| The distributed power generation based on the renewable energy sources has been an important mean to promote the utilization of renewable energy sources and clean energy,and attracted more and more attention of researchers in worldwide.As the interface between the distributed power generation system and the power grid,the grid-connected inverters play an important role in safe running,stability and high-quality operation of the distributed power generation system.However,the low-frequency harmonics generated by dead time and the background harmonics from the grid voltages in practice deteriorate the injected grid currents from the grid-connected inverters.Therefore,it is important to study the methods of the harmonic suppression for the grid-connected inverters.Firstly,the theory,stability and harmonic rejection of the conventional repetitive control(CRC)is given,and then the performances of the cascade and parallel hybrid control schemes of RC and proportional integral(PI)control are compared.Though the parallel hybrid control scheme can suppress harmonics,the harmonic suppression need further enhance.Secondly,an active damping technique based on notch filter is proposed to neutralize the resonance of LCL filter.The active damping technique does not need extra current or voltage sensors and has a relatively good robustness for the grid-side inductance fluctuation in weak grid systems.Thirdly,a novel proportional integral multi-resonant control(PIMR)scheme is put forward for a grid-connected inverter,and the proposed PIMR control scheme is composed of a repetitive control(RC)in parallel with a proportional gain,and therefore only a few parameters need to be designed for the purpose of replacing lots of parameters of conventional proportional multi-resonant controller.The proportional gain not only improves the dynamic response,but also enables RC to have a bigger RC gain,and consequently a faster convergence rate.Theoretical and stability analysis of PIMR have been given,and then a step-by-step systematic design procedure with consideration of the practical implementation aspects has also been provided.The simulation and experimental results have validated the feasibility and outstanding performance of the proposed control scheme.CRC in digital control can work perfectly when N is an integer,i.e.the sampling frequency can be divided by fundamental frequency completely without remainder.However,the grid frequency in the distributed power generation system changes obviously,and consequently,N is not an integral and the system performance will be degraded significantly.To solve this problem,fractional delay RC is proposed and then is used in PIMR construction.The performances of a finite impulse response(FIR)digital filter and an infinite impulse response(IIR)digital filter adopted to approximate the internal model of any ratio N are compared and fractional delay in PIMR construction based on FIR filter is studied when the grid frequency changes from 49.5Hz to 50.5Hz.Furthermore,a new PIMR with frequency adaptive capability is presented to settle the frequency variation.Simulation and experimental results verify the effectiveness on increasing the steady-state tracking performance and harmonic suppression.Limited by digital sampling,CRC with integer phase lead compensation cannot exactly compensate the system phase lag,which may result in unstable in the case of low sampling frequency(4 kHz and under 4 kHz).A fractional phase lead compensation repetitive control scheme is proposed to enable the phase lead step to be fractional,which can enlarge the stability region and improve the tracking accuracy.A Lagrange interpolation based on fractional lead filter is applied to approximate the fractional lead items.Meanwhile,the synthesis and analysis of fractional phase lead PIMR for a single LCL grid-connected inverter are given.Furthermore,simulations and experiments are provided to demonstrate the validity of the proposed fractional phase lead compensation scheme. |
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