Mechanism Analysis And Suppression Strategy Of Grid-connected Sub-synchronous Oscillation In Doubly-fed Wind Farms

With the large-scale new energy grid-connected operation,the power oscillations induced by series-connected capacitor compensation into the grid operation or the delivered by HVDC have caused the facus of attention in the field of power system simulation analysis and dispatching operation.In this paper,we study on the subsynchronous oscillation induced by the power dlivery via series capacitor compensation,and deeply revealed the oscillation mechanism and characteristics of subsynchronous oscillation?SSO?/subsynchronous control interaction?SSCI?in doubly fed induction generator farms.Finally,this paper aiming at the subsynchronous control interaction from the viewpoint of oscillation propagation path and rotor side converter control strategy,the corresponding power oscillation supression strategy is proposed.In order to study the mechanism of subsynchronous control interaction,a small signal strability analysis model of doubly-fed wind turbines,inverters,series compensated transmission lines and DC bus estabilished firstly.Then,through the eigenvalue analysis and the participation factor analysis of state variable corresponding the main modes,the distribution of the participation factors of the three main oscillation modes is obtained.Finally,the influence varibles of the SSCI are obtained as rotor-side controller's parameters,series compensation transmission lines and wind speed.According to the above eigenvalue analysis,the simulation model of the double-fed wind farm constructed by the electromagnetic transient simulation software—PSCAD/EMTDC,and it is used to simulate the different wind speed,series compensation and PI parameters of RSC Time domain simulation verification.The simulation results show that the smaller the wind speed,the greater the series compensation and the larger the proportional coefficient of the rotor-side converter,the smaller the integral coefficient,the more serious the oscillation of the subsynchronous control interaction.At the same time,through the comparative analysis,the PI parameters of the current loop are more sensitive to the SSCI than the PI parameters of the power loop.Aiming at the existing damping control strategies,the control strategies of doubly-fed wind turbine converter and the propagation path of oscillation source are seldom considered.In this paper,two SSCI damping control strategies:First,aiming at the subsynchronous component of SSCI source,which is mainly the output of RSC current loop,the SSCI can be suppressed by installing a band-reject filter on the dq-axis rotor voltage output by the current loop to restrain the harmonics at the resonance frequency point.Second,additional damping power is added to the power outer ring through a phase-compensated hybrid additional damping controller—?35?P_s?and?35?Q_s?.The difference between angular frequency of the fan shaft system and the steady frequency difference is applied to three phases of delay,filter,phase shift control the angular difference frequency is converted to a positive damping power variable,making the system quickly resume stable operation.The time-domain simulation results show that the first additional damping control strategy has adaptability and robustness to different wind speed,series complementarity and RSC inner loop parameters,and can effectively reduce the risk of SSCI.The second additional damping control strategy,the effect of suppressing SSCI can be achieved by changing the frequency range of the band-stop filter,the number of cascades leading and lagging?changing the phase compensation angle?.The above research results provide the theoretical basis for subsynchronous oscillation verification for the calculation of the mode of national economy and the operation of large-scale wind farms,and provide technical guidance for improving the capability of large-scale wind power transmission.