Mains frequency
Yes the shapes of the waveforms for voltage and current, especially
deviation from sinusoidal currents and phase shifts due to generation -
transmission system - loads, are of great interest to the utilities in load
planning. Of very great interest to the utility industry in the past
decade, are the waveforms associated with various fault conditions, as
these can be used to analyze the type of a failure and work around it - no
purpose reclosing a breaker into a dead short, but reclosing into other
kinds of faults can be fine. Looking at I and V in the area around the
fault can allow this determination as well as help localize the fault from
timing. This allows huge capacity and cost savings if done right - roll the
truck to the right place on the 10 mile branch rather than search the
entire 10 mile branch. The folks that sell these services and equipment, I
know they advertise that they are doing fault analysis at way faster than
the megasample/sec level. Also they care about synchronizing measurements
taken at geographically diverse locations thus they care about accurate
timing.
But all the above is looking at details far finer than basic phase and
frequency of the 60Hz fundamental that is "the grid". The tau of 0.2
seconds that Tom mentions, is the ballpark a time nut cares about -
anything above that is drift, anything below that is hash.
On Wed, Mar 12, 2014 at 1:15 PM, Tom Knox actast@hotmail.com wrote:
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@rubidium.dyndns.org
To: time-nuts@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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Hi Tom,
On 12/03/14 18:15, Tom Knox wrote:
So we know there are deviations in line freq. But it seems strange in
this era of very accurate and inexpensive freq references.
May seem strange, yes.
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?
The rotating transformers will lag different amounts depending on the
load. You balance the frequency by balacing the generation with the
load. A higher load than generation causes frequency to go down. A
higher generation than load causes frequency to go down. The generators
is then within their synchronous region be running synchronously, but
not quite synchronous, their phase angle may shift depending on load and
strength of the network. When phase diverge, the network fall apart, not
often without a black-out as a result. Remember the 2003 NE blackout?
On top of that, there is inter-area modes with oscillations, there is
forced oscillations (such as from broken generators), control algorithm
re-balancing and then all the nice transients from transformer
steppings, cap tripping and tripping breakers for a line. Then the load
changing patterns.
There is a fair amount of GPS clocks being in use, but it doesn't change
the behaviour of the grid that directly. It is used as reference to
measure phase-vectors, frequency and ROCOF (rate of change of frequency,
what we call linear frequency drift) which is used as input for the
control of the power-grid.
PS. Where are you, no Tom Knox in Knoxville ;-)
Cheers,
Magnus