Research On Modular Multilevel Converter (MMC)

With the rapid development of power electronics technology, multilevel converter and its control strategies is becoming one of the hot spots in the field of high-voltage and/or high-power applications. Modular multilevel converter (MMC), features with the characteristics such as high modularity, low generation of harmonics, etc, is one of the next-generation multilevel PWM converters intended for medium-or high-voltage utility grid without line-frequency transformers. So the study on some key issues of MMC, experimental system design and its application in power quality controls, especially for the negative-sequence balancing, has been conducted in depth in this dissertation.By controlling the role of circulating current as current controlled voltage source (CCVS), the circulating current model of MMC was proposed. Analysis of the key factors affecting the circulating current shows that circulating currents cannot be completely eliminated, but can only be suppressed by means of control. The existing of the phase-to-phase circulating current component, which produced by redistributing the active power among three phases through the common dc-bus, shows that MMC is suitable for the need to achieve negative-sequence balancing applications. Based on the circulating current model, a universal circulating current suppressing strategy has been proposed and adopted for improving the power quality of leg current, suppressing the common-mode voltage and the low frequency oscillation appeared in the common dc-bus current.Detailed DC capacitor voltage has been developed using the switching functions concept. Moreover, a harmonic elimination technique based on the modified switching function is applied for suppressing the low-order harmonics in leg currents, which will further reduce the leg inductor requirement. On the basis of derivation of its accurate mathematical model, the current decoupling control theory is proposed, which provides the theorial fundamental for the further research on the steady-state and dynamic-state performance of MMC.The carrier phase-shifted SPWM (CPS-SPWM) is more suitable for MMC control compared with other multi-carrier SPWM techniques. So, this dissertation proposes a double-modulation CPS-SPWM (DM-CPS-SPWM) according to the output characteristics of MMC, and the implementation of DM-CPS-SPWM in MMC has been explained in detail. Moreover, a new power banlance control schemes of carrier disposition SPWM (CD-ROT-SPWM) technology based on carrier rotating diposition concept is proposed. The new CD-ROT-SPWM technology ensures the output power balance among different power units.Two pre-charging control methods without any external DC voltage sources are proposed to solve the DC capacitor voltage initialization issue. By analyzing of the key factors of the DC capacitor voltage imbalance phenomenon, a combination of the individual voltage control and the averaging voltage control strategy is proposed for DC capacitor voltages balancing control, which steady-and dynamic-state performance is verified by both the subsequent simulation and experimental results.Based on the analysis of the design principles of the main circuit component parameters, two sets of experimental prototypes and a set of DSP and FPGA-based digital master-slave control system have been designed and constructed. Representative experiments have been carried out on the experimental prototypes, and the testing results have verified the research work in this dissertation.The concept of M-STATCOM is introduced firstly. The study on the working principle and control scheme under two different conditions are conducted. Simulation and experimental results show that M-STATCOM can not only be controlled in static var generating condition for reactive power regulation purpose, but also be suitable for the comprehensive compensation of all the power quality problems such as negative-sequence balancing, reactive power compensation and harmonics suppression. M-STATCOM shows good prospects in electrical traction lines, medium-or high-voltage power distribution system, etc.