Dynamic performance of voltage balancing and circulating current suppression control for modular multilevel converter

Abstract

Global power consumption has increased by approximately 3% each year over the past 15 years. The growing demand for energy has stimulated the spread of clean and reliable renewable energy networks and power grid interconnections throughout the world. For example, in Europe, there are 23 High Voltage Direct Current (HVDC) Transmission lines under construction which are scheduled for completion before 2024. The Modular Multilevel Converter (MMC) is one of the most attractive candidates for the HVDC transmission system converter technology. Its high flexibility and controllability make it an attractive option for HVDC transmission. However, the higher initial investment and the unfavourable conditions for using associated DC circuit breakers have always been a barrier to further installations. Since ABB successfully developed the HVDC DC circuit breakers in 2012, there is increasing interest in DC grids using the MMC HVDC transmission system. However, one of the common problems existing in the HVDC transmission system is the control of the capacitor volt-age in each submodule of the MMC. However, in the transmission systems, especially in the renewable energy systems, there are disturbances existing. The conventional voltage balancing control is weak to the disturbances, such as power and sampling frequency changes. Therefore, the proposed voltage balancing control in this thesis has improved the responding time and precision of the control. It determines the charging state of each submodule by deriving the capacitor voltage variations, thereby ensuring the voltage of each capacitor is within pre-defined range regardless the disturbance. In later study, both simulation and experimental results have shown the proposed control approach has strong immunity to the sampling frequency noise compared to the conventional control. However, even with the proposed voltage balancing control, the capacitor voltage difference cannot be eliminated entirely. They will cause circulating current flowing among the phases of the circuit. Therefore, causing unnecessary pressures to the affected components. The circulating current suppression control pro-posed in this thesis can eliminate the AC component of the circulating current, by regulating it according to the power going through the converter. It gets rid of the two PID controllers and abc-dq transformation which are commonly used in conventional circulating current control approach. The simulation and experiment results have shown the suppression of the proposed control approach regarding the AC components in the circulating current, and the fast response time taking effect within one control cycle. In this thesis, both proposed control approaches are presented with simulation results and validated with the scaled down experiment model.