Predictive Control For Cascaded H-bridge STATCOM Under Unbalanced Grid

With the rapid development of new energy and the promotion of policies,new energy power generation will be merged into the grid.In order to solve the problem of "discarding wind and photovoltaic" caused by the insufficient capacity of new energy power absorption,high voltage AC/DC transmission and distribution is included in the development planning.Considering the wind power,photovoltaic and other new energy have the character of volatility,intermittent,in order to improve the stability of AC/DC hybrid power grid and efficient transmission and distribution,reactive power compensation devices,such as STATCOM,has been widely applied in new energy power generation.In order to improve the dynamic performance of STATCOM,and to be able to run normally under unbalanced grid,this thesis take the CHB-STATCOM as research object,study the predictive control in the application of STATCOM,and the control strategy under unbalanced grid conditions.The finite state control set of model predictive control(FCS-MPC)has the advantages of simple theory and good robustness,it is easy to deal with multi-constraints and nonlinear systems.However,it is difficult to apply the control method in the high voltage STATCOM engineering with multi-level and multi-switch state.In this paper,based on the basic principle of FCS-MPC,the effective calculate way of optimal solution with one single controlled variable and multiple targets control was given,combining with the optimization of current and voltage-hierarchical control,required resource of calculation in FCS-MPC got greatly reduced.Based on the comparative analysis of operational resources,he proposed scheme was verified to have the advantage on computing resources.In the case of unbalanced power grid,the realization of three-phase dc voltage balance control is analyzed and the application of FCS-MPC in STATCOM is extended.The continuous state control set of model predictive control(CCS-MPC)control the continuous modulation wave,resulting need of PWM modulation,and therefore the switching frequency is fixed to facilitate the design of the filter.Due to difficulty of CCS-MPC in dealing with the multiple targets control,this thesis establish the objective function of the different control target by weighted superposition,combine the the optimal solution calculation strategy of FCS-MPC with CCS-MPC,it can realize the multi-objective control,and improve the dynamic performance of dc voltage balancing.After dealing with the dc voltage ripple,the effect on the performance of current harmonic was effectively suppressed.Similar to FCS-MPC scheme,this thesis not only analyzes three clustered dc voltage balancing solution implemented in the CCS-MPC,but also put forward a kind of dc side voltage separation control,according to the relationship between clustered dc voltage and the output voltage on the dq0 shaft,used in limitation of negative-sequence current under unbalanced conditions,which can effectively improve the dynamic performance of the overall voltage control loop and the current harmonic performance.Based on the Matlab/Simulink simulation and experimental platform,several control methods of FCS-MPC and CCS-MPC proposed in this thesis are validated,the simulation and experimental results show that the proposed methods are reasonable and effective.