| The advantage of DC transmission in the aspects of transmitting large capacity power long-distantly, interconnecting power system with DC transmission as well as submarine cable transmission and asynchronously interconnecting the both ends of AC system etc. is significantly prominent, to which more and more attention has been laid. Several DC transmission lines have been built and more DC transmission lines are now being under design in China. Significant effects on the running of AC power system that has been existed previously will be produced inevitably accompanying the establishment and operation of DC transmission system. This dissertation studies the theory and realization of AC and DC hybrid system supporting with each other by coordinated control under the strategy of DC active power emergency control as well as its impacts and function for maintaining the stability of system. The stability of AC and DC hybrid system is also studied when DC defaults or small disturbances happen after ultra-high voltage multi-feed DC system being connected into AC power system. The theories of relay protection characteristics of AC and DC hybrid system as well as fault location for hybrid transmission line are investigated. The relative protective and monitoring device and faults distance measuring plug-in card are developed. At last, the distribution characteristics of DC quantities in hybrid system are studied and effects on DC quantities distribution caused by soil properties are analyzed. The main research contexts are as follows.Based on DC frequency modulation and DC active power modulation, the principle and realization method of AC and DC hybrid system power supporting with each other by coordinated control under the strategy of DC active power emergency control is investigated, and also the stability impacts on AC and DC hybrid system after DC power emergency control being introduced is studied. Taking the concrete three double-aisled lines of HongGou-BanQiao, ZiYang-BeiPei and LongQuan-LongWang in southwest hydropower AC-DC hybrid system for example, the choice of control signals needed to be used to coordinate and cooperate with DC system is investigated when the system lost stability caused by the faults of three-phase permanent tripping taking place. The configuration principles of the third defense line as well as its coordinated control strategy for maintaining system stability are studied. The system stability is studied when the send out DC system and backbone lines which lie in the vicinity of inverter station run in the modes of foundation way and extreme way separately after the ultra-high voltage multi-feed DC system being connected into AC power system in SiChuan electric network. The principle of reactive power voltage characteristics of transmission line and the transfer function control model of generating set are analyzed. The security and stability of system is studied when the send out lines or generating set go wrong.Takeing Sichuan AC-DC hybrid power grid as the study object, the effect on relay protection of AC transmission line caused by the connection of DC system is theoretically analyzed in the first place, and the validation test is completed based on RTDS, through which the effects as well as corresponding protective measures under different protective theory of AC system are obtained when multi-feed DC transmission system is introducted. Based on the theory of two-terminal travelling wave, an algorithm of faults location for hybrid transmission mixed by overhead lines and cables was proposed. The simulation model of fault transient travelling waving for hybrid lines which has embedded the algorithm through some two-terminal travelling wave device for fault range measuring is established to verify its validity. Result shows that the location error of this algorithm is no more than 1km with repeatability. Combined with relay protection formore, the implementation scheme of adaptive reclosing contral system for hybrid transmission line was constituded. The faults distance measuring plug-in card on basis of travelling wave was developed, which posseses the function of travelling wave sampling, time synchronization, travelling wave faults distance measuring, communication and evevts warning and so on simultaneously.The distribution characteristics of DC quantities in hybrid system is studied, and effects on the distribution of ground surface potential and direct current caused by stratified soil structure with monopole mode working is calculated and analyzed respectively. Based on actual project parameters applied in De-Bao DC transmission item, the simulation model of DC transmission under monopole earth return mode of operation was built by CDEGS simulation software in the situation of stratified soil structure. The sensitivity to ground surface potential and direct current for soil parameters is studied. Compared with the second and third layer of soil, the variation of soil resistivity as well as its depth of the first layer soil has a greater influence on DC quantities(that is ground surface potential and direct current distribution) while ground surface potential is less affected than direct current distribution by the soil parameters. |
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