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.

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.

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.

Yes.

I have done relay testing and yes it does trip. When injecting secondary current, I used that equation to determine timing.

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.

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]

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.

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)