Experimental Validation of a Dynamic model of a SRM used in superconducting bearing flywheel energy storage system J.L. SILVA NETO 1 , R. DE ANDRADE Jr 1,2 , L.G.B. ROLIM 1,2 , A.C. FERREIRA 1,2 , G. G. SOTELO 2 , W. SUEMITSU 1,2 1 Department of Electrical Engineering, 2 COPPE/Electrical Engineering Program Federal University of Rio de Janeiro P.O.Box 68504, CEP 21941-972 BRAZIL luizneto@dee.ufrj.br , ferreira@ufrj.br , rolim@dee.ufrj.br Abstract: - This paper presents a dynamic model of a flywheel energy storage system with superconducting magnetic axial thrust bearing (SMB) and a permanent magnet radial bearing (PMB), which uses a switched reluctance machine (SRM) as motor/generator. The SMB was built with Nd-Fe-B magnet and YBCO superconducting blocks, refrigerated with liquid nitrogen. The developed model can take SRM non-linearities into account, by including the inductances as a function of the angular position and phase current. Simulations of the power electronics and SRM show that the system can work up to 30,000 rpm supplying the required energy during disturbances. The control and power circuitry are described. An experimental validation of the model is performed. Key-Words: - flywheel, superconducting magnetic bearings, switched reluctance machine, energy storage. 1. Introduction In modern production plants, momentaneous voltage disturbances, such as 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 to the energy quality problems. In order to mitigate voltage sags dynamic voltage restorers can regulate the voltage at the load bar avoiding the 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]. The above reasons have motivated the launch of a joint project between the Federal University of Rio de Janeiro (UFRJ) and a local energy distribution company, concerning the development of a flywheel-based energy storage device for custom power equipments. Two major potential applications are under consideration for further development: the compensation of voltage sags and a short-term uninterruptible power supply, to provide ride- through capability to critical loads under momentary fault conditions. The performance degradation of the electrochemical batteries is directly related to the number of charges and discharges cycles. Furthermore, they are expensive and not environmental safe [7]. On the other hand, 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, for example. 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 [7]. 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 are increased too, due to the aerodynamic drag and the losses in the bearings. The aerodynamic drag can be reduced with a partial vacuum in the flywheel container, and the bearing losses with magnetic bearings [2-4]. Active magnetic bearings (AMB) are expensive and demand an energy consumption that have to 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 does. The SMB will fail if its cooling system fails and the superconductors warm up, but this 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 of efforts to bear the flywheel and motor and the SMB gives stability to the whole set. Other significant aspect of the energy storage 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. It is usual to use a permanent magnet synchronous machine in flywheel projects [2,4], but the permanent magnet machines always have idling losses related to the 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]. The SRM can work at very wide IEEE ISIE 2006, July 9-12, 2006, Montréal, Québec, Canada 1-4244-0497-5/06/$20.00 © 2006 IEEE 2492