> > 24hr period from 3:30am to 3:30am should be accurate to one
> > or two cycles, +/- 1/25th second.
>
> That sounds a lot more tolerance than I expected - having spoken
> to generator people, I understood that they try to get the 24hr error
> as low as possible - zero in relation to their reference is their target,

It's not that a day is any shorter or longer according to Transpower.
What he's saying is that because of frequency variations during any
given day, you can't predict with absolute certainty when the last cycle
of the day will be. IOW it might not be exactly at the precise time it
was yesterday. So the inaccuracy can be on the part of the measurer

For example, you'll have heavy load during meal times, eg 6-8pm,
and the frequency will go down perhaps 1Hz. In a city like Auckland,
say there's a huge jam on the motorway and many people get home
and start cooking (if at all) later than they would have done. This results
in load pattern different from the day before, which might have been a
public holiday, for argument's sake, and evening power demand was
down

Same thing when industry starts in the morning. Any load fluctuation
(perhaps the S. Auckland steel mill has a line down for maintainence)
will cause a change in the frequency pattern

> > But he reckoned 3:30am is about when they'd put out the
> > 4,320,000th and final cycle for the day.
>
> Yes, I suppose they need to tweak it at a time when there is
> the smallest variation in load, so they probably start working
> towards the correct cycle count at, say, midnight, with the
> appropriate delta to get it right by 03:30

Exactly. Masuring from 8am to 8am would probably result in a
lot more daily variation between those two times for the reason
above

> Since the Cook Strait link is DC, do the two islands have their
> own controls for their respective grids, I wonder ?

Wouldn't be surprised if they both had access to something
the same atomic clock

I asked at Meridian (a power retailer) and he said he wasn't
aware of any synchronisation. The regulations allow for some
quite wide tolerances

http://www.electricitycommission.govt.nz/pdfs/rulesandregs/rules/rulespdf/Pa
rt-C-1-Sep-05.pdf

Generally electricty flows from South to North, although it has
gone the other way once or twice

> I presume you don't have any sort of radio time standard there,
> like MSF, DC77, and whatever the Colorado one is called, there?

That's certainly something to look into. Cycle counting is easy
enough to do, but there's no actual guarantee you picked the
right time to start. There are times when it's more likely you'd
get a good result though

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