Research On Inverter Control Technology Based On Fractional-Order Repetitive Control

In recent years,power electronic inverter,as an interface connecting different power devices,has been widely used in various distributed generation systems,and has become an indispensable part of modern power system.There are diode uncontrolled rectifier circuits or thyristor phase control circuits in the power system,and the resulting harmonic pollution will seriously harm power quality and even affect industrial production and human life.In order to solve the harmonic problem caused by power electronic inverters,various control schemes are applied to the inverter,and repetitive control is a very effective control scheme.Repetitive control is a control scheme based on internal model theory,under the condition of constant signal frequency,digital repetitive controller can track periodic signal without static error or eliminate disturbance,so it is very suitable for harmonic control in power system.However,in actual working conditions,the working frequency often varies,which leads to non-integer delays in conventional digital repetitive control.Fractional delay will lead to the power quality deterioration of controlled inverter,such as the increase of output voltage/current Total Harmonic Distortion(THD),which seriously affects the control effect.Therefore,in order to make the inverter still have excellent control performance when the frequency varies,this paper studies the fractional delay problem existing in the conventional digital repetitive control in detail.Firstly,the general control methods of inverter are introduced,and the advantages and disadvantages of various control schemes are compared.The drawbacks of conventional repetitive control under fractional delay are analyzed from the control principle,the fractional-order repetitive control scheme is proposed,and its basic idea and specific implementation method are explained.The fractional-order repetitive control system is described.Then,based on the above research works,the conventional selective harmonic repetitive control(SHRC)is improved,the new generation SHRC(NG-SHRC)and fractional-order NG-SHRC(FO-NG-SHRC)are proposed.NG-SHRC has a more streamlined controller structure than conventional SHRC,and it is more convenient to design and use.FO-NG-SHRC is the combination of NG-SHRC and Farrow structure fractional delay filter based on Taylor-series expansion.Compared with the conventional FO-SHRC,FO-NG-SHRC consumes less computational resources,and can update the filter coefficients in real time to approximate the fractional delay of any value,so as to selectively eliminate the harmonics of any frequency.It is worth noting that FO-NG-SHRC provides a general controller structure for various integer-order/fractional-order RCs.Simulation results verify the effectiveness and superiority of the proposed FO-NG-SHRC.In some inverter applications,the frequency variations are often concentrated in a small frequency range,such as 50±0.5Hz.However,the fractional-order repetitive control needs to update the coefficients in real time during applications,which consumes certain computational resources of the hardware controller.Based on this,the order-expandable SHRC(OE-SHRC)and high-order SHRC(HO-SHRC)are proposed.OE-SHRC conforms to the standard RC internal model structure,that is,it has the ability to expand the order,so it can be further expanded to HO-SHRC.By setting the value of the weight coefficients reasonably,HO-SHRC can have good frequency adaptability when small frequency variations occur.It is worth noting that HO-SHRC does not need to change any controller parameters when the frequency varies,which reduces the computational burden of the controller and provides a general controller structure for various first-order/high-order RCs.Simulation results verify the effectiveness and superiority of the proposed HO-SHRC.Finally,in order to fully verify the effectiveness and superiority of the proposed control scheme,an experimental platform of fractional-order repetitive control inverter system based on d SPACE 1103 is built.Experiments were carried out for the proposed FO-NG-SHRC and HO-SHRC respectively.The experimental results fully prove that both FO-NG-SHRC and HO-SHRC have good frequency adaptability when fractional delay occurs caused by frequency variations.