[time-nuts] Features of a Precision Clock?

[Dr Bruce Griffiths]

Dean Weiten wrote:
> Hi there,
>
> Having worked with the folks who operate the power utilities (I designed
> protective relaying and recorder electronics for several years), I can
> advise that they do take the long-term accuracy of their power
> seriously.  However, the short-term is not a big concern, and in fact,
> they cannot control it all that well.
>
> It turns out that power flow on an AC line requires a phase difference
> between end points (as opposed to a DC system where it is resistance
> that counts).  The resistance of the line is not important.  This is
> because power transmission lines are almost pure inductive reactance -
> in power systems terms, the line angle (impedance angle) is generally
> near to 90 degrees.
>
> Systems are connected at multiple points, like a mesh of rubber bands
> connecting weights and support points.  Some of these points are heavier
> (down) or pull stronger (up), some have stronger bands, some have very
> weak bands.
>
> When the load changes, or when a line opens or closes, the phase angles
> of the power through all these interconnected ties will shift to
> establish a new equilibrium.  In so doing, your power will advance or
> retard somewhat.  If you have a clock running on the phase of AC power,
> your clock will gain or lose a bit of time.  It is unclear whether you
> will ever be corrected - the new equilibrium might just be a fact of life.
>
> The prime movers of the systems (generators) are almost all physical
> moving devices, like hydro-electric (water dams) or thermal (coal,
> natural gas, or nuclear powered turbines).  When they are loaded down,
> they slow down - and when less loaded, they speed up.  This isn't as bad
> as it sounds - the rest of the system rolls along at the "system
> frequency", and the generator's slight frequency change actually becomes
> a phase change, which, as per above, changes its power output.  Then the
> generator gets back into sync, but with a phase angle different than before.
>
> As you can imagine, it is a challenge to maintain tight control of the
> phase, with all the changing conditions on the power grid.  In the case
> of our utility (Manitoba Hydro), they keep power system clocks at the
> big "24 by 7" staffed power stations and in the main control room, and
> will, under their rules of operation, tweak things slightly over time. 
> I am not certain of the rules of operation, or of the way they tweak
> things (generator bias?), but could find out from friends and
> colleagues, if you wish.
>
> Here in Manitoba, we are blessed to have much of our power supplied from
> the hydro-electric generators in the north, through a DC link.  It turns
> out that this is economical above a certain distance and power level -
> related partly to the "skin effect" (yes it becomes important, even at
> 60 Hz).  At the south end of the link, we have a DC-to-AC inverter
> system (huge - pretty impressive), fibre optic fired thyristors
> (equivalent to triacs?  SCRs?).  We can change the firing angle on a
> cycle-by-cycle basis, adjusting the power flow in and out, and exerting
> extraordinary control of the system phase.  We use it to stabilize the
> system more than for power frequency correction, but I assume that this
> could be done too, just unsure of the algorithm.
>
> Of course, the system is a lot more complex than I describe it here,
> with phase shifting transformers, tap changers, and more modern
> back-to-back DC links, wind generation at the distribution (lower
> voltage) level, etc.  More complex than I understand, to be sure.  But
> those are the basics.
>
> Regards,
>
>
> Dean Weiten,
> Elecsys Solutions,
> Winnipeg, Manitoba.
>
>
>
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>   
When the length of the transmission line approaches a quarter wavelength 
at the power line frequency (1250 km/780 miles @ 60Hz) it acts somewhat 
like an open wire RF transmission line. The line no longer acts like a 
lumped component and it also acts like a somewhat inefficient antenna 
radiating at the grid frequency.

Some generators are kept continuously spinning and synchronised to the 
mains but generating little power. These spinning reserve generators are 
necessary to stabilise the grid against load fluctuations, they can very 
quickly supply power when required.

Here in New Zealand where the length of each of the 2 main islands 
approaches a quarter at the 50Hz grid frequency a submarine 
bidirectional dc link connects the power systems of the 2 islands. Both 
mercury vapour phase controlled rectifier and SCR inverters are used.

A thyristor is another name for an SCR, triacs are not used in high 
power circuits, inverse parallel SCRs are used where a triac like 
function is required.

Bruce