Download II. Flywheel Energy Storage System

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Flywheel Energy Storage System Tests under
Induced Faults
Rubens de Andrade, Jr., Guilherme G. Sotelo, Antonio C. Ferreira, Luis G. B. Rolim, Walter I.
Suemitsu, Richard M. Stephan, José L. da Silva Neto, and Roberto Nicolsky

Abstract—This paper presents test results of a flywheel energy
storage system (FESS) prototype. The bearing system set is
composed of a superconducting magnetic thrust bearing (SMB)
and a permanent magnet bearing (PMB). The SMB was built
with Nd-Fe-B magnet and YBCO superconducting blocks. The
PMB has the function of positioning radially the switched
reluctance machine (SRM) used as motor/generator and reduce
the load over SMB. The SRM drive is responsible to convert
electrical into mechanical energy, and vice versa. The prototype
still operates at low speeds, but the power electronics and SRM
drive showed that the system can work at high speed, supplying
the required energy during disturbances. The performed tests
with the FESS prototype show the supply energy to the grid when
a disturbance occurs.
Index Terms— Flywheels, Superconducting magnetic bearings,
High-temperature superconductors.
I. INTRODUCTION
pinning inside of superconductor, a drawback is the need of
cryogenic refrigeration, but there are recent developments of
innovative design for the cryogenic insulation that can
minimize the refrigeration costs [2].
A flywheel coupled to an electrical drive consists of a
flywheel energy storage system (FESS), which can convert
electrical to kinetic energy and vice versa. In a previous work
[3] it was shown the development of a FESS with
superconducting magnetic bearings designed to compensate
voltage sags. The FESS bearing system was designed to be
Evershed type, with a SMB as the thrust bearing and a PMB
for radial positioning and to reduce load over the SMB. The
simulation of the power electronics that has been designed and
mounted showed that the FESS is able to compensate voltage
sags.
This paper describes the FESS tests. In these tests the FESS
was able to supply energy to the grid and after recharge
drawing energy back. It is also show the measurements of
levitation force and radial restoring force of the PMB.
A
FLYWHEEL stores kinetic energy; the amount of stored
energy is proportional to the inertia moment of the
flywheel and the square of its angular velocity. Therefore,
increasing the flywheel angular velocity may increase the
energy stored per volume in the flywheel, but it also increases
the idling losses [1]. The idling losses come mainly from the
air drag and bearing losses. The air drag losses can be reduced
putting the flywheel in a vacuum enclosure and bearing losses
using magnetic bearings. There are several types of magnetic
bearings that can be used to minimize the bearing losses:
permanent magnetic bearings (PMB), active magnetic
bearings (AMB) and superconducting magnetic bearing
(SMB). PMB are less expensive, but they are not able to
provide a stable suspension in all dimensions and can only be
used as an auxiliary bearing. AMB are the most used, but
require complex active control that is sensitive to
electromagnetic disturbances. SMB are self-stable due the flux
Manuscript received August 25, 2006. This work was supported in part by
the CNPq under Grant 479557/04-7 and FAPERJ.
R. de Andrade, Jr. is with the DEE/Poli/UFRJ, Federal University of Rio
de Janeiro, Rio de Janeiro, RJ 21945-970 Brazil (phone: 55-21-2562-8031;
fax: 55-21-2562-8088; e-mail: [email protected] ).
G. G. Sotelo and A. C. Ferreira are with PEE/COPPE/UFRJ, Federal
University of Rio de Janeiro, RJ 21945-972 Brazil (e-mail: [email protected],
[email protected]).
J. L. Silva Neto, L. G. B. Rolim, W. I. Suemitsu, R. M. Stephan, and R.
Nicolsky are with the DEE/Poli/UFRJ, Federal University of Rio de Janeiro,
RJ 21945-970 Brazil (e-mail: [email protected], [email protected],
[email protected], [email protected], [email protected] ).
II. FLYWHEEL ENERGY STORAGE SYSTEM
A. Prototype
Fig. 1 shows a photograph of FESS prototype that is in
development. It is composed of an Evershed type bearing in
order to minimize the bearing losses, a switched reluctance
machine (SRM) as the motor/generator and a flywheel to store
kinetic energy. The SRM is driven by a power electronics
converter, which is not shown in the picture. This converter
will be responsible for interfacing the FESS to the power grid.
The system will be placed in a vacuum chamber, with pressure
of about 1 bar, to reduce the aerodynamic drag.
B. Superconducting Magnetic Bearings
The superconducting magnetic bearing used in these tests
consists of rotor of Nd-Fe-B magnets mounted in the flux
shaper configuration [3] attached to SRM axis and a stator
with YBa2Cu3O7- (YBCO) superconducting blocks. The
stator consists of nine YBCO seeded melt textured blocks, 28
mm diameter and 10 mm high, attached on the top plate of the
chiller. The superconducting blocks are maintained in vacuum
and refrigerated by the contact with the top plate of chiller.
The chiller is sealed in order to allow the liquid nitrogen flow
inside it.
The superconductors are Field Cooled (FC), which means
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Fig. 3. Measurement of the vertical attraction force as a function position
made for the permanent magnetic bearing showed in Fig. 2.
Fig. 1. The picture shows the flywheel energy storage system with the
vacuum enclosure open.
that they are cooled with de permanent magnet rotor at
specified distance from the superconductors. This procedure
reduces the levitation force, but increases the axial and radial
stiffness of the bearing [4].
