Variable Exponent Based Integral Reset Control For Grid-Connected PV Systems Embedded With Multilevel Inverters

As one of the typical representatives of new energy,light energy can be converted into electrical energy with a wide range of uses and easy transmission through the power generation system composed of photovoltaic panels and inverters.The inverter is the core equipment of the photovoltaic power generation system,and its performance directly affects the efficiency and reliability of the entire power generation system.Multilevel inverters are widely used because of their advantages of lower voltage stress,lower electromagnetic interference and switching loss,and less output voltage harmonic content,which can effectively solve the problems of traditional two-level inverter topology.Nowadays,to ensure accurate tracking,stable output,and good dynamic performance of the inverter,constantly optimizing the topology of the inverter as well as applying advanced control theory has become a hot spot in academic and industrial research.The existing control methods for grid-connected photovoltaic multilevel inverters can be divided into three modules,and the algorithms in each module are: Maximum Point Tracking(MPPT),decoupling control,and pulse width modulation(PWM).In order to further adapt the control algorithm to the requirements of high power,high voltage and high performance of the inverter,this paper proposes a Variable Exponent based Integral Reset Control(VEIRC).Researches of this manuscript are as follows:(1)Improvement of the MPPT controller module: Take the switched-diode cascade multilevel inverter as the research object,and adjust each cascade in the switch-diode cascade multilevel inverter by adjusting the light intensity of the photovoltaic power supply in the unit.The distribution characteristics of the maximum power point(MPP)were analyzed,and the maximum power point scanning algorithm(MPPS)was proposed to design a portable maximum power point scanner(MPPSer).The principles of the MPPSer are clarified as follows.First,the MPPSer disconnects the injection of energy on the AC side during the current commutation,and connects all the PV panels together.Then a controlled voltage source(CVS)in the MPPSer applies variable voltage at both ends of the PV panels to obtain the characteristic curves,including power-voltage(P-V)and current-voltage(I-V)curves.Finally,the global maximum power point(GMPP)is calculated by predefined formulas.The practicability of the MPPS algorithm and the MPPSer has been verified through simulations and experiments.(2)Improvements of the PWM generator module: According to the established gridconnected circuit of the photovoltaic cascade multilevel inverter,the Variable Exponent based Integral Reset Controller(VEIRCer)is proposed according to the commonality among various nonlinear control algorithms for inverters.The VEIRCer uses a general formula to describe the modulating waves and carriers,and replaces the sampling of the output voltage of each cascade module in the multilevel inverter with the Q(k-1)state of the SR trigger.Compared with several commonly used integral reset control methods such as One Cycle Controller(OCCer)and Energy Balance Controller(EBCer),it reduces the sampling,simplifies the calculation,and realizes more functionalities,such as reducing the impact on the inverter system,suppressing the over-voltage caused by the filter and the interferences from the DC sources.The performance of the controller is confirmed through simulation and experiments.(3)Further promotion of the VEIRC algorithm to the adaptive VEIRC control: An adaptive VEIRC control algorithm is further proposed.The values of parameters are not given in advance using the adaptive VEIRC control algorithm.Both of the MPPSer and VEIRCer are applied in order to achieve better control results of the photovoltaic grid-connected system.To achieve the adaptive parameters,the integer parts of parameters are obtained using a fuzzy control algorithm,while the fractional parts are obtained with a multi-objective optimization algorithm.The prove and analysis of the stability and the dynamic performance of the adaptive VEIRC algorithm verify its theoretical and practical significance of both the dynamic response regulations and the steady-state error regulation characteristics.In summary,this paper is based on the photovoltaic grid-connected system embedded with the switched-diode cascaded multilevel inverter,and proposes a VEIRC control algorithm.Both the MPPT and PWM modules are significantly improved to achieve adaptive parameters,which enhance the performance of the control algorithms as well as the outputs of the inverters.