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ICSET 2008
Wind Power Integration in Isolated Grids enabled by Variable
Speed Pumped Storage Hydropower Plant
Jon Are Suul, Kjetil Uhlen, Member IEEE, Tore Undeland, Fellow IEEE
Abstract— This paper presents aspects of control and
operation of a variable speed pumped storage power plant for
integration of wind power in an isolated grid. A topology based
on a synchronous machine and a full scale back-to-back voltage
source converter is suggested for variable speed operation of
the pump-turbine. With this topology, variable speed operation
can be obtained in both pumping mode and generating mode,
and reactive current can be controlled independently of the
active power flow and the operation of the pump turbine. By
utilizing the controllability of the variable speed system, power
fluctuations from wind turbines can be compensated to limit
the influence on the rest of the grid. At the same time the
pumped storage can be controlled to take part in the frequency
control of the system and also to control grid voltage or flow of
reactive power. The operation of the proposed system is
illustrated by simulations based on the situation on the Faroe
Islands, where controllable energy storage could allow for
higher penetration of renewable energy in the power system
and by that reduced dependency on power generation based on
fossil fuels.
Keywords— Pumped storage, wind power, isolated grid,
power balancing, reactive power, energy storage, synchronous
machine, voltage source converter.
I.
INTRODUCTION
Integrating large shares of fluctuating renewable energy
sources into electric power systems requires sufficient range
of power control in production units and loads. In many
cases, additional energy storage will be needed to ensure the
power balance and maintain stable operation of the grid [1].
Islands and isolated power systems will be the first locations
where it will be relevant and possible to achieve sustainable
energy systems based entirely on environmental-friendly
and renewable energy sources, and can therefore serve as
good examples and test-cases. [2]. With increasing share of
fluctuating renewable energy sources in an isolated power
system, the use of pumped storage hydropower plants as
energy storage can be among the most relevant alternatives
with respect to feasibility and cost when suitable locations
are available [3]. A few examples of such systems have been
put into operation, and others are under development [4]-[9].
J. A. Suul is with the Department of Electric Power Engineering,
Norwegian University of Science and Technology, 7491 Trondheim,
Norway (Phone: + 47 73 55 03 85; fax: + 47 73 59 42 79; e-mail:
[email protected]).
K. Uhlen is with SINTEF Energy Research, 7465 Trondheim, Norway
(e-mail: [email protected] ).
T. Undeland is with the Department of Electric Power Engineering,
Norwegian University of Science and Technology, 7491 Trondheim,
Norway (e-mail: [email protected]).
Operation of conventional pumped storage units with
constant power in pumping mode will mainly help to
improve the energy balance of an isolated power system.
One way of introducing additional controllability so that the
pumped storage can contribute to the steady state frequency
control of the system will be to use several separate pumps,
so that the load in pumping mode can be controlled in steps.
The most flexible and effective solution will be variable
speed operation, such that the input power in pumping mode
can be controlled continuously within an allowable range of
operation [3]. If a pumped storage hydropower plant with
reversible pump-turbine is designed for variable speed
operation, the controllability of the system can be utilized to
balance power fluctuations from renewable energy sources
both in pumping and generating mode [10].
The development of variable speed pumped storage
power plants have until now been mainly focused on units
for energy storage, load balancing and stability enhancement
in large power systems. For such applications, the doublyfed asynchronous machine has been preferred, to limit the
needed rating on the power electronic converter required for
obtaining variable speed operation [11], [12]. Variable speed
operation can be even more important in an isolated grid
than in a larger power system, since introduction of
fluctuating power production in a small system can represent
more challenges to the grid operation. For lower power
ratings and applications in isolated grids, full-scale converter
topologies can be relevant for control of pumped storage
units. The highest flexibility in control of active and reactive
power can be obtained with a full-scale voltage source
converter (VSC), but few applications of this topology for
pumped storage units have been reported. The main
objectives of this paper are therefore:
1. To present a configuration for variable speed operation
of a pumped storage hydropower plant based on a
synchronous machine with a full-scale back-to-back
voltage source converter, and to discuss aspects of
control and operation of this topology.
