Modeling and Stability Analysis of a Direct-Drive Direct-Grid Slip-Synchronous Permanent-Magnet Wind Generator

In order to simplify and increase the reliability of wind generators, a new type of a direct-drive directly grid-connected wind generator known as a slip-synchronous permanent-magnet generator (SS-PMG) is proposed. It consists of two integrated generating units, i.e., a directly turbine-connected slip PMG unit and a directly grid-connected PM synchronous generator unit. The main aim of this paper is to determine if this generator is stable under transient wind turbine and grid conditions. A transfer function model is developed to evaluate the dynamic performance of the wind generator system. The transient response of a single 15-kW grid-connected system is evaluated. It is shown that the generator is very stable under transient turbine torque conditions. Furthermore, it acts as a filter to prevent voltage flickering due to oscillatory turbine pulsations. A very important design aspect to take into account is the torque ripple characteristics of the slip PM generator as torque ripple can be transferred to the grid current under certain conditions. Simulated and dynamically measured results are shown for a series of 15-kW SS-PMG prototypes.