The Research On Modulation Algorithms Of High Voltage Direcrt Current Transmission Based On Multilevel Modular Converter

In multilevel modular converter system,switches of sub-modules are controlled to synthesis terminal output voltages.Compared with the traditional converter,the output voltage of step response of the structure is reduced.Output waveform has high quality.The converter displays excellent fault handling ability.By control of the converter,system active and reactive power can be adjusted independently.Then the system can achieve asynchronous networking,distributed power synchronization and isolated power supply.This thesis makes a research of basic operating principle of MMC and mathematical model is established.Using the corresponding control and modulation algorithm makes the structure to get better application.It is introduced in detail as followed.(1)This thesis makes a research of basic operating principle of MMC and mathematical model is established.The existence of circulating current is deduced.A circulating current suppressor is used to curb the circulating current in order to reduce the current pressure imposed on switches and the consequences of common DC bus low frequency oscillation.(2)Voltage balance control of MMC is the key technology for its normal operation.Therefore,voltage balance control methods of sub-modules are studied.Proportional-integral method and sorting algorithm are adopted to balance voltages.(3)Based on the nearest level modulation(NLM)algorithm,this article introduces the concept of voltage deviation correction,which superimposes correction value of sub-modules caused by inconsistency of system parameters and adopts voltage sorting balance algorithm to equalize voltage.The scheme solves problems of voltage balance failure between arms due to varying converter parameters which uses traditional algorithm.(4)This thesis proposes a fast space vector algorithm in the application of MMC.It only needs simple addition,subtraction and shift operation to realize sector location and time calculation of reference vectors,which reduced the computational complexity and resource usage of digital controller greatly.Combined with sorting algorithm,it can achieve excellent control of MMC.(5)Considering the practical and requirements of resistant to high voltage and current,the number of sub-modules demanded is large.The paper adopts technology of automatic generation with m language to program,21 modules and 62 modules being generated automatically.Simulation and experimental results verify the correctness and validity of the algorithm.Furthermore,it shows strong robustness with the proposed algorithm.