Accelerated starting by motoring a grid-connected small wind turbine generator

Abstract

In recent years, an increasing proportion of small wind turbines have been connected to the electrical grid. With appropriate switchgear and control, it should be possible to use grid power to operate the generator as a motor for faster starting of the turbine. The artificial case of a step increase in wind speed from zero is considered. It is shown that motoring yields an energy gain for all wind speeds typical of the operating range of small turbines. This paper proposes short-period motoring of a permanent magnet generator in a 5kW wind turbine system connected to the grid with a backward very sparse matrix converter which has bidirectional power flow capability. Motoring produces torque in the same direction as the aerodynamic torque on the turbine which in turn accelerates the turbine more quickly towards the maximum power point. Interestingly, the overall result is that more energy is delivered to the grid by short-term motoring than in the case where the turbine is accelerated aerodynamically without motoring. In the proposed system, the very sparse matrix converter controls the generator torque through field orientation where d-axis current is set to zero while the q-axis current is used to control the generator torque and thus control the turbine speed in both motoring and generating modes. The switching signals of the converter are formed based on the principle of space vector modulation. Simulation results, with and without an electromagnetic starting torque, indicate that the proposed technique increases the overall energy efficiency of small wind turbine systems while providing fast starting.