87 Differential tripping main breaker??? 5

[rockman7892]

I have a problem where a SEL 387 differential is tripping our main 5kV breaker. The differential zone is between the secondary bus of our substation transformer and on the load side of our main breaker right after the breaker. The cable that is between the differential zone is 9 runs of 1000MCM for a distance of 1000ft.

The SEL 387 keeps tripping the main and indicating an 87 trip for phases B and C only. The only thing that we did different was added an additional small load in the plant to this breaker however the breaker was closed when we added this load and did not trip. It was not until we opened the main breaker ( unrelated reason) and then tried to close it again that it tripped on differential. After it tripped we tried to close this main again an it tripped on the B and C phase again.

I'm confused b/c nothing that we do on the load side of the breaker (in the plant) should effect the differential zone since what goes in must come out. I'm thinking that there is a problem with the cable run between the substation transformer, or something wrong within the main breaker.

Any ideas???

 

[davidbeach]

What do the relay event reports show. Post a one-line and the event files (CEV format preferred) and maybe somebody can suggest something.

 

[edison123]

Check polarity of CT's. May be they are reversed.

 

[slavag]

Hi
As Edison said, polarity of CT's it's first reason of false operation in your case:
1.Load was lower than setting.
2.With additional load, sum. load still was lower than setting.
3. Inrush current in time of open/close of main CB was enough for tripped CB.
Are someone provided stability check of the protection system?

Please send us SLD, as David asked, please with all CT's technical data, maybe some CT's was in saturation

Best Regards.
Slava

 

[rockman7892]

Attached is a single line for this relay. It isn't the best SLD, and i'm working on getting another one posted. The breaker I'm referring to is labeled SECT 1. You can see the two sets of CT's where are at the breaker and right after the switch on the transformer secondary bus.

I dont think it has anything to do with the CT's b/c like I've mentioned in the post it has worked fine in the past. When we energize the cables to the line side of the breaker everything is fine with the relay. It isn't until we close the breaker that the relay trips.

I'll try to get the relay files from the utility when they come out tomorrow.

 

[jghrist]

The SLD shows only one set of CTs connected to the 87L that trips SECT1. Where is the other 87L winding connected? The SLD shows a second 87L linked to the first by fiber optic and shows one CT lead going off the page. Where is it connected and what does the fiber optic link do? Is there no transformer differential? I would expect a transformer differential zone to include the transformer and the main breaker.

I would not rule out CT polarity or winding connection problems. It may be that without the added load, the error did not cause enough differential current to operate the relay. The new load may have a high enough inrush to trip only on energizing.

CT wiring is the most common problem with differential trips.

 

[davidbeach]

It would have been nice if you would have said SEL-387L in your first post rather than SEL-387. The 387 is a transformer differential and the 387L is a line differential; very different application. Where is the connection of the upper relay and its CTs? What is the line that goes off to the right about 1/3 up the diagram?

Please post an event report from one, or better yet both, of the relays showing the trip.

 

[slavag]

Additional Q about SLD.
What is a some connection before main CB ( right side with a truck symbol)?
Regards.
Slava

 

[ANGIBEW25]

Hi Rockman 7892:
I agree with slavag I would check the settings of this relay and confirm the CT specs and wiring. You may have a rolled CT, but more likely the additional load is exceeding the relay set points during inrush.
I would test the following:
1. SEL relay for proper pick-up at required set points.
2. CT, ratio, polarity and wiring to relay. Make sure that all the CT's are on the same tap.
3. Cable tests. Do a Quick Megger check phase-ground and phase-phase. You do not have to diconnect any cabling to do this test in this scheme, Just rack-out the c/b and open the isolating switch. If the cables indicate no fault you then have to concentrate on the relay and the CT circuitry.
Good Luck.

