COMPANY TO WATCH:
Efficient, high-temperature silicon carbide switches could slash power losses from silicon-based FACTS controllers by more than 50 percent. Cree leads a US $3.7 million project with the U.S. government’s ARPA-E high-risk energy R&D fund to engineer 15- to 20-kilovolt silicon carbide power modules ready for grid-scale power flows.
The world’s most sophisticated FACTS controller keeps New York City lit when lines upstate start to max out. The system, completed by the New York Power Authority and the Electric Power Research Institute in 2003, pulls hundreds of megawatts from NYPA’s congested Albany circuit and pushes it onto the Catskills line, a feat that has yet to be rivaled.
The first FACTS controllers emerged in the 1970s as an improved means of balancing the two types of power that coexist on AC networks: active and reactive power. Active power, the familiar watts consumed by lightbulbs and toasters, is the product of voltage and the component of an alternating current that is in phase with the voltage. The component that is out of phase multiplied by the voltage gives the reactive power, which is measured in volt-amperes reactive or VARs (or more commonly, megavars).
Reactive power is a necessary evil: It does no work, and yet you have to add it to move active power. Reactive power results when electricity flows through an inductor or a capacitor, which causes the current to lag (the inductor) or lead (the capacitor) the voltage. When the current lags, engineers refer to it as negative reactive power; when it leads, they call it positive. It’s the negative sort that tends to occur in lines, transformers, motors, and even some generators. Too much negative reactive power will cause the voltage to “sag,” a condition that can damage electrical equipment. And the damage can spread: Inadequate reactive power support during peak periods is a common contributor to cascading failures—including the 2003 blackout that toppled grids from Ottawa to Baltimore.
Typically, utilities compensate for the negative reactive power caused by inductors by injecting positive reactive power into the system. Traditionally, there are two ways of doing that: by patching banks of capacitors into a circuit to convert some of its megawatts into megavars, or by tuning the generators in conventional power stations to produce current waveforms that lead voltage. FACTS got started as a more dynamic solution, and it has become increasingly relevant as deregulation has progressively turned the electricity business into a kind of promiscuous dating game, whereby supply is married and remarried frequently to match demand, sometimes on an hourly basis, and without much regard for the capabilities of the transmission assets connecting those scattered centers of supply and demand.
By means of such matchups, FACTS allows system managers to send more power over a line than it could otherwise support. The increase can be as high as 50 percent, says Ram Adapa, a technical manager for EPRI. Stability enhancement accounts for part of the boost, allowing grid operators to operate lines closer to their thermal limits.
The heart of a modern FACTS controller is an array of solid-state switches, often coupled with capacitors. Typically, the solid-state switches open to tap power from the line and charge a capacitor; then the switches fire in sequence to create a synthetic AC waveform with precisely the needed phase difference between current and voltage. That waveform is then applied to the grid. By precisely varying the phase difference, the FACTS controller can add or subtract reactive power in fine increments.
Impressive as it is, such dynamic voltage regulation is the simplest of the FACTS grid control modes. FACTS innovators went further in the 1990s by exploiting newly developed high-power semiconductor switches that could switch at frequencies higher than the standard 50- or 60-hertz AC cycles.
With relatively advanced switches, such as insulated-gate bipolar transistors, FACTS controllers could simultaneously regulate voltage and surgically remove a variety of glitches in the AC signal. One such FACTS device, the static synchronous compensator, or statcom, has played a decisive role in the more than tenfold rise in wind power capacity worldwide over the past decade. A Siemens-built statcom, for example, is stabilizing flows from the world’s largest offshore wind farm, completed this past September, whose 100 3-megawatt wind turbines should feed enough energy to the United Kingdom’s grid during the year to supply more than 200 000 homes.
Massive wind farms barely raise an eyebrow today, but Charles Stankiewicz, executive vice president at power equipment manufacturer American Superconductor Corp., says that just a few years ago they still rattled many transmission engineers, who viewed their gusty, noisy power signal as a threat to grid stability. Stankiewicz suggests that many of the more than 70 wind farms stabilized by his firm’s FACTS controllers might never have been erected without FACTS technology. “It would have been one more excuse that allowed the electric utilities that maybe weren’t inclined to accept wind power to basically say, we’re not going to do it,” says Stankiewicz.
The most advanced FACTS go beyond stabilizing a line to reducing its apparent impedance, so grid managers can actually push more power down it. This application went commercial in 1998, when Brazil commissioned a pair of 1000-kilometer, 500-kilovolt lines to link its northern grid, replete with Amazonian hydropower, to the southern grid serving its coastal population centers. A FACTS controller near the northern end of the line simultaneously drives power and damps down destabilizing feedback signals.