2. To investigate control strategies for the pumped
storage unit that can enable higher wind power
penetration in an isolated grid by compensating for
active and reactive power fluctuations, and to illustrate
the operation by simulations based on the power
system of the Faroe Islands.
399
c 2008 IEEE
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II. PROPOSED CONFIGURATION FOR VARIABLE SPEED
PUMPED STORAGE POWER PLANT
Some of the early investigations into variable speed
operation of reversible pump-turbines were based on
synchronous machines with thyristor-controlled current
source converters feeding the stator of the machine [13].
Only few plants with this topology have been constructed,
and the majority of existing variable speed pumped storage
units is based on doubly-fed asynchronous machines [12].
For applications in small isolated power systems, the ratings
can be much lower compared to the large units with doublyfed machines. With the continuous development of forcecommutated semiconductors and high-power VSC drives,
configurations with a back to back VSC for controlling a
synchronous machine can therefore be relevant for variable
speed operation of pumped storage units in isolated grids
[10], [14], [15]. A schematic layout of this proposed
configuration, including an overview of the suggested
control system is shown in Fig. 1. The machine is
considered to be a salient pole synchronous machine with
static excitation system that can be operated both by the
converter and with direct connection to the grid like a
traditional power station, as indicated by the bypass switch
in the figure.
A. Operational characteristics of proposed configuration
The configuration presented in Fig. 1 provides full
flexibility in control of active and reactive power on the grid
side, and the system can operate with variable speed both in
pumping mode and in generating mode. The main
motivation for introducing such a configuration will be to
allow for controllable power in pumping mode, but the
controllability and the speed of the response when delivering
power to the grid can also be improved by variable speed
operation. By controlling the reactive current component
from the converter, also the grid voltage or the reactive
power flow in the grid can be controlled by the presented
configuration. This ability can be utilized even if the pumpturbine is not in operation, and the grid side converter will
LC-filter
Grid
C1
then be operating as a STATCOM.
The configuration in Fig. 1 is also introducing extra
redundancy to the basic operation of the system compared to
a doubly fed asynchronous machine, since the power plant
can be operated directly connected to the grid in generating
mode. That also means that the power station can be used
for a traditional black start of the system while the converter
is out of operation. On the other hand, the presented
configuration can make it possible to operate the pumped
storage with variable speed even without any controlled
production units based on synchronous generators operating
in the system. In such a situation the converter for the
pumped storage will have to control both the voltage and the
grid frequency in stand-alone mode
B. Description of control system
The main structure of the control system for operating the
suggested configuration in pumping mode is included in Fig.
1. The figure shows how the grid side converter, connected
to the main transformer through a LC-filter, can be
controlled by a traditional voltage oriented vector current
control system in a synchronously rotating dq-reference
frame. The estimate of the voltage phase angle used for the
park transformation is obtained by a PLL that is also
tracking the grid frequency and the voltage components in
the rotating reference frame [16]. The d-axis of the rotating
reference frame is aligned with the grid voltage vector, and
the q-axis is leading the d-axis by 90°. The current
controllers can be PI-controllers in the rotating reference
frame with feed-forward from measured grid voltage and
decoupling terms depending on the filter inductance and the
grid frequency [17], [18]. To avoid oscillations in the LCfilter, an active damping routine can be added to the
function of the current controllers [19]. The output from the
current controllers is divided by the DC-link voltage to
decouple the current controllers from the dynamics of the
DC-link. After transformation into phase coordinates and
adding third harmonic injection, the reference voltages are
used for PWM modulation of the switches of the converter.
