Control of a Small Wind Turbine in the High Wind Speed Region

Abstract

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.

Control and emulation of small wind turbines using torque estimators

“…If the estimated turbine torque is below the generator rated torque, the output of the controller is disabled by setting it to zero. When the turbine torque surpasses the generator torque the rotor speed must be decreased and the controller delivers a negative speed command, ∆ω * rm , that is added to the current MPPT command (7). The ramp generator is stopped in this case to avoid conflicts between MPPT control and torque control ("hold" block in Fig.…”

“…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 activated.…”

“…The method proposed in this paper overcomes the limitations of the method proposed in [7], a turbine torque estimator is used for this purpose. The estimator allows fast and accurate detection of high wind speed conditions, decreasing the required braking torque/current compared to [7].…”

“…The estimator allows fast and accurate detection of high wind speed conditions, decreasing the required braking torque/current compared to [7]. Furthermore, precise information on the produced torque allows to operate the generator at its torque limit, therefore increasing the power generation.…”

“…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…”