IEEE Industry Applications Magazine septemberoctober 2018 1077-2618182018I eee18 [629232]

IEEE Industry Applications Magazine œ september/october 2018 1077-2618/18©2018I eee18
Variable-Speed Operation
of Hydropower Plants
A LOOK AT THE PAST, PRESENT, AND FUTURE
Digital Object Identifier 10.1109/MIAS.2017.2740467
Date of publication: 18 June 2018
In Th Is arTIcle, T he sTaTe of T he ar T In
the variable-speed operation of hydropower plants
is reviewed, with a focus on pumped-storage hydropower. r elevant literature is reviewed to
address the benefits of variable-speed opera –
tion for both power systems and hydropower facilities. Two main configurations to enable variable-speed operation—the doubly fed
induction machine (D fIM) and the con –
verter-fed synchronous machine ( cfsM)—
are discussed and compared. The article addresses the technology and energy
policies of the past, present, and future and points out how the motivation,
services, value, and technology of
variable-speed hydropower plants have been subject to consider –
able change.
By Mostafa Valavi and Arne Nysveen
©istockphoto.com/goce

september/october 2018 œ IEEE Industry Applications Magazine19
Pumped-Storage Plants
Variable-speed hydropower genera –
tors (motor–generators in pumped-
storage facilities) do not need to
operate at a constant rotational speed because they are no longer directly
connected to the grid. In conventional
hydropower plants, to produce the grid frequency, the rotational speed
of the generator and turbine must
be constant. In the case of pumped-storage plants, operation at a con –
stant speed means that the pumping power cannot be properly adjusted. But variable-speed operation can offer
many advantages. This article aims to
address the opportunities and chal –
lenges of variable-speed hydropower facilities. The most
relevant application is pumped-storage hydropower, which
is the focus of this work. In addition, variable-speed op –
eration in small hydropower and high-voltage dc ( hVdc)-
connected plants is discussed.
Pumped-storage hydropower facilities are the most
efficient and practical large-scale energy storage systems,
with typical overall efficiency in the range of 70–85%
[1]–[3]. In production mode, the plant operates as a con –
ventional hydroelectric plant. In pumping mode, electri –
cal energy from the grid is consumed to pump the water from the lower reservoir to the upper one. In a pumped-storage facility, a motor–generator is used to work either
as a generator in the production mode or as a motor in
the pumping mode.
for the hydraulic system, the following two configu –
rations can be employed: 1) a reversible pump–turbine (usually of the francis type) and 2) a separate pump and
turbine (i.e., a ternary system). It is most common to use
reversible pump–turbines in pumped-storage hydropower
plants (such as the one shown in f igure 1). In this config –
uration, the direction of rotation must be reversed when
the pumping mode is switched to the production mode
and vice versa. a s will be discussed later, in the design
of reversible pump–turbines, priority is normally given to
the pumping operation. The emphasis of this article is the
variable-speed operation of reversible pump–turbines.
a way to bring additional flexibility to hydropower
plants is to use ternary systems, in which a separate pump and turbine can work simultaneously. In this con –
figuration, a pump and turbine are connected to the
motor–generator so that there is no need to reverse
the direction of rotation when the mode of operation changes. h ence, they can offer a quicker transition time
between modes and a faster response. In addition, both the pump and turbine can be optimized, leading to a high –
er hydraulic efficiency.
The operation of ternary systems in hydraulic short
circuit also makes it possible to regulate the pumping power. In contrast to reversible pump–
turbines, the drawbacks include high investment costs, larger space
requirements, mechanical complexity,
and high operating and maintenance costs. as indicated in [ 3], both vari –
able-speed and ternary systems are considered to be advanced pumped-storage hydropower technologies. In
pumped-storage plants with relatively
low heads, a mechanically complex pump–turbine solution (i.e., the Deri –
az turbine [5]) with adjustable blades can extend the operational range and enable the regulation of the
pumping power; its use, however, has
been limited.
as a mature technology, conventional pumped-storage
facilities have been used mainly for balancing the power production and load demand in the grid. Typical opera-tion includes working in the pumping mode during off-
peak hours (normally at night) and in the production
mode during peak hours. The flexibility of pumped-storage plants allows large thermal and nuclear power
plants to operate most efficiently at their peak produc –
tion. In many countries, this was the main motivation for
the development of pumped-storage technology in the
1970s [3], [6].
