Wind Turbine Simulation Based On Switched Reluctance Motor

In the research and development of the wind generation system under the laboratory condition, it is necessary and important to simulate the wind turbine. As the wind mill equipment is expensive and inconvenient, a motor drive is usually used to emulate the wind turbine characteristics. It drives the generator in the wind generation experiments, in order to develop maxmimum power tracking algorithm and test the grid integration equipment performance. For this reason, the wind turbine simulator (WTS) is very important in wind generation studies.The present wind turbine simulators are mostly based on Direct-Current(DC) motor,Asynchronous motor or Permanent Magnet Synchronous Motor(PMSM). The DC motor is not reliable enough for the maintenance of its brushes and slip rings. It is also unsuitable for the simulation of large capacity wind turbines. Asynchronous motor is complex to implement good torque characteristics and the vector control is influenced by the time-varying rotor coefficients. In addition, its low speed output is small, thus results in the vibrations and unstability. The PMSM is costly in the machine material and is also not suitable in simulaion of the large capacity wind turbines. As the development of the wind generation technology, the servo motor used in wind turbine simulator is ought to be durable,reliable and modulated well in the a wide speed range.The switched reluctance motor (SRM) is solid and reliable due to the simple structure of the rotor: brushless and windingless, thus overcomes the drawbacks of DC motor. Its converter is more reliable than the asynchronous motor. Its torque output is large in low speed scope, potentially better than the asynchronous motor in an overall speed range. Also the motor is much cheeper than the PMSM. In addition, it has several control items, adaptive to flexible control and could acquire the mechenical performance as good as the DC motor through approriate control. All these priorities make the SRM a good choice in wind turbine simulation. However, there have been few researches on the wind turbine simulator adopting switched reluctance motor. At present, most of the wind turbine simulators are implemented by the way of torque control, which is hard and complex to realize. It is necessary to make a thorough study in the WTS and explore some new schemes to simplify the WTS structure in the wind generation studies.This paper focuses mainly on the control of the switched reluctance motor to simulate the wind turbine, and has done all-around study, from simulation to experiment, from theory to field practice. The detailed research work is as follows:1. This paper summerized and analyzed the present technology of wind turbine simulation, explored the possibility of the wind turbine simulation based on the switched reluctance motor. The advantages of SRM based WTS is also presented.2. The paper proposes an WTS:The system differential equation is solved in every time stamp to acquire the reference rotational speed the real turbine will arrive at the next time point. The speed commands are sent instantly to SRM controller, which regulate the SRM to transit to the reference speeds. A simulation error model is established to measure the WTS adopting speed regulation scheme. The wind turbine simulation error in function of the simulation step and feedback rotational speed is studied through MATLAB numerical analysis. The 4th Runge-Kutta method is adopted in the solving equation work to reduce the simulation error.3. A fuzzy logic algorithm combined with PI controller is designed to implement the speed regulation scheme based on switched reluctance motor. Because the SRM is highly nonlinear due to the double salient structure and saturated magnetic circuit, as well as completely dependent on switches in normal work, the accurate model is hard to obtain. The WTS in the speed regulation scheme can be realized in the form of fuzzy logic of the chopped current control of SRM. A PI control scheme is added to the fuzzy algorithm in order to eliminate the steady error. The SRM tracking system is set to typical II system to determine the simulation step and the simulation error is measured by accounting the numerical results. Through digital simulation of the WTS in a single step in MATLAB/Simulink environment, the fuzzy PI controller is proved to have a better speed tracking performance than the normal PID controller.4. A wind turbine simulator based on the DTC (Direct torque control) of switched reluctance motor is proposed. In order to emulate the wind turbine dynamically, a turbine inertial compensation method is deduced according to the system mechanical equations and applied to the proposed WTS. The wind turbine model is implemented in the real-time software simulator and the reference torque is acquired and sent to motor controller, which make the SRM track the reference torque in the DTC manner.5. Based