Reclosing into unsynched generation 1

[HamburgerHelper]

How is this usually handled? We have a situation were a breaker reclosed into a line that tripped out on a temporary fault. There was tapped generation on the line. The generator supported the voltage on the line but it was below the dead line voltage and the relay reclosed into the generator out of sync. The generators are supposed to trip themselves out. I don't know how they do this but I am guessing on overspeed? How is this usually handled to avoid closing back into thing out of sync?

 

[marks1080]

Trip the generator from the line protection?

But I am surprised the generator didn't take itself out.

 

[ScottyUK]

Normally an embedded generator will have a vector shift or rate-of-change-of-frequency (ROCoF) relay to disconnect it in a loss-of-utility situation. This doesn't apply to large central generating plant, but it does apply to relatively small / medium machines connected on the distribution (as distinct from transmission) system. These relays are awkward to set, or at least it's not easy to arrive at reliable settings without a fairly good model of the system and the generator.

 

[davidbeach]

We stall the reclosing until the line goes dead.

 

[HamburgerHelper]

I was thinking that same thing. Whatever process got interrupted is interrupted. If power comes back in 5, 20, 30 seconds or 2 minutes, it makes no difference.

 

[FreddyNurk]

Normally under voltage protection is also present on the generator, and it would be sensible to think that if the generator voltage is lower than the recloser dead line voltage then it would trip. It is possible that the time setting on the under voltage on the generator is set longer than the allowable reclose time, thus the reclose happened before the generator could trip off.

In my experience though, the generator won't stay up at reduced voltage for long enough for that to happen. It does depend on the size of the generator though, I haven't normally dealt with sets larger than about 2MW.

ScottyUK is correct though, under voltage is part of what I would consider normal set protection rather than interconnect protection, and the vector shift / ROCOF is needed for such situations, often mandated by the utility.

EDMS Australia

 

[davidbeach]

A lot depends on the generator/load balance in the island. Our first reclose is also less than 30 cycles so the generation needs to be gone fast if the reclose is to happen on schedule without stalling. I'm not aware of any incidents where we had any reclosings" due to generation not going away soon enough. "situation

 

[waross]

When I read the title of this thread, Reclosing into unsynched generation, the first word that came to mind was:
"BANG!"
The second thought was;
"Don't do it."
There is more than one solution, but whatever it takes, don't close a switch into an unsync'ed voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stevenal]

So is this a utility distribution feeder breaker on a line that had a tapped generation in addition to customer load? "Don't do it" kinda leaves all the customers in the dark. Maybe the reclose can be slowed, or the dead line condition detected, or a trip signal supplied. In the end, the utility needs to serve it's customers and the generator owner is responsible for protecting their equipment.

 

[xnuke]

IEEE 1547a-2014 calls out the clearing time the generator is allowed to remain interconnected when abnormal voltages occur. The defaults are based on percentage of nominal voltage:
[ul]
[li]V < 45% - 0.16 s[/li]
[li]45% ≤ V ≤ 60% - 1 s[/li]
[li]60% ≤ V < 88% - 2 s[/li]
[li]110% < V < 120% - 1 s[/li]
[li] V ≥ 120% - 0.16 s[/li]
[/ul]

Clearing times are adjustable if allowed by utility for low voltage ride through, but there are also maximum limits for these as well. Consult the standard for more information.

As far as reclosing, IEEE 1547-2003 is pretty clear:

"The DR shall cease to energize the Area EPS circuit to which it is connected prior to reclosure by the Area EPS."


xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[crshears]

I didn't see where the OP said anything about the voltage level of the equipment under consideration...and where I work that has a profound effect on what steps are taken to prevent out-of-phase re-synchronization.

Here are some bare-bones descriptions, starting at the bottom and working up:

Our 4 kV distribution connected customers are required to have local loss-of-connection schemes to remove themselves from service, whether this be frequency excursion, under/overvoltage, overcurrent, or whatever. Typically, no communication paths of any kind are employed to provide remote tripping of the generator for feeder faults, although there are, of course, exceptions [there are ALWAYS exceptions]. Also typically, customer generation re-synchronization is prevented by supervisory schemes that block re-connection until stable voltage and frequency have been present at the customer site for a minimum of five minutes.

The connectivity confirmation of distribution-connected generators on our 13.8 / 27.6 / 44 kV networks is commonly provided by means of bi-directional comm paths, such that operation of a feeder breaker for any reason sends a trip signal to the generator's circuit breaker to open it, disconnecting the generator from the feeder; feeder circuit breaker reclosure back at the transformer station is supervised by receipt of a "distributed generator end open" [DGEO] signal from that customer...and if there are multiple distributed generators on the feeder, DGEO signals must be received from all of them before reclosure is permitted.

At 115 kV and up, the schemes can become quite sophisticated, but can also be all over the map; communication paths for remote/transfer tripping are generally, but not always, provided, and where they are not, almost invariably no breaker reclosure is even provided for the circuit breakers at the generating facility's terminal of that circuit; instead, that terminal of the circuit will be re-loaded by manual closure of breakers under the supervision of a synchro-check relay. Sometimes reclosure schemes are provided but are normally in the blocked state, their enabling being reserved for such occasions as planned outages where, due to station configuration, prompt re-energization of load-only equipment is feasible and desirable.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]