Today, such plants in the grid can play much a greater
role than that. a s intermittent renewable energy sources
such as wind and solar become more important, advanced
pumped-storage hydropower may be the enabling technology that allows for the higher penetration of
renewable energies into the grid. Because the gen –
eration of these variable renewables is difficult to pre –
dict, flexible energy storage capacity is needed to improve
FIGURE 1. The runner of a 240-MW reversible Francis pump–turbine
in the Limberg II pumped-storage plant, Austria. (Used with
permission from [4].) Pumped-storage
hydropower facilities are the most efficient and practical large-scale energy storage systems, with typical overall efficiency in the range of 70–85%.

IEEE Industry Applications Magazine œ september/october 201820
their grid integration. c onventional pumped-storage
facilities with constant rotational speed are not capable of
providing the high degree of flexibility that a power sys –
tem needs in this case. The variable-speed operation of
pumped-storage hydropower plants can bring additional
flexibility to the power system while offering a variety of
valuable ancillary services. In addition to the power system, the hydropower facility itself could benefit substantially
from variable-speed operation, through, e.g., improved effi –
ciency and an extended operating range.
This article reviews the state of the art in the vari –
able-speed operation of hydropower plants and is an
extended version of [7]. The status of the technology is reported, and future trends are discussed.
Benefits of Variable-Speed Operation
The development of variable-speed pumped-storage plants dates back to the early 1990s in Japan, where pioneering
achievements took place and the world’s first such facili –
ties were successfully commissioned. The main reason
for their development was to reduce the number of large
thermal plants operating as reserves during the night
and take advantage of the great flexibility offered by variable-speed pumped-storage facilities for frequen –
cy regulation [8].
The main advantage of variable-speed operation for
pumped-storage plants is the ability to control power in
the pumping mode. h ence, such plants can contribute to
frequency control in the pumping mode as well as in the
production mode. a s will be addressed in this section,
variable-speed pumped-storage facilities can also offer ancillary services to support the reliable and stable opera-tion of the grid.
The application of variable-speed hydropower technol –
ogy is, however, not limited to pumped-storage plants. In the case of h Vdc-connected hydropower facilities
[9], because the frequency of the generator is not tied to the grid, the operation of the plant can be optimized by adjusting the rotational speed. In such plants, it can
be advantageous to employ variable-speed technology. another application that could benefit from variable-
speed operation is in small hydropower systems [10],
where head and flow variations can be considerable. With
variable-speed technology, it is possible to replace rather
mechanically complex turbines with simpler ones, while maintaining a sufficiently high efficiency.
Benefits for the Power System
The flexibility and stabilization of the power system can be greatly improved with the ancillary services that hydro –
power plants with variable speed can provide. o ffering high
dynamic control, they increase the controllability of the power system. one obvious advantage of pumped-storage
facilities is their ability to adjust the pumping power and hence contribute to frequency regulation. In fixed-speed systems, the pumping power cannot be properly
varied, so one way to increase flexibility is to use mul –
tiple pumps. The variable-speed solution, however, has
distinct advantages over the multiple pumps solution; for
example, the capability for load balancing is much better,
and there is no need for frequent start/stop sequences.
Variable-speed pumped-storage plants are also able
to compensate in the production of variable renewables and improve their integration into the grid [3], [11], [12]. still, nondispatchable production of these renewables is
not the only challenge for this integration. The variable renewable sources do not provide inertia in the same way that classical units do, and, as a result, grid stabil –
ity problems can be a limiting factor for high penetra –
tion of renewables [13], [14]. The stability of the grid can be greatly improved by the ancillary services provided
by variable-speed pumped-storage plants, and there
are many papers and reports that discuss this [3], [8,] [14]–[19]. The variable-speed hydropower approach is
claimed not to need any power system stabilizer (P ss)
functionality [20]. a comparison between variable-speed
hydroelectric systems and conventional systems with P ss
in terms of dynamic behavior is presented in [19]. Iso –
lated grids are, in general, more sensitive in responding to frequency deviations; hence, pumped-storage facilities
could play a key role in ensuring the safe operation of
the grid [12], [21]. In addition, flexible pumped-storage plants, particularly those equipped with variable-speed
technology, can greatly reduce the amount of wind
power curtailment [3], [22].
