Control and Emulation of Small Wind Turbines Using Torque Estimators

Abstract

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

Accurate Rotor Speed Estimation for Low-Power Wind Turbines

“…Speed command ω * rm in Fig. 5(a) is obtained from the electrical power using the control strategy described in [10]. An operating temperature of 60 • C is considered in the generator.…”

“…The wind turbine is emulated by a vector-controlled induction machine drive. Turbine inertia is emulated using the method described in [10]. The load drive includes an incremental encoder, which will be used to assess the accuracy of the proposed method.…”

“…The relevant boost converter parameters are shown in Table III. The turbine control strategy followed in this paper is described in [10]. The turbine speed is controlled to follow the command provided by the MPPT algorithm.…”

“…boost converter) current can be used for this purpose. The rotor speed is accurately estimated using a polynomial approximation function using boost current and rectifier voltage (2) in [10]. Coefficients of this function are obtained in a commissioning stage and stored in a table, which is later accessed during normal operation of the turbine.…”

“…However, neglecting the load current effect in the rotor speed estimation results in large rotor speed estimation errors at high loads. An accurate speed estimation model using both the rectifier DC voltage magnitude and the boost converter current was proposed in [10]. Soft-stall is implemented through a mechanical torque observer using the speed estimate as input.…”