Research On The Control Of Three-level Grid-Connected Photovaltaic Inverter

The survival and development of mankind cannot be separated from energy.The effective use of energy can promote the progress of society and the improvement of people's living standards.At present,the energy used in human activities is mainly fossil energy.The limited reserves will eventually face depletion.The energy crisis and environmental pollution have attracted the attention of many scholars at home and abroad.Solar energy has been widely used due to its abundant reserves and clean and pollution-free advantages.Inverter technology is one of the main technologies of photovoltaic power generation.Neutral Point Clamped(NPC)three-level inverter has large output power and low harmonic wave content,and achieved a wide range of applications in photovoltaic grid-connected control.In this paper,the topology of the NPC type three-level inverter,the midpoint potential control method and the photovoltaic grid-connected control strategy are studied.The photovoltaic system is a coupled nonlinear system,and the parameters of the system are greatly affected by external conditions.The NPC type three-level inverter has been widely used in photovoltaic grid-connected control due to its unique advantages.The neutral-point balancing problem is an inherent characteristic that directly affects the quality of electrical energy output by the inverter.In this paper,the mathematical model of NPC type three-level inverter is established,and the Space Vector Pulse Width Modulation(SVPWM)technology used in this paper is determined.A reduction method is adopted in the judgement of sectors.By analyzing the influence of each vector on the midpoint potential,the midpoint potential can be adjusted by adjusting the action time of the positive and negative small vectors.Based on the characteristics of photovoltaic inverter,this paper uses Fuzzy Logic Control optimized with Particle Swarm to control the action time of positive and negative small vectors,so that the fluctuation of midpoint potential is in an acceptable range.In the photovoltaic grid-connected control strategy,a double-loop control strategy is adopted,that is,the outer current loop uses a fuzzy-PI controller to control busbar voltage,the inner current loop uses the improved slide mode and alterable structure control to decouple the active power and the reactive power current,so that photovoltaic grid-connected control is achieved.Traditional PI control and state feedback controllers,although able to achieve good control of the inverter,do not take into account the uncertainty of the inverter parameters and the influence of external interference,it has poor disturbance suppression effect and poor robustness.The control method in this paper has a good suppression effect on the uncertainty of the inverter parameters and external interference,and can meet the power quality requirements of photovoltaic grid-connected control.In MATLAB,the NPC Three-level Grid-connected Photovoltaic Inverter system is simulated and verified.The simulation results show that the midpoint potential control method based on fuzzy logic control has good control effect,the fluctuation of the midpoint potential is within the allowable range of the grid,and the double closed loop grid-connected control strategy can adapt well to the environmental changes,the anti-disturbance performance is good,and the power quality of the inverter output is also within the allowable range of the gridconnected.