Wacky Weekend: NERC to relax power grid frequency strictures

Matthew Huff mhuff at ox.com
Sun Jun 26 15:03:17 UTC 2011

Another couple of reasons to use a delayed transition ATS:

1) Motor lock. Delays on HVAC equipment never get triggered if the system never goes offline. Having a correct "open" period allows the motors to spin down, and start back up on the delays that are programmed keeping them from being synchronized

2) Allowing transformer fields to collapse. Even in phase, without a delayed transition ATS you can end up with a partially collapsed transformer field with a new field being created at non-ground state. This can cause a transient back wave that can snap circuit breakers. Yep, this one happened to us a few times before we switched to a delayed ATS, was a PITA to debug and resolve.

-----Original Message-----
From: Seth Mattinen [mailto:sethm at rollernet.us] 
Sent: Saturday, June 25, 2011 8:49 PM
To: nanog at nanog.org
Subject: Re: Wacky Weekend: NERC to relax power grid frequency strictures

On 6/25/2011 16:43, Paul Graydon wrote:
> On 6/25/2011 12:32 PM, Seth Mattinen wrote:
>> For open and closed transitions you'll most certainly want to sync to
>> utility to transition between the two. For the delayed transition model
>> it'll stop at the intermediate "open" point for a configurable amount of
>> time during which the load is disconnected from everything (i.e. let all
>> the motors spin down first).
>> ~Seth
> Take a guess what the datacenter our equipment is currently hosted in
> uses.  Yet another reason to be glad of a datacenter move that's coming up.

Also depends on the operator, so ask to see their xfer switches and how
they're programmed if that's a concern. All of the non-residential
models in that link for three-phase have motor/load disconnect signaling
capability. If the operator is clued enough to use it then it's all
good: shut off signal to motor/compressor loads, phase sync and switch,
signal reconnect after delay. But if they're not... run away.

Even with the delayed transition models the "hold open" delay can be too
short and end up re-energizing the motors too quickly. There's always
plenty of ways to f*ck things up good.


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