IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005
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Voltage Sags Compensation Using a Superconducting
Flywheel Energy Storage System
Rubens de Andrade, Jr., Antonio C. Ferreira, Guilherme G. Sotelo, José L. Silva Neto, Luiz G. B. Rolim,
Walter I. Suemitsu, Márcio F. Bessa, Richard M. Stephan, and Roberto Nicolsky
Abstract—This paper presents a voltage sag compensator, which
uses a flywheel energy storage system with superconducting magnetic axial thrust bearing (SMB) and a permanent magnet radial
bearing (PMB). The SMB was built with Nd-Fe-B magnet and
YBCO superconducting blocks, refrigerated with liquid Nitrogen.
The magnets are assembled with magnetic flux shapers in order
to increase the levitation force and the stiffness. The radial PMB
is used to positioning the vertically arranged switched reluctance
machine (SRM) used as motor/generator. Simulations of the
power electronics and SRM show that the system can work up to
30,000 rpm supplying the required energy during disturbances.
Index Terms—Flywheels, high-temperature superconductors,
magnetic levitation, reluctance motor drives.
I. INTRODUCTION
I
N modern production plants, voltage sags can cause slow
output, production interruption or damage to machines. Energy customers with continuous process involving fragile materials, like paper forming, food processing, etc, are likely to
avoid prejudices with interruptions due the energy quality problems. Dynamic voltage restorers can compensate voltage sags
avoiding these prejudices, but some of these devices draw the
energy necessary to compensate the voltage sags from the same
grid that they will compensate. This strategy can cause a system
failure in some cases. In order to avoid this failure, these equipments may employ energy storage devices as electrochemical
batteries and flywheels [1].
As well as the performance degradation directly related to the
number of charges and discharges cycles; electrochemical batteries are expensive and not environmental safe [2]. Flywheel
energy storage systems (FESS) coupled to an electrical machine
can be used to store mechanical energy, which can be used in
a dynamic voltage restorer. FESS are environmental safe and
do not degrade the performance with charges and discharges
cycles. To surpass the electrochemical batteries, FESS need to
have better energy volume density, life-cycle cost and FESS
monitoring require less effort than for batteries [2]. The FESS
has the burst risk in case of bearing failure.
The energy stored in a flywheel is proportional to the moment
of inertia and the square of angular velocity. Then increasing
the flywheel angular velocity may increase the energy density
of FESS [3]. When the flywheel velocity is increased the idling
losses increase too, due to the aerodynamic drag and the losses
in the bearings. These losses can be reduced respectively with
partial vacuum in the flywheel container and magnetic bearings
[2], [4]. Active magnetic bearings (AMB) are expensive and demand an energy consumption that must be computed as a loss.
Superconducting magnetic bearings (SMB) are one alternative
to AMB, they are entirely passive and do not suffer from catastrophic failure as the AMB. The SMB will fail if its cooling
system fails and the superconductors warm up, but this warming
up will be a gradual process [4]. As the price of superconductors
is still high, the overall system cost may be reduced if the SMB
is used together with a permanent magnetic bearing (PMB) in
an Evershed arrangement. In this case the PMB does the most
to bear the flywheel and motor, and the SMB gives stability to
the whole set.
Another significant aspect of the energy storage device is concerned with electromechanical energy conversion between the
flywheel and the electrical system. In order to use most of the
energy stored as kinetic energy in the flywheel, the electrical
machine has to have an electronic control. It is usual to employ
a permanent magnet synchronous machine in flywheel applications [2], [4], but the permanent magnet machines always have
idling losses because of eddy currents induced by the magnetic
field on the stator. This can be avoided using a switched reluctance machine (SRM), which has a null idle magnetic field [5],
[6]. The SRM can work at very wide speed ranges: from zero
up to several ten thousand rpm; it is fault tolerant and has extremely low idling losses. Its robustness leads to reliability.