L1
Vg
3
2
SM
Ig
VDC
PWM
VSM
3
PWM
2
I SM
3
n T
if
2
vD , E ,ref
Tg
PLL
id ,q
vd ,q
fg
id ,ref
DC-link
controller
n
vg , dq
Current
V/Q
i
V , Qgrid
controllers q ,ref controller
Pwind
\ s , ref
pref
Power
pset
im ,ref
T
id , q , ref
if
\s
vd ,q ,ref
vd ,q
id ,q
v f ,ref
n i f id ,q
'i f , ref
[
iD , E
T
Current
controllers
\s
Power factor
control
im
control
\s
Flux control
im
im,ref
il ,ref
Torque
control
vd ,q , ref
fg
Grid side
measurements
pref
vD , E
vD , E ,ref
\ d ,q
[ Current estimation
im ,l
&
\ s Flux calculation
f ref
if
n
Fig. 1. Suggested configuration for variable speed operation of pumped storage with control system for operation in pumping mode
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Since the d-axis is aligned with the voltage vector, the input
reference to the d-axis current controller is generated by an
outer loop DC-link voltage controller that is maintaining the
power balance of the system. The q-axis current reference
can be generated by an outer loop controller for grid voltage
or flow of reactive power.
The grid frequency from the PLL is also used for the
power control of the pump turbine. Different structures for
controlling the power flow of the pumped storage system in
pumping mode, and for generating the power reference to
the drive system of the synchronous machine are discussed
in [15]. The details of the drive system of the synchronous
machine are not of main importance to the characteristics of
the pumped storage system as seen from the grid if the
response is fast and precise. In this paper a similar vector
control structure as for the grid side converter is used, but
since the machine has salient poles, a stator flux oriented mlreference frame is used for the control of torque and flux
while a rotor oriented dq-reference frame is used for the
current controllers [10], [20]. Basically the same control
structure can be used for controlling the synchronous
machine drive in generating mode, but an additional speed
controller and the hydraulic control system of the turbine
will have to be included in the model.
III. COMPENSATING FOR WIND POWER FLUCTUATIONS IN
AN ISOLATED GRID
The controllability of a variable speed pumped storage
unit can be utilized to compensate for the influence of power
fluctuations in the output from renewable energy sources
like wind power, so that a higher share of fluctuating power
production can be allowed in the system. If the pumped
storage unit is located close to a dominating source of power
fluctuations like a wind farm, the variations in output power
can be compensated directly based on measurements of the
power flow. The pumped storage can be further utilized to
take part in the primary frequency control of the system, and
by that helping to improve the response of the system to
other disturbances and sudden changes in production and
load [10], [15]. With the presented topology the pumped
storage system can also be used for controlling the grid
voltage or the flow of reactive power in the system.
Diesel generators
6.6 kV
SM
Z1
66 kV
60 MVA
66 kV
Cd ,1
Cd ,1
2
2
5 MVA
60 MVA
2
22 kV
Z 2 3 km cable
Cd ,2
Load
10 MW
5 MVA
Load
Cd ,2
Wind farm
690 V
IM
3.3 kV SM
Hydro
22 kV
25 MVA
2
PQgrid
1 km cable
Zl 3
22 kV
CIM
Cd ,3
Cd ,3
2
2
PCC
PQwind
22 kV
3.3 kV
SM
Pumped storage
Fig. 2. Simplified grid model representing the main part of the power
system on the Faroe Islands
be considered relevant for practical implementation [10],
[14]. A few wind turbine units have been introduced to the
system, but if the dependency on fossil fuels for the
electricity generation is going to be reduced by utilizing the
available wind resources, there will be a need for energy
storage and more controllability to stabilize the system [10],
[21]. The most challenging situations for this system will be
operation at minimum load when there is a high average
power production from wind turbines, and this situation will
be the focus for this investigation
The operation of the proposed variable speed pumped
storage system is illustrated by simulations based on a model
in PSCAD/EMTDC of the main parts of the power system
on the Faroe Islands, including the proposed converter
topology and the corresponding control system of the
pumped storage unit. The basic configuration of the isolated
grid, including the voltage levels and the ratings of
difference units is shown in Fig. 2 while some more details
are given in Table 1. The system is simulated for 80 seconds
and the wind speed input used for simulation of the wind
farm is based on use of the Kaimal power spectra developed
for PSCAD simulation from [22], [23]. The power output
from the wind turbine model is not changing much with the
operation of the pumped storage, and can be considered
equal to the power series given in Fig. 3 for all investigated
situations. After 40 seconds of simulation, the hydropower
plant is tripped without reconnecting to the system.
