Subj : 3.5 weeks to being la
To   : Arelor
From : TassieBob
Date : Mon Jul 25 2022 18:35:38


 Ar> Say you have a Hospital which takes its power from the grid, and which
 Ar> has a diesel generator for emergencies. The basic state is for the
 Ar> switch to stay in position A so power passes from the grid to the
 Ar> radiodiagnostics equipment. THe switch stays in position A as long as
 Ar> the main grid is providing tension.

The medium scale installations I've been involved with typically did this with a couple of mechanically interlocked air circuit breakers.  Mechcanically interlocked so that even if the controller fails, or one gets locked up in the "on position the other can't fire.

The mains is monitored on all phases and when it fails a sequence is initiated to request diesel start, wait for diesel availability, open mains ACB, wait, close generator ACB.  Then the reverse when mains returns, with the generator itself having a run-down/cool-down timer after it receives the "shut down" signal.

Of course you can fire multiple mains/generator ACB's, and have multiple generators paralleled for larger requirements.

At the larger end of town a planned islanding will see the generators start and slowly take the load over 30 seconds or so to avoid dropping huge loads off the grid.  Similarly when going back on-grid the gens will ramp down generation and a graceful take-up by the grid.

 Ar> Then again, actual critical installations are more complex than that.
 Ar> A diesel generator takes a good bunch of seconds to kick in so

You might be surprised how quickly they can start and take load...  Some of the sites I worked at had, using diesel rotary UPSs had a window of about 15 seconds from mains fail to loss of power, and the diesels would be fired and on-load WAY faster than that - IIRC they could accommodate a failed start, followed by a successful start, and still be on-load in time.  That said, they usually started first time :-)

 Ar> generally you want a set of inline batteries somewhere to provide
 Ar> power while the generator is warming up, for example.

A "good" critical facility will have block heaters on the generators so they can safely go from not running to full noise pretty quickly.


--- GoldED+/LNX 1.1.5-b20220504
 * Origin: TassieBob's BBS (21:3/169)

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