Flywheel energy storage systems (FESSs) improve the quality of the electric power delivered by wind generators, and help these generators contributing to the ancillary services. Presently, FESSs containing a flux-oriented controlled induction machine (IM) are mainly considered for this kind of application. The paper proposes the direct torque control (DTC) for an IM-based FESS associated to a variable-speed wind generator, and proves through simulation and experimental results that it could be a better alternative. This DTC application entails two specific aspects: 1) the IM must operate in the flux-weakening region, and 2) it must shift quickly and repeatedly between motoring and generating operation modes. DTC improvement for increasing the FESS efficiency, when it operates at small power values, is discussed. Some aspects concerning the flywheel design and the choice of the filter used in the FESS supervisor are also addressed.

This paper presents a comparative study between the axial-flux and radial-flux permanent-magnet synchronous generators for a 3 kW wind turbine application. This study emphasizes the most cost-effective solution to be implemented by the industrial partner. There are three constructive types of electric generators considered in this work. All are based on permanent-magnet synchronous generators (PMSGs), but one has an axial-flux topology, the other one is a radial-flux outer-rotor machine and the third one is a radial-flux inner-rotor machine. Finite-element field analysis was made for the three PMSGs in order to compare their efficiency and active materials estimated cost. The results show that the axial-flux PMSG is the best solution for micro-wind turbine application.

Following a series of articles presenting the modelling and simulation of an autonomous variable- speed hydropower station feeding an isolated load or connected to a power grid ([2], [3]), this paper is aiming to validate on the test bench the solutions proposed earlier. The studied system is composed of a doubly-fed induction generator (DFIG) linked mechanically and electrically (through back-to-back power electronic converters in the rotor circuit) to a permanent-magnet synchronous machine (PMSM) which may recover or supply the slip power. One of the advantages related to this system is that the PMSM and the power converters are designed for only about 25% of the plant nominal power.

This paper investigates two topologies of ferrite-permanent-magnet synchronous machines for use as direct-driven generators in micro-wind power applications, i.e. the axial-flux double-sided inner-stator permanent-magnet (AF-PM) generator and the radial-flux spoke-type interior-permanent-magnet (RF-IPM) generator. The comparative design analysis of AF-PM and RF-IPM micro-wind generators based on finite-element field analysis proves that both topologies represent a good-performance and cost-effective solution of choice for micro-wind power applications.

Variable speed hydro power plants allow to compensate and to smooth the fluctuating wind power when feeding isolated loads. This is shown experimentally in this paper with the help of a 3 kW laboratory test bench able to emulate a hybrid wind/hydro power system. The test bench is equipped of a doubly fed induction generator based small hydro power plant emulator associated with a fixed-speed wind generator emulator. This kind of wind generator is often used in small or medium power isolated networks. Tests show that the frequency and voltage are well controlled in spite of load variations.