In variable-speed systems, the output power can be
controlled by the converter; by contrast, in convention –
al hydropower facilities, the turbine governor has this responsibility [3], [8], [23]. The result can be very fast and
high dynamic power control that can be used to improve
power system stability. In this regard, an interesting fea-ture offered by variable-speed operation is instantaneous
power injection [16], [18], [24], [25], as shown in f igure 2.
In this case, because the rotational speed does not need
to be constant, a large amount of active power can be
injected into the grid by reducing the rotational speed
0
–0.1
–0.2
–0.3
–0.4
–0.5
–0.6
–0.7(p.u.)
00.20.40.60.8 11.21.41.61.8 2
Time (s)
FIGURE 2. The measurement of instantaneous power injection (the
flywheel effect) in a 3.3-kVA prototype DFIM. The per unit (p.u.)
active power of the machine stator terminals is in red, and the network is in green. (Used with permission from [18].)

september/october 2018 œ IEEE Industry Applications Magazine21
(the flywheel effect). This can be particularly advanta-
geous during disturbances, allowing a reduction of the
grid’s spinning reserve [16].
an example illustrating the role of variable-speed pumped-
storage plants in providing grid support during disturbances is the operation of the o kawachi hydropower facility in
Japan during an earthquake disaster in 1995 [3], [8]. It was
reported that the variable-speed unit absorbed power dis –
turbances in random spikes and satisfactorily contributed to maintaining grid stability.
as mentioned previously, one motivation for introduc –
ing variable-speed operation in pumped-storage plants is to provide frequency control. This feature can reduce the number of thermal plants needed to operate at night as
a reserve for frequency regulation. flexible operation of
pumped-storage facilities can create a steadier operation
profile for thermal units and reduce their startups/shut –
downs and ramping costs [3]. as reported in [8], for the
case of Japan, the most attractive evaluation factor for the adjustable speed operation is a reduction of the thermal
power units to be operated for automatic frequency con –
trol at night.
as an alternative to large-scale pumped-storage plants,
relatively small variable-speed pumped-storage facilities
are being developed as closely as possible to wind farms, thus providing a decentralized energy storage capacity. as
claimed in [14], this can be a key storage technology for future smart grids.
Benefits for the Hydropower Plant
Variable-speed operation can offer distinct advantages for hydraulic machinery and overall power plant perfor –
mance. a n important benefit is improving hydraulic effi –
ciency, especially for reversible pump–turbines. h ydraulic
machines are optimized for a single operating point (the
best-efficiency point), which is a function of the head, dis –
charge, and rotational speed. a t a fixed speed, deviations
in the head and discharge can lead to reduced efficiency and increased vibration and cavitation problems. Thus,
only limited variations of the head and discharge are allowed. This is why hydraulic efficiency can fall rather
sharply at partial loads in a fixed-speed system. While the
operating range of fixed-speed pump–turbines is limited to a ratio of about 1.25 between the maximum and minimum
head [26], [27], this can be extended, using variable-speed
technology, to as high as 1.45, as claimed in [27]. Variable-speed operation offers a new degree of freedom to improve
efficiency at each point of operation [25], [28], [29].
