IEEE Power & Energy Magazine - September/October 2020 - 55

generator units that are online to provide frequency regulation during the early morning hours when pumped storage
units are being operated in pump mode. These two issues
focused attention on the cost to operate plants that are used
primarily for frequency regulation, load following, and AGC.
Government industrial policy regarding energy spurred
electric utilities in Japan to take action and reduce dependence on imported petroleum products. Both Kansai Electric and Tokyo Electric concluded that the capability of the
doubly fed machine to regulate frequency in the pumping
mode, along with the capability to pump at less than full
load, were key factors in their decision to pursue adjustablespeed hydro development.
In the early 1990s, Kansai Electric and Tokyo Electric installed adjustable-speed pumped storage units. The
motivation for this was their need to reduce the amount
of imported fuel used by combustion turbine-driven generators for frequency regulation during periods when
pumped storage units were operating in pump mode. It
was realized that, if pumped storage units could regulate
frequency in pump mode, then combustion turbine generators would no longer be required to provide frequency
regulation service. It was also recognized that other plant
and system benefits would be realized with adjustablespeed pumped storage units.
The first adjustable-speed units were at the Yagisawa
and Okawachi pumped storage plants. The installation at
the Yagisawa plant was an 85-MW unit that was converted
from a single-speed, 150 r/min, synchronous motor/generator to a doubly fed induction motor/generator with speed that
is adjustable over the range 130-156 r/min. The Okawachi
plant is a four-unit pumped storage plant with two 400-MW,
adjustable-speed motor/generator units with a speed range
of 330-390 r/min.
Since the initial units were installed in Japan, there now
are more than 30 adjustable-speed pumped storage units in
operation worldwide. Most of the units are in Japan, and
others are installed in Europe. To date, there are no gridscale adjustable-speed pumped storage units in the United
States, although several new pumped storage projects are in
the feasibility and licensing phases, and some may include
adjustable-speed units.

3) speed/power control system, and 4) rotor brush assembly.
Figure 3 shows the major components of a DIFM adjustablespeed motor/generator.
The stator of a DFIM is similar in construction to that of
a conventional synchronous machine; it has a multipole configuration with the number of poles determined by the nominal synchronous speed. Selection of the synchronous speed
is linked to pump/turbine and hydraulic design characteristics. A doubly fed induction motor/generator uses a solid
smooth air gap rotor with laminated construction instead
of a salient-pole rotor, as is used in single-speed synchronous machines. A three-phase sinusoidal magnetic field is
established on the DFIM rotor with ac provided by a voltagesource frequency converter.

Organization Activities
IEEE Power & Energy Society
In 2013, the IEEE Power & Energy Society (PES) Power Systems Dynamic Performance Committee approved the scope
for a task force on Advanced Pumped Storage Modeling.
The IEEE task force scope includes three advanced pumped
storage technologies: 1) DFIM, 2) converter-fed synchronous
machines, and 3) ternary units. A common characteristic
of these hydro storage technologies is that they can provide
ancillary services in both pump and generation modes.

System
Specification

Main
Transformer

Main Circuit
Main
Circuit Breaker
Generator-Motor
Specification

Exciter Circuit
Generator-Motor
Armature Circuit

DFIM Technology
DFIM pumped storage units and single-speed, synchronous
machine pumped storage hydro units are both designed to
store and supply electric energy using pump/turbines and
motor/generators. In many ways, a pumped storage unit with
a DFIM motor/generator is like a pumped storage unit with
a single-speed, synchronous, salient-pole motor/generator. In both cases, the mechanical/hydraulic, pump/turbine,
wicket gates, and turbine governor controls are basically the
same. However, there are four major equipment differences
between a DFIM and a single-speed synchronous machine,
including the 1) rotor construction, 2) rotor excitation system,
september/october 2020

Exciter
Transformer

ac Exciter
(Static
Frequency
Converter)

Generator-Motor

Pump-Turbine

figure 3. The construction of an adjustable- (variable-)
speed pumped storage system. (Source: CIGRE, Working
Group A1.34, "Testing voltage of doubly-fed asynchronous
generator-motor rotor windings for pumped storage system," Elektra, no. 306, October 2019. Courtesy of CIGRE.)
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IEEE Power & Energy Magazine - September/October 2020

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