IEEE Power & Energy Magazine - September/October 2020 - 54

While the pumped storage units were in pump mode, the
load-following and AGC services could be provided by other
generating units with load-following capability. An example of a pumped storage plant that was built to accommodate the needs of nuclear plants is the largest U.S. pumped
storage plant, the 3,003-MW, six-unit, open-loop Bath
County project in Virginia, operated by Dominion Generation/First Energy, as described in the DOE Global Energy
Storage Database (see the "For Further Reading" section)
(Figure 2).

Adjustable-Speed Pumped Storage
The next major technological hurdle to be addressed
involved the technical-economic limitations associated with
fixed single-speed pumped storage units and the motivation
to go to adjustable speed. A 1981 IEEE article includes
a discussion about the benefits and applications of twospeed motor/generators as well as adjustable-speed doubly
fed induction motor/generators (see the "For Further Reading" section).
This article by Gish et al. describes the results of a
study and model tests on the potential applicability of a
doubly fed machine for relieving the constraints inherent
in hydropower applications. Although the literature contains many theoretical studies of the machine, this article
reports a more pragmatic examination of its characteristics
and potential under a restricted set of conditions, where its
use may be most economically advantageous. The primary
function of the machine in this type of application is to
allow a modest range of speed adjustment, sufficient to
maintain the best turbine efficiency despite load and head
variations. In so doing, cavitation and draft tube surging
are also relieved.
Two basic approaches for continuous adjustable-speed
hydro operation have emerged.
✔ Control of the machine shaft speed through the magnetic field of a doubly fed machine: This approach is
discussed in this 1981 article and explains how a doubly
fed machine can be used. The article indicates that the
"application to pumped storage projects, conventional
hydro generation projects, and rapid start emergency
standby applications" appears particularly attractive.

figure 2. The Bath County pumped storage project, Virginia. (Source: Dominion Energy; used with permission.)
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ieee power & energy magazine

✔ Direct conversion of all electric energy interchange

between the machine and the bulk power system: This
approach involves unit-connected-generation, staticfrequency-converter, and load-commutated-inverter
pumped storage units.
The two approaches are based on necessary speed control being
accomplished entirely by electrical means and the fact that
speed is continuously adjustable over a given operating range.
There are other methods whereby operation at other than
a fixed, single synchronous speed can be accomplished.
These include the following:
✔ Pole switching: With pole switching, one machine is
constructed in such a way that the windings can be
physically switched. Switching the winding results in
a machine that has two possible numbers of electrical
poles. In this way, an ac machine can be operated at
either of two discrete speeds.
✔ Tandem machines: The tandem machine approach
is based on the idea that a single shaft has the
prime mover and two electric generators mounted
on it. Each generator is designed to operate at a
discrete synchronous speed.
✔ Mechanical transmissions: Mechanical gears, clutches, and mechanical transmissions can be used to accomplish speed changes.
Although pole switching, tandem machines, and mechanical transmissions allow operation at more than one speed,
they do not have the capability of continuous speed adjustment. Mechanical transmissions require constant maintenance and have high losses. Evaluations of the benefits
and operating capabilities of DFIMs have shown that pole
switching, tandem machines, and mechanical transmissions
do not exceed the benefits and operational capability of the
DFIM approach.

The Japanese Utility Experience
Japanese utilities have pioneered the application of adjustable-speed technology to pumped storage hydro plants. In
1994, the Electric Power Research Institute, with support
from the NHA, funded a study of Japanese adjustable-speed
pumped storage development. The study was published in
1995 (see the "For Further Reading" section).
As a first step, a trip was undertaken to meet with engineers at Tokyo Electric Company and Kansai Electric Company as well as with two manufacturers: Toshiba and Hitachi. Visits were made to the Yagisawa pumped storage plant
and the four-unit Okawachi pumped storage plant, which
was then under construction. Since then, Japanese utilities
have constructed several pump storage plants with adjustable-speed pumped storage units.
Two interrelated factors have influenced the development
of adjustable-speed machines at Japanese utilities. One is the
need for frequency regulation capability as base-load nuclear
plants are added. The second is the reduction of imported
fuel (oil and liquid natural gas) for diesel and gas turbine
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IEEE Power & Energy Magazine - September/October 2020

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