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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

    ; superconductor, a drawback is the need of cryogenic Abstract—This paper presents test results of a flywheel energy refrigeration, but there are recent developments of innovative storage system (FESS) prototype. The bearing system set is design for the cryogenic insulation that can minimize the composed of a superconducting magnetic thrust bearing (SMB) refrigeration costs [2]. and a permanent magnet bearing (PMB). The SMB was built with A flywheel coupled to an electrical drive consists of a Nd-Fe-B magnet and YBCO superconducting blocks. The PMB flywheel energy storage system (FESS), which can convert has the function of positioning radially the switched reluctance electrical to kinetic energy and vice versa. In a previous work [3] machine (SRM) used as motor/generator and reduce the load over

    SMB. The SRM drive is responsible to convert electrical into it was shown the development of a FESS with superconducting mechanical energy, and vice versa. The prototype still operates at magnetic bearings designed to compensate voltage sags. The low speeds, but the power electronics and SRM drive showed that FESS bearing system was designed to be Evershed type, with a the system can work at high speed, supplying the required energy SMB as the thrust bearing and a PMB for radial positioning and during disturbances. The performed tests with the FESS to reduce load over the SMB. The simulation of the power prototype show the supply energy to the grid when a disturbance electronics that has been designed and mounted showed that the occurs.

    FESS is able to compensate voltage sags.

    Index Terms— Flywheels, Superconducting magnetic bearings, This paper describes the FESS tests. In these tests the FESS High-temperature superconductors. 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. I. INTRODUCTION

     FLYWHEEL stores kinetic energy; the amount of stored

    A energy is proportional to the inertia moment of the II. FLYWHEEL ENERGY STORAGE SYSTEM

    flywheel and the square of its angular velocity. Therefore, A. Prototype increasing the flywheel angular velocity may increase the Fig. 1 shows a photograph of FESS prototype that is in energy stored per volume in the flywheel, but it also increases development. It is composed of an Evershed type bearing in the idling losses [1]. The idling losses come mainly from the air order to minimize the bearing losses, a switched reluctance drag and bearing losses. The air drag losses can be reduced machine (SRM) as the motor/generator and a flywheel to store putting the flywheel in a vacuum enclosure and bearing losses kinetic energy. The SRM is driven by a power electronics using magnetic bearings. There are several types of magnetic converter, which is not shown in the picture. This converter will bearings that can be used to minimize the bearing losses: be responsible for interfacing the FESS to the power grid. The permanent magnetic bearings (PMB), active magnetic bearings system will be placed in a vacuum chamber, with pressure of (AMB) and superconducting magnetic bearing (SMB). PMB about 1 ？bar, to reduce the aerodynamic drag. are less expensive, but they are not able to provide a stable

    suspension in all dimensions and can only be used as an B. Superconducting Magnetic Bearings auxiliary bearing. AMB are the most used, but require complex The superconducting magnetic bearing used in these tests active control that is sensitive to electromagnetic disturbances. consists of rotor of Nd-Fe-B magnets mounted in the flux SMB are self-stable due the flux pinning inside of shaper configuration [3] attached to SRM axis and a stator with YBaCuO (YBCO) superconducting blocks. The stator 237-？Manuscript received August 25, 2006. This work was supported in part by consists of nine YBCO seeded melt textured blocks, 28 mm the CNPq under Grant 479557/04-7 and FAPERJ. R. de Andrade, Jr. is with the DEE/Poli/UFRJ, Federal University of Rio de diameter and 10 mm high, attached on the top plate of the Janeiro, Rio de Janeiro, RJ 21945-970 Brazil (phone: 55-21-2562-8031; fax: chiller. The superconducting blocks are maintained in vacuum 55-21-2562-8088; e-mail: randrade@dee.ufrj.br ). and refrigerated by the contact with the top plate of chiller. The G. G. Sotelo and A. C. Ferreira are with PEE/COPPE/UFRJ, Federal University of Rio de Janeiro, RJ 21945-972 Brazil (e-mail: ferreira@ufrj.br, chiller is sealed in order to allow the liquid nitrogen flow inside sotelo@coe.ufrj.br). it. J. L. Silva Neto, L. G. B. Rolim, W. I. Suemitsu, R. M. Stephan, and R. The superconductors are Field Cooled (FC), which means Nicolsky are with the DEE/Poli/UFRJ, Federal University of Rio de Janeiro, RJ 21945-970 Brazil (e-mail: silvaneto@dee.ufrj.br, rolim@dee.ufrj.br, that they are cooled with de permanent magnet rotor at walter@dee.ufrj.br, richard@coe.ufrj.br, nicolsky@if.ufrj.br ).

