Analysis and design of the modular multilevel converter for secure systems

This thesis investigates the operation, dynamics and design of modular multilevel converters (MMCs) for stable and reliable operation. The internal dynamics and control schemes of the conventional MMC is analysed with consideration of passive component tolerances. Detail qualitative and quantitative analysis of MMC internal dynamics with different control strategies, is performed. Inter-arm passive component tolerances lead to fundamental and higher odd-order harmonics in the common-mode loop, which may cause external oscillation in the dclink. Considering the technical characteristics of the half-bridge SM (HB-SM) and fullbridge SM (FB-SM), a T-type MMC (T-MMC) consisting of two SM-based stages is proposed in this research. The first stage of the T-MMC is a conventional MMC; and the second stage is a series-connected flexible AC transmission system (FACTS) device. The reliability and stability of the converter are significantly increased, whereas flexible operation with various modes is attained. Also, converter ac and dc fault-tolerant capabilities become feasible. Two bypassing approaches are presented to reduce the conduction losses of the FB-SMs; therefore, the converter normal mode operation efficiency becomes similar to that of the conventional MMC. The T-MMC integrated with energy storage elements (ESEs) is studied, and it is found that the T-MMC based energy storage system (ESS) can not only isolate faults but maintain continuous power for the normal side, which greatly improves stability and reliability of the connected systems. A T-MMC based multi-terminal high voltage dc (HVDC) network is studied, indicating the effectiveness of the T-MMC in power system scenarios. The presented investigation and design are supported by theoretical analysis, simulation, and experimentation.This thesis investigates the operation, dynamics and design of modular multilevel converters (MMCs) for stable and reliable operation. The internal dynamics and control schemes of the conventional MMC is analysed with consideration of passive component tolerances. Detail qualitative and quantitative analysis of MMC internal dynamics with different control strategies, is performed. Inter-arm passive component tolerances lead to fundamental and higher odd-order harmonics in the common-mode loop, which may cause external oscillation in the dclink. Considering the technical characteristics of the half-bridge SM (HB-SM) and fullbridge SM (FB-SM), a T-type MMC (T-MMC) consisting of two SM-based stages is proposed in this research. The first stage of the T-MMC is a conventional MMC; and the second stage is a series-connected flexible AC transmission system (FACTS) device. The reliability and stability of the converter are significantly increased, whereas flexible operation with various modes is attained. Also, converter ac and dc fault-tolerant capabilities become feasible. Two bypassing approaches are presented to reduce the conduction losses of the FB-SMs; therefore, the converter normal mode operation efficiency becomes similar to that of the conventional MMC. The T-MMC integrated with energy storage elements (ESEs) is studied, and it is found that the T-MMC based energy storage system (ESS) can not only isolate faults but maintain continuous power for the normal side, which greatly improves stability and reliability of the connected systems. A T-MMC based multi-terminal high voltage dc (HVDC) network is studied, indicating the effectiveness of the T-MMC in power system scenarios. The presented investigation and design are supported by theoretical analysis, simulation, and experimentation