If the rotational speed can be adjusted, it is possible
to reach an acceptably high efficiency even in the case of large head and discharge variations. In addition, vibra-
tion and cavitation problems can be greatly reduced, and the operating range can be extended. The efficiency gain
depends strongly on the plant’s operational conditions and
hydrology. e fficiency improvements are expected to be
higher at sites with large head variations and partial-load operation. It is also natural to expect greater efficiency gains in plants with single units compared with those hav –
ing multiple units. a s noted in [30] and [31], a hydraulic effi –
ciency improvement up to 10% can be achieved. In the case of the 400-MW o kawachi pumped-storage facility, an aver –
age efficiency gain of 3% is reported [8]. f igure 3 shows the
typical range of efficiency improvement for hydropower plants when variable-speed technology is employed.
as mentioned previously, it is most common to use
reversible f rancis pump–turbines in pumped-storage
plants. In this case, a single hydraulic machine operates
as both the pump and turbine. Because the hydraulic
parameters are different for these two modes of opera-tion, the hydraulic machine cannot have the best effi –
ciency characteristics for both modes. They are normally first designed and optimized as pumps and then work with a reduced efficiency in the turbine mode [6], [14],
[15], [17], [30], [33] because optimal speed during pump –
ing is normally higher than that during turbine operation
[6], [14], [30]. This problem can be solved with variable-
speed operation so that both the pump and turbine can
operate at their optimal rotational speeds to reach their maximum efficiencies.
In small and run-of-a-river hydropower plants with
low heads or no reservoirs, the discharge rate may vary significantly. h ere, mechanically complex Kaplan turbines
100
90
80
70
60Efficiency (%)
60 70 80 90 100 110 120 130
Rated Head (%)
100
90
80
70
60Efficiency (%)
60 50 40 70 80 90 100 110 120
Rated Power Output (%)(a)
(b)
Single-Speed Range
Adjustable-Speed Range
FIGURE 3. The turbine efficiency versus (a) the rated head and
(b) the rated output power. (Used with permission from [32].)

IEEE Industry Applications Magazine œ september/october 201822
can be used to maintain an acceptable efficiency amid the
wide range of head and discharge rates. But an alternative
solution is to use simpler and cheaper propeller turbines
equipped with variable-speed technology [10], [34]–[36]. This technology could help in developing small hydropower
projects where environmental impacts are less significant
compared to large plants.
In addition to efficiency, flexibility in terms of choos –
ing the optimal speed provides additional advantages for the hydraulic machinery. The speed can be adjusted to avoid hazardous operating zones and reduce cavitation
and vibration problems. In fixed-speed systems, partial-
load operation and specific gate openings (normally approximately 40–60%) can cause pressure pulsations
and result in considerable vibration [3], [30]. This can be
reduced in variable-speed operation. f igure 4 [37] pres –
ents a comparison between the vibration signals of a
reversible f rancis pump–turbine with fixed- and variable-
speed operation in the Yagisawa pumped-storage plant.
reduced vibration and cavitation problems can potentially
lead to less maintenance and an increased life span. In addition, because of improved cavitation behavior, less submergence may be necessary, which leads to a reduced civil engineering cost [14].
a further advantage of utilizing variable-speed technolo –
gy in pumped-storage facilities is that there is no need to use additional equipment for the pumping startup. In conven –
tional plants, frequency converters (previously pony motors)
are needed for pumping startup and synchronization.
It should be noted that variable-speed technology does
not normally allow a full range of variation (0–100%) in the rotational speed and hence the pumping power. The
main limiting factors are the cavitation and stability con –
siderations related to the hydraulic pump–turbine [3], [14].
still, considerable improvement in the operating range is
feasible, even with a limited allowed speed variation [3].
The typical power variation range in the pumping mode
is approximately 30–40%.
Challenges
In general, the most important hurdle for pumped-storage
facilities is profitability. new plants need a great deal of
investment, take a relatively long time to construct, and
could have a negative environmental impact; moreover, new project sites might be limited [1], [23], [38]. r egarding
the initial investment, while pumped-storage plants were cost-effective in the past compared to flexible gas tur –
bines, they are no longer economically competitive [3]. (In
the case of n orwegian hydropower, however, it is claimed
that, because of existing reservoirs, the investment costs
for new plants can be greatly reduced [39].)
In addition, gaining revenue from energy arbitrage
(i.e., pumping and producing when the electricity price is low and high, respectively) is no longer guaranteed for
pumped-storage facilities [3], [40]. In europe, for instance,
the price difference between peak and off-peak electric –
ity in the power markets has not been large enough to ensure plants’ profitability [38], [41]. In addition, pumped-
storage facilities today have to cope with a more dynamic operational routine, resulting in higher maintenance
requirements [38].
