Hiwassee Dam’s Unit 2 Pumps It Up

After five years of inactivity, Hiwassee Dam’s groundbreaking Unit 2 reversible generator/turbine unit is fully back in action, allowing TVA even greater flexibility to generate electricity—or remove it from the grid when needed to help balance base load.

JULY 7, 2016—In 1952, TVA conducted an experiment at Hiwassee Dam, designing a reverse-drive generator/turbine there—Unit 2—that could pump water from below the dam up into the reservoir above it. The dual function could generate 85 megawatts of electricity when acting as a turbine, or in pump mode could take electricity off the grid when demand for power dropped quickly.

It was the first of its kind in the United States, and at the time the largest and most powerful in the world. So important was Hiwassee Unit 2 that it was designated a National Historical Mechanical Engineering Landmark by the American Society of Mechanical Engineers in 1981.

But you haven’t heard much about it—especially lately. That’s because Unit 2 has been in hibernation. When the unit’s breakers experienced a failure on July 5, 2011, TVA chose not to restore its pumping abilities. The unit could still generate power through traditional hydro generation means—but the reversibility was no longer possible.

However, after five years of inactivity, Hiwassee Hydro Unit 2’s pump capabilities are now fully functional again.

Scott Davis, hydro technician IV, conducts final checks on Hiwassee Unit 2 as it gets ready to pump after restoration of its full capabilities; system engineer Lakesha Lee looks on.

“The decision came this year to restore its pump capability because hydro provides flexibility in meeting demands,” explains Michael Richards, plant group manager. “With  Watts Bar Unit 2 coming on with its large base load, it was a good time to return this unit to service.”

Kenny Mullinax, vice president, Western Coal and Hydro Generation, affirms Hiwassee Unit 2’s renewed importance. “With system turndown becoming more important with the addition of a large base load unit to the portfolio, having additional pumping capabilities is extremely important,” he says. “It is of significant help to the balancing authority to regulate the grid.”

The project was coordinated across TVA teams, and came in ahead of schedule and under budget, according to construction manager Brenda Byers. Repairs began April 17 and ended June 15, at a cost of $1.3 million.

The restoration project included not only repair of the breakers but included replacing a transformer, exciter, exciter transformer and repairing the wicket gate brakes.

Hiwassee Dam in North Carolina was a TVA construction project that begin in 1936, one of 16 dams it built in that period. Unit 1 went online in 1940 and is strictly a generator, without pumping capability. Raccoon Mountain is also a pumped storage site, with four units with a capability of pumping 1,500 MW. It came online in 1978.