Research On A Novel Gradationally Controlled Modular Multilevel Converter And Its Application

Topologies of modular multilevel converter(MMC)with modular design are simple.It can be applied not only to various DC transmission fields,but also to renewable energy generation fields such as grid-connected PV system.With the development of power and voltage levels,the number of sub-modules(SMs)that make up the MMC increases.The hardware cost of the system is high,and higher requirements are placed on the computational capability of the controller and the performance of the capacitor voltage balancing controlled algorithm.To solve these problems,a novel gradationally controlled modular multilevel converter(GC-MMC)is proposed combined with the strategy of the gradationally controlled multilevel inverter.Related research is as follows:1.The strategy of gradationally controlled multilevel inverters is used in this paper to improve MMC.GC-MMC is proposed and studied its operating principle.In addition,a novel voltage balancing controlled algorithm is proposed to stabilize the capacitor voltage of each SM at the binary proportional relationship.The result of simulations verifies the feasibility of GC-MMC and the effectiveness of the capacitor voltage balancing controlled algorithm.2.The strategies of the valve-level and system-level are studied and a two-terminal DC transmission simulation system based on the GC-MMC is designed.A dual-loop control strategy based on d-q decoupling is used.It not only inherits the operating characteristics of the MMC-based DC transmission system,but also increases the level of the output voltage at the AC side when using the same number of SMs,which reduces the system hardware cost.3.The topology and operating principle of the grid-connected PV system are studied.A grid-connected PV system based on the GC-MMC is designed and built for simulation verification by the MATLAB/SIMULINK.Compared with VSC and MMC,the system based on GC-MMC has the advantage of improving the waveform quality of the output voltage.The harmonic and hardware cost are reduced by the system.4.The protection and optimization about the DC transmission system based on GC-MMC are studied.When a bipolar short-circuit fault occurs in the system,GC-MMCs can reduce the short-circuit current by blocking FBSMs.After optimizing GC-MMC,the level of output voltage can be increased by adding more FBSMs connected in series.