Study On The Architectures And Power Imbalance Control Method For MV PV Cascaded Converters

In the utility-scale PV station,the conventional centralized structure contains a bulky low-frequency transformer,that increases the system and the open-circuit losses.Meanwhile,this structure limits the further improvement of system capacity.Compared with the centralized structure,the medium-voltage power electronic system(MVPES)using a medium-frequency transformer as galvanic isolation could achieve a smaller size and smaller weight because of higher frequency.The modular design of the MVPES could also improve the reliability and decrease the manufacturing cost.In this paper,research is carried out on the architecture synthesis method and power imbalance control method for photovoltaic medium voltage power electronic converter.Firstly,the background of the solar energy and MVPES is introduced.Some of the existing MVPES are discussed.The power imbalance problem is introduced.The control methods for power imbalance are also reviewed.Then the synthesis method of MVPES architecture is studied.The synthesis method is divided into two steps.The first step is to synthesize the power conversion chains(PCCs),and the second step is to synthesize the complete structure based on the PCCs.The article introduces the relevant assumptions and basic ideas of the synthesis method.The computer algorithm realization of the synthesis method and the preliminary deduction result are also given.The synthesis and evaluation results indicate that the two-stage structure prevails on the efficiency and cost aspects.One of the major challenges in implementation of two-stage structure is power mismatch problem.Thus,the reactive power control method for the modular power imbalance is studied.The minimum required reactive power to ensure the normal operation of cascaded PV converter is calculated.When the power imbalance is relatively slight,only some of the power modules are controlled to output reactive power.Under serious power imbalance,all power modules are controlled to output reactive power and the module apparent powers are controlled to be the same.The simulation results are given and a prototype of a three-module cascade converter was designed to verify the proposed scheme.The final chapter is the conclusion of this paper and outlook for the future research.