[time-nuts] Simple AC mains zero-cross detector

Magnus Danielson magnus at rubidium.dyndns.org
Sat Dec 20 09:48:09 EST 2014


On 12/20/2014 09:24 AM, Charles Steinmetz wrote:
> Gary <nuts at lazygranch.com> wrote:
>> I try to minimize dangerous voltages. Anyway, the filtering reduces the
>> slew, so you can't have it both ways.
> Starting with 120v gives you 10x the slew rate that starting with 12v
> does, whatever filtering you use.
>> If by post processing you are averaging, then you certainly have lost
>> frequency variation data. Averaging is a filter.
> You will not lose grid frequency variation data unless you average the
> 60 per second samples for *extremely* long periods of time, because the
> grid frequency is generated by rotating machinery weighing many tons
> that can only change frequency very, very slowly.  As I noted before,
> the simple system I described resolves frequency to better than 0.01 Hz
> in one cycle, so very little averaging is needed to achieve better
> resolution than anyone really cares about.  As long as the averaging
> function is more agile than the actual grid (and it will be under all
> practical conditions), all actual grid frequency variations will be
> preserved.

As you look careful on the phase variations, you will find that you have 
forced oscillations being pushed onto the network, some extending into 
several hertz and in one case a wind-farm had a 13 Hz forced oscillation 
being pushed out on the power grid. Also, there is inter-area 
oscillations creating resonant modes on the power-grid. These can either 
be fed from generators injecting energy into the mode or cause 
variations as breakers, transformer tappings or change of load occurs.

Things have been discovered when looking deeper into the phase 
variations with faster speeds.

Besides doing wide-area monitoring, starting to use these observations 
to steer stabilizers have been discussed and tested. The development 
goes quickly in the power-grid world at the moment.


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