Subj : 3.5 weeks to being la
To   : Arelor
From : TassieBob
Date : Sun Jul 24 2022 13:13:18


 Ar> Backup power systems that are worth anything must have a transwer
 Ar> switch to cut you off the grid while the backup power is enabled.

For small scale backup power systems like you'd have at home (generators, etc), yes.  Lose power, start up a genset, and fire the auto-transfer switch to disconnect mails and connect the generator (or do it manually).  In this instance there is a definite break before make when changing sources.

Home scale storage systems though are intended to operate with the grid and feed in when the batteries are full, and pull excess from the grid when the batteries are insufficient.  These systems run in sync with the mains, and will drop off the grid if the mains fails.

In larger scale backup power systems, such as might be used in a datacentre environment, the generators are sometimes setup to parallel with the mains to facilitate a slow controlled drop or increase in consumption.  Most utilities don't like it if you drop 20MW of load in a fraction of a second, or try dumping 20MW of load back on the grid in a similar timeframe (and their supply contracts at that level includes clauses to that effect).  The last one I worked on was a 30 second ramp up/down if I recall correctly.

 Ar> This is also to protect your premises. If your backup power is working
 Ar> at a given phase, it is not isolated from the grid, and the grid
 Ar> suddenly comes back, chances are the electric wave won't be in sync.

Yeah, if you try and connect two out of phase sources together then it can end badly.

I've seen this happen with cheap in-rack automatic transfer switches where the normally on relay welds itself closed. If the other source isn't in sync then when the ATS tries to change over - b00m.  In a good datacentre the A & B feeds to a rack should normally be in sync, even if they're off different power systems - but there are fault conditions that can cause them to get out of sync.

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

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