Despite the investment issues, there have been chang –
es, mainly in global energy policies, that favor flexible pumped-storage plants. f or example, with the high pen –
etration of variable renewable sources, there comes an
increasing need for energy storage capacity in the grid. In
addition, the ancillary services offered by variable-speed
pumped-storage facilities are becoming more important in supporting the reliable operation of the grid. Within a
well-defined market, pumped-storage plants can gain
revenue from such services. h owever, most competitive
markets currently do not pay for some of the advanced
services offered by pumped-storage facilities [1], [3], [41].
While the added value of pumped-storage plants derived
from their ancillary services is not well defined in the Unit –
ed states, some areas of europe have stronger ancillary
service markets [1], [3]. e stablishing a market for these ser –
vices makes variable-speed pumped-storage facilities more
10 20 30 40 50 60 70 80(MW)
Penstock Pressure
Draft Tube Pressure
Shaft Deflection
Head Cover Vibration
10 20 30 40 50 60 70 80(MW)
Penstock Pressure
Draft Tube Pressure
Shaft Deflection
Head Cover Vibration(a)
(b)
FIGURE 4. The pressure pulsation and vibration of a reversible
pump–turbine with (a) fixed-speed and (b) variable-speed
operations. (Used with permission from [37].)

september/october 2018 œ IEEE Industry Applications Magazine23
economically attractive. The value of such plants depends
strongly on the level of renewable energy penetration in
the grid. In [3], a detailed economic study is presented to
assess the value of pumped-storage facilities in the United states. It is shown that their value increases with higher
penetration of variable renewables in the grid. This indi –
cates that pumped-storage plants will become increasingly valuable in future power systems.
note that the previously mentioned challenges are
related to the development of new pumped-storage plants in general. Including variable-speed technology will only
slightly increase the total investment costs, while provid –
ing many advantages. a s reported in [3], the incremental
costs for incorporating variable-speed capability are in the range of 7–15%, mainly due to the increased costs for
electrical equipment. c onsidering the challenges regard –
ing the development of new pumped-storage facilities,
upgrading existing plants to variable-speed technology is
gaining attention [23], [42]–[44].
When adopting variable-speed technology in hydropow –
er plants, several considerations must be carefully weighed.
●a more detailed analysis is needed in the design phase of a facility’s hydraulic and electrical equipment, and
the effects of speed variation should be thoroughly
investigated.
●Because the speed can be varied, there is an increased risk of mechanical resonance in the system that should
be considered.
●for variable-speed systems, more space is required to
accommodate the power electronics converter and the associated cooling equipment.
●In electrical machines, converter-fed operation could
bring more complexity and potentially greater losses,
vibration, and insulation problems [9], [45], [46].
●another challenge, as will be discussed in the “D fIMs”
section, is the operation of a D fIM-equipped power
plant in the event of grid failure.
Technology Evaluation
In conventional pumped-storage hydropower facilities,
the electrical machine is directly connected to the grid.
hence, frequency and rotational speed are constant. a
method to provide double-speed characteristics in the
past was to use pole-changing synchronous machines [6],
[33]. These machines are capable of changing the number
of active poles and are equipped with two separate wind –
ings in the stator, corresponding to each pole number.
They are heavier and more complex. needless to say, this
technology is not attractive today because its ability to provide adjustable speed is very limited.
To realize variable-speed operation, two solutions are
available: D fIMs and cfs Ms. Briefly stated, the cfs M
provides superior performance, but the need for a full-
rated converter is a main drawback. This makes the D fIM
more attractive in high-power applications because the
converter itself can be considerably smaller. This section discusses and compares these two solutions and also addresses technology status and future trends.
DFIMs
as shown in f igure 5, the D fIM stator is directly connect –
ed to the grid, while the rotor windings are connected via
a power electronic converter, using slip rings. a detailed
description of D fIM systems, including features related
to pumped-storage hydroelectricity, can be found in [47].