This paper describes a FESS with an Evershed type bearing
and a switched reluctance machine. Preliminary results of low
speed tests carried out in a prototype will be presented and the
characteristics of the bearings will be described.
II. FLYWHEEL ENERGY STORAGE SYSTEM
Manuscript received October 4, 2004. This work was supported in part by the
CNPq under Grant 472020/01-3 and Light.
R. de Andrade, Jr., 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, Rio de Janeiro 21945-970, Brazil
(e-mail: randrade@dee.ufrj.br; silvaneto@dee.ufrj.br; rolim@dee.ufrj.br;
walter@dee.ufrj.br; richard@coe.ufrj.br; nicolsky@if.ufrj.br).
A. C. Ferreira, G. G. Sotelo, and M. F. Bessa are with the PEE/COPPE/UFRJ,
Federal University of Rio de Janeiro, Rio de Janeiro 21945-972, Brazil (e-mail:
ferreira@ufrj.br; sotelo@coe.ufrj.br; bess@uninet.com.br).
Digital Object Identifier 10.1109/TASC.2005.849627
A. Design
Fig. 1 shows the FESS prototype that is in development. It is
composed of an Evershed type bearing in order to minimize the
bearing losses, an SRM as the motor/generator and a flywheel
to store kinetic energy. The system will be placed in a vacuum
, to reduce the aerodychamber, with pressure of about 1
namic drag. The Evershed bearing system is composed of an
SMB and a PMB.
1051-8223/$20.00 © 2005 IEEE
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005
Fig. 1. Scheme of the flywheel energy storage system that is under
development showing: 1) vacuum enclosure, 2) PMB, 3) aluminum shim, 4)
SRM rotor, 5) stator and PMB support, 6) flywheel, 7) SMB rotor, 8) YBCO
blocks, and 9) superconductors’ chiller.
Fig. 3. Levitation force measurements for both SMB rotors, the measurements
start at 55 mm, after the zero field cooling of the superconductors, goes until 0.5
mm and then returns to 55 mm, in order to shown the hysteresis of the levitation
force.
magnetic field of the bearing, reaching 74.5 N/mm for a cooling
distance of 3 mm, which results in a gap of 1.9 mm. From these
measurements the bearing damping was also obtained and it is
a low damping ( 0.01) in all cases. Dynamic measurements of
linear bearings with flux shapers showed similar results [8].
C. Permanent Magnetic Bearing
Fig. 2.
The SMB permanent magnets rotors: (a) SMB-A and (b) SMB-B.
B. Superconducting Magnetic Bearing
The superconducting magnetic bearing was constructed with
(YBCO) superconNd-Fe-B magnets and
ducting blocks. There are two SMB under test: SMB-A and
SMB-B. Both of them are assembled with 16 square blocks
36 mm and 20 mm high, mounted in
of YBCO, 36 mm
. The per4 4 array and cooled with liquid nitrogen
manent magnets rotors, Fig. 2, are assembled with magnetic
flux shapers, in order to increase the levitation force and the
stiffness [5], [7]. SMB-A uses a set of brick shaped magnets,
, assembled in a ring arrangement, Fig. 2(a). SMB-B uses 3 rings of Nd-Fe-B, magnetized
radially, and 4 rings of steel as flux shapers, Fig. 2(b). It is
difficult to get Nd-Fe-B rings magnetized radially; the rings
were fabricated and magnetized in sections. The steel rings
smooth the magnetic field enough to avoid magnetic drag in
the bearing [7].
The levitation forces measurements of the SMB are shown in
Fig. 3. The SMB rotor B has about the same levitation force as
SMB rotor A for gaps smaller than 10 mm, but rotor B rotor is
smaller (130 mm outer diameter against 175 mm) and lighter
(1.4 kg against 4.9 kg) than rotor A. This is due to the greater
intensity and radial gradient of magnetic field in the rotor B [7].
