LL fault on distribution feeder 1

[Prot123]

Hi,
What is your experience with clearing LL faults at the end of rural distribution feeder with low fault current. The problem is with fault level so low, there is no discrimination against load current. Also, what are the concerns with such a low LL fault going undetected? If this fault evolves flashover to ground then earth fault protection will clear it eventually. Some of my colleagues seem to not worry about the LL fault because of this? What are your views?

 

[davidbeach]

That’s why reclosers and line fuses get installed in long feeders.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Prot123]

Hi Davidbeach.
Even though reclosers are installed there wont be any overreaching protection or upstream backup unless you install few reclosers.
Also what are your views on not clearing or catering for end of line LL fault?

 

[davidbeach]

You really need to know how you’re going to clear that fault. I suppose letting it burn into something clearable, or burn itself out, but that’s certainly not an approach I’d take. Sounds like way to get hit with billion dollar lawsuits.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Prot123]

Thanks David. Your view is consistent with my thinking.

 

[beyond86]

What about voltage dip in this case? You can try to use confirming signal from UV protection or voltage negative sequence protection.

 

[RRaghunath]

Fault is fault whether the fault yields high current or low current and shall be cleared soonest. This is the only way one can keep a distribution system safe for the consumers and general public.
If the fault level for LL fault is as low as the full load current or even lower, one way of detecting it is use of Voltage dependent overcurrent function. This is available in most of the current day numerical relays and can easily be enabled / set if voltage signals are extended to the relay, in addition to current signals.

 

[cranky108]

I think a line recloser is a way to go. Special relay types just complicate things.

If you have voltage problems and need capacitors or voltage regulators, you need a line recloser.
Also good for small wire branches.

After all is the goal to be cheap, or serve the customer.

 

[Prot123]

VT may not be the solution as it is difficult to descriminate the fault relying on a bus VT.
NPS is a good way to detect low LL fault. What about low LLL fault?
Using a recloser is an excellent option; however, multiple reclosers are needed to provide remote backup during hardware failure.

 

[beyond86]

What about low LLL fault?

In this case you can combine NPS and symmetrical UV. When LLL fault occur, all phase voltage decrease symmetrical.

 

[davidbeach]

You can't rely on "NPS" aka negative sequence overcurrent. Blow a large tap fuse near the substation and you may well have more negative sequence current at the substation than a fault at the end of the line could produce. A long enough feeder, particularly starting in a community and then wandering to heck and gone through the countryside, could have more load current (particularly cold load) at the substation than it has fault current at the most remote end. Just can't solve that puzzle without multiple other devices on the feeder, whether fuses, reclosers, or some sort of hybrid device like the trip saver.

I’ll see your silver lining and raise you two black clouds. - Protection Operations