Research On Control Strategy Of Parallel Photovoltaic Inverters And Harmonic Circulating Current Restrain Method

Photovoltaic power generation is an important form of distributed power system, which has broad prospects for development. Inverter is the core equipment of the photovoltaic power generation system. With the increasing demand of humanity for electricity and increasingly high requirements of power system capacity, performance and reliability, single photovoltaic inverter device has not reached the requirements. Therefore, it is very important to research on the photovoltaic inverter parallel system. In this paper, two single-phase photovoltaic inverter paralleled system is taken as the research object and the control strategy of parallel inverter and parallel system harmonic circulating current control methods are studied.Firstly, a two-stage no isolation type structure is selected as the photovoltaic inverter main circuit topology, and equivalent model of a single inverter and the equivalent model of the two inverters in parallel operation are established, through the equivalent model, a theoretical analysis of the circulation of the paralleled inverter system is carried out, then we discuss the inverter parallel operation optimization control strategy to improve the traditional droop control method, namely, in order to realize the decoupling between the active power and reactive power, a new droop variable is introduced before the traditional droop control. The active power and reactive power of each inverter are reasonably distributed. Secondly, we analyze the causes and effects of harmonic circulating current in paralleled inverter system. For the wireless paralleled inverter system with nonlinear loads, a method based on decoupling control of parallel system harmonic circulating current suppression method is proposed. On the basis of the improved droop control scheme,we increase the harmonic power sharing control and decouple the harmonic power from the total apparent power, adding a harmonic power controller to suppress harmonic circulating current by reducing the size of the harmonic power. In order to quickly change the tracking compensation of active power and reactive power and harmonic power, instantaneous reactive power theory is introduced, which can quickly and effectively detect and calculate the active power, reactive power and harmonic power, so as to enhance the flow ability of parallel system and dynamic characteristics. Finally, hardware and software part of the control system are designed in details, including the main circuit of the device parameters and selection, DSP control circuit and the corresponding software design, then we analyze and compare the characteristics of parallel system before and after the addition of harmonic power sharing control through MATLAB/ Simulink simulation platform and the model experiment.Simulation and experimental results show that the proposed harmonic circulating current suppression method based on decoupling control of parallel system can obviously inhibit the harmonic circulating current between parallel inverter system, improve the quality of inverter parallel system output voltage, accurately allocate the power of each inverter, and effectively improve the performance of paralleled inverter system with nonlinear load.