Research On Fault Analysis And Protection Strategy Based On VSC-MTDC

VSC-MTDC(Multi-terminal DC Transmission System based on VSC), whichhas the wide application in renewable energy generation, high voltage DCdistribution system and distributed generation, is the trend of VSC-HVDC. Comparedwith two-terminal system, VSC-MTDC is more flexible and reliable. In this thesis,operation issues relevant to VSC-MTDC are investigated by means of theoreticalanalyses and simulation validations. The research on control strategies, DC-side faultanalysis and protection strategies against DC faults are the key work.Firstly, VSC-MTDC in parallel and VSC-MTDC in series are compared inoperating characteristics. Furthermore, three kinds of converters including two-levelconverter, three-level diode-clamped converter and MMC (Modular MultilevelConverter）are compared. The mathematic model of Multi-terminal DC System basedon MMC is deduced. Based on the mathematic model, the converter-level controlstrategies for the converters linked to different kinds of AC system are designed.Afterwards, the DC-side fault characteristics were analyzed in order to designthe proper protection strategy. The DC-side faults include pole-to-pole fault, singleline-to-ground fault and the DC open-circuit fault. The fault analysis was divided intotwo categories, and relative simulation model were built in PSCAD/EMTDC.Finally, with an eye to the future trend of protection strategy of VSC-MTDClines and the drawbacks of the existing protection methods, a boundary protectionstrategy based on wavelet analysis were proposed in this paper. A four-terminalmeshed MTDC model was built in PSCAD/EMTDC to validate the feasibility andeffectiveness of the protection strategies against DC faults proposed by this thesis.The simulation results were post-processed using Matlab. The simulation resultsshow that the system under fault can achieve fast recovery by using the protectionstrategies against DC faults. The protection strategies improve the fault ride-throughcapability of multi-terminal DC transmission system.