A. Case study
The power system on the Faroe Islands is taken as an
example for a case study of an isolated system that can
significantly benefit from a combination of wind power
production and a variable speed pumped storage power plant
[10], [15], [21]. The number of inhabitants on the Faroe
Islands is around 48000, and the electric power system is
currently dominated by diesel generator units supplying
around 60% of the annual electricity consumption. The
minimum load of the system is in the range of 14 MW,
while the maximum load can reach 70 MW, and a pumped
storage power plant in the power range around 10 MW can
TABLE 1 SYSTEM PARAMETERS
Wind farm
Hydropower plant
Diesel generators
Pumped storage
- 10 MW aggregated model
- Induction generators directly
connected to the grid
- Constant capacitors for reactive
power compensation
- Power set-point 0,8 pu = 4MW
- Static droop; 25 pu = 2.5 MW/Hz
- 18 MW aggregated model
- Power set-point 0.7 pu = 12,6 MW
- Static droop; 25 pu = 9 MW/Hz
- Power control range 4-12 MW
- Static droop; 25 pu = 5 MW/Hz
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10
Constant power
Load following
Droop with derivative term and load following
1.05
8
7
1
6
Pump turbine speed [pu]
Wind power output [MW]
9
5
4
3
0
10
20
30
40
Time [s]
50
60
70
80
Fig. 3. Power output from the wind farm
0.95
0.9
0.85
B. Power control
The possible influence on the rest of the power system by
controlling the power input to the variable speed pumped
storage power plant is illustrated by using three different
control strategies based on [15]:
1. Constant power input to the pumped storage system as
a reference case
2. Load following, where measured power fluctuations
from the wind farm are directly compensated by the
pumped storage power plant
3. Droop control based on grid frequency with a
derivative term for higher transient frequency response
and to damp oscillations in the system, combined with
load following.
The response of the diesel generators in the system to the
wind power series from Fig. 3 is shown in Fig. 4. It can be
seen that with constant power to the pumped storage, the
diesel generators have to cover most of the power
fluctuations from the wind farm, resulting in a wide power
operating range. The diesel generators will in this case also
have to cover all the loss of production when the small
hydropower plant in the system is tripped. Controlling the
pumped storage power plant to balance the power
fluctuations from the wind turbine, it can be seen that the
diesel generators are relieved from covering most of the
power fluctuation, but that they still have to cover all the
loss of production when the hydropower plant is tripped.
0.8
20
30
40
Time [s]
50
60
70
80
Fig. 5. Speed of pump-turbine with different power control strategies
Adding a frequency droop to the power control of the
pumped storage system, the diesel generators are relieved
also from some of the steady state frequency control, and the
derivative term added to the frequency control is damping
the remaining power oscillations in the system.
The response in speed of the pump-turbine of the pumped
storage is shown in Fig. 5. This figure shows how the short
term power fluctuations from the wind farm are filtered by
the large inertia of the generator and the pump-turbine, so
that mainly the slower power variations are reflected in the
speed of the system. The grid frequency shown in Fig. 6
show how the rest of the system is relieved from the
influence of the power output from the wind turbines when
the fluctuations are compensated by the pumped storage
system. It can also be seen how the frequency response of
the power system is improved when the pumped storage is
used for frequency control. The results in Fig. 5 and Fig. 6
indicate how the control of the pumped storage can limit the
necessary operating range of the diesel generators, and by
that also limit the fluctuations in grid frequency. This can
allow for having less diesel generator capacity on line, and
since the remaining units in operation can be operated at a
Constant power
Load following
Droop with derivative term and load following
17
50.3
16
50.2
Grid Frequency [Hz]
15
14
13
12
11
50.1
50
49.9
49.8
49.7
10
49.6
9
49.5
8
10
Constant power
Load following
Droop with derivative term and load following
18
Diesel power [MW]
0
0
10
20
30
40
Time [s]
50
60
70
49.4
80
Fig. 4. Power output from diesel generators with different power control
strategies for the pumped storage power plant
0
10
20
30
40
Time [s]
50
60
70
Fig. 6. Grid frequency with different power control strategies for the
pumped storage power plant
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80
higher average load, the efficiency of the diesel generators
can be increased, and this will lead to further reduction in
the fuel consumption of the electricity supply.
