Refined Model For The Control And Protection System Of Uhvdc And Analysis Of The Mutual Interaction Between AC And DC Systems

With an increasing number of long-distance and high-power ultra high voltage direct current(UHVDC)projects being constructed and put into operation,the circumstance of ?strong direct current(DC)systems connect to weak alternating current(AC)systems? has been gruadully formed in the AC/DC power grid of our country.The mutural interaction between AC systems and DC systems poses many threats to the stabilization of the power grid.Establishing refined models to simulate the dynamic characteristics of the AC/DC power gird is the foundation to tackle these problems.However,there exsist remarkable differences between the UHVDC control and protection system of simulation models presented in existing studies and that of practical UHVDC projects.This makes it difficult for UHVDC models to simulate the dynamic characteristics of practical UHVDC projects.As a result,it is of great necessity to establish a refined model for the control and protection system of UHVDC and research on the mutual interaction between AC and DC systems.In this context,this thesis establishes a refined simulation model for the control and protection system of UHVDC.Then the mutual interaction between AC and DC systems are researched considering two aspects: the commutation failures of DC systems caused by AC faults and the stabilization problems of AC systems caused by the power impact of DC systems.The main progresses are as follows:(1)Aiming at the problem that exsisting control and protection simulation models for UHVDC are either too simplified to simulate the characteristics of practical UHVDC projects or too modularized for reserchers to conduct further studies,this thesis establishes a refined and open-style simulation model for the control and protection system of UHVDC.Based on this model,an electromechanical and electromagnetic transient simulation model for the AC/DC power gird is built.Simulation results are used to verify the accuracy of these models.(2)Aiming at the problem that most studies of commutation failures are based on basic models for high voltage direct current(HVDC)systems,this thesis analyzes the mechanism of commutation failures and proposes a method to calculate the probability of commutation failures considering the fault instant of the AC system and the effect of commutation failure prevention control(CFPREV)which is widely used in practical HVDC systems.Simulation results are used to verify the correctness of the proposed method.(3)A control scheme which coordinately alters the firing angles in the rectifier side and the inverter side of HVDC after AC faults to prevent commutation failures is proposed.Simulation results show that compared with the exsisting DC current predictive control(DCCPC),the proposed method has a better effect on the mitigation of commutation failures.(4)The influence of HVDC voltage dependent current order limiter(VDCOL)parameters on the stabilization of the receiving AC system is analyzed.A practical method to optimize the VDCOL parameters in a HVDC system is proposed and verified by simulations.