Study On Parallel Inverter System Without Output Transformers

This dissertation is mainly devoted to the analysis of circulating current's characteristics and the control technologies of parallel inverter system without output isolating transformers. Based on comprehensive analysis and summarization on the principle and present situation of inverter parallel control technologies, the reasons and methods for detecting and restraining DC circulating current in parallel inverter system, the characteristics of circulating current and parallel control strategies of parallel system consists of double close-loop controlled inverters, the output characteristics and circulating current's characteristics of parallel SPWM inverters are deeply studied in this paper. For parallel inverter system, the imbalance of amplitudes and phases of each module's output voltage usually results in large circulating current, which reduces the reliability of parallel system. Aiming to improve the disadvantages of inductor-based circulating current restraining method proposed before, a novel circulating current restraining method based on coupled inductors is studied in this paper. This method has such advantages as suitable for any parallel inverter system with n modules, better performance on restraining circulating current (compared to uncoupled inductors) and no influence on the steady voltage accuracy of parallel system. In this paper, the reasons of DC bias contained in the output voltage of double close-loop controlled inverter are discussed, and the methods for detecting and digital adjusting DC bias with high precision are also analyzed. Then the conception and calculation of inverter equivalent output resistor is introduced, and the reasons and methods for detecting and restraining DC circulating current in parallel system based on double close-loop controlled inverters are also studied. Experimental results prove that the detecting and restraining methods mentioned above have good performance on restraining DC circulating current of parallel system. The expression of equivalent output impedance is deduced through the close-loop transfer functions of double close-loop controlled inverter, and the models based on equivalent output impedance for double close-loop controlled inverter and its parallel system are also established. Based on these models, the influences of main circuit and control parameters on the output characteristics of inverter are analyzed, and simulation and experimental research on the characteristics of circulating current are carried out from the point of view of equivalent output impedance when the amplitudes and phases of each module's output voltage are inconsistent. Based on the analysis of circulating current characteristics of parallel inverter system, an improved parallel control strategy based on real and reactive power is presented and verified by simulation and experimental results. Aiming to overcome the defect of traditional strategy based on droop characteristics, an improved parallel control principle based on frequency-droop and amplitude-droop scheme suitable for double close-loop controlled inverters is presented in this paper. The influence of dead-time on the output characteristics of SPWM inverter and the circulating current's characteristics of parallel SPWM inverters is deeply studied, where the SPWM inverter is based on instantaneous output voltage feedback control technology (single close-loop controlled inverter). Based on the analysis of the properties of harmonic voltages caused by dead-time, the linear model of close-loop controlled SPWM inverter with dead-time is established and the output characteristics of SPWM inverter under different voltage regulator, different dead-time and different load are also studied. At last a deeply research on the influences of dead-time on the circulating current's characteristics of parallel SPWM inverter is carried out in this paper.