Abstract-Soft-stall control of small wind turbines is a method to protect the generation system and/or load from excessive wind speeds and wind gusts without discontinuing power generation. Soft-stall can be activated due to either an excess of the power and/or torque/current. This paper proposes a method to improve the existing soft-stall methods for over torque/current protection using a turbine torque estimator. In addition, this paper also proposes two methods to emulate the wind turbine inertia without communications between the load drive (wind turbine emulator) and the generation system controller. This will allow the evaluation of the proposed methods in working conditions. I. INTRODUCTIONThe increasing electrical energy demand has boosted the interest in renewable energy sources due to economical and sustainability reasons. The development of the energy conversion technologies has also brought opportunities to small scale consumers to produce electricity to cover all or part of their electrical energy needs, normally using photovoltaic panels and small wind turbines, the second being considered advantageous in terms of power density and cost.To become attractive to private consumers, small wind turbines should be able to operate unattended under a broad range of weather conditions. Self protection of the wind turbine under high wind speed situations is mandatory in this case. When the power produced by the turbine exceeds load or generator rated powers, the turbine must be operated at a reduced efficiency to avoid damage either to the load or to the generator. A variety of methods to decrease the turbine efficiency under high wind speed have been proposed, including pitch, furling and stall control, mechanical brakes and electric brakes [1], pitch and furling being only possible in variable pitch wind turbines. The electric brake is the preferred option for small wind turbines due to its simplicity and low cost. However, activation of the electrical brake produces a high torsional torque in the turbine shaft and large currents in the generator windings what stresses the system significantly. A negative temperature coefficient resistor (NTC) crowbar has been proposed to mitigate those problems [1], [2]. Nevertheless, the activation of the crowbar discontinue the power generation. Furthermore, the occurrence of successive start and stop cycles can stress or even damage the turbine.To avoid the electric brake activation various soft-stall methods have been proposed [3]-[6]. These methods limit the power produced by the wind turbine by reducing its efficiency. To achieve this goal, the turbine speed is decreased, and therefore operates with a non-optimal tip speed ratio (TSR).In [7] a soft-stall method using the generator and/or power converter current limit (or alternatively by generator torque limit) instead of the rated power of the connected load was proposed for generator protection. This method also allows automatic reconnection of the system if the crowbar has been

Abstract-This paper proposes a new soft-stalling control strategy for grid-connected small wind turbines operating in the high and very high wind speed conditions. The proposed method is driven by the the rated current/torque limits of the electrical machine and/or the power converter, instead of the rated power of the connected load, which is the limiting factor in other methods. The developed strategy additionally deals with the problem of system startup preventing the generator from accelerating to an uncontrollable operating point under a high wind speed situation. This is accomplished using only voltage and current sensors, not being required direct measurements of the wind speed nor the generator speed. The proposed method is applied to a small wind turbine system consisting of a permanent magnet synchronous generator and a simple power converter topology. Simulation and experimental results are included to demonstrate the performance of the proposed method. The paper also shows the limitations of using the stator back-emf to estimate the rotor speed in permanent magnet synchronous generators connected to a rectifier, due to significant d-axis current at high load.

Abstract-This paper proposes a new soft-stalling control strategy for grid-connected small wind turbines operating in the high and very high wind speed conditions. The proposed method is driven by the the rated current/torque limits of the electrical machine and/or the power converter, instead of the rated power of the connected load, which is the limiting factor in other methods. The developed strategy additionally deals with the problem of system startup preventing the generator from accelerating to an uncontrollable operating point under a high wind speed situation. This is accomplished using only voltage and current sensors, not being required direct measurements of the wind speed nor the generator speed. The proposed method is applied to a small wind turbine system consisting of a permanent magnet synchronous generator and a simple power converter topology. Simulation and experimental results are included to demonstrate the performance of the proposed method. The paper also shows the limitations of using the stator back-emf to estimate the rotor speed in permanent magnet synchronous generators connected to a rectifier, due to significant d-axis current at high load.

Small grid-tied wind turbines based on permanent magnet generators often use a cost-effective power converter topology consisting of a passive rectifier, a boost converter, and an H-bridge inverter. Speed or position sensors are rarely used due to cost issues. Model-based estimators relying on electrical magnitudes are used instead. However, such estimators are parameter sensitive, which limit their accuracy. Further concerns arise if these parameters change with the operating condition of the machine, mainly due to temperature. Speed sensorless control using the rectifier voltage ripple is analyzed in this paper. This technique provides good dynamic response and does not depend on machine parameters. Simulations are provided for speed and power tracking comparison with an accurate modelbased speed estimation method operating at non-rated parameters. They show the speed accuracy and power tracking capability of the proposed method are similar to that provided by a speed sensor. This is translated into a 0.9% power increase when the model-based speed estimator shows 9 % of error. Experimental results are carried out to test the effect of current and temperature in the estimation, showing temperature insensitivity and some distortion due to fast current transients. A speed estimation accuracy of zero mean error and 1.7% standard error is experimentally obtained in the regular operation of the wind turbine.

This paper analyzes a cost-effective modification of the power topology commonly found in small wind turbine systems based on a passive rectifier and a boost converter. The boost converter inductor and the input filter capacitor often placed at the rectifier output can be replaced by the generator phase inductance. Different controller structures have been proposed for this low-cost inverter, but they have been focused in the converter itself rather than in the overall turbine control. Moreover, only steady-state behavior has been demonstrated. This paper proposes a control structure only requiring retuning of the boost current controller found in systems equipped with boost inductance; other control loops remaining unchanged. The inductorless converter dynamic performance is studied and compared with the conventional topology in terms on current and torque control capability. The system efficiency, including the losses distribution in the generator, is analyzed. Simulation and experimental results are presented to demonstrate the technical viability of this proposal.