Through frequency control of the rotor current, it is pos –
sible to have variable-speed operation while the stator
frequency and voltage remain constant. In the D fIM, the
stator frequency ( ,fs in hertz) is a function of both the
rotational speed ( ,n in revolutions per minute) and the fre –
quency of the rotor current ( ,fr in hertz):
, fnNf120sp
r#! = (1)
where Np is the number of poles. The ± before fr depends
on the rotational direction of the magnetic field produced
by the rotor currents. a ccording to (1), it is possible to vary
the rotational speed in a certain range (above or below
the synchronous speed) while having constant stator
frequency (50 or 60 h z). This variable-speed operation
can be realized by controlling the frequency and voltage
applied to the rotor windings. To have around ±10% of
speed variation, the converter rating does not normally
exceed 30% of the rated power. This is the main advan –
tage of a variable-speed system with a D fIM, which
makes it the preferred configuration for high power rat –
ings (higher than 100 MW). Therefore, the cost and losses of the converter can be greatly reduced compared to the
cfsM solution with a full-power converter.
Until recently, the cycloconverter-driven D fIM was
the most common technology in variable-speed plants. The thyristor-based cycloconverter produces a lower-
frequency three-phase output from the grid. The state-of-the-art technology, however, is the voltage source
converter (V sc) [18], [33], as shown in f igure 5. The V sc
Pump–
Turbine
Pump–
TurbineDFIM
ac
acGrid
Griddcdc
ac
ac dcdc
CFSM(a)
(b)
FIGURE 5. (a) A DFIM configuration and (b) a CFSM configuration.
(Power transformers are not shown.)

IEEE Industry Applications Magazine œ september/october 201824
offers many advantages compared
to the classical cycloconverter solu –
tion, as noted in [1], [6], [16], [18], and [33]: a simpler structure, the ability to regulate reactive power (the cyclo –
converter absorbs reactive power, making compensation necessary), an improved ability to control the
machine during faults, no need for
an additional frequency converter during startup, and a much lower
total harmonic distortion. In terms
of power electronic devices, both transistor- and thyristor-based equip –
ment can be used [6], [48]. The latter—e.g., integrated gate-commu –
tated thyristors (IG cTs)—is claimed
to be best suited for high-power applications [20], [23], [38], [49].
for providing variable-speed capability in pumped-
storage plants, the focus has been on the D fIM configura-
tion, both in industry and in academia. h owever, as will
be addressed later, variable-speed hydropower systems
with D fIMs have a considerable number of drawbacks
compared with those utilizing cfs Ms.
●The use of slip rings is a major drawback.
●The rotor is mechanically more complex and expen –
sive than that of a synchronous machine.
●Because of the rotor complexity, the maximum D fIM
speed is normally lower than that of a comparable syn –
chronous machine.
●It has been claimed [49] that using D fIMs for sites with a
maximal head higher than 600 m could prove challenging.
●The D fIM startup procedure can be difficult [33],
[49], [50]; because of the limited starting torque, a time-consuming and costly dewatering procedure may
be necessary.
●The performance of D fIMs during grid failure and in
meeting grid code requirements, specifically low-
voltage ride through ( lVrT), can be challenging [24],
[33], [49], [51], [52]. In this situation, the rotor voltage
dynamically increases, and a special protection scheme
must be designed to protect the converter [24], [52]. In
the worst case (i.e., severe short circuit faults), a crow –
bar must be activated, short-circuiting the rotor wind –
ings, to protect the converter against the overvoltages before the circuit breakers switch off the drive. In this case, it is impossible to control the D fIM [33]. Improv –
ing the l VrT capability creates additional complexity
and costs. Demanding new grid codes may require the power rating of the converter to be increased [38], [49].
for upgrading existing plants to variable-speed technol –
ogy, the synchronous generator’s rotor and exciter system must be replaced with the new D fIM rotor. In some cases,
it might be possible to keep the stator. It should be noted, though, that if fractional-slot windings are used in the sta-tor, operation as a D fIM could be
problematic because of the subhar –
monics and reduced air gap length. In
this case, one probable consequence
would be a considerable increase in the vibration level, as reported in [53].
