[time-nuts] Daft idea with the National Grid

[Magnus Danielson]

Hi,

On 2021-02-08 12:31, Hal Murray wrote:
> andy.g4jnt at gmail.com said:
>> I can probably measure the grid frequency to a few micro Hz   over a period
>> of tens of seconds. So I make a continuous recording of this, averaged over
>> say 10 second periods. Now take a 7kW load (the maximum reasonably possible
>> on a domestic circuit) and switch it on and off at intervals of perhaps 10
>> minutes, precisely timed so it can be correlated with the frequency log.
>> That 7kW load will be about 0.2ppm of the average for that for the whole of
>> the UK.    By post processing, and some deep correlation covering days worth
>> of cycles of load on-off with the frequency, I wonder if it would be possible
>> to see the loading, the mean frequency changing by a few uHz. Not sure what
>> the time constant of the grid control is, but for* small signals* I doubt it
>> can be faster than a few minutes. 
> I'd expect an ADEV type pattern.  For long tau, the wander in the system will 
> dominate.  We have lots of long term data so should be able to plot that part 
> of the graph.  For short tau, the noise will dominate.  I don't have any data 
> below 10 seconds.  But I do have a TICC.  I wonder if the middle is the 
> classic V or has some wiggles.
ADEV is a poort estimator for this. ADEV was meant of noise power
estimation, but power-grid is mainly systematics
(pseudo-random/systematic) and feedback-loops on to counter that. There
is other tools in the toolbox for that. There is a wealth of tools to
cover that.
> How much energy is in the rotating turbine and generator?  That should provide 
> a lower limit on the time constant of the control loop.
Only some loops are automatic, and often those are local. That is,
protection switches break lines on over-voltage, over-current,
over-frequency-deviation, over-ROCOF (Rate Of Change Of Frequency) etc.
Regular read-out of the state is done and risk calculation is done. Then
the overall state is monitored by these protein computers supervising it
all. The overall feedback loop is thus fairly slow but kind of works.
The read-out rate has classically been an issue, especially as you want
to see two or three readouts being consistent, and you get an update
very 5 s or so.
>
> If the ADEV pattern is a clean V and the signal is 2 levels, I'd expect the 
> optimal modulation pattern is a homework problem for the right course.  We can 
> synchronize the transmit and receive clocks with GPS.
Modulating over long distances on powergrid? Can be done, but not on my
electical bill.
>
> This feels like I have to use something like 8B/10B with lots of transitions 
> so the receiver can track the system frequency?
>
>
WSPR is more like it.

Cheers,
Magnus