Evaluating the performance of different machine types to operate a Flywheel energy storage system

Abstract

In this thesis, the performance of 3 different drives to operate the flywheel energy storage system (FESS) were analysed. These machines are the permanent magnet synchronous machine (PMSM), the squirrel cage induction machine (SCIM) and the doubly fed induction machine (DFIM) (also referred to as slip ring/wound rotor induction machine). The vector control model for the 3 systems were built using PSIM.9 in order to prove the functionality of the 3 machines for the flywheel application as different vector control topologies as well as different methods for speed control of the machines were used. For PMSM and SCIM, field weakening is required to reach higher speed above rated speed while keeping the back emf voltage and the power constant, while for DFIM a slip range of +/- 0.3 (+/- 30 % of rated speed) can be used. The speed ranges of the 3 machines as well as their voltage and current rating while operating in the constant power region was examined to determine the energy storage capability as well as the converter rating, cost and losses. Some improvements for reducing the converter rating and thus the losses and cost for the DFIM were proposed by reducing the full speed range used to be -20% to 30% of the rated speed. Also, another improvement for the PMSM design was proposed for reducing the amount of injected magnetizing current required for the field weakening approach. This was done by using smaller magnets, and this will reduce the overall current so smaller current rating for the converter can be used and thus also reducing the losses and the size of the converter