Reasearch On Novel Protection Shcemes For UHVDC Transmission Line

Ultra High Voltage Direct Current (UHVDC) transmission line is main backbone ofpower system, the important mission of UHVDC transmission line is the electric energytransmission from the source of energy to the power load center. The high performanceprotection scheme for UHVDC transmission line can detect line faults quickly, itguarantees safety operation of the UHVDC system and the adjacent ac system. So theUHVDC transmission line protection has the important role in the security of the powersystem.The dissertation focuses on the response features of UHVDC transmission line faultsand novel protection schemes for UHVDC transmission line. The main contributions areas follows:(1) The UHVDC numerical simulation model is presented in this dissertation and thedetailed structures and parameter configuration of the UHVDC transmission system modelare also introduced comprehensively, the reliability of the UHVDC transmission systemmodel is verified.(2) The effect of long distance UHVDC transmission line on differential protection isanalyzed and an improved differential protection scheme with distributed capacitivecurrent compensation is proposed. Comparing the performances of differential protectionwith/without distributed capacitive current compensation, the result shows that thereliability and sensitivity of DC differential protection are effectively improved withdistributed capacitive current compensation.(3) Two problems facing traditional differential protection for UHVDC transmissionline: the misoperation under external faults and the refuse operation under internal faults.In the dissertation, a novel differential protection scheme for UHVDC transmission line ispresented. With parameters of DC transmission line, the shunt branches are corrected andthe harmonic currents are removed, the reliability and sensitivity of DC differentialprotection are effectively improved.(4) The relation between the parameters of DC transmission line and the variation of transient energy at two terminals of the UHVDC transmission line are analyzed and theresponses of the variation of transient energy under different faults are investigated. Atransient energy protection scheme for UHVDC transmission line is proposed. It can notonly identify internal fault and external fault correctly and quickly, but also can responseto the high ground resistance fault.(5) Based on the boundary characteristic of the UHVDC transmission line, theresponses of characteristic harmonic currents at two terminals of the UHVDCtransmission line are analyzed under various faults. The periodic characteristic harmoniccan be detected under internal fault. According to that, a new characteristic harmoniccurrent protection scheme is proposed and the influence on the performance of theproposed protection by two factors including fault resistance and fault location is alsodiscussed.(6) VSC-based HVDC is a more flexible HVDC transmission mode, the active powerand reactive power can be decoupled and controlled independently in VSC-HVDCtransmission system. The responses of the first carrier frequency harmonic current at twoterminals of VSC-Based HVDC transmission line are analyzed under various faults. Acarrier frequency protection scheme for VSC-Based HVDC transmission line is proposed.By the protection scheme, the internal fault and external fault can be identified correctly.(7) The RTDS system and fault recorder data from UHVDC converter substation areemployed to verify the proposed protection schemes. The presented protection schemescan trip correctly under internal faults, so the proposed protection schemes in thisdissertation are reliable and feasible.