Research On Diode-clamped Grid-connected Inverter With Variable Topology

At present,the development of renewable energy can solve the source shortage and environmental pollution issues has been formed a broad consensus.Photovoltaic power generation with its rich reserves,convenient to access and other advantages gained wide attention.However,photovoltaic power generation has shortcomes such as sensitive to environmental factors,severe power fluctuations and high generation costs.Therefore,it is of great theoretical and practical value to carry out research on the new inverter topology,modulation strategy,control strategy,optimization method.Meeting the characteristics of PV power generation promotes the optimization of the comprehensive performance of photovoltaic power generation system.In this paper,a variable topology diode-clamped grid-connected inverter is proposed.It improves the efficiency of the inverter in the overall working range.At the same time widens the power generation range.The paper carried out research work including the structural principle analysis,loss and grid-connected current THD modeling,control strategy designing,simulation and experimental verification.Firstly,the working principle of the proposed new topology is analyzed.The principle of topology switching is described.The modulation strategy based on carriers cascade for the two working modes are designed.The current flow path and the DC side capacitor operating characteristics of the inverter in two working modes are analyzed in detail.It lays the foundation for the subsequent loss and grid curren t THD modeling.Secondly,the average loss and the THD expression of the two modes towards the DC input voltage in a fundamental cycle over the entire operating range are derived.After selecting the specific system parameters,the distribution curves of the loss and THD in the two working modes are given.Based on that,the two models are compared and analyzed in order to design the topology switching strategy.Thirdly,based on the analysis of the principle of current predictive control strategy,the closed-loop control strategy of two working modes is obtained.Then the switching strategy between the two modes is given and the switching timing selecting principle is analysed.The simulation model based on MATLAB is built and the performance of the system is verified by steady state,dynamic and topology switching simuliation.Finally,an inverter verification platform based on DSP + FPGA is designed and built.The parameters of hardware circuit is given,and the software program of DSP and FPGA is written.A detailed experimental study on the proposed topology was carried out to verify the proposed modulation strategy,loss and THD modeling,current control strategy and mode switching strategy.The results show the variable topology grid-connected inverter related theory is correct.