Cascaded Distributed MPPT Structure And Control Strategies For DC Microgrid System

Cascaded distributed maximum power point tracking(MPPT)structure is one of new architectures in photovoltaic(PV)system.In mismatch cases,cascaded distributed MPPT structure can enable each PV module to work at their maximum power points,thus improving the generating efficiency of PV system greatly.So it is important to study the output performance and control strategy of cascaded distributed MPPT structure.In order to analyse the output performance of cascaded distributed MPPT structure in some detail,the effect of environmental change on the maximum voltage of each module optimizer is considered,and accurate computations of bus voltage and current corresponding to the maximum output power range are proposed in this paper.Moreover,the range of bus current for the safety of system is clarified.Output performances of each module optimizer and the cascaded distributed MPPT structure are validated through simulation analysis,besides the calculation methods about key pionts at the output performance curve are also verified.In distributed MPPT photovoltaic system,the dc-bus voltage needs to be adjusted with the variations of temperature and illumination to improve system output power.However,a wide dc-bus voltage variation brings undesirable influence on post-stage devices and complicates the control circuit.Although taking system safety into consideration,the fixed dc-bus voltage seems to be a good solution,such configuration suffers from power loss issue under some mismatch conditions.In order to reduce the power loss and maintain dc-bus voltage stability simultaneously,an optimal fixed reference voltage selection method is proposed.The specific steps of the method are analyzed through a simulation example,the comparison between the fixed dc-bus voltage configuration and traditional dc-bus voltage configuration are given,and the validity of the method is verified.For the dc micro-grid system with PV unit,batteries and loads,control strategy of cascaded distributed MPPT structure is analyzed.When system switches from MPPT mode to voltage regulated mode in PV unit,a seamless switching control method is proposed without increasing communication circuit in this paper.Simulation results show that cascaded distributed MPPT structure can greatly improve output power of PV unit under mismatch conditions,the seamless switching function of the proposed control method is also verified,and system units can cooperate with each other well with designed energy management control strategy,guaranteeing the system in safe,stable and efficient state.Finally,the experimental platform of cascaded distributed MPPT structure is built,and the correctness of the theoretical analysis and simulation analysis about structure's output performance is verified by experimental study.