Research On DC Voltage Control And Smooth Switching Strategy Of Soft Open Point

With the large scale access of distributed power sources,the power flow of the distribution network is becoming more and more complicated,the problem of voltage limit violation is increasingly prominent,and the reliability of power supply and the level of power quality have been reduced.As a new type of flexible power distribution equipment,the soft open point has broad application prospects,and it has more flexible power flow regulation capabilities than the tie switch in the traditional distribution network.In order to improve the system operation stability and reliability,this paper focuses on the DC voltage control and smooth switching strategies of soft open point.Firstly,the basic working principle of the soft open point is analyzed,the distribution topology and terminal converter main circuit topology of soft open point are introduced.On the basis of establishment of soft open point mathematical model,the inner loop current control,outer loop constant active and reactive power control,constant DC voltage and reactive power control and constant AC voltage and frequency control etc.basic control of soft open point are researched.Secondly,to reduce the influence of system disturbance on the DC bus voltage of the soft open point,the disturbance rejection scheme with feedforward of the disturbance observation value is adopted,and a DC voltage disturbance rejection strategy based on the improved linear extended state observer is proposed.The simulation results show that the proposed strategy can effectively suppress the DC voltage fluctuation caused by system disturbance.And in order to maintain the stability of the DC voltage of soft open point under the feeder fault,a DC voltage coordination strategy and control terminal selection method for soft open point is proposed based on the master-slave control and distribution network information.Finally,to realize the smooth transition of the soft open point working mode in case of performing multiterminal coordinated control,the causes of shocks and oscillations during the mode switching are analyzed.The steady state current compensation is used to reduce the difference between the controller states before and after the switching,and the smooth switching strategy of soft open point is proposed.Moreover,the phase smoothing control structure was designed by adding off-grid locking and grid-connected pre-synchronization links on phase locked loop.Simulation results show that the strategy proposed in this paper can be adopted to realize smooth switching of soft open point under various conditions.