Research On Single-phase Transformerless Common-ground-type Five-level PV Grid-connected Inverter With Low Leakage Current

With the development of economy and society,non-renewable energy is increasingly consumed and causes a series of environmental problems.Therefore,clean energy solar energy is favored by all parties,and photovoltaic grid-connected inverters have developed rapidly as the core of solar power generation.The single-phase transformerless photovoltaic grid-connected inverter removes the isolation transformer on the basis of the traditional inverter,which reduces the size,increases the power density,and reduces the loss.However,there is a direct electrical connection between the photovoltaic cell and the grid,and it is formed with the earth.The electrical circuit generates leakage current with the change of common mode voltage,which may cause equipment failure or even threaten personal safety.At the same time,increasing the output voltage platform order can reduce the current harmonics in the grid connection process.This paper mainly studies the topology and control strategy of a five-level grid-connected inverter that suppresses leakage current.First,a corresponding equivalent circuit model is constructed for the common mode voltage of a single-phase transformerless photovoltaic grid-connected inverter,the basic principle of leakage current generation is explored,and the effectiveness of traditional inverters and common ground inverters in suppressing leakage current is analyzed.Principle,through simulation,it can be known that the parasitic capacitance of the common ground inverter can completely eliminate the leakage current.Then a new type of five-level common-ground grid-connected inverter is proposed,which can completely eliminate leakage current and reduce grid-connected current harmonics.Its working mode and working principle are analyzed,and the performance of traditional inverters is compared with simulations.The cost comparison has obvious advantages.Then research the related control strategies of single-phase transformerless photovoltaic grid-connected inverter system,mainly analyze the relevant principles of the phase-locked loop,outer loop voltage loop and inner loop current loop in the control system,and focus on the model predictive control The strategy is applied to the current controller of the inner loop current loop,and optimized and compensated by calculating the delay and the reference current delay.The grid-connected system using model predictive control is more flexible than the traditional PI and PR current controllers.,Simple control,good robustness,rapid response,strong anti-interference,and better control performance.Finally,a 1k W experimental prototype was designed,related parameters of the prototype and device selection were introduced,and then the main circuit,voltage and current sampling circuit,drive circuit and other related hardware circuits were designed,and the software algorithm was designed.Experiments have verified the leakage current suppression level and THD performance of the prototype,and proved the feasibility and superiority of the new topology and control strategy.