Research On Topology And Control Of DC-Module-Based Photovoltaic Power System

Solar power has become one of the most important renewable resources due to the energy crisis recently. Building integration is the tendency of photovoltaic generation system in the future. Photovoltaic power system for the residential and building integration application is sensitive to the mismatch of electrical parameters due to the partial shadows and different illumination condition. As a result, the yield of the system decreases remarkably. In the severe cases, the hot-spot problem can arise and PV modules can be irreversibly damaged. In order to maximize the energy conversion efficiency under the partial shadows and mismatch condition, a novel photovoltaic power system structure based on paralleled DC modules is proposed, system configuration, topology, characteristics of PV modules, MPPT and grid-connected control strategy of the proposed system are analyzed.Three kinds of DC-module-based photovoltaic power systems are compared comprehensively in the paper first. With the help of simulation, the paralleled DC-module-based structure is proved to have the highest efficiency and be the most suitable for the BIPV application. Suitable topologies of the DC-DC converter in the DC module and centerial grid-connected inverter are selected according to the main technical specifications of the system.Then, basic circuit principle of current-fed half bridge(CFHB) converter is expounded in detail. In order to overcome the problems about load range, soft starting and conversion efficiency, etc, improved CFHB topology with active clamping snubber is adopted, and the operation modes, as well as several correlative design issues are discussed thoroughly. Incremental conductance method is selected to realizing the MPPT control function of the DC-module. The effect of several control parameters such as tracking step size and sampling interval on the performance of INC algorithm is discussed. In addition, through the simulation modeling, causes for the variation of the output characteristics of the PV module under the partial shadows are analyzed, several traits of the characteristics curve under that condition are summarized.Finally, a control scheme constituted by double loops is designed and realized. For the inner grid-connected current loop, two different controller forms are studied. Considering the effect of output filter capacitor on magnitude, phase and low-order harmonics of grid current, capacitor current compensation is added to the normal PI control structure. In order to achieve zero steady-state error and compensation for the low-order harmonics, PR controller based on sinusoidal internal model is introduced. Analysis of control characteristics of the PR controller in the frequency domain is given, as well as the digital realization based onδ?1 operator and DFIIt. DC bus voltage regulator based on PI controller is adopted, guaranteeing the the power balance of the system. Furthermore, coordinated control strategy for the two stages is discussed briefly.