CFSMs
In this configuration, a synchronous machine is connected to the grid via a
full-rated converter. as shown in f ig-
ure 5, a back-to-back Vsc is used to
connect two ac sides using a dc link.
hence, the frequency of the motor–
generator does not need to be equal to the grid. since the machine is decou-
pled from the grid, a wide range of
speed and frequency variations is pos-sible. obviously, the drawback is the
full-rated converter, which could be very expensive and not practical for the high power ratings. converter losses could also be an issue. While the efficiency of the synchronous
machine is higher than that of the similar induction machine,
mainly due to lower rotor losses, higher converter losses make the cfsM less efficient than the DfIM system [54].
These problems limit the application of cfs Ms to hy –
dropower plants with power ratings below approximately 100 MW. This limit is expected to be pushed to higher
power ratings in the future with advances in semiconductor
devices (e.g., wide-bandgap semiconductors, such as silicon carbide) and converter topologies. It has recently become
possible to build a frequency converter with a rated power
in excess of 100 MV a [44], [49]. a s claimed in [49], using
modular multilevel converter technology with IG cT devic –
es, it is possible to design converters for very high power ratings (up to 500 MV a). In the future, progress in power
electronics may provide the opportunity for the cfs M to
become the preferred configuration, even in high power ratings, because of its superior performance over the D fIM.
The concept of variable-speed operation in hydropower
systems using the cfsM was introduced for hVdc applica-tion using current–source converters [9], [30], [46]. Because of the remote location of some hydropower plants, an hVdc
transmission solution might be preferred. an hVdc link
makes the hydropower generator independent of the ac grid,
and the requirement for having constant speed and frequen-
cy is lifted. This can result in the relaxation of some restric-
tions imposed by the stability requirement, such as minimum inertia [30]. It should be noted, though, that the main moti-
vation for the hVdc connection of hydropower facilities is
transmission benefits; variable-speed operation is a further advantage, leading to improved efficiency and extended
operating range. a detailed report regarding variable-speed
operation of hVdc-connected plants is presented in [9]. More
recently, hVdc-connected hydropower plants utilizing the
cfsM configuration with a Vsc are discussed in [55].The concept of
variable-speed operation in hydropower systems using the CFSM was introduced for HVdc application using current–source converters.

september/october 2018 œ IEEE Industry Applications Magazine25
In small hydropower facilities, the cfsM configura-
tion is normally preferred over the D fIM because of the
relatively low power ratings. In the proposed concept of
decentralized pumped-storage plants [14] with power rat –
ings up to 50 MW, cfs Ms are employed.
a pioneering project to use the cfs M system in pumped-
storage facilities is reported in [56], where a 60-MW cur –
rent source converter is provided between the existing
generator and the grid to enable variable-speed opera-
tion in a hydropower plant with large head variation. recently, a 100-MW cfs M system equipped with a V sc
started operation at the Grimsel 2 pumped-storage plant in switzerland [44], [57].
compared to the D fIM, the cfs M offers superior
performance and significant advantages. The startup is easier and faster and can be performed in water, thanks to the possibility of producing substantial torque at zero
speed [49], [57]. In addition, speed and power variations
can be larger [38], [49]. The cfs M system does not have
the limitations on maximum speed that the D fIM has,
and it can be used for sites with high heads and large head variations [49]. c ompared with the D fIM, a variable-
speed system with a cfsM offers good lVrT capability
and better compliance with grid codes [38], [49], [51], [58]. The converter could be used (while not connected to the machine) as a reactive current static compensator, sup –
plying considerable reactive power to the grid [49]. In addition, a cfs M configuration is more suitable for use in
upgrading existing plants. With respect to the design of
the synchronous machine, lifting the rotational speed and
frequency requirements could initiate a new generator design strategy to achieve optimized operation [59].
In the cfs M configuration (and in contrast to the
DfIM topology), a switch can be provided to bypass the
converter, if needed [12], [44], [49]. The most relevant case
is when the hydraulic efficiency gain due to variable-
speed operation does not compensate for the converter losses. for this reason, the converter is bypassed in the
production mode at Grimsel 2, as reported in [44].
