[time-nuts] Power lines and time
bill at iaxs.net
Sun Aug 21 18:48:31 EDT 2005
Here's the story on frequency regulation of power lines:
You can control a few synchronized generators to an external
time standard. You will not be able to get phase control
unless the electric loads are steady. Think of the load
variations as you would temperature variations, except the
load can pull the generator out of phase with the external
frequency. And you can't insulate a generator from its load.
Power companies bill on time-integrated power - watt-hour
meters in the US. Watt-hour meters are still mostly driven
by electric clocks, in a way. The frequency does matter.
Power networks, large or small, cannot be automatically
controlled for frequency - especially not by independent
controllers in each powerhouse. Instead, the network is phase
controlled by the use of synchronous generators. Any single
generator will exchange energy with the network to hold
itself in phase with the network.
If the loads on the network do not match the steam power from
the power plants, then the whole network will rise or fall in
frequency until it reaches a balance of power. This is why
under-frequency relays are used to disconnect plants and loads
from the network in order to save part of the network.
The balance of power is so important that network dispatch
centers monitor generated and consumed power, doing what they
can to maintain balance. They usually lose frequency during the
day when loads are large. As the loads fall off during the
night, excess generated power is used to raise the frequency.
The frequency changes do not exceed 0.1% in large networks.
The basis for controlling the frequency is a cycle-counting
clock that is compared to a time standard. The goal is to have
the clocks match once every 24 hours. This assures fair allocation
of the cost of power - it isn't something that the dispatchers
just do for fun on the night shift.
Based on this:
1. It is unlikely that any power network just lets itself go,
with no standard time/frequency to hold. The under-frequency
relays would make that hazardous.
2. Point measurements of line frequency are pointless. The
cycle count must be integrated over 24 hours. I don't know
what time the dispatchers aim for to match clocks. Back in the
fifties I was taught that the time of minimum activity is 4:30
AM, according to an Air Force study of the best time to bomb.
Traffic counters confirm this number.
3. Leap seconds are easily integrated into the cycle count
during the following 24 hours.
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