IEEE Power & Energy Magazine - September/October 2020 - 53

served as a prototype for design and construction of
subsequent pumped storage plants. The 59.5-MW capacity far exceeded the 25-MW ceiling predicted when
reversible units were first studied. Larger units meant
that pumped storage could better fulfill its promising
role in meeting peak demand. The compact physical arrangement also improved opportunities to build
pumped storage. Today, reversible pump-turbines
have almost completely supplanted the use of separate
pumps and turbines in these facilities. In addition, the
Hiwassee pump-turbine was the first reversible pumpturbine built in the United States using wicket gates
for control of turbine output power and improved
pump efficiency.
The combined pump and turbine at the Hiawassee plant
was the first of many to be installed in U.S. power plant systems. At the time, the plant was the largest and most powerful in the world. As a pump storage unit in the TVA's system,
it effected significant savings in the generation of electrical energy.

Pump Motor Starting
Another technological hurdle that was addressed was the
method used to start pumped storage units in pump mode.
In pump mode, it is necessary to bring the pump up to
speed while spinning in air. Once the pump has reached
a synchronous speed, the gates can be opened to allow
water to enter, followed by opening the main valve, which
allows the pump to move water from the lower to the
upper reservoir.
There were several ways to start units in pump mode:
across-the-line starting, back-to-back starting, pony motor
starting, or use of a separate small hydro turbine on the
shaft. Each method had its technical limits as well as system
dispatch and operating constraints.
Across-the line starting required a generator in a nearby
plant to be online and capable of meeting the starting current
requirement of the unit in pump mode. Back-to-back starting used water from the upper reservoir to operate a unit in
the plant in generation mode, which, in turn, would provide
a starting current to start the unit in pump mode. The pony
motor starting method still required a dedicated generation
source but had a lesser impact on the bulk power grid. Starting with a pony motor uses a wound rotor induction motor
mounted on the unit shaft and connected to the grid or an
in-plant generator that could power the motor, bringing
the pump up to synchronous speed and allowing the pump
motor to be synchronized to the grid. When the pump is
connected to the grid, the pony motor is disconnected. A
separate turbine involved additional expensive hydraulic and
civil works.
In addition to initial expenses and maintenance costs, the
across-the-line, back-to-back, and pony motor starting methods imposed significant operating constraints on pumped
storage operation. The motor-pump starting constraint was
september/october 2020

significantly reduced when solid-state technology made
variable-frequency starting available.
The development of high-capacity, advanced solid-state
devices made it possible to start units in pump mode with
variable-frequency converter-inverters and eliminated the
need for pony motor, back-to-back, and across-the-line
starting. With a variable-frequency controller, the input
power is drawn from the plant main bus at a fixed voltage
and system frequency. When the unit is being started as a
pump motor, the line-side converter acts as a rectifier and
feeds the machine-side inverter. The thyristors and controls of the machine-side inverter produce a field current
on the stator windings of the unit to be started. As the
frequency of the field current is increased, the pump motor
speed is also increased until the pump reaches a synchronous operating speed, and the pump motor is connected
to the grid.
The introduction of variable-frequency motor starting
resulted in several operational benefits that allowed pumped
storage plant operations to be more predictable and easier to
coordinate with system dispatch requirements. This measure
also allowed pumped storage units to change from pump to
generation mode, and vice versa, in minutes.

Pumped Storage and Nuclear Power Plants
In 2015, there were 40 pumped storage hydro plants in
operation in the United States, with a total installed capacity
of approximately 22 GW. Many of these plants were constructed from the 1960s through the 1980s to complement
large base-load nuclear- and coal-powered plants by providing fixed pump loads during the early morning hours, with
peaking power and load following in generation mode during the daytime peak.
One of the technical/economic factors that drove the
development of the Hiwassee Unit 2 reversible motor/
generator and pump/turbine on a common shaft was that
base-load nuclear power plants were being planned at the
time, and the Nuclear Regulatory Commission was concerned about two technical/reliability issues. One issue
was that nuclear units could not be cycled to follow the
daily load demand. The second issue was the need for
a sufficient dedicated power supply that could quickly
provide power to operate cooling pumps in the event of a
major system or plant event. A solution that was considered was to require that nuclear plants be connected with
a reliable transmission line to a pumped storage plant.
A pumped storage plant, with a sufficiently large upper
reservoir and reliable transmission connection, could provide power to operate cooling pumps for an extended period
while reactors were being shut down. The need to deal with
the cycling issue could be addressed by pumped storage
units providing load-following and automatic-generationcontrol (AGC) services when the units were in generation
mode during peak load demand and then operating in pump
mode during the early morning low-load period.
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IEEE Power & Energy Magazine - September/October 2020

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