The Research Of Capacitor Voltage Balance Control Strategy For Modular Multilevel Converter Based VSC-HVDC System

In the world, the traditional energy resource has restricted the development of world economy, and how to develop and utilize new energy sources such as wind power and solar energy has been the focus of national research. After AC power and H VDC transmission, the development of flexible HVDC technology provides a flexible and reliable solution for a distributed renewable energy power generation and ocean island power supply. Academic circles called the technology as VSC-HVDC (Voltage Source Converter-High Voltage Direct Current). At present, MMC-HVDC (Modular Multi-level Converter-High Voltage Direct Current) which is one topology type of VSC-HVDC develop rapidly, and its application is more and more widely. The capacitor voltage balance control strategy received extensive attention of scholars at home and abroad as one of the focus in the study of MMC-HVDC.This paper briefly introduces the development history of MMC-HVDC converter and its application field. Based on in-depth analysis of the topological structure characteristics, working principle and mathematical model, paper studied the ladder wave modulation strategy based on sine approximations, and successfully applied to the subject being studied modulation of the inverter.From the charging and discharging process of capacitor, this paper studies the inverter voltage balance principle and analyzes the main factors of capacitor voltage unbalance. Based on the theory, the Papers related mathematical model was deduced and has done simulations to verify the influence of the factors. Paper in-depth analysis deficiencies of the current capacitor voltage balance control strategy, and combining with balance control strategy and target, is proposed one control strategy called the voltage difference which are based on sorting algorithm.In order to verify the validity of the capacitor voltage balance control algorithm, the article set up49level simulation model by using PSCAD/EMTDC software. From the start, unlock, power conversion and steady state operation, the paper has carried on the simulation research. Through simulation, the article concludes that under the environment of high voltage large capacity of MMC-HVDC, voltage difference algorithm is more effective and reliable.Based on theoretical research, Paper has set up a platform MMC-HVDC physical model test, and the capacitor voltage difference value control strategy has carried on the experiment. Through the analysis of test results and data, it proved the effectiveness of the capacitor voltage balance control strategy. The experiment has reached the expected goal.