| Compared with fossil fuel and nuclear power, wind power is of cost competitive, environmentally clean, safe, renewable power sources, and being paid more attention recently. An actual turbine cannot extract more than 59.3% of the air kinetic energy according to Betz theory. In practice, this factor is less because of mechanical imperfections, but we also hope get maximum energy by adopting various methods.Recently, an increasing number of large-scale wind turbines are being developed with a variable speed-variable pitch (VS-VP) technology. The main objective of adopting a VS-VP technology is to improve the fast response speed and capture maximum energy, which means to obtain the maximum power at low wind speed and the rated power at high wind speed. But wind energy conversion systems are of strong nonlinear characteristics because of many uncertain factors. At the same time, the power generated by wind turbine changes rapidly because of the continuous fluctuation of wind speed and direction. On the other hand, in a renewable energy system, both power quality and reliability are two most vulnerable issues. The ordinary linear constant gain controller will cause overshoot or even loss of system stability, meanwhile the adaptive control method is not applicable in this case due to the complexity, such as algorithm, high order, coupling and strong nonlinear characteristics of large-scale VS-VP wind turbine.The control strategy is one of the most basilical renewable technology. According to different wind speed range, the wind generating set consists of four dynamic processes: starting, variable speed running, variable pitch running and braking. The main objective for starting and braking process is that the system has faster response speed in the most short time. Objectives for variable speed control system are summarized by the following general goals: to regulate and smooth the power generated, to maximize the energy capture, to alleviate the transient loads; Objectives for the variable pitch control are similar to the variable speed ones but only can be match a rotational power by regulating pitch angle.This dissertation has studied establishing system model and optimizing power control for large wind turbien. This paper describes a 1.5 MW variable speed-variable pitch wind turbine DFIG where control strategy has been used extensively to optimize the power output and enhance system performance. The validity of obtained result can be illustrated by the simulation research.The main results and contributions of this dissertation are listed as follows:(1) With a view to the fact that the wind turbine system is a very complicated nonlinear system consisting of wind blade, gear box, doubly-fed induction generator (DFIG), variable pitch mechanism etc. To place emphasis on analyzing DFIG and hydraulic variable pitch mechanism, we establishing reliability nonlinear model of DFIG wind turbine system using the electric motor convention and adopting the d ? q reference frame.(2) The calculation and analysis of the variable-pitch load are very important work for variable-pitch mechanism design. This Paper investigates the method of determining the reference frame, ascertains the means of classification load, and obtains the variable-pitch load for large horizontal axis wind turbine by analysing and calculating the incident coefficient out of plane, incident coefficient in-plane, chord, relatine wind speed, inflow angle, attack angle, pitch angle using matlab and VB.net.(3) The collective pitch control and the individual pitch are two primary variable-pitch methods. The design of electric individual pitch and hydraulic collective pitch are proposed, including theoretical calculation,analysis of the scheme and the selection of units etc. Using subsistent experiment condition, the electric individual pitch is analyzed by experiment and the hydraulic collective pitch is analyzed by simulation.(4) When the wind speed range varies from cut-in wind speed to rated wind speed, we adopt variable speed control method by adjusting the rotor speed in order to get optimal power. When the wind speed range varies from rated wind speed to cut-out wind speed, we adopt variable pitch control method by adjusting the pitch angleβin order to make generator work in the case of rated power. Considering that there exists the case that fuzzy control needs no accurate mathematical model and sliding-mode control can provide a good robustness and stability of system. In this paper a new kind of state controller is proposed by fuzzy sliding-mode control theory is established. This control strategy can eliminate the steady state error, compensate the nonlinearity and have excellent robustness.(5) In view of the fact that The main objective of adopting a variable speed-variable pitch technology is to improve the fast response speed and capture maximum energy, a kind of layered multi-mode optimal control strategy is proposed: bang-bang control strategy is adopted in starting and braking process; meanwhile, fuzzy control strategy is adopted in variable speed running process and adaptive proportional integral derivative PID control strategy is adopted in variable pitch running process. This control method can shorten the system response time, improve the wind turbine performance at low, rated and high wind speed.This work was supported by the Baiyulan Foundation for Science & Technology Talents, Shanghai (No. 2007B073), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (No. [2007]1108), the Postdoctoral Foundation of China (No. 2005038435), the Postdoctoral Foundation of Shanghai, China (No. 05R214133) and the Shanghai Educational Development Foundation (No. 200603).
|
PDF Full Text Request