Relay Electromechanical Reset 2

[ee_123456]

Hello All,

All the relay vendors have "Electromechanical reset" or "instantaneous reset" on their protective function reset curves.

GE Says "use when coordinating with other Electromechanical devices".
SEL says "use when there is an upstream electromechanical device".

My question is, what if you're trying to coordinate an SEL Reclosing Relay with an up-stream electromechanical relay. Are there any implications in using an instantaneous reset vs setting to electromechanical reset? What if that SEL relay is now also coordinating with down-stream fuses? What is the difference between the recloser using an inst reset versus a electromechanical reset?

 

[davidbeach]

If you have an EM relay and a numerical relay with Inst reset both looking at the same sputtering fault the EM relay will almost certainly trip before the numerical relay. Sputtering faults and Inst reset can result in staggeringly long trip times.

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

 

[ee_123456]

Dave, I guess what I'm trying to say is if your EM relay uses a delayed reset could that impact coordination with an inst reset down-stream numeric relay? What exactly could be a sputtering fault?

 

[bacon4life]

ee- If the upstream relay has an EM reset and the downstream relay has an INST reset, there will be a larger coordination margin that with two relays that have the same setting.

David-How often do you see intermittent arcs that have enough current to be above the relay pickup? Is intermittent arcing common enough that you would generally recommend to avoid using the instantaneous reset? It seems like all the distribution fault records I have looked at either had a fairly solid fault (>80% of bolted fault current) or a very high resistance (<100 A of current). I might just be lucky to live in a region that usually has wet, conductive soils.

 

[ee_123456]

That makes perfect sense. For example if an EM relay coordinated with down-stream Microprocessor.
Assume EM Trip in 2 second and Down-stream trip in 1 second. The EM Relay will have turned 50% of the way toward trip upon the down-stream relay opening.

Now on a second shot of the recloser, if the EM relay is slow (say 20 - 30 second reset). It would not have fully reset during the open interval. So it will keep timing from the stopping point and essentially trip faster since it was already at say 30-40% (after reseting slightly during the open interval).

I think the best option here is to match reset characteristics. This is so that the down-stream recloser can also use EM reset and essentially trip faster also to match the up-stream relay. Downstream of that there is just fuse so no other reset characteristic to consider.

 

[davidbeach]

Typical feeder 51G pickup set at 360A or 480A can chatter when dealing with a fault miles out and a tree bouncing on and off the feeder can take a long time hold long enough to trip with Inst reset.

At the moment I can’t think of any miscoordinations with EM relays but it could happen.

The worst example was a fault in a vault, water getting somewhere it shouldn’t and getting turned the steam, and the relay produced nearly 70 event files over about 5 minutes before the fault lasted long enough for a trip. A EM style reset would have gotten the job done far sooner.

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

 

[wcaseyharman]

On a long 12.5kV circuit the company was getting voltage/flicker complaints from a customer. When we looked in the event report it was filled with low amperage ground faults. If I remember right the pickup was 300 A and the faults were around 500A. The faults were never present long enough to trip. It ended up being a tree adjacent to the line, and when the wind would blow just the tips of the branches would brush the line. The tree was trimmed and the problem went away.