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Step distance relaying under double circuit faults
4

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

We had a cross country fault like 6 months ago. It was just treated as a very rare event and we moved on.
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

(OP)
Is it rare though considering the number of double circuit towers and trees/lightning strokes? One concern I have being that under a double circuit fault the voltage in the substation will get pulled down far more than for a single line fault, so what the relay sees will be untrue in relation to its CT input.

Thank you for the reference btw :)

RE: Step distance relaying under double circuit faults

We have had them as well. When towers have the same vertical phasing, I assume the same phase faults to ground in both circuits. If you analyze all the different combination, the analysis gets a lot harder. Parallel lines usually have significant zero sequence coupling, which is a big impact on ground relay settings. Some level of mis-coordination seems likely and acceptable for faults this rare.

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

(OP)
Common circuit towers have proven to be not as advertised lol :P For this rule I do not blame NERC, I have been well ahead on this for a decade. There have been many cases here and around the world where a tree topples into the line and the branches reach into the other circuit taking that out as well. Even then taking that tree out safely requires de-energizing the other circuit. Not pretty when the line serves a 100MVA radial load pocket.


If the phasing is swapped on the other circuit, does it help?


RE: Step distance relaying under double circuit faults

I think one of the early Northeast blackouts (some 50+ year ago) was attributed to a simultaneous fault to a double-circuit structure with 345 kV circuits. Even though it is rare, the risk is much higher as you go up in voltage. I'd be less concerned if it were two 115 kV circuits.

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

(OP)
Was this the 1977 ConEd black out btw? If I remember correctly breakers failing to re-close, inadvertent transfer trip, and lack of load shedding did them in. But still worth considering.



FWIW I came across this:


https://www.pacw.org/fileadmin/doc/SummerIssue08/l...

RE: Step distance relaying under double circuit faults

Phase and Ground Distance Relays may support communications-assisted tripping schemes. If applying this Distance Relay to Permissive Overreaching Transfer Trip (POTT) schemes with the assistance of a communications channel, the relay provides high-speed protection for faults anywhere along the protected transmission line.
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

(OP)
I could be really wrong here, but is this reversal not taken care of by directional current measuring in modern microprocessor relays?

Edit: I guess this would only take care of one bay, but not the other.

RE: Step distance relaying under double circuit faults

In a modern scheme you'd have both POTT and DUTT. When that first zone 1 happens it sends a trip to the remote end. Even without the DUTT the POTT logic in modern relays addresses the reversal problem by not transmitting immediately following a reverse fault.

RE: Step distance relaying under double circuit faults

(OP)
Well, in ideal modern situations. :P That and the fact communication can be lost in any scheme turning classic step distance into a last resort backup.

RE: Step distance relaying under double circuit faults

Set taking the parallel line into account there's no reason for simple stepped distance to get it wrong. But you have to model the mutuals and account for them in the fault studies. It may help to set k0 and k01 differently.

RE: Step distance relaying under double circuit faults

(OP)
Any tips on where to start modeling the mutual relation between the two lines? I will look into setting the undereaching zones at different points, but unsure how the theoretical aspect works out.


https://selinc.cachefly.net/assets/Literature/Publ...

RE: Step distance relaying under double circuit faults

There are commercial options, such as the Aspen Line Database, or you can roll your own. I have a Mathcad sheet that I built on the equations in Anderson's Analysis of Faulted Power Systems. However it's done, the calcs require knowledge of the full geometry; just knowing the equivalent spacing isn't sufficient.

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

(OP)
I can agree with all the above, however, Im lead to ask. How was this done in the 40s and 50s?

RE: Step distance relaying under double circuit faults

I would guess it wasn't. There were probably bigger fish to fry. Complete modeling of mutual coupling through out our system only happened in the last 10 years.

RE: Step distance relaying under double circuit faults

Mbrooke, do not worry about past times, worry about present, and even future, times.

RE: Step distance relaying under double circuit faults

(OP)
Thats what I was thinking imho, if they got millions of double circuit lines to work for over 80s without mutual modeling, they must have known something.

RE: Step distance relaying under double circuit faults

That, or they had very few of the specifically bad fault cases.

RE: Step distance relaying under double circuit faults

(OP)
That too lol. Probability (often very small) x consequence (often very large) = outcome (what eventually happens when least expected).

Question:

Quote (DavidBeach)

Even without the DUTT the POTT logic in modern relays addresses the reversal problem by not transmitting immediately following a reverse fault.

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

Hi Mbrooke,

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

Before widespread event recording, there wasn't much to go on for determining correct relay versus incorrect relay operation. Once we started analyzing high speed recordings, there were a number of events where the correct breakers opened, but some of the internal relay elements did not actually operate as intended.

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

(OP)

Quote (submonkey)

Hi Mbrooke,

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.


Thank you, this is a good info. wye-wye equivalent transformer?

Quote:

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.


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.

