Reclosing on Islanded system 1

[Mbrooke]

What a fault takes place on a normally islanded generating station- do you set relaying to reclose automatically or must it be done manually- primarily in a steam turbine system but diesels too?

 

[FreddyNurk]

The remote diesel powered systems I used to work on were set to auto reclose for phase overcurrent faults, lockout on earth fault (at least as far as I can remember).
They were set to automatically start up more generation than was required for the load though, although I don't recall the exact sequence, or whether they reclosed first on running capacity and waited until more capacity came on if a second attempt was needed, or just started more first. With the protection schemes they had in place they often used to black the station out for certain feeder faults, they were moving to voltage controlled overcurrent but I don't know what effect that had on keeping the station up.

There were a few rules changes regarding whether it was safe to reclose or whether it was straight lockout, but that wasn't to do with the capacity of the station, rather that was safety and operational issues.

EDMS Australia

 

[Mbrooke]

Neat

For an underground cable I could understand. But in overhead reclosing is a must.

 

[RRaghunath]

Reclose is preferred on single line to ground faults and not on phase faults in islanded system.
It is because a reclose on to fault that is close to generating station could cause generator to trip on out-of-step.

 

[Mbrooke]

Thanks RRaghunath.

Is the risk for out of step also present a the first 3 phase fault? And if so what is normally done to mitigate it?

 

[FreddyNurk]

RRagunath, is the reason for blocking reclose for phase faults because there's more than one source in the island, or is it related to system behaviour?
All of the sites I worked on were effectively single source, so no risk of reclose while other sources in the network might be doing things.


EDMS Australia

 

[RRaghunath]

Three phase fault close to generator causes sudden load throw-off on the machine (phase fault is mostly inductive). This could cause machine speed to go up. The impact is great, especially when the generator is not part of any large grid.
Faults are inevitable but reclose is a choice we make.
Reclose on ground faults shouldn't pose a problem.
Moreover, majority of faults in overhead lines are supposed to be ground faults anyway!

 

[FreddyNurk]

RRaghunath, that makes sense, although it wasn't relevant for us, machines were mostly high speed diesels, and load rejection was generally tested without too many issues. I'm sure it wasn't that nice for the machines, but they almost always responded fast enough that it wasn't a concern.

You're correct regarding majority of faults though.

EDMS Australia

 

[krisys]

The intention of auto re-closure is to clear the temporary faults happening due to the heavy wind, falling tree branches etc. That too when there is no attendants for the switching operation. The probability of single phase faults remaining for a moment is far higher than the three phase faults.

The auto re-close shall not be tried for the cable distribution system, as the faults are normally expected to be permanent in nature.