Research On Electromechanical Transient Modeling And Voltage Support Of VSC-HVDC Systems

Voltage source converter based high voltage direct current(VSC-HVDC)is a new generation of direct current transmission technology.Compared with line commutated converter based high voltage direct current(LCC-HVDC),it has many advantages such as no commutation failure and flexible control,which has attracted much attention from academia and industry.Considering the energy development strategy and power grids development trend in China,the impact of the VSC-HVDC technology on power systems mainly includes two aspects:(1)The emergence of hybrid multi-infeed HVDC systems;(2)Improve the consumption level of new energy grid-connected,and improve the security and stability of new power systems.With more and more VSC-HVDC projects commissioned,the operating characteristics of the entire AC/DC power system become more and more complex.Therefore,this paper studies the electromechanical transient modeling and voltage support of VSC-HVDC systems focusing on the above two aspects.The main contents and novel works are organized as follows:(1)The electromechanical transient model of power systems with VSC-HVDC is established.First,the fundamental equivalent circuit of the VSC is theoretically derived.Second,a practical power flow calculation method for power systems with VSC-HVDC is proposed.And a complete electromechanical transient simulation model is established.In order to further increase the simulation step size of electromechanical transient and improve the simulation efficiency of the system,the dynamic process of each internal link of VSC-HVDC system is quantitatively analyzed,and a simplified electromechanical transient modeling model of VSC-HVDC is derived.Finally,based on the proposed electromechanical transient simulation model,the stability calculation of Shanghai power grid is carried out,and the supporting effect of VSC-HVDC technology on conventional multi-infeed HVDC systems is preliminarily verified.(2)The steady-state operating area of VSC-HVDC in hybrid dual-infeed HVDC systems is analyzed.Based on the mathematical model of hybrid dual-infeed HVDC systems,a general calculation method for the steady-state operation area of VSCHVDC subsystems is proposed.This method can directly identify the safe operation area of VSC-HVDC subsystems on the P-Q complex plane.Furthermore,the impacts of the AC system strength,the interaction factor between VSC and LCC,the transmission power of LCC,the control mode of LCC and the system operating condition on the steady-state operating area of VSC-HVDC are qualitatively analyzed.Finally,the main limiting factors of the steady-state operation area of VSC-HVDC subsystems are clarified.From the perspective of improving the steady-state operating area of VSC-HVDC,the LCC inverter station should adopt the constant voltage control mode.(3)The power transmission capacity of hybrid dual-infeed HVDC systems is analyzed.The concept of the maximum power curve is extended to the hybrid dualinfeed HVDC situation,and a numerical calculation method for the power stability margin of hybrid dual-infeed HVDC systems is proposed.Based on the power stability margin index,the supporting effect of VSC-HVDC on LCC-HVDC is evaluated quantitatively.The quantitative relationship between the power transmission capacity and the effective short circuit ratio in hybrid dual-infeed HVDC systems is further verified.And the value range of the critical effective short circuit ratio is determined.Finally,the evaluation criteria for the stable operation of HVDC systems under different VSC control modes are given.(4)A general design method for strengthening regional power grids with high proportion of new energy using VSC-HVDC technology is proposed.The method mainly includes two parts: initial design of the scheme and technical comparison.In the initial design of the scheme,this paper focuses on the determination of location and capacity of VSC stations.From the two control dimensions of active power and reactive power,make full use of the technical advantages of VSC-HVDC to enhance the transmission capacity and voltage support strength of regional power grids.In the technical comparison,three evaluation indexes that related to the security and stability of power grids are put forward,namely line load rate,bus voltage level and system strength of the VSC-connected terminal.Finally,the optimal access scheme of VSCHVDC is determined based on evaluation indexes and comprehensive weight coefficients.