Mechanism Analysis And Mitigation Strategy Of DFIG Grid-connected Sub-synchronous Control Interaction

Sub-synchronous control interaction is a new type of sub-synchronous oscillation fault caused by the grid connection of doubly-fed wind turbines with fixed series compensation capacitors,causing a serious threat to the safe and stable operation of the power system.Moreover,doubly-fed wind turbines have a large number of nonlinear power electronic components and complex control system parameters.Therefore,it is somewhat difficult to analyse the mechanism of sub-synchronous control interaction in doubly-fed wind turbines and to study the suppression strategy.Currently,the analysis of the mechanism of sub-synchronous control interaction is mainly focused on the influence of the rotor-side controller parameters on doubly-fed wind turbines,while the influence of the grid-side controller parameters and the interaction mechanism between the rotor-side and grid-side controller parameters have not been studied in depth.The existing suppression measures concentrate on direct or indirect changes to the controller parameters to achieve the suppression effect.In this study,the mechanistic analysis of sub-synchronous control interaction and their suppression measures are investigated as follows.First,a mathematical model of a doubly-fed wind turbine grid-connected system is given.From the point of view of circuit properties and controller transfer functions,well-established analytical expressions for the DFIG impedance considering only the rotor-side converter control parameters and for the DFIG system impedance considering both rotor-side and grid-side converter controller parameters are derived,thus obtaining the conditions for the occurrence of sub-synchronous control interaction in the system.On the basis of the PSCAD/EMTDC simulation platform,the mechanism and pathways of the control parameters influence on the system impedance are revealed by the dynamic frequency sweep method in the case of operation without fixed series compensation capacitors.The validity and correctness of the analysis process is verified by comparing the effect of the control parameters on the system impedance through time-domain simulations under the condition with fixed series compensation capacitors.Then in order to address the problem of sub-synchronous control interaction in doubly-fed wind turbines,a strategy for suppressing sub-synchronous control interaction using an H-bridge cascade type static synchronous series compensator(HSSSC)device was proposed,the H-SSSC topology is presented and the SSSC operation mode is analysed.The H-SSSC control strategy for suppressing sub-synchronous control interaction is proposed in conjunction with the basic SSSC principles.It is verified through simulation results that the H-SSSC device can systematically suppress system oscillations caused by the sub-synchronous control interaction phenomenon and reduce the amplitude of output power oscillations.Finaly,in response to the shock disturbance caused by the H-SSSC device connected to the grid,a damping controller based on an improved whale algorithm was proposed to be attached to the H-SSSC device,where the input signal of the damping controller adopts the power difference between the two ends of the H-SSSC device.At the meantime,the whale algorithm was improved to take the minimisation of the power difference between the two ends of the access H-SSSC as the objective function,and the algorithm-optimised controller parameters are fed into the controller.The simulation confirms that the additional damping controller is effective in suppressing the shock disturbance brought to the system by the H-SSSC device.The optimised additional damping controller has a better dynamic response to all kinds of short-circuit conditions and improves the stability of the system.