1.03
1.02
Grid voltage [pu]
1.01
1
0.99
0.98
0.97
0.96
0.95
0
10
20
30
40
Time [s]
50
60
70
80
Fig. 7. Grid voltage at PCC with different strategies for control of voltage
or reactive power
voltage or reactive power controller. In Fig. 7 and Fig. 8,
one case is shown where reactive current from the converter
is filtered and used to generate the droop. It is seen from the
figures how this makes it possible to mitigate the short term
fluctuations in voltage and reactive power flow, while the
system has a droop characteristic for the longer term voltage
variations. The voltage source converter is providing full
flexibility with respect to control of voltage or reactive
power, and the functionality and response of the system can
easily be designed according to the most relevant control
objectives for a specific implementation.
IV. CONCLUSION
A topology for variable speed operation of a pumped
storage hydropower plant based on a full-scale back-to-back
voltage source converter driving a synchronous machine is
suggested for balancing of wind power fluctuations in an
isolated grid. With the proposed topology, the pumped
storage unit can also be operated with the machine directly
connected to the grid, running at constant speed like a
conventional power plant, such that the operation of the
system will be less dependent on the converter reliability.
Zero reactive current
Control of grid voltage
Control of reactive power to the grid
Grid voltage control with slow droop
3
Reactive power to the grid [MVAr]
C. Voltage and reactive power control
With the back-to-back voltage source converter, the
reactive current on the grid side can be controlled
independently of the active power flow and used for
controlling reactive power flow in the grid or for taking part
in the voltage control. In contrast to frequency, that can be
considered a global variable in steady state, the grid voltage
is a local variable, and the design of the control loops for
voltage or reactive power will therefore be dependent on the
configuration of the local grid. The control objective can
also be different depending on what kind of grid the system
is located in and what kind of challenges that are more
critical. The presented topology has the flexibility to easily
implement different control structures for voltage or reactive
power operating within the limit of total converter current.
In the investigated model, the grid is mainly consisting of
high voltage cables, and is therefore quite strong with
respect to voltage. Still voltage flicker can be a problem with
wind power integration, and mitigation of voltage
fluctuations can therefore be important.
The influence of different voltage control strategies on the
grid voltage, and the flow of reactive power, is illustrated
with the simulations shown in Fig. 7 and Fig. 8. The
simulations are carried out with the 3rd power control
strategy from section III B. and the results with four
different control strategies are shown in the figures:
1. Zero reactive current in the converter
2. Grid voltage controlled to 1.0 pu by PI-controller.
3. Reactive power flow to the grid from the point of
common coupling (PCC) in Fig 2 is controlled to 0 by
PI-controller.
4. Grid voltage control with PI-controller and droop from
filtered reactive current on the voltage reference.
With reactive current from the converter controlled to
zero, it can be seen that the voltage is fluctuating with the
power variations from the wind turbines, and that these
fluctuations in reactive power have to be provided by the
other generators in the grid. If the grid voltage is controlled
to 1.0 pu, the converter for the pumped storage is supplying
the reactive power consumed by the wind farm, and also
delivering reactive power to the grid to boost the voltage.
The presented figures also show how the reactive power
exchange with the grid can be controlled directly, and that
the converter in this way can easily compensate all the
reactive power fluctuations caused by the wind farm. The
voltage in the grid is also stabilized at a level close to the
rated value, depending on the characteristics and the
response of the rest of the system.