The stiffness, axial and radial, and damping of SMB-A was
measured in a previous work [5]. The stiffness, axial and radial,
increases strongly when the superconductors are cooled in the
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 [8].
Finite element simulation of the designed PMB [7] shows that
it will produce a levitation force of about 200 N at 5 mm gap and
a radial stiffness of about 24 N/mm.
III. MECHANICAL STABILITY
As the FESS is a rotating machine, it is important to know
its dynamical behavior when in operation. In this way, using
the finite element method (FEM), the prototype was modeled
considering its low speed test configuration, Fig. 4, supported
axially by a superconducting magnetic bearing and having
two radial ball bearings positioned above and below the reluctance motor. The initial tests were made at low speed, up
to 3000 rpm, and the simulation for this case obtained the
following natural frequencies of vibration system: 1st Mode:
383.01 Hz, 2nd Mode: 617.36 Hz, 3rd Mode: 1039.44 Hz, 4th
Mode: 2747.81 Hz. A Campbell diagram obtained for speed
values through 30 000 rpm showed a first critical speed around
18 000 rpm.
This prototype is currently under modification and the mechanical bearings are being replaced by PMB. These PMB
will be mounted with aluminum shims in order to improve the
overall damping. The rotor disturbances will cause a magnetic
flux change in the aluminum shims, which will generate eddy
currents that will damp these disturbances [8].
DE ANDRADE et al.: VOLTAGE SAGS COMPENSATION USING A SUPERCONDUCTING FESS
Fig. 4. Experimental test rig used in the low speed SRM tests. In the figure: 1)
the vacuum enclosure, 2) the motor support, 3) the SRM, 4) the SMB rotor, 5)
the YBCO blocks and 6) the superconductors’ chiller.
Both SMB and PMB have low stiffness [4], [8] that leads
to low resonance frequencies of the rotating set (SRM rotor,
flywheel, shaft and bearings rotors). The final goal is to reach
30 000 rpm; therefore the study of dynamic stability of the
rotating set is an important issue in the development of the
prototype.
The experimental tests indicated natural frequencies of the
flywheel support structure lower than the rotating ones, which
may cause system disturbances during operation, limiting the
operation speed. Thus, a new support structure design is being
considered.
IV. FESS MODEL AND EXPERIMENTAL RESULTS
Future developments of the proposed FESS rely heavily
on the possibility of working with virtual prototypes, which
will allow for testing several design optimizations without
actually building them. Therefore there is a need for models
that correctly represent all the system components. In this
section is presented a model developed to analyze the interaction between the energy storage system and the electric power
system to which it is connected. This model was implemented
in PSCAD/EMTDC software [9] and is able to represent the
main components affecting this interaction, namely: the SRM,
the power electronics circuit, the control circuit and the mechanical system. The model is first validated against test results
carried out in a prototype running at low speed and later is
used to predict the system operation at speed levels close to the
envisaged.
A. Switched Reluctance Machine and Mechanical System
The machine is modeled using the following set of equations
(1)
(2)
2267
Fig. 5. Current in one stator phase in the single pulse mode. The machine was
running at 1000 rpm with 9 V in the dc link. The tables were obtained from the
static simulation on FEM was used in the dynamic model.
(3)
where V is the coil terminal voltage, is the coil resistance and
is the current flowing through the coil. J is the combined mois the electromagment of inertia (SRM rotor plus flywheel),
is the load opposing torque and mechanical
netic torque and
losses. is an angular position measured in a reference frame
fixed to the stator.
B. Power Electronics and Control Circuits
The power electronics circuit consists of two converters. To
drive the SR machine a half-bridge IGBT-based converter is
used, allowing operation as motor or generator. The dc link is
connected to the network by a bridge PWM converter. 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 variable: the voltage across the dc link
capacitor. The main idea is to control the acceleration of the
SRM in proportion to the mismatch between the dc link capacitor voltage and a given reference value.
