at hand.
I know after 2003 NERC became interested in load encroachment with MHO circles, but from what I have read and concluded was that in the 1965 NE back out handled the gradual voltage collapse and frequency decline far better than the system did in 2003.
How much guessing and on average by how much is zone 1 zone 2 being effected?
RE: Step distance relaying under double circuit faults
And I would argue that despite that, power systems in North America and Western Europe performed very well. Theoretically in a system with generous reserve capacity, that risk can be argued as mitigated. But regardless it is worth knowing.
RE: Step distance relaying under double circuit faults
It is usually zero sequence mutual coupling
which has an impact. I think it would be
easiest to insert a transformer (or coupled
inductors) within the zero sequence equivalent
circuits. Sequence networks are always balanced, and so are considered "per phase".
The concept of wye wye does not really apply.
Regarding the quantitative impact of mutual
coupling - I won't give any numbers here for
fear that someone may actually use them. If you do a few setting studies for mutually
coupled lines, you will get a feel for the
typical values.
Also worth mentioning are cases where fault
current flows "forwards" on one side of a tower line, and "backwards" on the partner
line. In these cases the coupling effect is
much stronger than when one line is earthed.
It's important to understand the physical
arrangement of the lines, and where the fault
current flows. I usually look at our GIS maps
and sketch up the arrangement before commencing a study.
Thanks,
Alan
RE: Step distance relaying under double circuit faults
At 'A' the apparent impedance for a 1LG fault at 'C' was lower than for the same fault at 'B'. Hard enough to set the relays with a good model, without would have been much more difficult.
RE: Step distance relaying under double circuit faults
Ok, that makes more sense now, as an inductor. Transformer through me off, which would be poor in any case imho.
Post them but with a disclaimer if you feel comfortable. As far as I am concerned if someone else uses them without second thought it would be at their own risk/responsibility.
But from everything I gather all mutual coupling does is sway the single line equivalent impedance measurements somewhat rather than causing cataclysmic (for lack of better terms) under/over reach. Personally, to me my biggest concern appears to be sudden power flow reversals on the opposite line and closing into a 3 phase fault in front of the line.
RE: Step distance relaying under double circuit faults
Now, place a fault just in front of A1. Relay at A1 will see zone 1 and trip instantaneously. Relay as A2 will see a reverse fault and do nothing. Relays at B1 and B2 will both see a zone 2 fault and begin timing.
Three to five cycles later the breaker(s) at A1 will clear. B1 will continue timing toward a zone 2 trip. A2 will now see a forward, zone 3 fault and begin timing. B2 will now see a reverse fault and stop timing.
At 20 cycles, B1 will trip and the breaker(s) will clear 3-5 cycles later. As the fault clears, B2 will still be seeing a reverse fault and still be doing nothing while A2 is about 15-18 cycles into the 60 cycle delay and never trips.
The reversal only becomes a problem with a directional based communication aided tripping scheme such as a POTT scheme. In the example above, if there is a POTT scheme the relays at B2 will be keying permission. When A1 clears the fault, there will be some period of time in which the relays at A2 begin to see the fault as forward rather than reverse while still receiving the permissive signal from B2. If A2 relays will key permissive after the reversal (for us much more likely for a 1LG fault than for a 3LG fault) then A2 could trip based on that stale receive from B2 and its own key.
In the relays we use, zone 3 is inherently reverse, so the third forward zone is actually zone 4. The POTT logic in the relay includes a block on zone 3 and a drop out timer to hold onto that block for some amount of time (a few cycles) to allow the key from the remote end to drop out.
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
Thank you yet again, I had not considered slower tripping time on breakers that have been in service for some time. I always went buy the manufacture's stated trip/clearing times, but never thought about normal deviations during the in service life of the breaker.
The relays I will be using here have those polarity dependent high speed tripping contacts. Would you say they make any dent in the relay operating time itself?
I found the paper. I am reading it right now and its exactly what I've been looking. Thank you :)
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
In other areas of protection, we have been moving towards standardized settings to reduce human error. Creating custom settings or using multiple settings groups adds points of possible failure, so they are only used where there is a clear justification.
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
RE: Step distance relaying under double circuit faults
Ground it to get the trapped charges off the line to facilitate placing the working grounds around the work zone and then unground the ends.
RE: Step distance relaying under double circuit faults
https://www.youtube.com/watch?v=nsn-Tw_wzyg
As such it may be a good idea to take the other circuit out of service as well during such testing?
@David Beach- Adjacent as in a circuit sharing a common structure or anther line in the same easement? The sounds like a huge value. I am guessing that the parallel path might be more at play than induction- but thats me just guessing.