Analysis Of Subsynchronous Oscillation Characteristics And Damping Suppression Strategy Of Doubly-fed Wind Farm Based On Impedance Modelin

There are significant differences between the geographical locations of wind power generation bases and load centers in China,the wind power transmission line often shortens the distance between the two through series compensation capacitors,so as to realize massive,remote and Offsite grid connection transmission of wind power,However,the series compensation capacitor is easy to interact wi th the doubly-fed wind turbine controller module,causing the problem of sub-synchronous oscillation,Leading to a decline in the power quality of wind farms,poses a threat to the stable operation of wind power grid connected systems,Therefore,the research object of this paper is the grid connected system of doubly fed wind farms via series compensation lines,Study on the problem of sub-synchronous oscillation caused by the interaction between the series compensation capacitor and the doubly-fed fan controller module.Firstly,this paper is based on the topology of the grid connected system of a doubly fed wind farm through series compensation lines,an equivalent circuit model considering the influence of the inner and outer rings of the rotor side conv erter of a doubly fed fan is established,and characterized in the form of resistance,inductance,and capacitive impedance,Further,Based on the resistance,inductance,and capacitive impedance models of the series compensated grid connected system of a doubly fed wind farm,draw impedance frequency characteristic curve of grid connected system,put forward a method for determining the stability of sub-synchronous oscillations in grid connected systems based on damping stability criteria is proposed,and analyze the sub-synchronous oscillation stability of the system according to the damping stability criterion,In addition,based on the resistance,inductance,and capacitive impedance model,analyze the occurrence mechanism of the sub-synchronous oscillation of the double-fed fan grid-connected system;Secondly,by calculating the impedance sensitivity of each control parameter,quantitative evaluation of the influence of various control parameters on the sub-synchronous oscillation characteristics of a doubly fed fan connected to the grid through series compensated capacitors,and introduce the concept of relative impedance sensitivity,eliminate the parameter dimension,obtain the influence order of each control parameter,and determine the key parameters that affect the sub-synchronous oscillation of the system;At the same time,the coupling law of the effect of the proportional gain of the inner ring and the proportional gain of the outer ring of the fan rotor controller on the sub-synchronous oscillation is analyzed;Simulation model of grid connected system using doubly fed wind farm,the accuracy and effectiveness of the research results in the analysis of sub-synchronous oscillations in doubly fed wind farms are verified;In addition,based on the negative damping expression of the rotor-side controller,design of an additional damping controller considering the influence of parameters,and add it to the rotor controller of the doubly fed fan,simulation results demonstrate the effectiveness of the damping controller considering the influence of parameters in suppressing sub-synchronous oscillations of the system.Finally,a control strategy for suppressing the next sub-synchronous oscillation of grid connected systems under multiple operating conditions is proposed.According to the occurrence mechanism of the sub-synchronous oscillation,the strategy reconstructs the extended state observer with the double-fed fan rotor control equation as the underlying model,realizing real-time tracking and observation of sub-synchronous components in rotor converters The effect of the sub-synchronous component on the output voltage of the rotor is compensated by introducing the direct-insulation link and the proportional-phase shift link,thus effectively suppressing the sub-synchronous oscillation under different series compensation degrees.In addition,analyze the mechanism of sub-synchronous oscillation suppression through theoretical derivation.and verify the stability of the proposed controller using the Herwitz criterion,simulation model of grid connected system using double fed fans,comparative analysis of the suppression effects of the proposed control strategy on sub-synchronous oscillations with additional damping control and active disturbanc e rejection control,the results show that this strategy can effectively suppress sub-synchronous oscillations in grid connected systems under different operating conditions,improved robustness of the doubly fed wind farm system.