Research On Analysis And Suppression Measures Of Low Frequency Oscillation In DC Interconnected Power Grid

Large interconnected power systems are more economical,but low-frequency oscillations occurs occasionally.High voltage direct current(HVDC),as an important means of large capacity and long distance transmission,has obvious advantages in damping inter-area oscillations.In AC/DC hybrid systems,the high controllability of DC power can provide fast power support for AC tie-lines,increase system damping and suppress inter-area low-frequency oscillations.The inter-area low-frequency oscillations are completely eliminated during asynchronous interconnection,and the ultra-low frequency oscillations become the main stability problems.Firstly,the dynamic mathematical model of the system is established,including the prime mover and its governor,the traditional HVDC transmission system,the voltage source converter based high voltage direct current(VSC-HVDC)system and the load.Using PSCAD/EMTDC,the AC/DC hybrid simulation system and DC asynchronous networking system are built on the basis of typical four-machine two-zone system.In AC/DC hybrid systems,HVDC supplementary controllers are designed.The suppression effects of the HVDC supplementary controllers on forced oscillation are analyzed from three aspects:When the frequency of disturbance source is the inter-area natural oscillation frequency,the suppression effect will be the best;With the electrical distance between the disturbance source and the DC converter buses increase,the suppression effects will decrease slightly,but are still obvious;With the HVDC transmission power level higher,the suppression effect will be better.VSC-HVDCs are used more widely because of no commutation failures.The same design method of the HVDC supplementary controller is used to design the VSC-HVDC supplementary controller.The simulation results show that in the research of disturbance sources location and frequency,the suppression effects of HVDC supplementary controller on forced oscillation are the same as that of VSC-HVDC supplementary on forced oscillation;The suppression effects are almost independent of the HVDC transmission power level.In DC asynchronous interconnection systems,a transfer function identification method is proposed combined with damping torque analysis(DTA)method to accurately capture the damping characteristics of speed-governing systems.The influences of water starting time,hydro penetration,load characteristics,and speed-governing systems' control mode on ultra-low frequency oscillation are analyzed.From the analysis and simulations,large water starting time,high hydro penetration,small load frequency characteristic coefficient,and unsuitable control mode of speed-governing systems which worsen the damping of system may be the reason for ultralow frequency oscillation occurrence in Yunnan Power Grid.The ultra-low frequency oscillation can be effectively suppressed by adjusting the control mode of the speed control system or adding the GPSS.