Maximum Power Point Tracking Control Scheme for Grid Connected Variable Speed Wind Driven Self-Excited Induction Generator

Maximum Power Point Tracking Control Scheme for Grid Connected Variable Speed Wind Driven Self-Excited Induction Generator

ABSTRACT

This paper proposes a wind energy conversion system connected to a grid using a self-excited induction generator (SEIG) based on the maximum power point tracking (MPPT) control scheme. The induction generator (IG) is controlled by the MPPT below the base speed and the maximum energy can be captured from the wind turbine. Therefore, the stator currents of the IG are optimally controlled using the indirect field orientation control (IFOC) according to the generator speed in order to maximize the generated power from the wind turbine. The SEIG feeds a (CRPWM) converter which regulates the DC-link voltage at a constant value where the speed of the IG is varied. Based on the IG d-q axes dynamic model in the synchronous reference frame at field orientation, high-performance synchronous current controllers with satisfactory performance are designed and analyzed. Utilizing these current controllers and IFOC, a fast dynamic response and low current harmonic distortion are attained. The regulated DC-link voltage feeds a grid connected CRPWM inverter. By using the virtual flux orientation control and the synchronous frame current regulators for the grid connected CRPWM inverter, a fast current response, low harmonic distortion and unity power factor are achieved. The complete system has been simulated with different wind velocities. The simulation results are presented to illustrate the effectiveness of the proposed MPPT control scheme for a wind energy system. In the simulation results, the d-q axes current controllers and DC-link voltage controller give prominent dynamic response in command tracking and load regulation characteristics.

Introduction

Recently, the wind generation system is attracting attention as a clean and safe renewable power source. Induction machines have many advantageous characteristics such as high robustness, reliability and low cost. Therefore, induction machines are used in high-performance applications, which require independent torque and flux control. The induction machines may be used as a motor or a generator. Self-excited induction generators (SEIG) are good candidates for wind-power electricity generation especially in remote areas, because they do not need an external power supplies to produce the excitation magnetic fields [1-3] The excitation can be provided by a capacitor bank connected to the stator windings of the induction generator. Magnetizing inductance is the main factor for voltage build-up of the IG. The minimum and maximum values of capacitance required for self-excitation have been analyzed previously [4-7].

The three phase current regulated pulse-width modulation (CRPWM) AC/DC/AC converters have been increasingly used for wind energy system applications. Their attractive features include: regulated DC-link voltage, low harmonic distortion of the induction generator currents and controllable power factor and efficiency [8-9]. The current regulation of a SEIG in the synchronous frame has the advantages of fast dynamic current response, good accuracy, constant switching frequency and less sensitivity to parameter variations.

In wind generation systems, a variable speed generation system is more attractive than a fixed speed one because of the improvement in the wind energy production. In a variable speed system, wind turbine can be operated to produce its maximum power at every wind speed by adjusting the shaft speed optimally. In order to achieve the maximum power point tracking (MPPT) control, some control schemes have been studied. For example, a search-based or perturbation-based strategy [10,11] , a fuzzy- logic based control [12,13], a wind speed-estimation-based algorithm [14] has been applied.

In this paper, a variable speed high performance generation system using a SIEG is studied. The requirements of high dynamic performance is gained utilizing filed-oriented control (FOC) in which the dynamic model of the induction generator is simplified and decoupled. The FOC strategy studied in this context was developed by Hass and Blashke in Germany some thirty years ago. This technique improves the performance of the induction generator to a level comparable to that of a separately excited DC generator. Therefore, the FOC of an induction generator system permits a high performance dynamic response using decoupled torque and flux control. The FOC strategy can be classified into two types, the direct filed orientation control (DFOC) and indirect filed orientation control (IFOC). The IFOC strategy is simpler than the DFOC strategy. The orientation technique may be done for rotor or stator and/or air-gap flux. The rotor flux orientation is the best one because there is a linear relationbetween the electromagnetic torque and the stator torque current of the induction generator. Some control schemes have been studied using DFOC [15,16] and IFOC with stator flux orientation [17].

The IFOC of the SEIG for the WECS is composed of a dynamic model of the wind turbine, a dynamic model of the IG in the arbitrary and synchronously rotating reference frames, a dynamic model of the IFOC technique (decoupling controller), d-q axes current controllers, a voltage controller, coordinate transformations, a space- vector PWM (SVPWM) and AC/DC/AC CRPWM converters as shown in Fig. 1.

In the proposed wind generation system, the control scheme is based on the MPPT technique. The IFOC technique with the rotor flux of a SEIG is used for controlling the CRPWM converter while the grid connected CRPWM inverter is controlled utilizing the vector control technique. The current vector of the SEIG is suitably controlled according to the IG speed in order to optimize the wind turbine operation for various wind speeds. The IFOC of the SEIG allows for control of the d-q stator currents. So, the output voltage of the CRPWM converter can be regulated and we can maximize the efficiency. In the proposed control scheme with the IFOC of the SEIG, a dynamic model of the induction generator in d-q axes arbitrary reference frame is carried out. At field orientation control (FOC), λqr=0, dλqr/dt=0, dλdr/dt=0, and ωm=ωe, the IFOC is derived in the synchronous reference frame. Based on the transfer function of the IG at FOC, the proportional plus integral (PI) current controllers in the d-q axes are designed and analyzed to meet the time domain specifications: minimum overshot, minimum settling time and minimum steady-state error. After that, a PI voltage controller is designed to accomplish the specifications of the voltage control loop based on the dynamic of the DC-link and the SEIG. Also, this paper studies the design and control of the grid connected CRPWM inverter with the MPPT algorithm and unity power factor. Similarly, by applying the vector control technique to the grid connected CRPWM inverter, we can deduce the transfer function of the inverter with the grid at virtual-flux orientation control (VFOC) and the design of the d-q axes current controllers are accomplished. To verify the design of the controllers and system performance, the WECS is simulated starting with the wind turbine, the SEIG and then the AC/DC/AC CRPWM converter. The dynamic performance of the WECS has been studied under different wind velocities and MPPT. The simulation results are provided to demonstrate the effectiveness of the proposed control scheme.

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