Control Technique And Parameter Design Under Weak Grid Conditions Of Single-phase Grid-connected Inverter With LCL Filter

In recent years,with the increasingly serious energy crisis and environmental pollution,distributed generation has received wide attention and develops rapidly.The grid-connected inverter is the interface between the distributed generation system and the grid,and its control technique will directly affect the output power quality as well as the operation of grid.The control technology of single-phase grid-connected inverter with LCL filter is studied from three aspects,including filter design,current feedback control strategy and parameter design of current regulator under weak grid conditions.Filters are mainly divided into three types: single L filter,LC filter and LCL filter.By comparing the filtering performance of these three filters,it is clear that LCL filter can achieve the same filtering effect with smaller size and lower cost.According to the technical standards and requirements,the design process of LCL filter parameters is illustrated in detail.Comparative analysis of three current feedback control strategies is given.The output power factor of inverter-side current feedback control strategy,which is a kind of indirect current control method,is analyzed.In order to restrain the resonant peak,passive damping method and active damping method are adopted in grid-side current feedback control strategy to increase system damping,and the consistency on the principle of the two methods is illustrated.As for the split capacitor method,the reason of its resonance is discussed,for which the positive damping method is added.In order to eliminate the influence of grid voltage harmonics on the grid current,the grid voltage feedforward is derived based on the equivalent control block diagram.A method of current regulator parameter design based on active damping control strategy under weak grid conditions is proposed.Considering the influence of Phase-Locked Loop(PLL),the output impedance model of grid-connected inverter is derived,and the corresponding impedance stability criterion is developed.With the constraint of the impedance stability criterion and grid current steady-state error,the method of the current regulator parameters(including capacitor current feedback damping coefficient)design is discussed minutely by means of the root locus in discrete domain and Bode diagram.The simulation model and the experimental platform have been established,and the results verify the feasibility and effectiveness of the above theoretical analysis.