Quote:

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


How much guessing and on average by how much is zone 1 zone 2 being effected?

RE: Step distance relaying under double circuit faults

(OP)

Quote (bacon4 life)

Before widespread event recording, there wasn't much to go on for determining correct relay versus incorrect relay operation. Once we started analyzing high speed recordings, there were a number of events where the correct breakers opened, but some of the internal relay elements did not actually operate as intended.

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

Hi Mbrooke,

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

Under sufficiently perverse conditions you can get results such as I found several years ago. Is was setting the ground distance elements at 'A' for parallel lines to 'B'. The lines were in the 20 mile ballpark. About midway between was 'C'; and from 'C' to 'B' there were two more lines.

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

(OP)

Quote (submonkey)

Hi Mbrooke,

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.

Ok, that makes more sense now, as an inductor. Transformer through me off, which would be poor in any case imho.


Quote (Submonkey)

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.


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.


Quote:

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


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

2
The reversal shouldn't affect anything if the protection is just stepped distance. Start with two lines from 'A' to 'B', resulting in relays at A1, A2, B1, B2. Assume three zones of distance protection, with reaches of 85% of the line, 150% of the line, and 100%+ of the end of the next line out. Delays of 0, 20, and 60 cycles.

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

(OP)
Would the breaker clearing be 2 to 3 cycles? I know nit picking, but just want to see what you have in mind.

RE: Step distance relaying under double circuit faults

A cycle or so for the relay and 2-3 cycles for breaker time plus, maybe, a bit of slop. I like to list the longer end of the time range but I do see the occasional fault that is cleared in 3 cycles (relay + breakers) with a 3 cycle breaker. But I also see a 3 cycle breaker sometimes take an extra cycle or two if it's been closed for a very long time and the go just 3 cycles on the second trip.

RE: Step distance relaying under double circuit faults

MBrooke, you can find among SEL Application Guides that identified with number AG95-29, Dated on 20140611, and authored by Armando Guzman, Jeff Roberts, and Karl Zimmerman, Volume I, entitled "Applying the SEL-321 Relay to Permissive Overreaching Transfer Trip (POTT) Schemes. Read pages from 8 to 12 and you will find very clearly explained Current Reversals under "POTT Scheme Complications and Solutions.

RE: Step distance relaying under double circuit faults

(OP)

Quote (David Beach)

A cycle or so for the relay and 2-3 cycles for breaker time plus, maybe, a bit of slop. I like to list the longer end of the time range but I do see the occasional fault that is cleared in 3 cycles (relay + breakers) with a 3 cycle breaker. But I also see a 3 cycle breaker sometimes take an extra cycle or two if it's been closed for a very long time and the go just 3 cycles on the second trip.

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?



Quote (Piterpol)

MBrooke, you can find among SEL Application Guides that identified with number AG95-29, Dated on 20140611, and authored by Armando Guzman, Jeff Roberts, and Karl Zimmerman, Volume I, entitled "Applying the SEL-321 Relay to Permissive Overreaching Transfer Trip (POTT) Schemes. Read pages from 8 to 12 and you will find very clearly explained Current Reversals under "POTT Scheme Complications and Solutions.


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

(OP)
One final question. When both circuits fault, one is contributing to voltage drop which the relay is not accounting for- ie it is only aware of the drop caused by its own measured current (faulted circuit). Does this under any scenarios or phase fault combinations cause under/over reach?

RE: Step distance relaying under double circuit faults

(OP)
Do any of you guys lower your K0 factors to take into account when the neighboring circuit is out of service and grounded down for repair?

RE: Step distance relaying under double circuit faults

Nope. Should we be? Does the type of grounding make a difference? In some cases, there is a single ground at the worksite. In other cases, grounding switches are used resulting in two or three grounds on the line.

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

(OP)
Theoretically there may be a need since the mutual coupling properties change when the other other circuit is grounded down.

RE: Step distance relaying under double circuit faults

I think the concern is where multiple grounds are used causing a zero sequence path.

RE: Step distance relaying under double circuit faults

(OP)
Line grounded at both ends for servicing.

RE: Step distance relaying under double circuit faults

Suggest using single point grounding at the work site. Grounding the ends effectively energizes the parts of the line away from the grounds, and does nothing to enhance safety.

RE: Step distance relaying under double circuit faults

Under the right conditions, an out of service line grounded at both ends can see thousands of amps coupled into the line from adjacent lines in the right of way. On a 180 mile 500kV line, the middle line of three in the right of way, I modeled a fault at the bus at one end with both ends of the line grounded. With one of the ends faulted we get nearly 700A per phase, nearly 2100A of 3I0. With the other end faulted, the phase currents go to nearly 800A, nearly 2400A of 3I0. I don't have a sufficiently detailed model to calculate the standing currents that would exist simply because the lines aren't transposed.

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

(OP)
@stevenal- got it. There are times when I it is temporarily required:

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.

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