Study On The Control Strategy Of The Grid-Connected Converter At Low Switching Frequency

With the increase of the rated power,the switching frequency tends to be lower to reduce system losses and improve device output ability in the high-power grid-connected converter system.However,it also leads to performance degradation in the current control system.Therefore,in this paper,the two-level grid-connected converter is taken as the research object,and its mathematical model and control strategy are studied with considering the influence of time delay at low switching frequency.Firstly,the conventional mathematical model of grid-connected converter is established in different coordinates.Then the complex vector and transfer function matrix model of grid-connected converter are established in d-q coordinate at low switching frequency with explicitly considering the time delay,which reveals the reasons of system coupling.Secondly,the relative gain array(RGA)analysis method is introduced to analyze the static and dynamic coupling degree of L type grid-connected converter system with non-decoupling method at different switching frequency.Although the PI regulator can completely eliminate the static coupling of the system,the dynamic coupling is more serious with the decrease of switching frequency.The dynamic coupling degree of the system is improved,after the traditional feedforward decoupling method is employed,but it is still very serious.Therefore,an output feedback decoupling method is proposed to realize the decoupling of the system in this paper under the premise of keeping the main channel characteristics of the system,which is based on the idea of transfer function matrix diagonalization,where the control performance and parameter sensitivity of the system with different decoupling methods are compared by RGA analysis method and multiple input and multiple output(MIMO)root locus.Finally,the parameter design method of control system for the L type grid-connected converter at low switching frequency is given,and the simulation analysis is conducted in MATLAB/Simulink.Thirdly,on the basis of the complex vector model of LCL type grid-connected converter at low switching frequency,a passive damping series decoupling method is proposed,which is based on the idea of complex vector zero pole cancellation.Further more,a trap damping series decoupling method is proposed based on the active damping method of notch filter,where the parameter design method of the control system for the LCL type grid-connected converter at low switching frequency is given.Finally,the control performance of the coupling system and decoupling system is compared by the frequency characteristics and unit step response of the closed-loop system and it is verified in MATLAB/Simulink simulation.Finally,the experimental platform of L/LCL type grid-connected converter is built to verify the control performances of the two types of grid-connected converter system at low switching frequency.The experimental results verify the feasibility and correctness of the proposed theoretical analysis and method.