C. Permanent Magnetic Bearings
The PMB plays two roles in the FESS: radial positioning
and reduction of the load over the SMB. This PMB will act in
attraction in concert with the SMB. PMB by itself cannot
provide stability for a bearing system, as predicted by
Earnshaw’s theorem. The PMB tested, Fig.2, was designed
from finite element simulation [5]. The maximum levitation
force of this bearing, Fig. 3, is to high, 590 N at 1 mm of air
gap. The radial restoring force, Fig. 4, is linear and reversible
until 6.2 mm, for a larger displacement the PMB turns
instable. The maximum restoring force reaches 320 N at 6.2
mm.
III. SRM DRIVE
order to use the most of the stored kinetic energy in the
flywheel, the electrical machine has to be electronically
controlled. In this work a switched reluctance machine (SRM)
is used. The SRM can work at very wide speed ranges: from
zero up to several ten thousand rpm; it is fault tolerant and has
null idle losses. Its robustness leads to achieve a high
reliability.
The power electronics circuit consists of two converters, as
shown in Fig. 5. To drive the SR machine a half-bridge IGBTbased converter is used, allowing operation as motor or
generator. The dc link is connected to the network by a bridge
PWM converter, which is controlled according to Akagi’s pq
theory [6]. The objective of the control operation is to
determine the direction of the power flow. This is achieved by
regulating the dc link voltage. The flywheel shaft speed must
be controlled according to the instantaneous active power
demanded by the grid. In this work, the implementation of a
two-stage control strategy for the flywheel shaft speed is
proposed. Both stages are coupled through a common state
One significant aspect of a flywheel based energy storage
device is concerned to the electromechanical energy
conversion between the flywheel and the electrical system. In
Fig. 2. Permanent magnetic bearing.
Fig. 4.
Measurement of the radial restoring force as a function
displacement made for the permanent magnetic bearing of Fig. 2.
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Fig. 5. Flywheel energy storage system connected to the grid.
(a)
variable: the voltage across the dc link capacitor. Two
strategies can be employed in order to achieve the control of
the dc link voltage.
A. Strategy I
The main idea of this strategy is to control the acceleration
of the SRM in proportion to the mismatch between the dc link
capacitor voltage and a given reference value. If no power
flows between the flywheel and the grid, then the dc link
capacitor voltage remains regulated at its nominal value.
However, if the grid demands active power, the command will
act directly upon the network side converter, adjusting its
current. It causes variations of the dc link capacitor voltage,
which is compensated by the dc link voltage PI regulator,
which ultimately defines the operation of the machine as
motor or generator, and actuates on the SR machine driver.
There is however another speed PI regulator, with the main
purpose of adding a small offset to the grid converter average
real power (  p ), in order to bring the flywheel back to the
rated maximum speed after any transients. Its output signal
should be limited to values that do not cause excessive power
consumption from the grid.
(b)
Fig. 7. Flywheel injecting energy in the grid. Blue: vFA (50V/div). Green:
iFA (0.5A/div).
B. Strategy II
In this technique, on the other hand, the dc link voltage is
controlled by the network bridge PWM converter, which may
operate as an active rectifier taking energy from the grid, or as
an inverter delivering energy to the grid. The operation of the
two converters is coordinated. If an amount of energy must be
delivered to the grid (minus losses), the same amount of
kinetic energy stored on the flywheel is used to recharge the
capacitor. A similar action occurs to restore the nominal speed
of the SR machine
IV. TESTS
Fig. 6. Flywheel taking energy from the grid. Blue: vFA (50V/div). Green:
iFA (0.5A/div). Orange: vDC (50V/div).
In order to validate the flywheel energy stored system, tests
were carried out in the prototype presented in Fig. 1. In these
tests the current (iFA ) and the voltage ( vFA ) of the grid
converter, and the DC link voltage (vDC ) were measured. The
main components of the test rig are: 1.5 kW 6/4 SRM, bidirectional converter with a dc link capacitor and a
Superconductor Magnetic Thrust Bearing, which also works
as a flywheel increasing the system inertia.
Control Strategy II was adopted since it was the simplest to
implement . Nonetheless, the operation of the converters must
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V. CONCLUSION
A FESS prototype was connected to the grid as a shunt
compensator. This prototype uses a Evershed type magnetic
bearing, that is combination of SMB and a PMB. Two
possible control strategies for the FESS operation were
presented and discussed. Laboratory tests were carried out
using one of these strategies. The applied control strategy has
shown the ability to correctly vary the speed of the flywheel in
order to supply/absorb power to/from the grid. Due to the
mechanical design, this prototype is still limited to low
operating speeds.
Fig. 8. Angular velocity of flywheel as a function of time, when supplying
energy to grid, until 870 ms, and after drawing energy from grid to recharge.
be carefully coordinated to ensure that the dc link voltage
stays within an acceptable range. The FESS is connected to
the laboratory 60 Hz mains via an inductive reactance. Fig. 6
presents the case where energy is absorbed from the grid in
order to accelerate the SR machine – iFA opposed to vFA. Fig. 7
on the other hand, shows the case where a hypothetical fault
occurred on the line side, demanding energy to be injected into
the grid – iFA in phase with vFA. Fig. 7a shows the FESS
response to the fault, i.e. the flywheel is initially idling and
starts to inject power into the grid, while Fig. 7.b shows the
steady state operation during the fault. The variation on the
flywheel speed is presented in Fig. 8. After the fault, t=0s, the
FESS starts to decrease its speed in order to inject power into
the grid. When the fault was cleared the control system starts
to increase the motor speed in order to recharge the FESS. It
should be noted that the slope of the acceleration process in
Fig. 8 is merely illustrative of the control system operation. As
during the acceleration power is absorbed from the grid, in a
real application care should be taken in order to avoid
disturbances in the system.
ACKNOWLEDGMENT
The authors would like to thank Nilo F. B. de Mello for the
experimental support.
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