Research On Suppression For Wide Frequency Range Subsychronous Oscillation In Wind Farms

In recent years,wind power has increasingly joined the new energy generation and played an important role in the transformation of energy structure and improvement of environmental issues.The subsynchronous oscillation problem of wind power generation is a serious threat to the safety,economy and stability of its operation.The wind power subsynchronous oscillation mechanism is more complex than the traditional turbine’s and the oscillation has a wide frequency range characteristic.These characteristics put forward new requirements for the suppression of wind power subsynchronous oscillations,especially for wind farms connected to the grid via the HVDC system,whose special structure makes the design of the suppressor more difficult.The research and results in the paper are summarized below.(1)In order to research the subsynchronous oscillation problem of wind farms,complete linearized models of PMSG and DFIG including electrical systems,converters and control systems are established first.The time domain response curves of the analytical calculation results and the time domain simulation results of electromagnetic transients are compared under the small disturbance conditions.The results show that the theoretical analysis results and the time domain simulation results are consistent.It is proved that the small-signal models of wind turbines established in this paper meets the needs of subsynchronous oscillation research.(2)A wind farm subsynchronous oscillation suppression strategy and controller parameter optimization method based on the quadratic performance index are proposed.Since the performance index and the optimization conditions are independent of the oscillation frequency,it has a wide frequency range subsynchronous oscillation suppression capability.Firstly,the weight coefficients of state variables and control variables are determined with time optimal control as the goal.And the structure and parameter of the optimal feedback controller are designed with the goal of achieving the minimal value of the performance index function.Based on the eigenvalue distribution of the small-signal models of the wind turbines,a small-signal model order reduction method which retains the main oscillation modes of the system is designed.The optimal feedback controller is also designed based on the reduced-order small-signal model.Simulation shows that the controllers designed in this paper have good adaptability and suppression effect on the wide frequency range subsynchronous oscillations of PMSG and DFIG.(3)A coordinated control method with local state quantities feedback for both wind farm and VSC-HVDC is proposed for the subsynchronous oscillation problem of wind farm outgoing via VSC-HVDC.Firstly,a complete linearized model of the wind turbine and VSC-HVDC system is established.Since there is a certain distance between the VSC-HVDC system and the wind farm,the structure of the coordinated controller is designed with the constraint that only the local state quantities participate in the feedback.And the controller parameters are designed based on the improved quadratic index.It is verified by simulation that the coordinated feedback controller has good suppression effect on the wide frequency range subsynchronous oscillation of the DFIG and PMSG via VSC-HVDC outgoing.