| Solar photovoltaic power generation has inherent characteristics such as intermittence and fluctuations.It needs to be centralized and boosted before it can be connected to the grid.The problems of harmonic resonance and reactive power transmission in the traditional AC collection system are becoming more and more prominent,it will not only harm the operation of photovoltaic power plants and power systems but also seriously affect the transmission capacity of photovoltaic power plants.By comparison,the photovoltaic DC collection system does not have stability problems such as power angle stabilization or harmonic resonance in the AC collection system.Besides,the DC output of the photovoltaic power generation unit can be better connected to the DC-Grid by DC collection,which doesn’t need an intermediate DC-AC transformation link,reducing conversion losses and primary equipment investment,and has considerable economic benefits.Currently,the main research of photovoltaic DC boost collection system includes full DC cascade type and centralized series type.The two collection system are different in the topology structure,adaptation occasion,and DC converter requirements,but they all have the problem of series output voltage imbalance under uneven irradiation,the imbalance of output voltage between the converters is easy to cause the failure of MPPT control,and the system even can’t work normally in serious cases.First of all,analyzed the advantages and disadvantages of two kinds of photovoltaic DC boost collection system topologies and their respective suitable occasions,considering the different requirements of the two collection topologies for the DC converter,in this paper,the boost full bridge isolated converter(BFBIC)with variable topology is used in the full DC cascade collection system,while the combined converter is used in the centralized series collection system,that is,multiple BFBIC sub-modules are input in parallel and output in series(IPOS-BFBIC).Then introduced the working principles of BFBIC,established the small signal model of photovoltaic DC boost collection system and the closed-loop transfer function of the system is obtained.Subsequently,the problem of MPPT control failure caused by uneven illumination between photovoltaic arrays in the full DC cascaded collection system is analyzed.Because each collection unit of the full DC cascaded collection system is composed of multiple power generation modules,they are connected in series,the non-uniform illumination will result in the power imbalance between the photovoltaic array in the collection unit,further leads to the uneven voltage distribution of the corresponding converter.Due to the limitation of the duty cycle of the converter,when the output voltage of the converters is seriously unbalanced,the output voltage of some converters may not meet the requirements for stable operation,which will lead to the failure of MPPT control and converter operation.This paper presents a coordinated control strategy,the working mode of the converter is changed by changing the BFBIC topology,so that the converter has a wider gain ratio range,according to the degree of power imbalance among the photovoltaic arrays,the converter can automatically switch the working mode,so as to solve the MPPT control failure caused by the uneven voltage division between the converters.Finally,the problems of voltage and current sharing among sub-modules of the IPOS combined converter and the failure of the MPPT control caused by uneven lighting in the centralized series collection system are analyzed.If the voltage and current sharing of the submodules in the IPOS combined converter are uneven,it will reduce the working efficiency and service life of the combined system,a distributed voltage and current sharing control strategy is proposed in this paper,which can not only improve the effects of voltage and current sharing among the modules,but also improve the modularity of the combined system.Similarly,the uneven illumination between the collection units in the centralized series collection system will lead to voltage imbalance among the combined converters,due to the limitation of the duty cycle of the converter,when the output voltage of the combined converters is severe imbalanced,which may also cause MPPT control failure or system operation failure,this paper proposes a coordinated control strategy of the combined converter based on the switching control of the sub-modules.The output voltage range of the combined converter is changed by adjusting the number of sub-modules to solve the problem of MPPT control failure caused by uneven voltage distribution among combined converters. |
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