Step distance relaying under double circuit faults 4

[Mbrooke]

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?

 

[HamburgerHelper]

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.

 

[Mbrooke]

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

 

[bacon4life]

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.

 

[Mbrooke]

Common circuit towers have proven to be not as advertised lol 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?

 

[magoo2]

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!

 

[Mbrooke]

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/lessons_learned_summer08.pdf

 

[piterpol]

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.

 

[Mbrooke]

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.

 

[davidbeach]

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.

 

[Mbrooke]

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

 

[davidbeach]

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.

 

[Mbrooke]

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/...pedance_FC_20150513_Web.pdf?v=20151124-113324

 

[davidbeach]

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.

 

[Mbrooke]

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

 

[HamburgerHelper]

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.

 

[piterpol]

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

 

[Mbrooke]

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.

 

[davidbeach]

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

 

[Mbrooke]

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

Question:

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?