HVDC Transmission Line Protection Operating Characteristic Analysis And Improvement

HVDC(High Voltage Direct Current) transmission line is a crucial part in HVDC system. The accurate operation of line protection to detecting faults and disturbances is vital for safe and stable operation of HVDC system. Timing sequence mismatching of traveling-wave protection criterions, as well as which of traveling-wave protection between differential under-voltage protection, and inappropriate design of line longitudinal differential protection logic are existing problems in line protection. Therefore, we conduct the researches as following.Aiming at the timing sequence mismatching of traveling-wave protection which result in misoperation during lightning faults, this dissertation compares the timing sequence matching characteristic of traveling-wave protection criterions while ground faults and lightning faults applied on DC transmission line respectively, and the results show that only one high level signal occurs in the criterion logic during the former faults, while several occur in the latter. According this line, we propose to block the high level signal in the criterion logic for a specific period after which has been outputted by that logics. By this way, the misoperation of traveling-wave protection result from the repeatedly triggering of criterion logics would be avoided.In addition, focus on the selectivity problems between traveling-wave protection and differential under-voltage protection during ground faults applied on DC transmission line, by analyzing their operation logic and simulation results, we find that is because in the differential under-voltage protection, the blocking signal, RESSFP, is lag behind the operating time. Hence, an improvement proposal has been presented that introducing the traveling-wave protection operation signal into the differential under-voltage protection blocking logic. And the selectivity as well as reliability of the protection has been improved in view of the simulation results.Finally, since the line longitudinal differential protection would disoperation yet the pole protection would block the pole during the high-impedance ground fault, we take the principle of line longitudinal differential protection and its logic into consideration and conclude the problem is because the protection has been blocked by the “delay blocking logic” of which for 600 ms, that delay the operation time for more than 1.1s. Hence, we propose to introduce the AC voltage blocking criterion into the line longitudinal differential protection, and the simulation results in an actual HVDC transmission project verify the availability of this criterion.The research in this dissertation would provide a reference decision-making to operators when dealing with the DC transmission line faults, and provide academic bases for operation and design of HVDC relaying protection.This work is supported by the National Science Foundation of China(51377059), and the correctness and effectiveness of some achievements in this research has been validated by an actual DC project in EMTDC.