Research On Current Decoupling Technology And Delay Compensation Method For Grid-Connected Converter

The grid-connected converter has the advantages of bidirectional power adjustment and independent control of active power and reactive power.It is widely used in distributed new energy grid-connected power generation system,flexible AC transmission equipment and energy brake feedback AC motor drive system.As one of the key technologies of grid-connected converters,current control technology determines the overall performance of the system,which is necessary for further study.In this paper,based on the design of PI controller in the synchronous rotation coordinate system,the current coupling and delay compensation in the current loop are studied,which mainly includes the following aspects:Firstly,the three-phase half-bridge grid-connected converter is used as the research object,and its mathematical model is established under the three-phase stationary coordinate system and the synchronous rotating coordinate system.The scalar model intuitively reflects the cross coupling between d and q axis currents,but the system is presented as a two-input/two-output system,which is not conducive to theoretical analysis and controller design.In this paper,the mathematical model of the two-phase rotating coordinate system is expressed in the form of vector,and the vector model of the controlled object is obtained.At this time,the system is expressed as a single input/single output system,which makes it easy to use the root trajectory,frequency response and other analytical tools to study the current loop.Secondly,the coupling phenomenon is derived from the model simulation,and the influence of the coupling term on the current loop is pointed out.Two decoupling methods are proposed and discussed.Among them,the first decoupling method is the state feedback decoupling,that is,counteract the cross-coupling of the controlled object with the inductor current feedback,so the complex pole of the object becomes a real pole.The second is the complex coefficient vector current controller decoupling method.The principle is to design the zero point of the controller which can be offset directly by the complex pole of the controlled object,so as to achieve the purpose of decoupling.Considering the existence of errors in the parameter estimation of the grid-connected filter,the decoupling method of the complex coefficient vector current controller takes advantage of the robustness of the parameters by analyzing the current ring locus and the frequency response.In addition,in order to solve the delay problem of the digital control system and the delay of the zero order holder of the converter,the delay compensation problem of the current loop is studied deeply.In the case of delay,the two decoupling control methods are compared and analyzed.It is pointed out that the delay will affect both the stability and the dynamic performance of the system,and the complex coefficient vector current controller decoupling method has better control performance.In order to improve the stability of the current loop,the delay compensation of the two decoupling methods is studied respectively,and the design method of the optimal compensation angle is obtained.Finally,a 30kVA experimental prototype platform is built.In order to verify the rationality and correctness of the theoretical research above,two kinds of decoupling control methods are tested respectively.