Research On The Capacitor Voltage Control System Of Modular Multilevel Converter

In recent years, with the development of power electronic devices technologies, high voltage power electronic converters have been widely used in high power industry. Due to multilevel converter special structural advantages, it has gradually become the main form of high voltage and power application.Modular multilevel converter(MMC) is a new voltage source converter topology, which is widely used in high voltage DC transmission(HVDC), wind energy conversion device, high power motor drive and reactive power compensation device etc. Especially, MMC has become a more common type of converter in HVDC applications. Like traditional cascaded H-bridge multilevel converter(CHMC),it has the advantage of the highly modular structure and output voltage waveform preferred, which is beneficial to high voltage motor drive. At the same time, it can share the common DC bus, achieving a direct connection back-to-back in high power conversion applications.In this paper, we first introduce the circuit structure, modulation algorithm and basic working principle of MMC, then analyze the circulation characteristics of the circuit and its influencing factors. The analysis shows that the circulation characteristics can not be completely removed, and can be controlled by the control technology. In the analysis of the suppression method based on negative sequence coordinate transformation circulation premise, we put forward an improved circulation suppression method, which can restrain effect on the DC current ripple, and can effectively reduce the effect on bridge arm current. Meanwhile, based on the harmonic analysis, a harmonic suppression strategy is proposed, which can effectively reduce the middle and lower harmonics and the burden of the bridge arm inductance. It can be improved output voltage waveform for the capacitor voltage control system.As can be seen from the circuit structure of the MMC, a single sub-module is a half bridge structure.In order to prevent the switching device to open at the same time in the voltage balance control process, we must introduce the dead time in the control signal, which need for dead-time compensation to reduce the influence of dead time. Based on the research of the existing hardware compensation and software compensation, this paper proposes a dead-time compensation strategy based on CPLD. The simulation experiment verifies that this method can effectively improve the accuracy of dead time compensation.This paper focuses on the MMC sub-module capacitor voltage balance strategy. Firstly, we analyzes the capacitor voltage balancing control strategy existing literature proposed, which shows that the capacitor voltage unbalance causes is closely related to the circuit circulation, mainly as follows: the energy imbalance between the output DC voltage and DC bus voltage, the unequal total energy in the upper and lower bridge arm and the unequal average time of each sub-module. Based on the analysis of these reasons, a new control strategy for the voltage control of the bridge arm is presented, which is based on the closed-loop control algorithm. The simulation experiment shows that the method can effectively achieve the sub-module capacitor voltage balance.Finally, PSCAD/EMTDC-based the three-phase five-level system simulation circuit is built, the main circuit structure and software graphical user interface have also been given. Then, the simulation experiment results show that the capacitor voltage control system is effective and reasonable for the sub-module capacitor voltage balance.