In this season of discontent in the electricity business, only wind power seems to stand out as a global success story. While petroleum prices were convulsing in response to war and labor strife, and nuclear plants were stoking controversy in the Middle East and Asia, wind turbines were quietly becoming the fastest-growing energy source in the world. They now provide more than 31 000 MW of power, a total that has swelled by almost 30 percent in scarcely a year’s time and that keeps more than 200 million tons of carbon dioxide out of the atmosphere every year. Wind power’s ascendance has been so stunning that advocates are now rallying around an idea that would have seemed preposterous just a couple of years ago: that the wind could supply 12 percent of the world’s electrical demand by 2020.
Impressive as the gains have been, it isn’t quite clear yet that the wind can blow away the developed world’s fossil-fuel dependence. One of the most important reasons is that clean, renewable wind power comes with a serious hitch: while conventional power plants yield a steady stream of electricity, wind turbines often ply turbulent gusts and therefore spit out an irregular stream of electricity that is tough for power grids to swallow.
Now, though, high-tech solutions are at hand. Systems based on advanced power-electronics and energy storage devices are massaging and managing power flows from wind turbines, enabling them to contribute mightily to electricity grids without putting those grids at risk. Not only are the technologies making wind power more palatable to grid operators, they are even making it possible for engineers to finally harness wind energy’s tremendous potential in wind-swept, remote locales.
Perhaps nowhere is this potential so evident as in the state of Hawaii, whose isolated power grids could not otherwise risk taking full advantage of the archipelago’s abundant, renewable resource. In fact, with its lush, endless trade winds and growing commitment to wind power, Hawaii’s Big Island is emerging as a laboratory of the future of the technology. As wind power becomes a steadier and more reliable resource, it could help wean power producers all over the state from their dependence on costly imported oil.
But, for now, says Karl Stahlkopf, chief technology officer at Hawaii Electric Co., in Honolulu, even the existing wind farms on the Big Island—putting out just
10 MW, the equivalent of four state-of-the-art wind turbines—make grid controllers hop on days when the palm fronds fly.
The utility and its contractors plan to build what Stahlkopf calls an ”electronic shock absorber” to buffer the island’s power grids against the wind’s worst behavior. It’s a development that engineers elsewhere are following closely.
The reason is that the solutions to integrating modest levels of wind power on small, isolated grids today may foreshadow the installation of truly large-scale wind power in mainland networks five or 10 years from now. ”What’s happening in the Hawaiian Islands is a peek at the future,” says Bob Zavadil, an expert on wind power at the Arlington, Va.-based power systems analysis firm Electrotek Concepts Inc. ”They’re on the leading edge.” And that’s true not only of the technology but also of the new legal and regulatory conventions between utilities and independent power producers that will be needed before wind energy can truly thrive.
Reactive Power 101
Back on the mainland, wind farms have grown to dozens of turbines and hundreds of megawatts—rivaling the size of conventional power plants. To pave the way for installations like those, engineers had to grapple with the tendency of wind turbines to introduce voltage instability into electrical grids. That tendency follows from the intermittent nature of wind-generated electricity, which waxes and wanes in irregular episodes unrelated to the predictable daily ebb and flow of consumer demand.
To understand the instability issue—and its solution—you’ll need a few basics on the nature of power flows on utility grids. First, recall that these flows consist of both active and reactive power. The difference between them arises from the fact that the wave of alternating current on a power grid almost always leads or lags the voltage wave. In short, inductors, such as the coils in motors or transformers, cause current to lag voltage; capacitors cause current to lead voltage.
Active power is the familiar watts consumed by light bulbs and toasters; it is the product of voltage and the component of current that is in phase with the voltage. Reactive power, measured in volt-amperes reactive, or VARs, is the product of voltage and the out-of-phase component of alternating current. This out-of-phase power is consumed by energetic electric or magnetic fields—for example, in coils in inductive loads, such as motors, and also in the utility’s own transformers. Transmission lines themselves are also inductive. In the case of wind-generated power, this line inductance can be a critical factor, because the turbines are usually far from population centers, strung out across windy plains or even areas out at sea.
Utilities have to pay close attention to reactive power because it determines, along with active power, how the voltage declines, or ”sags,” on an electric network. Utilities must keep voltages very close to rated values because when they fall too low, motors and other inductive loads draw too much current, overheating the equipment and possibly damaging it and other utility gear as well.
Declining voltage on a network is a function of the consumption of both active and reactive power. In other words, if a big load consumes lots of reactive power, it will cause the voltage to sag intolerably unless that reactive load is adequately compensated for. Because these reactive loads are almost always inductive, causing current to lag voltage, utilities must feed them with reactive power in which the reverse is true—with current that leads voltage.
For this they have several options. They can supply reactive power right from the generators themselves, by producing current waveforms that lead voltage. They can also install banks of capacitors close to big inductive loads. As the load rises, relays switch on the capacitor banks to add reactive power.