Current Status and Future Trends
according to [1], out of 270 pumped-storage plants in the
world (either operating or under construction), 36 are
variable-speed systems, 17 of which are currently in opera-
tion. In e urope, 38% of the total pumped-storage capac –
ity planned to be installed by 2020 is for variable-speed
hydropower facilities [60]. Most of the operating variable-
speed pumped-storage plants are in Japan, with e urope
coming in second. currently, there is no variable-speed
pumped-storage plant operating in the United s tates, but,
as reported in [3], many of the approximately 50 proposed projects (in various stages of planning and licensing) are
considering variable-speed technology.
since the early 1990s, when the development of vari –
able-speed pumped-storage facilities started in Japan, the DfIM configuration has been the preferred technology for high power ratings. This is also the case for most of the plants under construction. The main reason, as mentioned
previously, is the reduced size of the converter compared to cfsM technology. a list of the existing and planned large
pumped-storage facilities with D fIMs can be found in
[3]. figure 6 shows the rotor of a 250-MW D fIM in the
linthal, s witzerland, pumped-storage plant (commissioned
in December 2015).
advances in D fIM systems have been related mainly
to the frequency converter. The development of variable-
speed pumped-storage hydropower facilities began with
thyristor-based cycloconverters. Today, the preferred topology is the V sc. With the use of modern semiconduc –
tor devices—e.g., IG cTs, insulated-gate bipolar transis –
tors, and injection-enhanced gate transistors—significant improvements have been achieved in the design and
operation of power electronic converters. regarding
electrical machines, D fIMs can now be used in sites with
higher rotational speed requirements. a s reported in [23],
the D fIM with the world’s highest rated speed of 576–
624 r/min was installed in a 340-MW pumped-storage
plant in Japan in 2007. The pump head is more than 700 m,
while the rotor weight exceeds 400 t.
In contrast to the established and mature D fIM tech –
nology, the cfs M configuration is still in its early devel –
opment stage for use in hydropower plants with high
power ratings. The experiences from the operation of a 100-MW cfsM system in Grimsel 2 have been promis –
ing [44]. figure 7 shows the full-rated converter of the
installed cfsM system in this power plant. e ven though
FIGURE 6. A motor–generator rotor of a 250-MW DFIM in the
Linthal pumped-storage plant, Switzerland. (Used with permission
from [61].)

IEEE Industry Applications Magazine œ september/october 201826
it is not yet economically beneficial to use full-rated con –
verters for high power ratings, the situation is expected to
change over the coming years. a dvances in power electron –
ics (including the development of silicon carbide devices)
make it possible to design converters with higher power
ratings [62]. In [49], it is claimed that extremely high power
ratings (up to 500 MV a) are achievable using modular
multilevel converters and IG cTs. for future variable-speed
hydropower systems, the cfs M configuration is expected
to be the preferred technology even for high power ratings.
Conclusions
This article discussed the challenges and opportunities for variable-speed operation of hydropower facilities and
addressed relevant applications of variable-speed tech –
nology in hydropower systems, with the most important
being the pumped-storage plant. The significant benefits
offered by variable-speed operation to both power sys –
tems and hydropower facilities were listed.
The D fIM and cfs M configurations to enable variable-
speed operation were compared. The cfsM offers supe –
rior performance compared to the D fIM, but the need
for a full-rated converter is a main drawback for this configuration. Because of advances in the field of power
electronics, the cfs M configuration is expected to be the
preferred technology in the future, even for plants with
high power ratings.
Acknowledgment
This work was supported by the n orwegian h ydro-
power centre ( nVKs) and norwegian research centre for
hydropower Technology ( hydro cen).
Author Information
Mostafa Valavi (mostafa.valavi@ntnu.no) and Arne Nys-
veen are with the norwegian University of science and Technology, Trondheim, norway. Valavi is a Member of the Ieee. nysveen is a senior Member of the Ieee. This article
first appeared as “Variable-speed operation of hydropower
Plants: Past, Present, and future” at the 2016 22nd Interna-tional conference on electrical Machines. This article was
reviewed by the Ias electric Machines committee.
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