Since pure integral effect in outer loop controllers can
lead to unintended interaction between different equipment,
steady state droop characteristics should be allowed for the
Zero reactive current
Control of grid voltage
Control of reactive power to the grid
Grid voltage control with slow droop
2
1
0
-1
-2
-3
-4
0
10
20
30
40
Time [s]
50
60
70
Fig. 8. Reactive power flow from PCC to the grid
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80
The back-to-back converter provides full flexibility in
control of active and reactive power, and the system can be
operated with variable speed in both pumping and
generating mode, while grid voltage or reactive power can
be controlled independently from the operation of the pumpturbine.
The operation and control of the suggested topology is
illustrated by simulating a grid model based on the power
system of the Faroe Islands. The presented results show how
the variable speed pumped storage can be controlled to limit
the influence of wind power fluctuations and at the same
time contribute to increased frequency response of the
power system. The simulation results therefore indicate how
the variable speed pumped storage can allow for a more
wind power to be introduced to an isolated grid without
undermining the frequency control and the instantaneous
power balance of the system. It is also shown how the
suggested topology can be utilized to control the grid
voltage or the flow of reactive power in the system.
Controlling the grid voltage, or compensating for the
fluctuating reactive power consumption of a wind farm, can
allow for a better distribution of reactive power flow in the
system and by that reducing the power losses. Voltage
control by the grid side converter can also mitigate possible
power quality problems related to voltage flicker and
improve the voltage stability of the system. The
controllability introduced by the suggested configuration for
a pumped storage power plant can in this way be used both
to compensate for the consequences of fluctuating power
production from wind turbines and to improve the general
operation of an isolated power system. This can make it
possible to allow for higher wind power penetration and
significantly reduce the dependency on fossil fuels for
electricity production in isolated power systems.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
ACKNOWLEDGMENT
Voith Siemens Hydro Power Generation, Trondheim,
Norway, have provided background information regarding
the proposed configuration and parameter values for the
pumped storage unit in the presented simulations.
[18]
[19]
V. REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
R. M. Dell, D. A. J. Rand: “Energy storage – a key technology for
global energy sustainability, Journal of Power Sources,” Vol. 100, No.
1-2, Nov. 2001, pp. 2-17
T. L. Jensen: “Renewable energy on small islands,” 2nd edition, Forum
for Energy and Development, Denmark, August 2000
J. S. Anagnostopoulos, D. E. Papantonis: “Pumping station design for
a pumped-storage wind-hydro power plant,” Energy Conversion and
Management, Vol. 48, No 11, November 2007
J. Taylor: “The Foula Electricity Scheme,” Proceedings of IEE
Colloquium on Energy for Isolated Communities, May 1988
International Scientific Council for Island Development, INSULA,
information page about the El Hierro project for 100 % renewable
energy supply, http://www.insula-elhierro.com/english.htm, Accessed
July 2008
P. Theodoropoulos, A. Zervos, G. Betzios, “Hybrid Systems Using
Pump-Storage Implementation in Ikaria Island,” In Proc. International
[20]
[21]
[22]
[23]
Conference on Renewable Energies for Islands – Towards 100% RES
Supply, Chania, Greece, 14-16 June 2001
C. Bueno, J. A. Carta: “Wind powered pumped hydro storage systems,
a means of increasing the penetration of renewable energy in the
Canary Islands,” Renewable and Sustainable Energy Reviews, Vol.
