Research On The Three-phase Four-legs Inverter For Power Generation

Because of the traditional energy exhausting increasingly and people pay more attention to environment increasingly, power system is facing a great change. Distributed generation system like solar energy, wind energy, micro turbine etc will gradually become the future development direction of power system.So people will pay more and more attention to the structure design and the control method of the key components for power generation systems—inverter.Generally, the three-phase inverters adopt three-leg structure. But when it is applied to the three-phase unbalanced loads condition, its three-phase output voltages will be asymmetrical. Three-phase four-legs topology is proposed to solve this problem. The topology is known to produce balanced three-phase voltages even under unbalanced load conditions. Comparing with the other three-phase four-wire inverters, its size and weight are smaller.Based on comparison of three-phase four legs inverter control strategies. this thesis introduces a Three-phase four-legs inverter control. which based on a study single-phase inverter. A double feedback loop control scheme, which is outer control loop and inner control loop, is proposed for single-phase power supply through sinusoidal inverter. The proposed outer control loop uses two PI controllers for the independent control as magnitude and phase to ensure a highly steady precising of the voltage output from inverted power supply. The inner control loop added a proportion of the controller, reduced the filter inductor, to ensure a fast response to the voltage output and reduce waveform distortion. A new algorithm is proposed that using of Correlation function to detect the phase of sinusoidal voltage output from the inverter. The simulation result is showing that the control system based on the proposed method has excellent dynamic and static characteristics to keep all voltage output waveforms highly sinusoidal under any loading conditions.As the existence of the forth leg and the coupling effect of the filter inductance, so that the control of the three-phase voltage became very complicated. The thesis proposed a new-decoupling control strategy for the above problem. In this thesis, the filter inductance is regarded as an inner resistance of the power source. A transformation relationship is proposed based on the idea of changing the inner resistance of the power source. Therefore the three-phase voltage can be decoupled entirely and the complex control problem can be changed into simple single-phase voltage control. We have adopted a3rd harmonic injection method and have improved the utilization of DC bus voltage which based on single-phase inverter control. At the same time, this method gives how to choose the value of the fourth leg’s inductance. In this thesis, the control strategy is simple and easy to control. The simulation results were shown that this inverted power supply fits any load and has excellent dynamic and static characteristics.The design of traditional inverter using only DSP to achieve many function like generating pwm, AD sampling, phase detecting and tracking, control strategy etc. It puts forward the high requirements to programming skills. If the interrupt and timing control processes improper, it is easy to cause the mutual interference even crash. This thesis is still using DSP for real-time data acquisition, calculation and control strategy, and Fully exert its advantages as high speed digital signal processor. While introducing FPGA, use hardware quickly generate SVPWM, make DSP and FPGA realizing division of labor and cooperation. This method makes the control system very simple, and benefit modular. While programming is simpler, the anti-interference ability is enhanced, the system is also easyer to maintain and expand.At last, a three-phase four-legs inverter is designed and the digital control based on DSP TMS320F2812and FPGA XC3S250E is realized. Experimental results are compared and analyzed.