on the approximate torque model, the magnetization and torque characteristic data, a dual hysteris loop control method is conducted to implement the DTC of SRM, in purpose of simulating the wind turbine in a torque control scheme. Both the torque and flux amplitudes are controlled in respective target hysteris loops, thus the direct torque control of SRM is carried out. Nowadays the high property control of SRM torque mainly incudes:torque sharing function(TSF),sliding mode variable structure control,iterative learning and neural network control.The methods are always complex, in need of large amount of off-line computation. The proposed direct torque control algrithm is based on control of dual hysteris loops of flux and torque, which borrows the DTC scheme of the asynchronous motor, and has a double on-off regulator. It locates the flux space vector, calculates the amplitude of the flux and motor torque, then choose the voltage space vector to decrease or increase the amplitude. Consequently, the flux and torque amplitudes are constrained in hysteris bands. The control structure is simple and robust and effective in the torque ripple reduction of SRM. The turbine model combined with the DTC tracking system of SRM is simulated in the MATLAB/Simulink environment. In constant wind, the simulation results show that the dual-hysterisis torque tracking system has good accordance with the real wind turbine characteristics and indicates a much less torque pulsation than the conventional chopped current hysterisis control.6. The wind turbine linearized model and the WTS linearized models both in speed control scheme and torque control scheme is deduced to compare the speed control scheme with the torque control scheme in frequency characterics. A simulation error model is established in frequency domain to evaluate simulation error for the two schemes. With respect to the power spectral characteristics of Van der Hoven modle for wind speed, the simulation error for both schemes is analized and compared. In conclusion, the torque control scheme of wind turbine simulator indicates a better tracking performance in high frequency simulation occation than the speed control scheme, while in the low-medium frequency scope the two schemes have the close simulation error.7. A test rig for the wind turbine simulator based on SRM is built. It is composed of a 10kW SRD (Switched Reluctance motor Drive) and a 14kW DC generator acting as the the load. Firstly, the fuzzy-PI controller of SRD is designed. In the experiments of WTS adopting the speed control scheme, the upper machine software implements the wind turbine model and sends the reference speeds to the SRD controller through RS232 port. The fuzzy-PI controller regulates the SRM according to the track the received speeds. The experimental results suggest that the fuzzy-PI controller of SRD is capable of speed tracking in WTS adopting the speed control scheme. In the experiments for the WTS based on DTC of SRM, The flux-current-position angle characteristics are measured and the flux-current-position angle characteristics are deduced by integrating the current i and differentiating the position angelθ。The charateristics are stored as data forms in the program and used to calculate the current fluxes and torques. The control program is made up of two parts:the wind turbine model implementation and the DTC algrithm of SRM torque track. The experimental results indicate that the WTS can perform the wind turbine characteristic under constant wind velocity with variable load. In the MPPT (Maximum Power Tracking point) test, the WTS characteristic also has a good identity with the simulated wind turbine.The thesis makes intensive study on the wind turbine simulator based on switched reluctance motor. The speed control scheme in WTS adopting the fuzzy PI control of SRM is put forward, as well as the direct torque control scheme of SRM in torque control scheme of WTS. The speed control scheme is simple, reliable and has a good stability. The torque control scheme of WTS is comparatively complex while the simulation error in high frequency domain is smaller than the speed control scheme. In conclusion, the torque control scheme of WTS is an ideal method. The wind turbine simulator based on switched reluctance motor indicates a reliable merit, high range speed regulation and good performance in low speed range. It is an effective servo-motor for the development of wind generation systems. The proposed WTS based on switched reluctance motor has provided a practical instrument for the wind generation study and it exhibits a prospect in large quantity and direct drive wind generation developments. Due to the intrinsic torque pulsation and heavy nonlinearity of switched reluctance motor, it is hard to modulate the torque and speed accurately. The torque pulsation minimization study is ought to be conducted to ensure and improve the performance of the wind turbine simulator in future work.