Improved MPPT Dynamics for Starting and Power Extraction of a Small Wind Turbine Employing a PMSG and a VSMC

Abstract

Small wind turbines usually do not have pitch adjustable blades, hence the aerodynamic starting of a small wind turbine is typically slow due the high angle of attack on the blades. Moreover, most small wind turbines employ a permanent magnet synchronous generator with a cogging torque which has a significant impact on starting. Also, conventional maximum power point tracking (MPPT) takes longer than necessary to return the turbine to its optimal operating condition when the wind speed changes. This dissertation presents a starting strategy that operates the turbine generator as a motor, in order to overcome turbine inertia and the generator cogging torque. When the optimal operating condition is established, operation is switched from motoring to generating. As a result, the turbine can start below the cut-in wind speed, and has a faster starting process as well as an increase in the energy harvested from the wind. Another contribution of this dissertation is short term motoring of the generator during dynamic wind conditions, with the intention that the turbine can more quickly reach the maximum power point operating condition when the wind speed is increasing. To control the motoring and generating operating modes, a bidirectional power flow converter is required. A very sparse matrix converter is a potential candidate, and is employed in a backward configuration where the inverter side of the converter is connected to the generator. Space vector modulation and field oriented control are employed. Experimental verification of the proposed approach involves the use of a 500 W permanent magnet synchronous generator with a very sparse matrix converter in a laboratory set-up. A theoretical extrapolation of the practical results indicate that a 5.7 kW wind turbine can accelerate to the maximum power point in under two seconds for 10~m/s wind speed, and at the same time, increase wind energy harvest compared to the case without the proposed motoring control strategy.