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 network. 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 error. 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, Bill Hawkins _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
