Need for a recloser in a distribution circuit 2

[Mbrooke]

Two questions in regards to this:

1) At what point (or conditions) do you need a recloser on an overhead line to cover for faults toward the end which may not be picked up by the substation breaker relaying? 556.5 AAC conductor @ 22kv through out the whole feeder.

2) Can an SEL-351 or SEL-551 be configured to do fast and slow curves (ie fuse saving to fuse blowing)? Or do I need a specific relay/pole mount recloser to accomplish this?

 

[davidbeach]

On long feeders fault current can fall off to the point that the substation relay is too slow or doesn't see it at all. If you're counting on the transformer protection to back up the feeder protection you'd want the recloser close enough that the transformer protection reaches a bit beyond the recloser.

You can set multiple 51 elements. The 551 (or is it the 551C?) has two; the 351 has a two for phase if you use both the 3-phase and the three 1-phase elements. To get two 51s for residual you need to wire the residual connection into the relay and then use 51N for one and 51G for the other. Supervise the faster of the two with the shot count wordbits; I'd leave the slower one active at all times.

 

[Mbrooke]

Off the top of my head I believe both the 551 and the 551C have two. (51P1T and 51P2T) The Quick set is showing me two "residual ground time over-current" elements on the 351, this means I can skip 51N altogether?

 

[davidbeach]

If there are now two then you're good. The time I used that trick was several versions ago.

 

[Mbrooke]

Version 7 appears to finally have it. One more reason SEL keeps getting better.


For those who know- is there a general rule of thumb when a distribution lines gets to long?

 

[bacon4life]

A line 4 miles would be long to me, but others might routinely have lines several tens of miles long. If you set fuse saving, be sure to include the breaker/recloser operating time when looking at the fuse curves. It takes either pretty low fault current or large fuses for fuse saving to actually work. As we started getting oscillography of distribution faults, I was amazed to see how often a fuse would open in the moments between the relay tripping and the breaker opening.

 

[Mbrooke]

Normally when I do fuse saving, I do it for both the ground and phase curves, but now that I think about it, would doing the ground curves only be sufficient enough?

 

[bwen08]

To answer your original question, we install reclosers to feeders when they facilitate our needs. Reasons include: lack of sensitivity on the breaker, mainline splits, energized work requirements, or sectionalizing to reduce exposure. We serve an area of mostly rural but also have some larger towns, our feeders vary from a few spans for primary metering to our longest 24.9kV of 50 miles and 12.47kV of 30 miles. Our longest two feeders have about 2 inline reclosers each. We also have a service territory where the fault current is only 160A at the point of ownership due to the neighboring utility's substation being about 60-70 miles away.

As for fuse saving, we've gone the fuse blowing route to reduce the blinks on our system which does result in more truck rolls for temporary faults. Depending on the curves you use for the fast trips, the ground will get most of the operations but you'll still want to check the phase to provide complete coordination. To go along with bacon4life's comment, add the operating time and some delay for your breaker/recloser to the curves and you'll likely see some fuses clearing while the breaker is opening.

 

[Mbrooke]

@Brian: does your utility use K or T fuses? I see an even split between POCOs but never fully understood the reasoning. I personally am a T guy.


Also, when I calculate minimum current for an end fault on a feeder for the determination of a recloser, I just use the minimum anticipated fault current at the substation plus the line impedance (R and X) components? What drives my question being that while the feeders in question are average length, normally open switches can significantly extend the line length. I'd like to either have the line crews covered, or have warnings in the switching procedure that to much back-feeding creates a "blind spot".

 

[HamburgerHelper]

"Often state-of-the art Type IV WTGs only inject a symmetrical current under all operating conditions including balanced and unbalanced faults. Hence, the negative and zero sequence components of the current during a fault are non-existent. However, it should be noted that the technology allows Type IV WTGs to contribute negative sequence current, if required."

http://www.pes-psrc.org/Reports/Fault Current Contributions from Wind Plants.pdf

 

[bwen08]

We are a T Link utility for taps and inline fusing. Our crews will typically only take holds on feeders when they are in their normal configuration; when switching has caused major changes, we'll review the coordination before they take a hold to verify the reclosers are operating as expected. The vast majority of our system is radial so having an abnormal configuration is few and far between.

In our experience, we've found faults to be bolted the majority of the time and on the rare occasion high impedance. We'll typically use 30% of maximum available fault current for the minimum fault current at each line segment for coordination review.

 

[Mbrooke]

WTGs? Sorry, brain freeze.

@brain: Thank you for your input, and the 30% number. The systems I deal with are far from radial, even if no recloser or auto loops are present, there are plenty or normally open gang switches. A short line can grow in lenghth for a number of reasons. Your ground pickup is 1/3 the phase value?