Wacky Weekend: NERC to relax power grid frequency strictures

Alex Rubenstein alex at corp.nac.net
Sat Jun 25 23:23:47 CDT 2011


> It ismy understanding also that most commercial grade gensets have
> built into the ATS logic that when utility power comesback online, that
> the transfer back to utility power is coordinated with the ATS driving
> the generator until both frequency and phases are within a user
> specified range?


Well, that depends.

If you have a open-transition ATS, where there is a 'neutral' (read: not connected to any source) position, it doesn't matter (much). Well, it matters a little. There is really two types of open transition.

Something called "open transition" will provide a transfer by going closed-open-closed (in both directions). The issue is the open portion of that transfer can be very short; sometimes only a few cycles at 60 hz. If you have an electric motor connected as load (fan, compressor, whatever), if the sources are out of phase, it can be an interesting event for said motor. Typically, a open transition switch will wait until the phasing is 'close enough' (usually programmable by way of degrees.). We have an old russ electric ats somewhere that is happy at about 15 degrees +/-.

There is also a type, "delayed transition", which is closed-open-wait-closed. Wait is typically programmable, it may be 500 msec, it might be a minute. It's up to the user. This is regarded as the safest type of switch (imho) because you do not run the risk of any of the above mentioned badness. However, in a datacenter scenario, you do have a battery hit (ranging from tens or hundreds of millisecond to many seconds depending on what you want). How good is your UPS and battery plant? Will your fans inertia keep air moving for a little while? All things to consider.

If you have a closed-transition switch, typically the retransfer from emergency to normal is closed-closed, meaning that emergency gen, normal utility, and load are all connected together for a short while. Typically in the tens or hundreds of msecs. Anything longer than that kinda falls into the cogeneration category. That is another discussion.

At least here in JCPL territory (northern NJ), closed transition is frowned upon. Too much risk, they think. They are correct, really, but the risk is mostly yours. If you lock to the utility out-of-phase, you will surely lose and they will surely win. The fault you create that they will see will probably not hurt them. Unless it is extraordinarily large and you are very close to the nearest substation. You must really trust your utility and your transfer gear and your generators to do this. Personally, I'm not a huge fan of this, but that is just religion.

Personally, I like delayed transition, and that is what we do on anything recent. Short, usually, like 3 or 5 seconds. If anyone wants a demonstration, let me know. Long enough for motor controls to say "oh, hey, we lost power so let's do a nice soft restart of motors" and compressor controls can do delayed restarts as well. Works quite well, in practice.

Much is overlooked in this discussion, as to things people should do about ATS and UPS programming.. but it is outside of the scope of NANOG unfortunately. Perhaps we need a NADCOG or something. 

What does this have to do with the whole 60 hz discussion and clocks? Not much. Other than I will have to rely on the cell phone more and the microwave less for time.









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