Externally summed CTs for bus protection
Externally summed CTs for bus protection
(OP)
Starting this not to steer off another thread, but I inquire 2 questions:
1. At what point or fault levels do engineers typically move away from externally summed CTs for (over-current) bus differential protection? C800 class CTs of concern here FWIW.
2. Can externally summed bus protection CTs be used in networked circuits?
1. At what point or fault levels do engineers typically move away from externally summed CTs for (over-current) bus differential protection? C800 class CTs of concern here FWIW.
2. Can externally summed bus protection CTs be used in networked circuits?






RE: Externally summed CTs for bus protection
There's at least a couple of way of interpreting that question. If you're using an overcurrent relay as a bus differential, everything is summed external to the relay, so that may not really be what the question is.
If you're talking a differential relay that produces restraint and operate currents, either electrically or numerically, you have to be careful what you sum up externally. You need to have every source on its own CT but you can parallel up loads. The problem with paralleling a source and a load is that for an external fault on that load circuit your restraint current will be much lower than it would be if every CT had its own input. Throw in a bit of error, and that missing restraint current may be allow the relay to operate for an external fault.
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
That may be a bit simplified, but 20 years ago we'd pick just the "right" relay for the application and have lots of different relays to purchase, design around, and maintain spares for. Now it's believe to be less costly to have as few different designs as possible and some applications get a design that would have been considered gross overkill 20 years ago. Part of it is compliance obligations, part of it is simply trying to get more out of each protection engineer than 20 years ago and there's no time to "optimize" each design, nor do we see the point.
I'd much prefer to get really good at a handful of standard designs, portions of which may be omitted at any given location, than to come up with just the right design every time.
So, if I need a full out low impedance bus diff design I see absolutely no value in also having an overcurrent based design. Would an overcurrent relay work some places? Sure, undoubtedly; but the savings would be illusionary.
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
Being SEL I am sure you will get your moneys worth. Unless you are dealing with profound saturation the relay should restrain on external faults but trip immediately on internal ones. However, FWIW I do know of utilities that have modernized their relaying from EM to microprocessor but kept the high impedance bus protection. Here is one example of a paper by SEL where a utility actually chooses electromechanical relays for bus protection despite replacing everything else. See page 10, descriptions numbered 2 and 7:
https://cdn.selinc.com/assets/Literature/Publicati...
RE: Externally summed CTs for bus protection
There are papers on the advantages of both low and high impedance relays, and the answer is it depends on you, your application, your people who work on it, etc.
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
My understanding is that when a CT produces less current, there is less current to cancel out the current coming from all of the other feeders combined. Ie, if bays 1-9 produce 90 amps and the saturated bay CT only produces 80 from saturation, 10 amps will flow through the relay when in theory it would be close to zero for a none saturated CT.
RE: Externally summed CTs for bus protection
If you expect that under some cases you will see error current, that should be taken into account in your calculations, such as slope or setting.
RE: Externally summed CTs for bus protection
I see SEL is now referencing IEEE C37.110. See the CT Requirements section of the 487B-1 manual. Looks like they may have a program available as well.
Your understanding is correct. With a low impedance relay, all currents flow into the relay and the differential current is summed in the relay. The input currents are used for restraint. As cranky said, you can set your slope (Iop/Irt) to allow for the 10 amps of Iop in your example.
RE: Externally summed CTs for bus protection
But what should I consider in setting if I do not have restraint current?
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
Question. In places where saturation is unlikely (such as a distribution bus with 1200/5 C800 CTs 10,000amp and under short circuit) is over current differential protection still ill advised?
RE: Externally summed CTs for bus protection
Vs = (1+X/R)·If·Zb
I would recommend low impedance with restraint or high impedance. If you are concerned about cost, consider a fast-bus trip scheme instead of bus differential. If you have microprocessor relays on the main, bus tie, and feeders, you might achieve 3 cycle operation without any additional relays or CTs.
RE: Externally summed CTs for bus protection
I can confirm from commissioning these schemes, the 3 cycle time (between relays) is easily achieved.
RE: Externally summed CTs for bus protection
That scheme has some obvious advantages and it was well sold; but that was 20 years ago. There are some very subtle ways for it to fail; at the moment I don't feel that sharing would be appropriate but it seems to have built in several traps. Don't (just) look at first cost; look at the costs (both tangible and non-tangible) of an extended outage simply because some corner case behaves radically differently from what everybody is more familiar with.
RE: Externally summed CTs for bus protection
I know long runs in the CT circuit (higher ohms) decrease saturation, but, how does low burden play into this?
Thanks, a wealth of wisdom as pure usual :) Considering your experience, practicality and advanced knowledge in the field of protective relaying I am going to take your word on fast-bus. Having decentralized bus bar protection for 12kv and above has never sat right with me, I actually prefer having more relays.
If you feel comfortable saying, what do you use for bus protection at the 12-34.5kv levels?
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
The problem with that scheme with the 251 is that it keeps cropping up as a way to save money.
What I don't find is papers on how to best install the fibers for arc-flash protection.
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
Make that increase.
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
https://cdn.selinc.com/assets/Literature/Publicati...
https://cdn.selinc.com/assets/Literature/Publicati...
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
Appears to also be available through Amazon.
Why would you need a "line angle" for a bus? I imagine that you could calculate the impedance as though the bus were a line and use that. Pretty sure you'll get something over 80 degrees.
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
RE: Externally summed CTs for bus protection
You should keep the impedance low to prevent saturation of the CT.
Except in the high impedance differential.
RE: Externally summed CTs for bus protection
Once I have CT saturation values and other parameters, should I let them extensively dictate my 487B settings or be more casual? I heard a member mention on here that the 487B makes a trip decision before saturation can take place?
RE: Externally summed CTs for bus protection
The problem with saturation is not failure to trip, it is the possibility of tripping for out of zone faults. Tripping decision speed doesn't come into play, since it can still make the wrong decision on the very next processing interval, or the next.
RE: Externally summed CTs for bus protection
http://www.eng-tips.com/viewthread.cfm?qid=424564