| Modular multilevel converter DC transmission is the main form of flexible DC transmission engineering practice at present.For the flexible DC transmission system with a large number of power electronic switching devices,although electromagnetic transient simulation has high accuracy,its solution speed is slow and many parameters are required,compared with electromagnetic transient simulation,electromechanical transient simulation has fast calculation speed and requires less calculation resources.For a foreseeable period of time,electromechanical transient simulation is still the main method of security analysis of large power grid.On the basis of the analysis of control strategy and mathematical model of converter,this paper studies the electromechanical transient simulation of passive network connected to AC power grid through MMC-HVDC.The specific contents are as follows:Firstly,based on researching the operation principle of MMC,the mathematical model of MMC is built,the equivalent circuit of MMC AC side is got,and the control strategy of MMC-HVDC system is analyzed,which lays a foundation for the proposal of the control strategy and electromechanical transient simulation model of MMC-HVDC system connected to passive network.Secondly,the control strategy of MMC-HVDC system connected to passive network in dq rotating coordinate system is proposed and verified by simulation.Based on the decoupling control strategy including outer power control and inner current control adopted by MMC connected to rectifier side,the MMC controller connecting passive network is designed.The model of MMC-HVDC connected to passive network was built in PSCAD software,and it was verified that the control strategy proposed was effective under different working conditions.Then,the electromechanical transient voltage source model and current source model of passive side when MMC-HVDC is connected to passive network are built.The complex dynamic process of MMC is ignored,only retaining passive network and inner and outer loop controllers,building the electromechanical transient voltage source model of MMC-HVDC passive side.Afterwards,simplifying the dynamic process of the inner loop controller on the basis of the voltage source model,building the electromechanical transient current source model of MMC-HVDC passive side.The operating characteristics of two electromechanical transient models and electromagnetic transient accurate model were compared under different working conditions,verifying the effectiveness of electromechanical transient models under different degrees of disturbance.Finally,the influence of inner loop controller parameter on electromechanical transient models’ operation results is studied in diverse operating conditions.It is concluded that the influence of parameters kp on simulation results of the two electromechanical transient models is related to degree of disturbance for passive system,and the stability of electromechanical transient voltage source model is sensitive to the change of kp,it is necessary to be combined with correct engineering parameters for electromechanical transient modeling.The two electromechanical transient models built in this paper have certain reference significance for the control and modeling of passive network connecting to large power grid through Flexible DC power grid in practical engineering. |
