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 Post subject: Question about relay curvesPosted: Mon Apr 22, 2013 8:12 am

Joined: Tue Jun 19, 2012 4:45 pm
Posts: 8
I have a question about the curves from some SEL relays. It's probably a dumb question, but I will ask anyway.

I've attached a U3 curve from an SEL 351 relay plotted in SKM. The CT Ration is 240/1, PU is at 6.6 and TD is 5. I'm curious why there is a gap between the PU (1584A) and where the curve actually starts. I am sure it's a simple, easy answer. But I don't know it.

Also, is there a place where I can get the different curves to try to plot things by hand? Kind of for educational purposes. Very retro like.

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 Post subject: Posted: Tue Apr 23, 2013 1:00 pm
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 620
The relay instruction manual (down-loadable from SEL) has the formulas used to plot the curves. If you do the math, I think you'll find that t= infinity at PU=1. Curves usually start at 1.5XPU.

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 Post subject: Posted: Wed Apr 24, 2013 5:31 am

Joined: Tue Jun 19, 2012 4:45 pm
Posts: 8
Thank you for the answer. I have a follow up question then...

I looked through the instruction manual and I was able to find the information on the curve calculation.

For U3, it is

Tp = 2 x (.0963 + 3.88/(M^2 - 1))

If the pickup is 1584A and the current is, say 1900A, then the trip time would be:

M= 1900/1584 = 1.2

Tp = 2 x (.0963 + 3.88/(1.2^2 - 1)) = 17.8s

Will it actually trip in that region where the curve isn't shown?The multiple, in this case is just 1.2.

Thanks. I am just really trying to get the math and the understanding behind all of this.

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 Post subject: Posted: Thu Apr 25, 2013 8:10 am
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Joined: Tue Jan 13, 2009 5:00 pm
Posts: 620
Yes.

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 Post subject: Posted: Mon Apr 29, 2013 6:32 am
 Sparks Level

Joined: Fri Apr 15, 2011 7:43 am
Posts: 178
I have done relay testing and yes it does trip. When injecting secondary current, I used that equation to determine timing.

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 Post subject: Posted: Mon Apr 29, 2013 9:43 am

Joined: Fri Sep 10, 2010 7:48 pm
Posts: 17
Location: Idaho
You may also want to look at the SKM software display settings to extend the line, I know with ETAP software this is an optional setting.

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 Post subject: Posted: Mon Apr 29, 2013 2:33 pm
 Sparks Level

Joined: Mon Jun 07, 2010 9:40 am
Posts: 119
SKM sometimes has a mind of it's own and ignores its settings that should control that display, especially between pickup and the start of a curve, but the relay itself will actually trip(eventually) for all values over it's pickup. [Assuming you have programmed the relay to trip for that element, which is a whole different issue]

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 Post subject: Posted: Mon Oct 28, 2013 8:54 am

Joined: Sat Feb 27, 2010 4:14 am
Posts: 3
SKM Powertools has a setting that stops the plotting of the curve over a certain time which I think is by default 1000 or 10,000 seconds. Check in your options or preferences (I don't have it open at the moment). I think you can increase this. Problem is that it is not realistic in some ways to have a curve plotted this far. I infer that there is a curve there somewhere, but the time my vary.

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 Post subject: Posted: Mon Oct 28, 2013 10:02 am
 Plasma Level

Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1714
Location: Scottsdale, Arizona
pazzjl wrote:
I'm curious why there is a gap between the PU (1584A) and where the curve actually starts. I am sure it's a simple, easy answer. But I don't know it.

The gap is a carry over from the old induction disk overcurrent relays. If an induction disk relay was set at 1584 Amps, when the current reaches 1585 amps the disk would not begin to rotate - it took a bit more current. There was friction and inertia that affected when the induction disk would actually go into motion and begin timing out so this was left undefined on the TCC (a.k.a. the gap in the curve)

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