[time-nuts] Mains frequency

Magnus Danielson magnus at rubidium.dyndns.org
Wed Mar 12 13:41:12 EDT 2014

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 ;-)


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