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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.

    specified distance from the superconductors. This procedure work a switched reluctance machine (SRM) is used. The SRM reduces the levitation force, but increases the axial and radial can work at very wide speed ranges: from zero up to several ten stiffness of the bearing [4]. thousand rpm; it is fault tolerant and has null idle losses. Its

    robustness leads to achieve a high reliability. C. Permanent Magnetic Bearings The power electronics circuit consists of two converters, as The PMB plays two roles in the FESS: radial positioning and shown in Fig. 5. To drive the SR machine a half-bridge reduction of the load over the SMB. This PMB will act in IGBT-based converter is used, allowing operation as motor or attraction in concert with the SMB. PMB by itself cannot generator. The dc link is connected to the network by a bridge provide stability for a bearing system, as predicted by PWM converter, which is controlled according to Akagi’s pq Earnshaw’s theorem. The PMB tested, Fig.2, was designed theory [6]. The objective of the control operation is to from finite element simulation [5]. The maximum levitation determine the direction of the power flow. This is achieved by force of this bearing, Fig. 3, is to high, 590 N at 1 mm of air gap. regulating the dc link voltage. The flywheel shaft speed must be The radial restoring force, Fig. 4, is linear and reversible until controlled according to the instantaneous active power 6.2 mm, for a larger displacement the PMB turns instable. The demanded by the grid. In this work, the implementation of a maximum restoring force reaches 320 N at 6.2 mm. 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. Two III. SRM DRIVE strategies can be employed in order to achieve the control of the One significant aspect of a flywheel based energy storage dc link voltage.

    device is concerned to the electromechanical energy conversion

    between the flywheel and the electrical system. In order to use

    the most of the stored kinetic energy in the flywheel, the

    electrical machine has to be electronically controlled. In this

     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)

    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 (b) Fig. 7. Flywheel injecting energy in the grid. Blue: v (50V/div). Green: FA(；p), in order to bring the flywheel back to the rated i (0.5A/div). FA maximum speed after any transients. Its output signal should be

    limited to values that do not cause excessive power

    consumption from the grid. operate as an active rectifier taking energy from the grid, or as

    an inverter delivering energy to the grid. The operation of the B. Strategy II

    two converters is coordinated. If an amount of energy must be In this technique, on the other hand, the dc link voltage is delivered to the grid (minus losses), the same amount of kinetic controlled by the network bridge PWM converter, which may 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

    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 (i ) and the voltage ( v ) of the grid FAFA

    converter, and the DC link voltage (v ) were measured. The DC

    main components of the test rig are: 1.5 kW 6/4 SRM,

    bi-directional 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 Fig. 6. Flywheel taking energy from the grid. Blue: v (50V/div). Green: FAbe carefully coordinated to ensure that the dc link voltage stays i (0.5A/div). Orange: v (50V/div). FADC 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

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    ONCLUSION V. C

    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.

    ACKNOWLEDGMENT

     The authors would like to thank Nilo F. B. de Mello for the Fig. 8. Angular velocity of flywheel as a function of time, when supplying experimental support. energy to grid, until 870 ms, and after drawing energy from grid to recharge.

    REFERENCES

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