10, No. 4, August 2006, pp. 312-340
D. Al. Katsaprakakis, Pr. Dimitris, G. Christakis, “A Wind Parks,
Pumped Storage and Diesel Engines Power System for the electric
power production in Astypalaia,” in Proc. of European Wind Energy
Conference, EWEC 2006, 27. Feb. – 2 March 2006, Athens, Greece
A. Ceralis, A. Zervos: “Analysis of the combined use of wind and
pumped storage systems in autonomous Greek islands,” IET
Renewable Power Generation, Vol. 1, No. 1, 2007, pp. 49-60
J. A. Suul: “Control of Variable Speed Pumped Storage Hydro Power
Plant for Increased Utilization of Wind Energy in an Isolated Grid,”
MSc. Thesis, Norwegian University of Science and Technology,
Department of Electrical Power Engineering, 2006
T. Kuwabara, A. Shibuya, H. Furuta, E. Kita, K. Mitsuhashi: “Design
and Dynamic Response Characteristics of 400 MW Adjustable Speed
Pumped Storage Unit for Ohkawachi Power Station,” IEEE
Transactions on Energy Conversion, Vol. 11, No. 2, June 1996, pp.
376 – 384
J. Lanese, A. Powers, H. Naeff, “Selection of Large Variable Speed
Pumps for the Domenigoni Valley Reservoir Project,” in Proc. of the
1995 International Conference on Hydropower, San Francisco, USA,
25-28 July, 1995, Vol. 2, pp. 1902-1912
R. J. Kerkman, T. A. Lipo, W. G. Newman, J. E. Thirkell, “An Inquiry
into Adjustable Speed Operation of a Pumped Hydro Plant, Part II –
System Analysis,” in IEEE Transactions on Power Apparatus and
Systems, Vol. PAS-99, No.5 Sept./Oct. 1980
Ø. Holm, Departmental Manager - Small Hydro Power, Voith Siemens
Hydro Power Generation, Trondheim, Norway: Personal
communication, spring 2006
J. A. Suul, K. Uhlen, T. Undeland, “Variable speed pumped
hydropower for integration of wind energy in isolated grids – case
description and control strategies,” Proceedings of Nordic Workshop
on Power and Industrial Electronics, NORPIE 2008, Espoo, Finland,
9-11 June 2008
A. Timbus, R. Teodorescu, F. Blaabjerg, M. Liserre, A. Dell’Aquila,
“Independent synchronization and control of three phase grid
converters,” in Proceedings of International Symposium on Power
Electronics, Electrical Drives Automation and Motion, SPEEDAM
2006, 23-26 May, 2006, pp. 1246-1251
V. Blasko, V. Kaura, “A New Mathematical Model and Control of a
Three Phase AC-DC Voltage Source Converter,” in IEEE
Transactions on Power Electronics, Vol. 12, No. 1, January 1997, pp.
116- 123
F. Blaabjerg, R. Teodorescu, M. Liserre, A. V. Timus, “Overview of
Control and Grid Synchronization for Distributed Power Generation
Systems,” IEEE Transaction on Industrial Electronics, Vol. 53, No. 5,
October 2006
O. Mo, M. Hernes, K. Ljøkelsøy, “Active damping of oscillations in
LC-filter for line connected, current controlled, PWM voltage source
converters, in Proceedings of the 10th European Conference on Power
Electronics and Applications, EPE 2003, 2-4, Sept. 2003 Toulouse,
France
M. Alaküla, “On the Control of Saturated Synchronous Machines,”
PhD-thesis, Lund Institute of Technology, Lund, Sweeden, 1993
P. A. Mikladal: “Sustainable energy in the Faroe Islands – the role of
hydropower,” Ministry of the Interior, http://www.lmr.fo/, Available
from http://www.docrenewableenergy.info/, Accessed February 2008
P. Sørensen, A. D. Hansen, P. André, C. Rosas, ”Wind models for
simulation of power fluctuations from wind farms,” Journal of Wind
Engineering and Industrial Aerodynamics, Vol. 90, No. 12-15, Dec.
2002, pp. 1381-1402
M. Þ. Pálsson, T. Toftevaag, K. Uhlen, I. Norheim, L. Warland, J. O.
G. Tande, “Wind farm modelling for network analysis – Simulation
and validation, ” in Proc. Of European Wind Energy Conference
EWEC 2004, London, UK, 22-25 Nov., 2004
404
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