LOP Concerns with SEL 311C/311L Relays

[thermionic1]

I was recently doing a relay upgrade on an unusual station that had 2 lines, 2 buses, but only one set of VT's. We were told by system protection that there was a concern about LOP mis-operations on the line that was staying in service. A protection outage was requested to pull all of the trips from the other line, being radial fed with current diff in place as well as Z2 timer at remote end disabled.

I didn't get a straight answer from the protection engineer, and he transferred jobs in the middle of the project, so I pull the trips on my end. No LOP targets, no LOP SER, nothing. A few weeks later at a different station with 3 lines and a single set of VT's we had a similar but different challenge which was to replace the VT secondary terminal blocks in the house. We did the X winding (primary relays) then the Y winding (secondary). Still had an outage request to have trips off during the cutover. Same relays in play and still no LOP alarms, targets or SER records.

Is this a real issue or was the outgoing protection engineer misguided? He emphasized the 311C as being troublesome. IIRC, the 311C and 311L are identical other than the diff board, or at least that's what SEL said years ago when it came out.

Has anyone else experienced mis operations due to loss of a VT signal?

 

[davidbeach]

Yep,assume identical response for both the 311C and 311L for anything not related to the 87L.

We set overcurrent elements supervised by LOP do deal with protection while in an LOP condition. Performance isn't great, but the faults will get cleared. Perhaps over-cleared. But unless/until there is a fault everything can stay in service. Better to carry the load with a risk of overtripping than preemptively dropping the load.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[wcaseyharman]

In 2011 we had a pretty exciting event dealing with an LOP misoperation on an SEL relay. I think it may have been a 311C, but I’d have to dig back in my emails.
The substation was getting rebuilt, and the short piece of bus where the PTs were was de-energized. It was thought that the LOP would keep the lines from tripping once the OTs were de-energized. My memory tells me there was a problem with how SEL determined LOP in the relay and the LOP asserted, dropped out for a moment and reasserted. When LOP dropped out it tripped a radial line that serve a hydro plant that had one unit at full load. Due to the antiquated relaying on the hydro unit the load at the end of the line was not tripped while the unit went into overspeed and it blew up a bunch of stuff including multiple VFDs.
I had thought that SEL had updated the firmware for that relay after the event, but this was all 12 years ago.

 

[System Protection]

I saw a relay trip when a potential was interrupted during construction. The LOP asserted but not before the relay tripped. Pretty sure it was a 321. I no longer have access to the relay event unfortunately and there are two different LOP methods available on the 321 and I don't remember which one it was using.

I also saw an event from an adjacent utility where the original LOP logic in a 321 asserted for a real 3LG fault and prevented its operation. The advanced LOP logic would not have asserted if I recall correctly.

 

[bacon4life]

Thread

https://www.eng-tips.com/viewthread.cfm?qid=459352

has some info on troublesome aspects of the SEL LOP logic.

 

[thermionic1]

Thanks for the responses. wcaseyharman the rebuild situation you described was very similar to the station I was working at. We pulled the trips at the local end, then proceeded with switching. No unusual events occurred. This however, was the first time protection has asked for trips off during switching, which annoyed the switchmen, as they arrived at 5 AM and I got there at 8. Imagine switchmen having to wait....

The previous station I was at, we were upgrading all 20 year old relays in a 8 breaker ring. An overzealous relay tech in training pulled the VT box knife switches for the wrong bus section. I was outside at the time doing other things but received a call from dispatch about LOP on an adjacent section. Our LOP indication to SCADA is time delayed for 5 seconds. The relays were a mix of 321 and 311C. Neither tripped, just the alarm.