Hey Kids, What Time Is It?

Those of you of a certain age (what age is that?) will recall that ancient call. But for the rest of us that may become a reality thanks to a new government program. And what will that program do? It will let the frequency of the grid wander to better accommodate intermittent sources of power like solar and wind. And why is solar intermittent you ask? Clouds. Solar can go from full power to low power and back to full power with the passing of a cloud. And depending on wind speed that can happen rather quickly. With that going on it is tougher to keep the frequency constant. And keeping the frequency within one cycle of the 5,184,000 cycles that are supposed to happen in 24 hours requires co-ordination.

Regulation of power system frequency for timekeeping accuracy was not commonplace until after 1926 and the invention of the electric clock driven by a synchronous motor. Network operators will regulate the daily average frequency so that clocks stay within a few seconds of correct time. In practice the nominal frequency is raised or lowered by a specific percentage to maintain synchronization. Over the course of a day, the average frequency is maintained at the nominal value within a few hundred parts per million.

Think of the grid as a huge rotating machine with what amounts to electrical “shafts” between every generator and load. If the speed of the generator and load differ greatly the shaft will break.

The primary reason for accurate frequency control is to allow the flow of alternating current power from multiple generators through the network to be controlled. The trend in system frequency is a measure of mismatch between demand and generation, and so is a necessary parameter for load control in interconnected systems.

Frequency of the system will vary as load and generation change.

So frequency will go up or down depending on supply and demand. A drop in supply (increase in demand) causes the frequency to go down. An increase in supply (decrease in demand) causes the frequency to go up. On an instantaneous basis this is held quite close to keep out of phase current (caused by generators and loads at different frequencies) to a minimum. Phase current requires bigger wires and transformers but delivers no power to the load. Expense without revenue. This is bad for business.

The frequency is generally changed (synchronized) at night when loads are lowest so the amount of phase current is minimized. Obviously it is cheaper and requires less co-ordination to let things drift a little. But when it comes to time a little means a lot. A five minute a month drift will put you off by an hour in a year. (that hour a year is about .01% accuracy). To keep it to 6 minutes a year requires that the grid average frequency be .001% accurate.

What kind of devices will this change affect?

A yearlong experiment with the electric grid may make plug-in clocks and devices like coffeemakers with programmable timers run up to 20 minutes fast.

The group that oversees the U.S. power grid is proposing a change that has the potential to disrupt electric clocks in schools, hospitals and other institutions, according to a company presentation obtained by The Associated Press. It may also mess with the timing of traffic lights, security systems, sprinklers and some personal computer software and hardware.

The biggest disruption will be in traffic flow control. What happens when you end rush hour traffic control twenty minutes early? Well, the electric power guys will be saving money and those stuck in traffic will be losing it.

With everything so interconnected seemingly insignificant changes in one part of the system can have huge effects in other parts of the system. For instance what about getting your alarm clock wake up at the right time to get to work on time? If you use an alarm clock like this one, Sony ICF-C318 Automatic Time Set Clock Radio with Dual Alarm you will be resetting your clock frequently despite the fact that the clock automatically adjusts for daylight savings time. So how do you get the right time? There is your computer or cell phone of course. But if you want something on your wall I like this clock: La Crosse Technology WT-3102B 10-Inch Atomic Analog Clock. It is synchronized to WWVB which is the time standard for North America (good to plus or minus one second or better – depending on some technical details). And what do you know? The frequency it broadcasts at (60,000 Hz) is exactly 1,000 times the desired North American line frequency to within one part in a thousand billion or better. Which is one second in about 30,000 years. Close enough for most work. Our most accurate atomic clocks are about a million times better – currently. And scientists and engineers are working constantly to improve the clocks. In fact of all the fundamental quantities we can measure – length, mass, time – time is the quantity we can measure most accurately although it is the most ephemeral.

Cross Posted at Power and Control