Research On Coordination Control Strategy Of Hybrid Cascaded Multi-terminal HVDC System

The line commutated converter based high voltage direct current(LCC-HVDC)has the advantages of large transmission capacity,low cost and high reliability,but LCC-HVDC has the risk of commutation failure,and the converter station needs a large number of reactive power compensation devices.VSC-HVDC(voltage source converter based high voltage direct current)avoids the risk of commutation failure fundamentally,and has the advantages of independent control of active power,reactive power and small floor area.Due to the development of modular multilevel converter(MMC)technology,the capacity of VSC is more and more large,but at present MMC still faces the problems of high cost and low transmission capacity.In order to combine the advantages of LCC and MMC,hybrid HVDC system has become an important research direction of HVDC.In this thesis,a hybrid cascaded multi terminal DC transmission system with long-distance and large capacity advantages,which uses LCC at the sending end and multiple MMCs in series at the receiving end,is studied.The basic structure and operation principle of the system are analyzed,and the steady-state mathematical model of the system is derived.Then,the basic control is designed for the hybrid cascaded multi terminal DC transmission system proposed above The system model is built in PSCAD /EMTDC simulation platform,and the start-up,steady-state and AC fault characteristics are simulated.The simulation results show that the control strategy designed in this thesis makes the system have good steady-state and transient characteristics.Finally,in order to solve the problem that the basic control strategy can not take into account the power requirements of the AC system connected to each converter station on the inverter side,an additional power control strategy is designed for the system based on the basic control strategy.In addition,the fault control strategy is designed to solve the problem of large fluctuation of DC voltage and DC current caused by additional power control strategy and the risk of LCC commutation failure.The additional power control and fault control strategy are simulated and verified.The simulation results show that,compared with the basic control strategy,the additional power control strategy can realize the independent control of the active power of each converter station on the inverter side,making the scheduling more flexible.At the same time,the fault control strategy based on the additional power control strategy can effectively reduce the risk of commutation failure and improve the fault recovery characteristics of the system.