Novel superconducting device provides essential voltage support
Nearly two decades ago, the discovery of a new class of superconductors was announced to the sort of fanfare rarely bestowed on scientific breakthroughs. At a famous March 1987 meeting dubbed "the Woodstock of physics," expectations were high that these materials, which could conduct electricity with no resistance at temperatures attainable with liquid nitrogen cooling, would revolutionize everything from tiny sensors to magnetic-levitation trains.
Because superconductors carry much more current for a given volume, transmission cables, motors, generators, and transformers would be not only more efficient but also much more compact. Magazines like Time and Newsweek hailed a coming technological utopia, with illustrations showing details of the good life that, they suggested, was less than a decade away.
Now, almost 19 years after that meeting of the American Physical Society in New York City, many of the anticipated applications haven’t made it past the pilot-project stage. To be sure, hopes for the superconductors remain high, and there have been several encouraging developments: motors using the high-temperature superconductors have been successfully tested for the U.S. Navy, and some highly specialized instruments are on the market. But to date there has been no commercial installation of cable, generators, or transformers.
Instead, it appears that the first superconducting machine to be commercially used in a power system will be one nobody had thought of previously: a superconducting synchronous condenser. Not exactly a household item, a synchronous condenser is basically an electric generator that is optimized and operated to act like a capacitor or inductor, providing the grid with reactive power, an abstruse but absolutely vital element in any power system relying on alternating current.
In any modern-day power grid, the power delivered can do useful work only when current and voltage are perfectly in phase with each other, so that reactive power is neither added nor subtracted. But rotating equipment like motors and generators, in particular, tends to cause current to lag behind voltage, as energy transfers to the magnetic fields of their coils. Left unchecked, this lagging can cause voltage on a network to plunge and ultimately bring down an entire system—the way one did on 14 August 2003, when a huge chunk of the U.S.-Canadian grid collapsed.
Reactive power is what’s required to counteract lagging currents and sagging voltage, and if it isn’t supplied quickly or efficiently enough, networks crash or equipment suffers. Indeed, reactive-power-supply problems are among the chief culprits in an overall power-anomaly and -disturbance problem that costs the United States alone between US $119 billion and $188 billion a year in lost economic activity, according to a 2001 report by the Electric Power Research Institute, in Palo Alto, Calif. Such staggering losses add up to 1.2 percent to 1.9 percent a year of the country’s gross domestic product, the report noted.
In simplest terms, there are two ways of providing reactive voltage support. Banks of capacitors, outfitted with sensors and the latest in semiconductor-controlled switches, can be used to make current lead voltage, opposing inductive effects. Ten years ago, it was widely expected among power system technologists that the latest such devices would enable grid controllers not only to sustain voltage but to steer current through the grid along optimal paths by regulating reactive inductance and capacitance. A big demonstration device, a static compensator, was installed for testing at a Tennessee Valley Authority (TVA) substation in Knoxville, Tenn., in 1995.
The compensator performed pretty well, but grid specialists began to grumble that it was too expensive and cumbersome to perform the role envisioned by proponents of futuristic transmission systems. Perhaps an alternative way of making reactive power was better, after all. This is the synchronous condenser, in essence a generator rigged and operated to act like a capacitor. Like any electric motor or generator, it has stator coils, which don’t move, and rotor coils, which move when their magnetic fields interact with those of the stator coils.
The synchronous condenser has one more key component—an exciter—which controls the amount of current feeding the rotor coils. When the exciter stimulates the rotor, it induces reactive power in the stator coils to match the reactive power being consumed on the grid.
Consider the machine under normal, routine conditions. The condenser’s inductive stator coils, connected to the grid, consume reactive power. To keep the machine in phase with the grid, the exciter pushes just enough current into the rotor coils to induce compensating reactive power in the stator—that is, the electromotive force induced in the stator coils is equivalent to the reactive power being consumed by the same coils. In effect, the condenser is idling, staying in step with the grid.
But now suppose that it is late morning on a hot day, and thousands of people are turning on their air conditioners. As all those compressor motors get connected to the grid, the voltage sags. To keep up, the current flowing to the rotor coils via the exciter can be increased until the condenser is overexcited, to use the technical term, to provide exactly compensating reactive capacitance.
Because of this innate flexibility and high sensitivity to external conditions, the synchronous condenser is "the way God meant for the job to be done," says Mike Ingram, a TVA research engineer, who asks that his "somewhat southerly way of speaking" be indulged [see photograph, " Nature’s Way"].
But even this setup has a serious drawback: resistance in the condenser’s rotor coils slows and damps its responsiveness, and the loading and unloading that occur during voltage sags put a lot of thermal stress on its components. Enter SuperVAR, a synchronous condenser in which the rotor coils are made from the high-temperature superconductors that first set imaginations aflame after their discovery in the 1980s. (VAR stands for voltamperes- reactive, the measure of reactive power.)
