[time-nuts] Re: UTC, 60 Hz
bill at iaxs.net
Thu Jul 28 03:45:34 EDT 2005
TVB said, "If there's a power
industry person in time-nuts we'd love to ask you
a few questions."
I'm not a power industry person, but I've researched the
problem over the years. Here are some of the results.
The generators are all synchronous machines. The ones in
one power plant rotate with very little phase angle
difference between them. Long transmission lines act like
springs, so that phase angles in different plants can be
different, maybe by tens of degrees.
If you had one isolated generator, perhaps the power plant
for a small island, you would have a speed control loop for
the turbine (or engine) throttle that controls power to the
generator shaft. Then you would have a second controller
that could integrate frequency error in a way that trims the
speed control to precisely hold the frequency.
Changes in power delivered to the load by the generator cause
the speed control to change the power delivered to the generator.
If there is a step change in power, like the 5 minute shutdown
of industry at noon on Armistice Day in the 1920s, the power
difference between turbine and generator causes the rotor to
accelerate or decelerate. The speed controller changes the drive
power quickly but not immediately. Stability requires less than
infinite gain. The slower integrating frequency control gradually
brings the frequency error back to zero.
One form of integrating control consists of a person looking at
the powerhouse clock and a standard like WWV. As the error grows,
the person fine-tunes the speed control. Changes may be made a few
times per day.
If the generator is part of a power network then it is not possible
to use integrating frequency control, because you can only have one
of them in a system. Two or more will not track and the result is
wasted power as they fight each other for control. One generator
can't possibly affect the thousands of generators connected to a
Power distribution networks have dispatchers in regional offices.
As I understand it, the dispatcher watches the time error as well
as the balance of power flows. The dispatcher tells a power plant
how many megawatts to put into the line, not what speed to run.
One of the factors in adjusting the megawatt balance is the time
The US is divided into 3 or 4 independent zones that are isolated
by very high voltage DC transmission lines. Maybe it was 3 - East,
West and Texas.
About 15 years ago, I used a frequency input on a control system to
compare and plot the difference between power line and crystal time.
The power line lagged during the day as industrial loads exceeded
the dispatcher's requests for power and caught up again in the early
morning hours. The swings seldom exceeded +/- six seconds in MN.
I don't know if they do this, but adding a one-second change in UTC
would be no problem at all. Of course, it would take several hours.
Hope this was useful,
More information about the time-nuts