> > According to Transpower, NZ's power generator, measuring a
> > 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

The above was what I got verbally. This is a written answer I
received a short while ago. The phrase "even a stopped clock
is right twice a day" comes to mind ;-)))

Maybe I'll take a closer look at the "pips" or colour burst to
fine-tune a PIC

======================

The short answer to your question is "no", there is not a set time of day
that you could start a counter on and be guaranteed 4,320,000 cycles
between then and a point 24 hrs thereafter. A slightly longer, and
hopefully more helpful answer, is as follows:

The terminology used for what you are referring to is the 'frequency time
error' or 'time error' for short, which is a measure of the average
frequency over a 24 hr period. Under the Electricity Governance Rules
(EGR), we are obliged to try and keep the 'time error' within 5 seconds
of NZST (which means the number of cycles per day should be 4,320,000
+/- 250). The EGR also obliges us to attempt to ensure that the 'time
error' crosses the zero point at least once per 24 hr period, but this does
not occur at any fixed time of day

While we will normally manage to meet these obligations in a given 24 hr
period, this is not something we can guarantee as there are factors outside
of our control that will influence this (for example the tripping of a large
block of load, or of a major generator can significantly distort the 'time
error' for a much longer period than would normally be expected). We would
therefore urge a degree of caution to anyone intending to use the system
frequency to implement any time-critical process as the accuracy is measured
in terms of seconds rather than milliseconds. Depending on what you are
doing however, this level of accuracy may suffice.

If you have not already considered this, another factor that can influence
the 'time error' as seen at any given point is the manner in which you
measure the system frequency. One of the more common methods with
micro-processors is by 'zero-crossing' detection (where the frequency is
calculated by detecting each time the AC waveform crosses the voltage
zero). While simple to implement, this method is prone to errors caused by
distortion in the voltage waveform, which can be caused by such things as
voltage phase shifts or harmonics. The quality of the voltage signal you are
receiving can therefore also be something you need to take into account.
Being in the vicinity of industrial loads with variable speed motor drives
and inductive or capacitive loads, or being on a connection prone to
phase-phase or phase-earth faults are typically factors that can distort the
voltage waveform. Again the degree of accuracy you require would be the
major factor.

-----

Hope this helps Joe.

Regards, Team Online


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