As Ingram sees it, SuperVAR is nature’s absolute best way of creating reactive power. SuperVAR responds to a voltage sag almost instantaneously and, with very little excitation current, can induce much more current than its conventional counterparts. Thermal stresses are minimal or nonexistent.
Conceived by TVA’s operations chief, Terry Boston, and manufactured by American Superconductor Corp., in Westborough, Mass., the first SuperVAR machine was installed for testing at a Tennessee steel mill at the beginning of last year. By all accounts, its performance has been outstanding, and American Superconductor expects TVA to soon place an order for two follow-on units. These two units will let engineers see what SuperVAR can do in larger, more complex transmission systems.
Bumping along in Ingram’s old pickup truck, we pull into Hoeganaes Corp.’s steel plant near Gallatin, Tenn., outside Nashville. Overhead, power lines are coming in from a 988-megawatt coal-fired power plant 8 kilometers away. Big graphite cylinders wait in boxes—replacement electrodes for the plant’s arc furnace, in which scrap steel is melted down to make powder, primarily for the auto parts manufacturers that are legion in this part of the country. There’s metal scrap mixed in with the gravel on the plant’s driveway, which doesn’t make the ride any smoother in Ingram’s modest truck.
The SuperVAR machine itself, at first glance, isn’t much to look at: it’s just a shipping container sitting on a flatbed truck. In the substation just behind it, I hear birds chirping—"faux birds," that is. The sounds are recorded songs of distressed birds played to scare away real birds that might fly into the station and short out circuits, explains Ingram, a lanky, thin-lipped Alabamian with a dry sense of humor.
We are joined at the machine by Charles ("Chuck") Stankiewicz, vice president and general manager of American Superconductor’s power electronics unit in Middleton, Wis., which analyzes transmission grids and sells SuperVAR machines. Also on hand is David Madura, product manager and chief engineer for the SuperVAR machine at American Superconductor, as well as other representatives of the company and TVA. Stankiewicz has flown in from Wisconsin and Madura from Massachusetts, and Ingram himself has just driven 3 hours from Chattanooga, to show off and explain what they all obviously think is a very hot item [see photo, "Exciting the Grid"].
This steel plant is an ideal place to test the SuperVAR, because its huge arc furnaces put enormous and fast-varying inductive loads on the grid, consuming large quantities of reactive power erratically. The arc can vary between extinction, at zero current, to the short-circuit condition, caused by scrap contacting the electrode, says Ingram. Physical movement of both the solid charge pieces and the melt cause variations in the arc length—fluctuations that can occur many times a second. Superimposed on these effects are variations caused by mechanical vibration of the electrode and its supporting structure.
Conceptually, the SuperVAR machine is simple enough. The rotor is a large thermos bottle, about 2 to 3 meters long, containing liquid neon at 27 degrees above absolute zero. In principle, a SuperVAR could be built to run on liquid nitrogen at 77 degrees above absolute zero, but current densities would be lower and the number of wire windings greater. The neon comes from a cryocooler in which the primary loop runs on gaseous helium. Inside the thermos are the rotor coils, made from American Superconductor’s first-generation superconducting wires.
SuperVAR can work two ways. It can provide compensating reactive power continuously to the grid as needed. A sensor provides readings of how much current is lagging voltage, and the control unit tells the exciter to inject the needed compensating reactive capacitance. Or the voltage can be set at a suitable level in the rotor, so that it spontaneously adds the compensating reactive power when the grid voltage starts to drop.
The SuperVAR setup at the mill is designed, says Ingram, to provide accelerated test conditions. Running on a 13.8-kilovolt circuit that feeds Hoeganaes’s 50-megavoltampere arc furnace, the SuperVAR machine has been in operation for 3000 hours. It has gone through more than 2300 melt cycles, in which 140 million kilograms of steel have been melted. That’s the equivalent, says Ingram with a note of irony, of melting 100 000 Porsche Boxsters.
Hoeganaes, the owner of the mill where SuperVAR is being put through its paces, bills itself as one of the world’s largest makers of ferrous metal powders. As we stand next to the SuperVAR machine, a scrap bucket carrier rolls into the mill, destined for the arc furnace. It takes 60 tons of scrap to make a standard load, and a melt takes place—the big electrodes dipping down into the furnace—about once an hour.
Stepping into a building behind the trailer, where monitoring devices and controls are located, we wait for the next melt. After a while, there’s a big rumble, and voltage levels dive drastically—by as much as 20 percent—on the meters we’re looking at. We step back to the trailer to listen for the SuperVAR to kick in, but the machine is almost noiseless. Nevertheless, the whole structure is vibrating from the enormous, instantaneous torquing of the SuperVAR in response to the melt operation going on in the adjacent building. It’s a punishing environment, and it’s exactly what SuperVAR’s designers had in mind for this initial test.
