The U.S. electric grid operates at 60 Hz (and much of the rest of the world at 50), right? Well, not exactly. The operating frequency of the U.S. grid generally varies between approximately 59.95 and 60.05 Hz. Fall below those frequencies and safety systems may step in, shutting off electrical equipment for certain customers make sure the system stays up and running. That, of course, is not something utilities want happening very often.
It turns out, however, that it may be happening more than it needs to because grid operators intentionally slow down the frequency on occasion for the benefit of electric clocks. If something happens during one of these intentional slow-downs that also reduces the frequency—a power plant going offline due to mechanical problems or a loss of a feeder line in a storm, for example—the total frequency drop may be large enough to fall below the allowable parameters and kick off shutdowns.
So why mess with the frequency? It’s done to avoid confusing electric clocks, more specifically, electric clocks that get their timing information from the power grid, ticking along at 1 second per 60 Hz. While clock designers assume a 60 Hz power supply, the grid typically runs at a slightly higher frequency to increase its ability to recover from those situations that drop the frequency. This makes clocks run fast, gaining as much as what could be 20 minutes a year were nothing done. So a “time monitor," that is, a person charged with managing the time error, directs generators to temporarily set their operating frequencies below 60 Hz to, in effect, reset those clocks. And then problems can occur. In fact, a recent study of approximately five years of frequency drops below 59.95 Hz determined that 97 percent wouldn’t have happened had there not been a time error correction underway.
Andy Rodriquez, director of standards development for the North American Electric Reliability Corp. (NERC), explains it this way: “When you’re driving a car, you try to stay in the center of your lane; you may drift left and right, but you keep bringing it back to center. That’s the way the grid should operate.”
But, he said, in order to keep the clocks running properly, the power grid is driven a little differently. It’s as if, Rodriquez says, a driver “makes a mistake that moves him to the left of his lane; he then overcorrects and drives off to the right to net it out.” That keeps his average path in the center (but makes driving a lot more dangerous).
This is the way the electrical systems around the world have operated since the at least the ’30s, Rodriquez said. But does sacrificing reliability in order to keep grid-based clocks on time make sense in today’s world, where people are more likely to check the time on their cell phones rather than glancing at a wall or other clock that runs on grid-time?
The NERC, the organization certified by the Federal Energy Regulatory Commission to set reliability standards for the power grid, plans to find out. Starting soon, possibly this month, time monitors, at the NERC’s instruction, will stop calling for timing corrections. The experiment will run at least a year. And the NERC will basically see how many people are bothered enough that they’re inspired to complain to their local utility operators. They’re anticipating that that number won’t be too high, given that the vast majority of clocks people use today get timing information from somewhere other than the grid. Most typically, it comes from services provided by the National Institute of Standards and Technology (NIST). NIST, with help from the U.S. Naval Observatory, maintains the official U.S. time clock, and distributes that timing information over the Internet, radio services, and the telephone. Cell phone providers get their time information from NIST or from the GPS network.
I did a quick inventory of the clocks in my house to see if NERC is right. My bedside clock sets itself from the Internet, the old VCR/DVD combo picks up its timing information from the local PBS affiliate, the wall clock runs on a battery, a few watches tune into the NIST’s radio signal, and my cell phone runs on Verizon time. I found one clock that I occasionally look at that operates from electric grid—the clock on the coffeemaker, which is usually wrong anyway, given its controls are right next to the on/off button and I’m often accidentally advancing it an hour. Rodriquez indicated that other appliances, even some without noticeable clocks, do use the electrical grid for timing, but are unlikely to be negatively affected—my microwave might, perhaps, heat my lunch for .003 minutes less than the 2 minutes I programmed it for. I think I’ll be able to cope.
Tekla S. Perry is a senior editor at IEEE Spectrum. Based in Palo Alto, Calif., she's been covering the people, companies, and technology that make Silicon Valley a special place for more than 30 years. An IEEE member, she holds a bachelor's degree in journalism from Michigan State University.