Subsynchronous Oscillation Characteristics Analysis Under All Operation Regions Of D-PMSG And Mitigation Strategy Of Grid-connected Direct-drive Wind Farms Via VSC-HVDC System

With the rapid development of new energy power generation technology,the installed capacity of wind power continues to increase.Existing research shows that there is a risk of subsynchronous oscillation(SSO)when wind farms are connected to the grid via VSC-HVDC.Among the existing scenario,grid-connected direct-drive wind farms via VSC-HVDC system is an important scenario for the development of offshore wind power in the future,and its stability can not be ignored.This paper focuses on the SSO problem of the grid-connected direct-drive wind farms via VSC-HVDC system,and the following researches are carried out:In view of the subsynchronous oscillation caused by grid-connected direct-drive wind farms via VSC-HVDC system,existing research focuses on the maximum power point tracking operating region of wind turbine.In addition to the maximum power point tracking operating region,the direct-drive permanent magnet synchronous generator(D-PMSG)has power limiting operating region and high wind speed region.Firstly,a small-signal model of the grid-connected system is established.Then,all operation regions of D-PMSG are analyzed based on the operation characteristic curve of D-PMSG,and the SSO modes in the system are analyzed by eigenvalue method under all operation regions.The theoretical analysis is verified by the PSCAD/EMTDC simulation.Finally,the influence of pitch angle control on system stability is analyzed.The results show that the higher wind speed and the higher output power can lead to the more stable system under all operation regions.The output power has a great impact on the damping ratio of SSO at low wind speed and has less influence on the damping ratio of SSO at high wind speed.The pitch angle control does not directly participate in system SSO interaction,but affects the stability of the system by changing the power flow.In practical operation,if the power grid has no restriction on the output power of the wind farms,the maximum output power of the D-PMSG should be maintained as far as possible to improve the stability of the system;If the power grid restricts the output power of wind farms,the output level of a single wind turbine generator should be improved by tripping measures.The grid-connected direct-drive wind farms via VSC-HVDC system is exposed to the potential risk of subsynchronous oscillation due to the interaction between direct-drive wind farms and VSC-HVDC system.A nonlinear control strategy based on state feedback linearization is proposed to mitigate subsynchronous oscillation,which is applied to wind turbine grid-side converter and VSC-HVDC system send-side converter.Firstly,the affine nonlinear mathematical model of wind turbine grid-side converter and VSC-HVDC system send-side converter is established.Secondly,the mathematical model is tested to prove that the condition of state feedback linearization is satisfied.Then the nonlinear control law is obtained and the corresponding control block diagram is given.Finally,eigenvalue analysis and a simulation model which is built in PSCAD / EMTDC are used to verify the effectiveness of the proposed nonlinear control strategy.The results show that the proposed nonlinear control strategy can effectively mitigate subsynchronous oscillation with the variation of wind speed,controller reference or other situation in the direct-drive wind farms via VSC-HVDC system compared with traditional PI control.