This thesis investigates the design, analysis, and operation of modular multilevel converters (MMC) for HVDC applications. Based on the operation principles of the MMC, the operation of MMC under asymmetrical arm impedance conditions is analysed using three equivalent sub-circuits at different freqeuncy. Detail analysis of the impact of asymmetrical conditions on the differential-mode current, the common-mode current and sub-module (SM) capacitor voltages, is performed. Based on the analysis, the corresponding control targets and an improved control strategy are designed to improve the operation performance. Considering the advantages of half-bridge based SM (HBSM) and full-bridge based SM (FBSM), a hybrid MMC (H-MMC) configuration consisting of FBSMs and HBSMs is proposed. By adopting the negative voltage state for some of the FBSMs, the output voltage range is extended to increase converter power transmission capability. By considering the relationships between the AC and DC voltages, AC, DC and arm currents, the ratio of the numbers of the FBSM to HBSM is analysed in order to maintain capacitor voltage balance and retain DC fault blocking capability. An equivalent circuit for the H-MMC is proposed, which considers each arm to be consisted of two individual voltage sources. This model is used to analyse SM capacitor voltage balancing and ripple. A two-stage selection and sorting algorithm is developed to ensure capacitor voltage balancing among the SMs. The proposed H-MMC is compared to other topologies in terms of power device utilization and power losses, and it shows that the H-MMC has higher device utilization and lower power loss than the conventional FBSM based MMC; Furthermore, The DC fault ride-through capability of the H-MMC are discussed. It is found that the H-MMC can not only isolate the DC fault, but also coniture operating at a wide DC voltage range from zero to rated value. Such two features of the H-MMC show the advantages in the hybrid configurations over the conventional FBSM and HBSM systems. Finally, two applications based on the proposed H-MMC are presented; one is a high power DC/DC converter with fault blocking capability for interconnecting large HVDC systems, and the other is a hybrid HVDC transmission system comprising a wind farm side VSC based on the H-MMC and a grid side LCC for transmitting wind power to AC grid.
|Date of Award||13 Jun 2015|
- University Of Strathclyde
|Sponsors||University of Strathclyde|
|Supervisor||Lie Xu (Supervisor) & Barry Williams (Supervisor)|