Dynamic Interaction Between AC And DC System With Closely Coupled Multi-infeed DC

The geographical distribution of power resource and consumption in China does not match.The construction of AC/DC power grid composed of HVDC and AC power grid makes large-scale power transmission become reality.The extensive application of HVDC and power electronic devices also brings a series of problems of stability and control of AC/DC power grid,especially its small disturbance stability,performing complex dynamic characteristics,which is related to the strength of AC power grid and the parameters of SVC control,et al,and has become the focus of attention of power systems all over the world.In order to discuss this problem,this paper first studies the work principle of HVDC system,static var compensator and AC/DC system control.The design of the structure and control of DC system must consider the security and stability requirements of DC system with power electronic devices;the reactive power compensation requirements of DC system and the SVC topology and control principle.Secondly,this research makes use of a four-terminal system with two DC links and one AC links designed by CIGRE.The model is characterized by the large electrical distance between the rectifier bus of two DC systems and the small electrical distance between the converter bus of the inverter side,which is a typical feature of the multi-infeed DC to AC/DC system in the near district.After that,a simulation model is built in PSCAD/EMTDC,including SVC.The operation results of the simulation examples are in line with expectations.Finally,based on the established model,the dynamic interaction characteristics of the system under small disturbances are studied.The effects of multi-DC feeding AC system strength change,SVC control parameters change on the system and the influence of the electrical distance of DC links receiving terminals on the subsynchronous oscillation are discussed respectively.The simulation results show that weak AC system or improper setting of SVC control parameters will lead to sub-synchronous oscillation of the system,and the electrical distance between DC links receiving terminals will affect the sub-synchronous oscillation characteristics.Therefore,in order to maintain the stability of the system,the AC system with multi-infeed DC should be maintained at a sufficiently high intensity,the control parameters of SVC should be adjusted within a reasonable range,and the appropriate electrical distance between DC links receiving terminals should be set.