Abstract: This paper analyzes the mathematical model of a megawatt-level doubly-fed induction generator (DFIG) wind turbine generator and establishes a dual-PWM converter control system model based on stator magnetic field vector control technology in MATLAB using a structured design method. Simulation results verify the effectiveness of the system in achieving maximum wind energy capture and active/reactive power decoupling control.
Keywords: Doubly fed wind turbine; Dual PWM converter; Vector control technology; MATLAB simulation
Simulation Analysis of the Mega-watt DFIG Wind Turbines system
Dong Hongxi1 Sun Zhiyi2
Abstract: In this paper, analyzing the mathematical model of MW doubly fed induction generator (DFIG) wind turbines. In the MATLAB, using a structured design method, a dual PWM converter control system model were established on the basis of the stator flux field vector control technology. Based on the simulation results, it was proved that the system is capable of capturing maximum wind powers, achieving the decoupled control of active and reactive powers.
Keywords: DFIG; dual-PWM converter; vector control technology; MATLAB simulation
introduction
Doubly fed asynchronous wind power generation technology has become an important development direction for wind power generation technology because it has the advantages of: adjusting the generator torque according to wind speed changes to obtain maximum wind energy at low wind speeds; and changing the blade pitch angle to maintain rated power when the wind speed is higher than the rated wind speed.
1. Working principle and mathematical model of doubly-fed wind turbine generator set
1.1 Working Principle of Doubly Fed Asynchronous Generator Set A doubly fed asynchronous generator consists of a wound-rotor asynchronous generator with slip rings and dual PWM converters. The stator windings of the doubly fed asynchronous generator are directly connected to the power grid. During operation, the relationship between the generator speed and the frequency of the stator and rotor currents can be expressed as: f1 = pn/60 ± f2. From this equation, it can be seen that when the generator speed changes, f1 can be kept constant by adjusting f2.
1.2 Mathematical Model of Doubly Fed Asynchronous Generator
This paper establishes a mathematical model of a doubly-fed asynchronous generator according to the conventions for electric motors. The voltage equation is as follows:
In the formula, subscripts 1 and 2 represent the stator side and rotor side, respectively; d and q represent the d-axis and q-axis, respectively. P is the differential operator, and ω and ωS are the synchronous angular velocity and slip angular velocity, respectively. The flux linkage equation is as follows:
Doubly fed generators employ stator field-oriented vector control technology, positioning the stator flux linkage Ψ1 along the d-axis, neglecting resistance voltage drop.
From the above equations, we can obtain the equations for electromagnetic torque and power.
(4)
When the generator is connected to the grid, the stator voltage remains unchanged, and Ψ1 also remains unchanged. As can be seen from equation (4), the electromagnetic torque of the generator can be controlled by the q-axis current in the rotor to achieve the purpose of speed regulation. The stator active power P1 is only related to the rotor current iq2, and the reactive power Q1 is only related to id2, thus realizing the decoupling control of active power and reactive power.
2 Simulation Model of Doubly Fed Wind Turbine Generator
The simulation modules for doubly-fed asynchronous wind turbine generators in MATLAB include: wind speed model, variable pitch wind turbine model, doubly-fed wind turbine generator model, dual PWM converter model, grid-side and rotor-side control system models, etc. [3]. Given the complexity of wind turbine generators, this paper only presents the simulation models of the grid-side and rotor-side control systems.
2.1 Vector Control of Grid-Side Converter
The vector control simulation model of the grid-side converter of the doubly-fed asynchronous generator is shown in Figure 2. It consists of a three-phase phase-locked loop, a 3/2 vector converter, a current regulator, and a 2/3 vector converter [4-6].
Figure 1. Simulation structure diagram of grid-side converter control system
2.2 Vector Control of Rotor-Side Converter
Figure 2 shows the simulation model of vector control on the rotor side of the doubly-fed asynchronous generator. It consists of a 3/2 vector transformation, a reactive power regulator, a torque controller, optimal rotor q-axis current calculation, a current regulator, and a 2/3 vector transformation.
3. Simulation Analysis
This paper establishes a simulation model of a wind power generation system with a rated wind speed of 11 m/s and a rated turbine power of 1.5 MW in the MATLAB software platform. Simulation results show that the wind turbine generator set established in this paper can track the maximum wind energy well when the wind speed changes, and can achieve good active and reactive power decoupling control. Figures 2 to 6 show the wind speed model, generator speed, active and reactive power components, respectively.
4. Conclusion
This paper analyzes the working principle and mathematical model of a megawatt-class doubly-fed induction generator (DFIG) wind turbine generator. A simulation model of a 1.5MW wind turbine generator was established in MATLAB using a structured approach. Based on a stator field-oriented vector control method, and employing dual PWM converters, the model simulates maximum wind energy tracking and decoupled control of active and reactive power in a large grid-connected wind turbine generator. The simulation results verify the effectiveness of the control system and have significant practical implications for its application.
References
[1] Ye Hangye. Control Technology of Wind Turbine Generator Sets [M]. Beijing: Machinery Industry Press, 2006.
[2] Hui Jing. New Energy Conversion and Control Technology [M]. Beijing: Machinery Industry Press, 2008.
Figure 2 Simulation structure diagram of rotor-side converter control system
[3] Sun Lei, Wu Tao, Fan Xiaowei, Mi Zengqiang. Simulation Analysis of Operating Characteristics of Doubly Fed Asynchronous Wind Turbine Generators [J]. Electric Power Science and Engineering, 2009.6
[4] Chen Kunming, Tang Tianhao, Chen Xinhong, et al. Simulation of control strategy for permanent magnet semi-direct drive wind turbine [J]. Journal of Shanghai Maritime University, 2008.12
[5] Hopfensperger B. Stator-flux oriented control of a doubly fed induction machine with and without position encoder [J]. IEE Proc Electr Power Appl, 2000, 147 (4): 241-250.
[6] Li Wenjing, Xiong Guangyu. Simulation of asynchronous motor vector control variable frequency speed regulation system based on MATLAB [J]. Journal of Electric Power Engineering, 2006.2
About the author:
Dong Hongxi (1983-), male, from Yuncheng, Shanxi Province, is a postgraduate student at Taiyuan University of Science and Technology, specializing in wind power generation control technology.
Sun Zhiyi (1959-), male, from Changzhi, Shanxi Province, is a professor and doctoral supervisor at Taiyuan University of Science and Technology. His research interests include control theory and engineering.
Mailing Address : P.O. Box 633, Taiyuan University of Science and Technology; Postcode: 030024; Mobile: +86 13653659874; Email: [email protected]
