Power Transmission Without the Power Electronics

GE's variable-frequency transformers transfer power between out-of-sync grids without the problems of semiconductor power electronics

PHOTO: General Electric

14 November 2007--During their low-resolution beginnings, digital music and photography delivered a jarring rendition of sounds and images, full of noticeable discontinuities. Today, you get the same sort of thing from semiconductor-power electronics devices used to manage power flows on high-voltage transmission grids, and those jarring oscillations can damage the high-precision turbines spinning in power plants. This costly problem inspired engineers at General Electric Co.'s Atlanta-based GE Energy division to go back to the future and create a new semiconductor-free-power control device: the variable-frequency transformer, or VFT.

U.S. electricity giant American Electric Power (AEP) started up GE's first commercially produced VFT this summer in Laredo, Texas, to pull in up to 100 megawatts of emergency power from Mexico. Montreal-based Hydro-Québec has been operating a demonstration VFT since 2004 to manage power exchanges with the eastern U.S. grid. By 2010, three more VFTs should be operating in New Jersey to push extra power into chronically undersupplied New York City.

VFTs and power electronics manage power flows by reshaping AC power waves. They can thus bridge grids whose 60-hertz frequencies are out of sync with each other, such as those in Mexico and Texas, or they can drive power from one line to another by synthesizing ac waves that are out of phase with respect to one another, exploiting a phenomenon known as ”phase angle” that is a grid operator's primary tool for directing power flows.

But VFTs and power electronics work their magic via radically different means. Power-electronics devices use semiconductor switching to convert ac to dc and then digitally synthesize new ac waves of whatever frequency and phase is needed. However, their synthetic ac is not a smooth sinusoidal wave but rather a series of steps. ”They're doing very fast switching and every time they do that there's a small spike that comes off the edge of that waveform,” says Paul Marken, GE Energy's applications engineer for the VFT. Managing those spikes, the ”harmonic pollution,” can require complex analysis and control schemes and costly filtering equipment.

VFTs, in contrast, skip the dc step, reshaping ac waves through a creative blending of century-old hydroelectric-generator technology and an ac transformer. From the outside it looks like a hydro generator, with an 11.5-meter-tall rotor poked through a 5.4-meter-diameter donut-shaped stator. (In fact, Marken says that most of the parts are supplied by factories that make standard 17-kilovolt hydro generators.) To grasp the difference, one must follow the wires: the rotor's coils are wired to a power line rather than simply forming an electromagnet designed to spin at high speed. The stator coils, meanwhile, are wired into a second line, turning the hydro generator into a transformer bridging the two lines. AEP's new VFT bridges two grids, because the stator is wired to an AEP line on the Texas grid, and the rotor links to a line from the Mexican national utility Comisión Federal de Electricidad.

Completing the picture is a motor to turn the rotor, because offsetting the paired coils in a transformer shifts the phase angle between them. By shifting the VFT's rotor in either direction, grid operators can thus dial in up to 100 MW of power flow in either direction. Power transfers are often achieved using semiadjustable phase-shifting transformers that on-site technicians can set to provide, on average, a dozen predefined angles. The VFT, in contrast, is infinitely variable and operates in real time. ”It's like a phase shifter with an infinitesimally small step. It's continuous,” says Marken. In operation, the rotor continually turns to maintain the desired phase angle even though the frequency of the two grids it connects tend to drift around the 60 Hz ideal.

PHOTO: AEP

Having 100 MW of power on demand eases a major headache for AEP, whose power lines in the Laredo area are long, weak, and prone to brownouts during the summer peak. Until now, AEP has had to keep small and expensive gas-fired power plants in Laredo idling and ready to prop up the voltage. It could get help from the stronger Mexican grid, but that would entail first blacking out some customers and then manually resetting substation relays to bring those customers back up as part of the Mexican grid. With the VFT, AEP has instantaneous access to surplus power without inconveniencing its customers.

”The dynamic stability that this thing is capable of providing to a system is just tremendous,” says David Kidd, AEP's senior planning engineer for transmission in Texas. Kidd adds that the lack of harmonic pollution made it easier to get support for the project from Comisión Federal de Electricidad. ”It was a big plus in this location that the VFT didn't produce any harmonics,” says Kidd.

The technology's next planned deployment is in the Northeastern United States where three VFTs operating in parallel will drive power through an underwater electric line from Linden, N.J., to the Bronx, the northernmost section of New York City. The line is being expanded by another GE division, Stamford, Conn.–based GE Energy Financial Services, to carry an extra 300 MW into New York City. Without the VFT's phase-angle control, the expanded line would actually suck power out of New York City. GE Energy says VFTs won out over semiconductor-based power electronics because there is already too much harmonic pollution on the New Jersey lines. Site preparation work began early this year, and the VFTs are scheduled to begin pushing power by the end of 2009.

Kidd sees more applications coming. For one thing, he thinks VFTs could play a role in much larger power imports from Mexico that his company and the Mexican utility are studying. Kidd says that 10 years from now there could be one or two major new transmission links across the border importing up to 3000 MW from Mexico. While power electronics devices are cheaper than VFTs at such large power levels and would probably provide the bulk of that new transmission capacity, Kidd says two or three VFTs might be needed to provide stability. ”It hasn't been tested yet,” says Kidd, ”but that's what we've been thinking about and exploring.”

About the Author

Contributing Editor Peter Fairley has reported for IEEE Spectrum from Bolivia, Beijing, and Paris.

To Probe Further

Click here to see a video of Hydro-Québec's VFT rotor in operation.

The rotor turns as the frequency of the Québec and New York grids drifts up and down around their 60 Hz standard frequency, speeding up or slowing down relative to each other. ”To hold the same phase angle, the rotor has to move around,” explains Marken.

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