Research On The Oscillation Characteristics And Suppression Strategies Of Direct-Drive Wind Farms With VSC-HVDC Systems

With the development of the wind farms with voltage source converter-based high voltage direct current transmission(VSC-HVDC)projects,the related oscillations become prominent at home and abroad.Practical oscillation problems may cause the wind turbine shutdown,equipment damage and power quality problems.In light of this,four aspects are studied in this paper:small-signal modeling,oscillation characteristics and mechanism,oscillation mitigation strategies,and equivalent modeling of the direct-drive wind farms with VSC-HVDC(DDWFV)systems.The main contents of this thesis are described as follows:(1)The small-signal model of three direct-drive wind turbines with VSC-HVDC system is built.Every direct-drive wind turbine represents the detailed model or equivalent model of single wind turbine.Considering the decoupling effect of the VSC-HVDC DC-side,the DDWFV system is divided into two subsystems at the DC bus of VSC-HVDC inverter:direct-drive wind farms via VSC-HVDC rectifier system and VSC-HVDC inverter connected to the AC power grid.The dynamic mathematical models of direct-drive wind turbine,VSC-HVDC and their interface are built.Based on the dynamic models of direct-drive wind farms,VSC-HVDC rectifier and their interface,the small-signal model of the direct-drive wind farms viaVSC-HVDC rectifier system is built;Based on the dynamic models of the VSC-HVDC receiving AC system and control system,the small-signal model of the VSC-HVDC inverter connected to the AC power grid is built.Finally,the small-signal model is verified through the PSCAD/EMTDC simulations.(2)Based on the eigenvalue method,inside-wind-farm/wind-farm-grid oscillation characteristics of the direct-drive wind farms via VSC-HVDC rectifier system are analyzed.From the point of view that the control system acts on the electrical system to form an unstable positive feedback loop,the instability mechanism is revealed.First,using the eigenvalue method,inside-wind-farm/wind-farm-grid sub-synchronous oscillation(SSO)characteristics of the direct-drive wind farms via VSC-HVDC rectifier system are analyzed.Then,the impact of the VSC-HVDC control and network characteristics on the wind-farm-grid SSO characteristics is analyzed.Finally,under the impact of the control system,the interactions among state variables are anaylzed.The results show that under the impact of the DC voltage control,the interactions among electical system state variables is prone to form an unstable positive feedback loop,which results in the instability of the direct-drive wind farms via VSC-HVDC rectifier system.Based on the right-half-plane zero limit principle and the Routh-Hurwitz stability criterion,the instability criterion of the VSC-HVDC inverter connected to the AC power grid is proposed,and the physical meaning of the criterion is revealed.Under different operation modes of the voltage source converter(VSC),the linearized transfer function models of the VSC-HVDC inverter station connected to the AC power grid are established.By investigating the zero/pole distribution principle of the open/closed loop transfer functions of the lineraized transfer function models,the system's instability criterion is proposed.In addition,the linearized relationship among the output active power,reactive power,terminal voltage amplitude and the dq-axis current injected into the grid of the VSC is studied.Then,from the perspective of the unstable positive feedback loop,the physical meaning of the instability criterion is revealed.The results show that when the AC power grid is weak,the sensitivity of active power to the d-axis current is negative.At this time,under the impact of the active power control,the interactions among the state variables of the electrical system will form an unstable positive feedback loop,leading to the instability of the VSC-HVDC inverter station connected to the AC power gird.(3)Through the feedback linearization sliding mode control(FLSMC),a SSO suppression method that takes into account the nonlinearity and uncertainty of the DDWFV system is proposed.The strategy is applied in the wind turbine grid-side converter and VSC-HVDC rectifier.The proposed FLSMC combines the advantages of the feedback linearization control(FLC)and the sliding mode control(SMC),The FLC transforms the nonlinear system into a linear system through coordinate transformation and feedback;The SMC is used to make up for the FLC's shortcomings of being sensitive to parameter perturbation or external disturbance by designing sliding mode surfaces and control law.The results show that the proposed FLSMC has a good SSO suppression effect under different operating conditions,and presents strong robustness to parameter uncertainties or external disturbances.(4)The applicability of the traditional capacity-weighted single-machine equivalent method for oscillation modes analysis and wind turbine parameters optimization is analyzed.The eigenvalue method is used to compare the oscillation modes and participation factors of the single-machine and two-machine equivalent models with the same capacity.Then,the damping coupling characteristics of the inside-wind-farm/wind-farm-grid SSO with the change of wind turbine parameters are analyzed.The results show that there are certain errors in the stability analysis while using the single-machine equivalent model,and the simple equivalence of the wind farm has limited contribution to the stability analysis and controller design of practical projects.Aiming at the defect that the traditional equivalent methods cannot reflect the inside-wind-farm SSO modes,a SSO equivalent modeling method based on the similar transformation theory is proposed.The proposed method has a rigorous theoretical basis.Based on the principle that similar matrices have the same eigenvalues,the 18N+8-order N-machine with the VSC-HVDC system is reduced to a 44-order two-machine system through the matrice' similar transformation.The results show that the proposed equvialent model can effectively reduce the dimension of the system while reflecting the inside-wind-farm/wind-farm-grid SSO modes,which makes up for the insufficiency of the traditional equivalent method that cannot reflect the inside-wind-farm SSO.