Besides running almost flawlessly so far, the SuperVAR has several inherent advantages over the standard synchronous condensers that do not rely on superconductors. Because there is no heating in its rotor, there are virtually no thermal stresses and no losses in its field coils. Also, thanks to the large air gap between the stator and rotor coils—a feature that is possible because the current density in the superconducting coils is so high—the machine is able to inject on demand lots of current and, hence, lots of reactive power.
Exploiting the large gap between rotor and stator, SuperVAR’s designers were able to surround the rotor coils with an aluminum conductive tube or shield. It’s this feature that enables the machine to react so vigorously to drops in reactive power on the grid. Basically, when there is a transient on the circuit running through the stator, currents in the shield spontaneously oppose what’s happening in the grid, says Bruce Gamble, providing a shorthand explanation. Gamble is American Superconductor’s director of engineering for supermachines.
So SuperVAR is expected to be a cost-effective source of dynamic reactive power. What’s more, its footprint is much smaller than those of equally rated conventional machines—a very important factor, especially in urban grids where substation space is scarce. In many places, because of the somewhat haphazard way the grid has been reorganized in the era of restructuring and deregulation, reactive power is not always provided efficiently.
To provide a sense of the potential market, Stankiewicz and Ingram note that TVA, with generating capacity of about 30 gigawatts, has 259 capacitor banks with a total rating of 9.4 gigavoltamperes-reactive (GVAR). Scaling to the whole United States, which has more than 700 GW of generators, there might be as much as 250 GVAR of transmission capacitors—all candidates for replacement by superconducting synchronous condensers. Of course, not every situation will be suitable for a SuperVAR-type device, but even a modest slice of the global transmission market would be a big market indeed.
If the market develops as SuperVAR’s boosters expect, it’s unlikely that American Superconductor will own it all forever. But the company was well positioned to devise the first prototype. The innovation was a happy conjunction, really, of several corporate developments. One was its commercialization of first-generation superconductor wire manufacturing technology, which the company accomplished by working closely with researchers at Oak Ridge National Laboratory, near Knoxville, Tenn. Two years ago, American Superconductor opened a production line for its first-generation wire in Devens, Mass., where it is turning out about 400 km of it each year.
Another development was American Superconductor’s acquisition of a leading firm in Wisconsin that makes the big semiconductor devices used in power electronics. That division now makes power electronic converters and devices based on insulated gate bipolar transistors (IGBTs) that are used not only in SuperVAR and other reactive power products but in fuel cellpowered buses to be tested in Winnipeg, Man., Canada, and in wind farms being built by Denmark’s Vestas Wind Systems AS, the world’s top windmill manufacturer.
Finally, American Superconductor has had considerable experience and success with rotating superconducting machinery in the context of the U.S. Navy’s motor demonstration program. The company has so far built and delivered a 5-MW superconducting motor for the Navy and is presently making a 36.5-MW propulsion motor. These motors have some important similarities with the SuperVAR machine. The Navy’s imprimatur, notes Ingram, was a critical factor in TVA’s decision to proceed with American Superconductor on SuperVAR.
Despite these encouraging signs, there could be some bumpy times ahead. For American Superconductor, there is the awkward fact that just as it has gotten its first-generation wire factory up and running, the industry seems prepared to shift to second-generation wire made from a different material. The second-generation wire should be less expensive and will hold up better in magnetic fields, so that it can be used at temperatures up to 50 degrees above absolute zero. Though American Superconductor is well fixed to capitalize on the transition to the second-generation wire, having developed key technology in this area as well, its investment in its first-generation production line may be largely a loss.
For the industry as a whole, despite some impressive successes, superconductor applications have not materialized as quickly or as smoothly as some expected or hoped. The first test of superconducting cable in a real-world transmission situation was unsuccessful, and development of superconducting transformers has been stalled by difficulties in coming up with wire configurations with sufficiently low losses in alternating-current conditions.
Still, as far as SuperVAR is concerned, prospects look good. At this writing, TVA is expected to place an order for two more devices, albeit at a precommercial price. A true commercial order should follow. If so, SuperVAR, coming from far behind, will be able to claim the distinction of being the first commercially successful piece of superconducting power equipment to be installed in a grid.
Superconducting Synchronous Condenser
Goal: Quickly provide large quantities of reactive power to support grid voltage—as much as double the rating for up to a minute from a single unit—and to supply controllable continuous-duty reactive compensation for optimizing grid-transmission effectiveness.
Why it’s a Winner: It promises to do the job more effectively, and possibly more economically, than other solutions, especially for industrial voltage flicker applications and voltage regulation of transmission lines with large isolated loads (mines and paper mills, for example).
Organizations: American Superconductor Corp. and the Tennessee Valley Authority.
Center of Activity: Westborough, Mass., and Gallatin, Tenn.
Number of People on the Project: 8 to 20.
Budget: Not available.