C. Model Validation
In order to validate the SRM drive model, tests were carried
out in the prototype presented in Fig. 4. The main components
are: 1.5 kW SRM with 6 stator and 4 rotor poles, bi-directional
converter with a dc link capacitor and a superconductor magnetic
thrust bearing, which would also work as a flywheel increasing
the system inertia. The coupling between the SRM rotor shaft
and the bearing is assumed to be rigid so they were represented
as a single inertia. The agreement between measurements and
simulated results is illustrated in Fig. 5, which shows the
current in one stator phase when the system is operating in
the single pulse mode. The machine was running in open loop
at 1000 rpm with 9 V in the dc link, opposing to the normal
closed-loop operating voltage ( 400 V). The linkage flux and
torque curves of SRM, as a function of current and angular
position, used in this model were obtained from motor FEM
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005
Fig. 6. Flywheel energy storage system simplified circuit.
where (for simplicity but without loss of generality) the connection to the power grid was modeled by a current source connected directly to the dc link (Fig. 6). The intention was to study
the performance of the control loop when energy is injected or
drawn from the link. The simulation results are shown in Fig. 7.
a 10 A current is solicited from the dc link by the
At
grid, imposing that the SRM operates as a generator. The situawhen the grid injects energy into the
tion is reversed at
capacitor at small rate in order to restore the reference speed.
V. CONCLUSION
The development of a FESS with SMB and SRM was described. The simulation of the power electronics projected to
control the FESS was validated for first tests and shows that this
system will be able to compensate voltage sags when operating
at speeds around 30 000 rpm.
ACKNOWLEDGMENT
The authors would like to thank G. C. Bordin,
A. da S. P. Corte Real, and S. L. P. C. Valinho for the
experimental support.
REFERENCES
Fig. 7. Result of the power electronics simulation showing the system
response to a voltage sag.
static simulations. The small differences observed in Fig. 5
are due, mainly, to the low dc voltage. Normally the forward
voltage drop in the power electronic devices is not significant
when compared to the dc link voltage, and then do not need
to be precisely modeled.
D. Operation at Higher Speed
In order to demonstrate the flywheel operation with the developed PSCAD/EMTDC model, a simulation was carried out
[1] G. Tanneau and D. Boudou, “Custom power interface,” in Proceedings
of CIRED2001, Jun. 2001, p. 2.48.
[2] A. M. Wolsky, “An overview of flywheel energy systems with HTS bearings,” Supercond. Sci. Technol., vol. 15, pp. 836–837, 2002.
[3] R. Hebner, J. Beno, and A. Walls, “Flywheel batteries come around
again,” IEEE Spectr., pp. 46–51, Apr. 2002.
[4] T. A. Combs, A. Cansiz, and A. M. Campbell, “A superconducting
thrust-bearing system for an energy storage flywheel,” Supercond. Sci.
Technol., vol. 15, pp. 831–835, 2002.
[5] R. de Andrade et al., “A superconducting high-speed flywheel energy
storage system,” Physica C, vol. 408–410, pp. 930–931, 2004.
[6] L. G. B. Rolim, A. C. Ferreira, G. G. Sotelo, and R. de Andrade Jr.,
“Flywheel generator with switched reluctance machine,” in Proceedings
of ICEM 2002, Brugge, Belgium, 2002.
[7] G. G. Sotelo, A. C. Ferreira, and R. de Andrade Jr., “Halbach array superconducting magnetic bearing for a flywheel energy storage system,”
IEEE Trans. Appl. Supercond., submitted for publication.
[8] R. de Andrade et al., “Performance of Nd-Fe-B and ferrite magnets in
superconducting linear bearings with bulk YBCO,” IEEE Trans. Appl.
Supercond., vol. 13, pp. 2271–2274, Jun. 2003.
[9] Manitoba HVDC Research Centre Inc., EMTDC/PSCAD Simulation
Manual.