Research On Control Strategy Of Flexible Tuning Of Half-wavelength AC Transmission

Ultra-long distance,large capacity,reliable power transmission technology is essential to take full advantage of wind energy inside or near to the Arctic Circle and solar energy around the Equator during the development of Global Energy Interconnection(GEI).Half-wavelength AC transmission(HWACT)technology,as a kind of point-to-point,ultra-long distance and large capacity transmission technology,is one of the effective technical ways to support transnational and intercontinental power transmission.However,there are still some technical problems with HWACT.This thesis mainly focuses on the artificial tuning and grid connection of HWACT.The proposed flexible tuning scheme could be used to compensate the electricl length,improve power factor and damp power oscillation during grid connection.HWACT usually requires that the electrical length of the transimission line between the sending and receiving ends is about one half of a power frequency wavelength.Otherwise,appropriate technology should be employed to compensate the electrical length in order to maintain the HWACT characteristics.This thesis proposes a novel ?type flexible tuning,consisting of two parallel tuners and one series tuner.In addition to basic electrical length compensation,flexible tuning has value-added function such as power factor compensation,power oscillation damping(POD)during grid connection and overvoltage suppression in transient process.Then the working principles of the parallel tuner and the series tuner are introduced in detail,and the main circuit topology of the cascaded H-bridge multilevel topology is identified for further study.After that,the nearest level modulation(NLM)is selected as the modulation strategy of the flexible tuning,and the capacitor voltage balance control algorithm based on NLM is used to maintain the balance of capacitor voltage among sub modules.Based on the topologies of the flexible tuning main circuit,the working principle and the expected functions,the corresponding control strategies are designed.The electrical parameters of the parallel tuner and the series tuner are designed and an HWACT flexible tuning simulation equivalent system is built in PSCAD/EMTDC and RTDS respectively,using the parameters of the existing UHV AC lines.The simulation results confirm that the proposed ? type flexible tuning scheme is feasible,the proposed control strategy is effective and the flexible tuning scheme can flexibly adapt to the changes of system and its parameters.Finally,the characteristics of point-to-grid connection of HWACT has been studied.Then the POD control strategy of the series tuner is designed,the series tuner with appropriate control shall improve the point to grid connection characteristics of the HWACT.