[time-nuts] Mains frequency

Tom Knox actast at hotmail.com
Wed Mar 12 13:15:50 EDT 2014

So we know there are deviations in line freq. But it seems strange in this era of very accurate and inexpensive freq references.
How much is related to the generation? It seems in this era of switching supplies and other complex loads that even if the power were perfect at the generator the phase/freq could vary widely across the grid as different parts of the sine wave are loaded in a non linear fashion. And could a small digital signal be added to the smart grid that would control switching supplies to correct rather then degrade the grid signal?

Thomas Knox

> Date: Wed, 12 Mar 2014 16:39:50 +0100
> From: magnus at rubidium.dyndns.org
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] Mains frequency
> Tom,
> On 18/11/13 23:15, Tom Van Baak wrote:
> > Magnus,
> >
> > I'm going to push back a bit on your mains sampling claim. Mostly, I'd like to see the results of the professional I-Q demodulated gear that you mentioned. Can you post raw data, or a sample plot?
> I don't have much of that myself. I do recommend you to check the 
> presentations of the NASPI conference (naspi.org). There is plenty of 
> plots there.
> > I agree that looking at power line voltage with 16- or 24-bits at 1 Msps is going to reveal interesting amplitude and phase noise information. But see how well a $1 PIC can do.
> Well, I should toss that over to the good folk at NIST doing 
> synchrophasor calibrations. Should I grab them now that we are in the 
> same room?
> Have a look at IEEE C37.118.1 for measurement methods.
> > Attached is a plot made using TimeLab + picPET just now. The picPET is fast enough to capture the zero-crossing of every 60 Hz cycle with 400 ns resolution; the TimeLab plots have tau0 of 16.67 ms.
> >
> > -- The blue trace was simply plugging a 9 VAC wall-wart into the picPET though a 10k resistor.
> > -- The pink trace was adding a 10 nF cap across the input.
> > -- The green trace was unplugging my laptop switching power supply from the same outlet!
> > -- The red trace is replacing the mains wall-wart with a hp 33120A set to 9VAC at 60 Hz, a tentative noise floor measurement of the picPET when used this way.
> >
> > My conclusions are that at least here in the US, or at least at my house, the short-term stability of mains hits about 5e-6, at about tau 0.2 seconds. The attached short-term plot is also not-inconsistent with the long-term plot at http://leapsecond.com/pages/mains/
> >
> > My other conclusion is that the picPET (a simple PIC-based time-stamping counter) is doing a pretty good job measuring this. Note, no software or data filtering was used. This is just raw serial/USB data going into TimeLab.
> Well, if you are happy with that, fine. But there are many things 
> happening on the grid which needs deep analysis and the tools for it has 
> been developed to provide both resolution and removal of noise which is 
> not part of the measurments. Just calibrating the trigger noise for a 
> PMU requires care, as the S/N required for a straight comparator for the 
> applications is several tens of dBs away from a good conditions, so they 
> have had issues with doing that.
> Doing your own time-stamping like you have done is naturally fun, but do 
> not confuse it with the experience and processing that have been shown 
> needed by an industry.
> BTW. WECC, who has a large network of PMUs, and that covers where you 
> have your house and measurement point, can't release detailed data to me 
> or you just for fun. It always needs to be cleared from a security point 
> of view.
> Cheers,
> Magnus
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