 

[davidbeach]

For those not familiar with the SEL-387L, it has no protection settings; it is just line differential in communication with its mate at the other end of the fiber optic link. CTs are absolutely critical, both must be connected at the same ratio and have the same performance characteristics as there is nothing to tell the relay(s) about the CTs.

My guess is that there is a CT problem and that load was low enough that it didn't trigger a differential, but the added bit of load was enough that now inrush currents cross the differential threshold. That's why I keep asking for an event report; it will clearly show why the relay trips.

 

[54494060]

Is it possible to disconnect the (added "small" load) and eliminate that possibility? May save some time and give you the necessary information to replace/recal your relaying.

As Iron sharpens Iron so does a man sharpen the countenance of his friends.

 

[slavag]

David,Thansks a lot for the explanation.
Could you please add for our education.:
Are possible watch diff. current on-line in thoses relays at the both sides?
Are protection curve include restraint curve?
Best Regards.
Slava

 

[slavag]

Intresting thread, intresting relay.
I see short manual of relay ( for full instruction manual registration needed). Nothing about commissioning. No data on the screen about diff currents and angles between phase currents on the both side of line.
What I do for stability test, from my point of view, only DR possible use ( if raley have this option)

 

[davidbeach]

I think that you can get a snap shot of the currents using the meter command, you can always trigger an event and get the full picture.

If I recall the principle of operation properly:

Each relay of the pair compares its current with the remote (other end of line) current, dividing one by the other. In an ideal world the result of that division is -1 as one current is in and one current is out. So, -1 is clearly a point of restraint. Then they expand that to the range of (if I remember right) -1/6 to -6 to account for ratio errors and minor tapped load. Then to account for angle errors, asymmetric channel delay, and various other errors, that band from -1/6 to -6 is rotated around the origin to cover the area from about 85[°] to 275[°]; forming a large "C". No other restraints and unlike any "historic" or "conventional" differential relay that I am aware of. The 311L uses the same principle and allows adjusting the settings, but the 387L is billed as the "Zero-Setting" relay (it has a total of 4 settings - Relay Id, Terminal Id, Transmit Address, and Receive Address, but no protection settings) and you get what you get.

Because of the range from -1/6 to -6 on the ratio of currents, you can have connection errors that won't cause a trip until load gets large enough. The OP is at the point that his connection error/load combination is just below the trip point, but during inrush it goes above the trip point, and there is next to no delay on the trip.

That's why an event report would provide so much information and allow us to figure out what the connection problem is.

 

[slavag]

David,thanks a lot again.
Now it's clear.
It's other principle, that I know.
Rockman, you have provide all needed tests again.
I fill, your additional loads it's this additional line on the right side of SLD.
Best Regards.
Slava

 

[rockman7892]

Thanks for all of your responses. Here is a better SLD that shows a more complete picture of the relaying scheme. Unfortunately I do not have the software to connect to the relay to obtain an event report. It is the utility's relay, so they are coming out first thing this morning to look at the relay. Hopefully they will be able to address the isssue, however I will try to obtain an event report to post as well.

 

[tem1234]

Hi rockman

Maybe i'm missing something, but there's a 25 kVA Txfo (station service) into the 387L differential protection zone! The current into this txfo get in the diferential zone but not get out. Can this be a cause of tripping?

 

[jghrist]

The station service transformer seems to be in the transformer differential zone in the new SLD, so this would not be a cause of the 387L trip.

The new SLD shows a 311L on one end and a 387L on the other end. I'm not familiar with the 387L, but if as David says, it uses the same principle as the 311L, I suppose that it might be compatible. I'd certainly get the advice of Schweitzer before assuming this. This is not likely to be the source of the problem; it would most likely either not work at all or not be a problem.

You can download software from the Schweitzer site to access the event reports. Or you can use any terminal program and relay commands. You do need a special cable to connect to the relay. A standard serial cable or null modem cable doesn't work.

 

[davidbeach]

No problem with a 311L-387L combination.

 

[slavag]

According to short manual, it's standard application 381L and 311L