Research On Modeling And Control Of Fractional-Order LCL Grid-Connected Inverte

Renewable energy distributed power generation technology has received heavy attention due to the growing problem of energy crisis.As the core equipment for gridconnected distributed power supply,most of the current researches on LCL-type gridconnected inverters still stay on the integer order,while the research shows that inductors and capacitors have fractional order characteristics by nature.As key components in LCLtype grid-connected inverters,their characteristics have a decisive impact on the static and dynamic performance of grid-connected inverters.In this context,this paper constructs fractional-order inductors and capacitors,extends the study of traditional integer-order LCL(IOLCL)filters to fractional-order LCL(FOLCL)filters,and establishes a single-phase LCL-type fractional-order grid-connected inverter(FOGCI).Based on this,this paper also proposes a class of FOGCI models without harmonic resonant peaks and designs a fractional-order weighted voltage feedforward strategy to enhance the stability of resonantfree FOGCI under weak grids.The main work of this paper is as follows:Firstly,the article introduces the research status of the basic topology structure and key technologies of traditional single-phase LCL-type Integer-order Grid-connected Inverters(IOGCI).It also elaborates on the relevant concepts of fractional calculus theory and its Laplace transform,as well as methods for implementing fractional-order operators.By using the improved Oustaloup algorithm and the theoretical basis of fractional-order component construction,the theoretical model of fractional-order capacitors and inductors based on the fractional-resistance chain circuit is derived in this paper.Subsequently,the mathematical model of FOLCL filter is derived and the characteristics of FOLCL filter are investigated.It is found that when α1=α2=α,the condition of FOLCL filter without resonance is α+β≠2(α1 is the order of inductor at inverter side;α2 is the order of inductor at grid connection side;β is the order of capacitor).According to this condition,a class of FOGCI model without resonance spikes is constructed by reasonably selecting the order of inductor and capacitor.Compared with the conventional IOGCI,the resonance-free FOGCI does not require active damping of capacitor current,which simplifies the control system design while omitting a current sensor.To make the FOGCI grid-control better,a fractional-order PI controller(FOPI)is used to form a "FOGCI+FOPI" fractional-order system.Compared with the single-phase IOGCI controlled by the Integer-order PI(IOPI)controller,the "FOGCI+FOPI" can obtain larger gain and phase margin,which improves the system performance.In addition,the fractionalorder feedforward control strategy of the grid voltage for FOGCI is derived,which greatly weakens the influence of grid voltage harmonics on the grid-connected current.Then,the mathematical model of single-phase resonance-free FOGCI under digital control is then derived,and it is pointed out that the actual grid voltage feedforward strategy will produce deviations due to the presence of digital control delays,leading to enhanced coupling between the inverter and the grid,which makes the grid-connected system unstable.To solve this problem,this paper proposes a fractional-order weighted voltage feedforward strategy.By multiplying the proportional term with different values of weighting factors to change the frequency characteristics of the inverter output impedance to make it match with the grid impedance,thus satisfying the stability condition of the gridconnected system under weak grid.Finally,in order to demonstrate the correctness of the model and control strategy proposed in this paper,circuit simulation and hardware-in-the-loop experiments are carried out on MATLAB/Simulink simulation platform and semi-physical experimental platform.