Research On STATCOM Based On MMC In Low And Medium Voltage Of The Distribution Network

Reactive power compensation is an important guarantee for the safe and economic operation of the distribution network, which is related to whether the users can get safe and high-quality power. STATCOM is the latest generation of dynamic reactive power compensation devices, which has the advantages of rapid response speed, continuous adjustment, remarkable effect of reactive power compensation, good performance of the output harmonic, convenient installation, suppressing voltage fluctuations and so on, which makes it a hot topic of modern reactive compensation technology. This dissertation researches the Static Synchronous Compensator(STATCOM) based on the Modular Multilevel Converter(MMC) systematically, including the analysis and design of main circuit parameters, the Pulse Width Modulation(PWM) control strategy, the control strategies of STATCOM based on MMC under pre-charge state and steady state, steady state characteristics analysis and control strategy research of MMC-STATCOM under the conditions of asymmetric bridge-arm, the design of hardware and software of the MMC-STATCOM system, simulation and experimental validation, etc.This paper analyzes the operating principles of MMC. On the premise of only consider fundamental components of bridge-arm voltage and current, the relationship between energy fluctuations of bridge-arm and capacitance value is derived, and design method of DC bus capacitor to suppress capacitor voltage fundamental frequency fluctuations is introduced. Considering the dynamic performances, bridge-arm current ripple and output power ability of MMC-STATCOM comprehensively, the design method of the reactive current-limiting reactor is put forward.To analyze the characteristics of the STATCOM based on MMC under steady state, this paper establishes the mathematical model of the system. Then the current feedback decoupling controller for the system is designed, which realizes decoupling and independent control of active power and reactive power. The multilevel pulse modulation technology suitable for MMC converter is studied. Of which, the carrier phase-shifted SPWM(CPS-SPWM) is more suitable for MMC control according to the output characteristics of MMC in steady state; the Nearest Level Modulation(NLM) is suitable for MMC in pre-charge state or the condition of more Sub-modules. In this paper, the DC capacitor voltage control strategy of MMC-STATCOM is studied, and a hierarchical control strategy based on CPS-SPWM modulation technique is proposed, which achieves the balance of system capacitance voltages.The uneven distribution of fundamental AC components between the upper and lower arms and fluxion of DC components and double-frequency components in AC side are due to asymmetric bridge-arm parameters of MMC. In this paper, the equivalent models of the AC side and DC side of a three-phase MMC-STATCOM are established under asymmetric bridge-arm parameters. Based on the equivalent sub-circuits at different frequencies, the effects of asymmetric conditions on MMC-STATCOM operation are analyzed in detail. A novel control strategy of suppressing the common-current of fundamental-frequency and double-frequency and keeping the voltage balance between the upper and lower arms is further proposed. Both simulation and experimental results validate the feasibility of the proposed design method.Finally, the experimental prototype of MMC-STATCOM is set up, and the circuit structure, working principle, software and hardware designs of the control system are studied systematicly. Experimental results show that MMC-STATCOM can achieve the function of capacitive and inductive reactive power, and meanwhile make the system get better steady-state and dynamic performance, which verifies the control strategy proposed in this paper is feasible and effective.