Cross-Country faults 1

[Variac200]

Are there any "standard" definition for a cross-country fault? Any references?
Is there any relationship between the faults ie. the occurrence of a second fault depends on the first fault? ( For eg. a phase to ground fault causing voltage rise in healthy phases which in turn may lead to some insulation breakdown somewhere in the system)

 

[davidbeach]

My working definition of a cross-county fault is one in which fault current flows between independent circuits. The fault may be direct, physical contact between conductors, or it may be indirect using ground.

As an example of the indirect is one we had a while back; feeder started out with about 2000A of A-phase current on an A-G fault. That 2000A was the maximum through the neutral resistor on the transformer. But then the A-phase current jumped to over 20,000A. Looking at just that relay's event report it seemed impossible. At just the moment the A-phase current increased, another feeder had a C-phase fault, also of around 20,000A, also supposedly limited by that same neutral resistor. Rather than seeing independent A-G and C-G faults, each relay was seeing one phase's component of an A-C-G fault. The transformer relay event reports looked just like they would for that A-C-G fault.

It is that "part of the fault" problem that makes cross-country faults a protection problem; the fault is pretty much always not the fault the relay is seeing.

 

[Variac200]

@Davidbeach,

Thanks for the response.I tend to agree mostly with your definition but I would also like to know if there is any "Standard" definition or better still if yours qualifies as such. And how different is it from "simultaneous fault".

I have also searched this forum and realised you have done much work on this subject. If possible, may I have as a guide and reference your paper on this subject?

 

[davidbeach]

Geesh, I did something a decade or so ago. Copies are probably available from the Basler.com web site, or perhaps in a WPRC archive. The "cross-country" faults that paper dealt with were on impedance grounded or ungrounded industrial systems.

 

[Mbrooke]

A cross country fault in my experience is one that occurs between separate circuits or transmission lines at the same time. The only time I have heard this term is when dealing with things like ungrounded delta distribution systems. Often in these systems one phase will go to ground trigger a GF alarm. Before the fault is attended to, a fault will occur on another phase on another circuit either due to bad whether (severe storm causing many lines to simultaneously fault) or marginal insulation breaks down elsewhere due to the P-G voltage rise on the other phases when the first one faults.

 

[davidbeach]

Broken conductor that falls into an underbuilt circut can produce a cross-country fault as can a snapped shield or guy that wraps around more than circuit's conductors. Another good one is when a fault on a lower circuit produces a plasma channel allowing a higher circuit to flash over into the lower circuit.

 

[Mbrooke]

But wouldn't that be a cross circuit fault? At least around me it is.

 

[crshears]

Cross-country fault??? Completely new term to me...

Had one once where the lowest phase of a 230 kV ckt dropped off of the tower arm due to a failed insulator retaining pin during a cross-wind, therefore no broken conductor or fault to ground on that phase...but the live conductor dropped on to the top phase of a 44 kV feeder that crossed underneath the 230, causing some fun and games due to simultaneous lock-outs of the 230 ckt and the 44 feeder...plus I suddenly had to be quite forcefully strident in getting my fellow operators to belay their attempts to pick up either pending ispection, since I was the only person connecting the dots between this pair of coincident events...

Would this occurrence meet the definition of a "cross-country fault"?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[marks1080]

At one of our stations: One feeder leaves the station and goes north. One feeder leaves the station and goes south. Legit fault on the feeder going south results in a trip of that feeder, as well as the feeder going north.

We found the cause to be earth resistance. The geography in the area meant that the southern feeder fault had an easier return path back to the station by going all the way around the station and finding the neutral path of the feeder that was going north. We called this a 'cross-country' trip. I believe the station was pretty much built on bedrock, but i never did get too involved in the geography analysis.

 

[davidbeach]

CR - yes, that would meet my definition.

 

[cranky108]

I don't know David, I could not find that paper. But then again I had trouble finding any papers on their web site.

I looked at WPRC papers and only found one on cross country faults in 2002.

 

[Variac200]

Thanks a lot for all the response. Very insightful and I beginning to appreciate the subject a lot better.

Still looks like different Utilities have a "unique" way of defining "cross-country" faults.
There are wide similarities; Minimum of two faults occurring either at the same time and same location or slightly different times and different locations (or circuits).