Impact Of Transition Resistance On Distance Protection Of The AC Transmission Line Near The MMC-HVDC System

MMC-HVDC system generally possesses the fault-ride-through capability when a fault happens in the nearby AC transmission line,and provides a certain short-circuit current to the fault point.The fault characteristics of the provided short-circuit current are affected by the control system of the MMC-HVDC system,which are different from the characteristics of the short-circuit current provided by the conventional AC system.Therefore,the impact of transition resistance on distance protection of the AC transmission line near the MMCHVDC system is different from that of the conventional AC system transmission line.Scholars have made a thorough study on the impact of transition resistance on distance protection of conventional AC system transmission line.However,there is still a lack of study on the impact of transition resistance on distance protection of the AC transmission line near the MMC-HVDC system,thus the impact is analyzed in the thesis.In section I,the external characteristics of MMC-HVDC system is studied.The topology,mathematical model,control system and protection configuration of the MMC-HVDC system are summarized.The external characteristics of MMC-HVDC system are studied under internal fault conditions of different fault locations and under the faults in the AC transmission line nearby,respectively.It is concluded that MMC-HVDC system is equivalent to symmetrical three-phase impedance under the most serious internal fault condition and current source with limited amplitude under nearby AC transmission line fault.In section II,the impact of transition resistance on phase-to-phase distance protection of AC transmission line near the MMC-HVDC system is studied.The theory related to distance protection is discussed.The analytical expression of the measuring impedance under phase-to-phase fault varying with the transition resistance is derived.The impact of transition resistance on phase-to-phase distance protection is then analyzed.The validity of theoretical analysis is verified via PSCAD/EMTDC simulation program.In section III,the impact of transition resistance on ground distance protection of AC transmission line near the MMC-HVDC system is studied.The analytical expression of the measuring impedance under grounding fault varying with the transition resistance is derived.The impact of transition resistance on ground distance protection is then analyzed.The validity of theoretical analysis is verified via PSCAD/EMTDC simulation program.In section IV,a current-ratio-based protection strategy for the AC transmission line near the MMC-HVDC system is proposed.The shortcomings of current differential protection principle under the fault of the AC transmission line near the MMC-HVDC system are analyzed.The current ratio protection principle is proposed.The ability of the proposed protection principle against transition resistance is analyzed under different fault types.The validity of theoretical analysis is verified via PSCAD/EMTDC simulation program.