Research On The Control Strategy Of Modular Multilevel Converter For STATCOM Applications

With the development of the modern power systems, its security, stable and economical operation problems are getting more and more attention, and reactive power balance is one of the key issues. Due to its excellent control performance and good compensation effect, STATCOM has become a focus in reactive power compensation field. Meanwhile MMC, as a new multi-level topology, also attracts much attention of domestic and foreign scholars. Therefore, this paper presents a new STATCOM, which is based on the MMC topology. Using carrier phase-shifting pulse width modulation(CPS-SPWM), this MMC typed STATCOM can effectively reduce the harmonic content and switching frequency. As easy to adjust its output voltage's level, the MMC typed STATCOM is suitable to expand to the high-power field.In this paper, the main research work is shown as below.(1) For MMC's specific topology, this paper details its working principle, and establishes its mathematical model, which lays a theoretical foundation for the following analysis of the modulation and the voltage-balancing method. Secondly, the basic principle of CPS-SPWM is analyzed in-depth, and a detailed modulation process is introduced. After simulating on Matlab platform, results show that the CPS-SPWM can effectively improve the equivalent switching frequency, and reduce harmonic content and the switching losses.(2) Through analyzing the MMC's basic structure, operating principle as well as CPS-SPWM in detail, this paper proposes a simple but effective method of balancing the capacitor voltages. On the basis of PI controller, by adjusting the capacitors' charge and discharge time, this method can effectively stabilize the capacitor voltage of each sub-module. In order to verify the effectiveness of the method, a 11-level MMC system is built on Matlab platform. and the results show that the proposed method can not only keep the sub-module capacitor voltage consistent, but also limit the leg circulation within acceptable range.(3) In this section, after analyzing STATCOM's reactive compensation principles, its mathematical model in three-phase abc coordinates and dq two-phase synchronous rotating coordinate is derived. According to the mathematical model, MMC typed STATCOM's control strategy based on PI controller and its equivalent circuit is put forward. In order to verify the feasibility and effectiveness of the proposed control strategy, MMC typed STATCOM system model is built on the Matlab platform. The results showed that, in the case of AC bus voltage's fluctuations, the system can quickly adjust reactive power, to maintain the bus voltage constant. Meanwhile, the harmonic content of the system is significantly reduced.