Decoupling Model and Control Strategy of Nonagonal Modular Multilevel Converter
Cheng Qiming1, Du Tingwei1, Cheng Yinman2, Lai Yusheng1
1. Shanghai Key Laboratory of Power Station Automation Technology College of Automation Engineering Shanghai University of Electric Power Shanghai 200090 China; 2. North Power Supply Branch Shanghai Electric Power Company Shanghai 200041 China
Abstract In the process of exploring new renewable energy, offshore wind power is becoming the focus of research in the field of new energy at home and abroad because of its strong stability, high annual utilization hours, small impact on topography, suitable for large-scale development and other characteristics. And the fractional frequency transmission is the most widely used transmission mode,nonagonal modular multilevel converter has attracted much attention because of its multi-port, direct AC-AC conversion capability, good low frequency performance and high reliability. However, nonagonal converter has the problem of strong coupling of electrical components with different frequencies, current research usually does not decouple the internal electric quantity but directly controls it, and the control effect needs to be improved. To address these issues, this paper proposes a decoupling method of the nine-sided converter. On this basis, the advanced control method is used to make up for the deficiency of the traditional control method, so as to improve the control performance of the system. Firstly, according to the topological structure of nonagon converter, the relationship between voltages and currents of the bridge arms is deduced in detail, and the decoupling mathematical model of nonagonal converter is derived. In this way, the complex system can be decomposed into several independent subsystems, and the coupling effect is reduced to a minimum, thus reducing the complexity of control design, simplifying the controller design process, and contributing to the adoption of advanced control methods, so as to optimize the system and adjust parameters. Then, PI control of N-MMC is proposed in this paper. However, when dealing with complex nonlinear nonagonal MMC system, the performance of PI controller will be limited due to its nonlinear dynamic characteristics, uncertainty and strong coupling. For example, the tracking speed and accuracy of the inner loop current will also be reduced. Therefore, for complex nonlinear nonagonal MMC, it is limited to overcome the shortcomings of PI control only through parameter adjustment, and it is necessary to consider nonlinear control methods to achieve better control performance. Then, nonlinear passive control (PBC) is proposed for the control of nonagonal MMC. Finally, the PI control and PBC control system simulation of the nonagonal MMC is built on the MATLAB platform, and a variety of working conditions including dynamic working conditions and fault working conditions are simulated. By comparing the effects of PBC control, PI control, and quasi proportional resonance (QPR) control without decoupling, semi-physical simulation is carried out on RT-LAB experiment platform to verify the feasibility and superiority of the decoupling model and control strategy proposed in this paper. The following conclusions can be drawn from the simulation analysis: (1) The decoupling model proposed in this paper can realize the decoupling of the frequency components of the bridge arm voltage and the bridge arm current of the nine-sided converter, and the PI and PBC control strategies designed on this basis can make the system run stably, and the system shows good control characteristics under various working conditions such as steady state, dynamic and single-port open fault. (2) By comparing QPR control, PI control and PBC control, it can be seen that the tracking speed under PBC control is faster, the dynamic stability and rapidity are better, and the harmonic pollution is lower. (3) Compared with M3C and hexagonal converter schemes, the nonagonal converter not only retains good low-frequency characteristics and the characteristics of direct AC-AC conversion, but also can realize the efficient interconnection of multi-ports and improve the integration of the system, which has great development potential in the field of fractional frequency transmission collection system and offshore wind power.
Cheng Qiming,Du Tingwei,Cheng Yinman等. Decoupling Model and Control Strategy of Nonagonal Modular Multilevel Converter[J]. Transactions of China Electrotechnical Society, 2024, 39(13): 4037-4051.
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2024, Vol. 39 
