Research On A Novel Feedforward Repetitive Control And Its Application In Grid-tied Inverter

The grid-tied inverter is the key interface equipment for distributed generation system to feed power energy to the grid,and its control performance directly affects the safe and reliable operation of the power generation system.In order to eliminate the harmonic pollution caused by the inverter output current harmonics to the grid,repetitive control(RC)has become a research hotspot because of its good harmonic suppression effect.Taking the single-phase grid-tied inverter system as an example,this paper studies how to further improve the harmonic suppression characteristics,dynamic performance and resistance to grid frequency fluctuations of RC.Firstly,the structures of traditional RC internal model and traditional odd-harmonic repetitive control(ORC)internal model are analyzed,and it is pointed out that their dynamic performance and harmonic suppression ability need to be further improved.Then,a feedforward channel is added to the traditional RC and traditional ORC internal model to form a novel feedforward RC internal model and a feedforward ORC internal model,and the frequency characteristics of traditional RC and feedforward RC,traditional ORC and feedforward ORC are compared and analyzed.It is concluded that the repetitive control internal models with feedforward channel have higher gain and wider resonant bandwidth at resonant frequency than the traditional repetitive control internal models without feedforward channel.Secondly,the feedforward RC internal mode and feedforward ORC internal mode are connected in parallel with proportional control,and the proportionalfeedforward repetitive controller and proportional-feedforward odd-harmonic repetitive controller are proposed,respectively,so that the feedforward repetitive control system can accommodate larger RC gain and enhance the stability of the system.At the same time,two kinds of feedforward repetitive control system structures are deeply studied,and their stability analysis and parameter design methods are given.In view of the weak ability of RC to deal with grid frequency fluctuation,this paper refers to the idea of quasi-resonant control,and establishes the mathematical equivalent relationship between feedforward RC internal model,feedforward ORC internal model and multi-quasi-resonant control through formula derivation,which is used to guide the parameter optimization design of feedforward repetitive control scheme under grid frequency fluctuation.For the problem that the harmonic suppression ability of the proportional-feedforward repetitive controller decreases significantly under the wide range of frequency variation,the fractional delay IIR filter is applied to the feedforward RC structure,and an adaptive proportional-feedforward repetitive controller is proposed,so that the proportional-feedforward repetitive control can still provide sufficient gain at the fundamental and harmonic frequencies under frequency fluctuations.At the same time,the simulation model of feedforward repetitive control system is built by MATLAB/Simulink,and the algorithm is verified on the experimental prototype based on d SPACE1202 of gridtied inverter.Finally,the fluctuation of grid frequency is simulated by simulation and experiments,and the steady-state and transient waveforms of the grid current of the traditional repetitive control,the proposed feedforward repetitive control,and the proposed adaptive feedforward repetitive control strategy are compared and analyzed.The results show that the conventional repetitive control system with parallel structure of repetitive control internal model without feedforward channel and proportional control can have good dynamic performance and high control accuracy.However,its performance decreases obviously under the changing grid frequency,and the proposed proportional-feedforward repetitive control system and proportional-feedforward odd-harmonic repetitive control system have better dynamic performance and harmonic suppression ability under normal frequency and also grid frequency variations,and the proposed adaptive proportionalfeedforward repetitive control and proportional-feedforward odd-harmonic repetitive control schemes still maintain excellent harmonic suppression characteristics under the wide range of frequency fluctuation from 49.5 Hz to 50.5 Hz.