Setting a KD-10 relay at MTA other than nameplate

[trat1208]

Hello,

Are there any good relay guys out there that can help me out with the following? I am trying to figure out how tap settings are determined for a KD-10 relay at a MTA other than nameplate. I know how to come up with settings at nameplete MTA using the formula ST/(1+-M).

So for a KD-10 with 75deg nameplate MTA where I need a specific reach at a 90deg MTA instead, I know that adjustments can be made to change the MTA and then the 90deg reach should relate to the ST/(1+-M) tap formula by some factor. How is this correction factor determined?

According to the manual (IL-41.490),
Z_setting = Z_desired at new MTA / (cos (MTA_nameplate - MTA_desired))

However, all of the examples given in the manual are for cases where the desired MTA is less than the nameplate MTA and I am not certain than this formula works for the example I have where I want a 90deg MTA on a 75deg relay.

I am not trying to set a relay, rather I am just trying to decipher some settings that I have on file for one of my stations. The settings I have show taps of T=5.8, S=2, and MTA=-0.03 for both PH-PH and 3PH elements on a KD-10 relay. This equates to 11.96 ohms at nameplate 75deg MTA. The settings say that this gives a desired reach of 12.38 ohm @ 90deg MTA. All of these values that I am talking about are secondary / relay values of course. Unless I am using the above formula wrong, I don't see how it gives me the correction factor that I am looking for.

Do these settings make sense? What math can be done to yield the correction factor that takes me from 11.96 @ 75deg to 12.38 @ 90deg?

Thanks

 

[davidbeach]

Plot the characteristic, does it look right? Does it cross the 90 degree MTA line at the right place?

 

[trat1208]

davidbeach,

If you are asking if the relay actually performs correctly, yes it does. From the last cal, at the settings mentioned above, the relay tested at ~12.38ohm with ~90deg MTA. I should have put that in my original post.

I know that these settings are proper. I am interested in how a protection engineer would come up with the tap settings given the non-nameplate MTA. There's got to be some calculations for determining tap settings with different MTAs.

 

[cranky108]

Why change the angle, you can calculate the desired reach at impedance of what you are protecting, and keep the relay at 75 degrees.

Besides I don't know of anything that has a 90 degree impedance line. Unless you are working with a super conductor.

The KD is generally used for protecting lines, generators, and maybe transformers, but none of those have a 90 degree impedance.

If you are concerned about load and the 75 degree angle, use a load encrochment to momentary disable the three phase unit.

 

[scrammblur]

I've only set a few KD relays, but boss has probably issued 2000 Kd-4 and KD-10 settings. I might have some notes that might help I'll look for some next week.

 

[trat1208]

Cranky,
I don't disagree with any of your comments, but I'm just trying to figure out the settings that are already there. I think that this is a very unique application. This is being used as part of an out of step (OOS) protection scheme on a generator, it will trip the unit if supervision by a fault detector relay is satisfied (from what I can tell set at about 125-150% rated amps). The KD relay is wired into the CT/PT's at the transmission interconnect breakers and looks back towards the GSU. I say that this is an OOS scheme because that is what the prints call it, to me it looks like a lead protection scheme for the HV leads from the GSU to the switchyard.

I am not sure how common a scheme like this is, as everything that I can find in relay textbooks refers to OOS schemes where the relays are connected at the generator CT/PT's looking out into the GSU and system. This scheme is standard on all of my company's units which are 1950/60's vintage and were all originally installed by the same utility. If you have any familiarity with such applications or how it relates to OOS conditions please let me know. To me this thing looks more like a lead protection scheme because the 90deg angle would be your expected fault angle looking towards the unit from the switchyard for a short run of overhead line.


scrammblur,
If you do have any info it would be much appreciated. I'm just trying to rationalize to myself how these tap selections are made.

 

[cranky108]

Most of our units are connected with a KD-11 on the high side of the generator looking out, with an offset looking back into the generator.

Haveing said that, I recall Blackburn one time saying that we as protection people should put relaying in the yard looking into the generator, because those generator people sometime do dumb things.

If you have a spare relay, maybe a test might be a good way of getting your answer.

 

[scrammblur]

From the KD-10 manual.. "The 90-degree setting is approached for all ranges when R3 resistor is open circuited for the 3φ unit or R2A and R2C for the phase-to-phase unit."

http://www05.abb.com/global/scot/sc...ce4291fea5c1256ed9003585a3/$file/41-490-j.pdf