Setting Line Relays Near Generation
Setting Line Relays Near Generation
(OP)
I'm wondering what others do when setting line relays at/near generation, looking onto the system away from the generation. In particular, setting backup protection - the traditional zone 3 or any overcurrent elements.
Historically we've essentially ignored the variability and set the relays like they were anywhere else in the system, everything in service and the generator impedance option set to subtransient. Well, it's easy enough to see that's probably right for a backup zone that will trip after 1 second. In the case I'm looking at at the moment, we have a 115kV line that has around 200MVA (nameplate) of generation behind it across 16 units at four plants. We might get most of that during the spring of a good water year, but by fall of a dry year it may be less than 70MVA of nameplate and only 4 or 5 units. Wildly different source strengths.
That 115kV line then terminates at a large station, on the low-side of a large (more than 300MVA) 30/115kV transformer. The standard criteria says that the third forward zone should be set to see past all that in-feed and reach the end of the longest line out of that station. That can wind up being further than the relay can reach.
Sequential tripping will get enough of the in-feed removed that the relay in question will reach as far as it needs to, but that can get rather ugly.
How do other people handle this type of situation?
Historically we've essentially ignored the variability and set the relays like they were anywhere else in the system, everything in service and the generator impedance option set to subtransient. Well, it's easy enough to see that's probably right for a backup zone that will trip after 1 second. In the case I'm looking at at the moment, we have a 115kV line that has around 200MVA (nameplate) of generation behind it across 16 units at four plants. We might get most of that during the spring of a good water year, but by fall of a dry year it may be less than 70MVA of nameplate and only 4 or 5 units. Wildly different source strengths.
That 115kV line then terminates at a large station, on the low-side of a large (more than 300MVA) 30/115kV transformer. The standard criteria says that the third forward zone should be set to see past all that in-feed and reach the end of the longest line out of that station. That can wind up being further than the relay can reach.
Sequential tripping will get enough of the in-feed removed that the relay in question will reach as far as it needs to, but that can get rather ugly.
How do other people handle this type of situation?






RE: Setting Line Relays Near Generation
We have some similar plants fed by a radial set of parallel 115 kV lines where we have to rely on sequential clearing. In the distant past, there was also a good likelihood of our generators tripping on low voltage prior to sequential clearing taking place. As we have improved the relaying, exciter and low voltage ride though capability, it is much less likely the generator would (incorrectly?)
RE: Setting Line Relays Near Generation
NERC went way overboard on the zone 3 bit a decade ago. A whole bunch of protection holes were created that took time to fill back in again; faults that would need to burn back toward the source before anything would trip. Bah humbug. Load encroachment was/is a much better solution to that mess than no remote backup.
RE: Setting Line Relays Near Generation
As there is enough time delay for Zone-3, the risk of cascade tripping is a lower probability.
RE: Setting Line Relays Near Generation
We've been using 87 more and more on HV lines and it makes things a lot easier.
RE: Setting Line Relays Near Generation
RE: Setting Line Relays Near Generation