When American Electric Power (AEP), Columbus, Ohio, designed its new energy delivery headquarters, power quality was at the top of its list of concerns. For a business operation with a PC on every desk, the risk of losing power; even momentarily; was unacceptable. After considering several alternatives, AEP's team selected a system that combines an uninterruptible power supply (UPS) with an advanced flywheel.

The Electric Power Research Institute (EPRI) describes this energy-storage technology as "an economical and elegant alternative to other methods of storing and providing premium power."

Combining a UPS rated at 150kVA with an Active Power CleanSource CS 200 flywheel rated at 200kVA, this integrated motor-generator-flywheel is 11 in. high and 31 in. in diameter and can produce 200kVA of power.

AEP's power distribution environment consists of one group of electrical panels for standard power and a separate group of panels served by the flywheel-based system for premium power. Each PC user has a designated orange electrical receptacle in their workspace for premium power delivery.

The UPS converts AC power to DC power. The flywheel, which is connected to the DC side of the converter, provides energy to the DC bus. Then, on the device output, the DC power converts back to AC power to serve the premium power circuits. This way, if there is any voltage sag or interruption on the source side of the device, the DC bus-supported by the flywheel can convert to a generator state and provide the required AC power.

AEP's UPS-and-flywheel solution employs one UPS for the entire installation, rather than using a separate UPS for each PC or even each floor of the four-story building.

"Having separate UPSs would have required more space and more maintenance," explained Harry Vollkommer, staff engineer for technology development and delivery at AEP. "We're more comfortable with a centralized approach. With individual UPSs, you'd have a higher kVA, because you'd be adding more capacity at each individual PC site."

The centralized solution costs about half as much as purchasing individual UPS devices for each desktop, according to the manufacturer. This approach also simplifies the hassle of reconfiguring offices and computer systems. Centralization also helps control costs because it requires less capacity to serve a diversified load. This is because not everyone is using their PC at the same time.

A key characteristic of battery-based UPS systems is their ability to meet power requirements for several minutes. A 200kV flywheel, on the other hand, may be able to supply power for only a minute or less. But since most power quality problems occur in less than a minute, the flywheel meets most users' needs.

According to the IEEE, when you look at building power served by distributed circuits, 80% to 90% of the disturbances are momentary. In AEP's case, with a 200kVA flywheel connected to a 150kVA UPS system, they have about 20 sec of backup power available at full rating. Because they generally operate at 60% of full rating, they really have 30 sec to 40 sec with the flywheel; more than enough to handle any potential voltage sag or momentary interruption.

It didn't take long to make PC users at AEP believers of the UPS-and-flywheel solution. During the system's first year on the job, it has maintained continuous power to the PC (with the longest interruption to the building lasting 35 sec, during a transfer to an alternate circuit).

The search for ways to deliver clean power to critical systems has generated a number of alternatives for storing energy and providing premium power, including superconducting magnets, batteries, and flywheels. More than 40 of these systems have been installed across the United States, demonstrating the viability of this technology.