Analyze Operating Characteristics Of Micro-grid With Micro-source Inverter Series In Asymmetric Conditions

Photovoltaic, wind power and other micro source in the micro-grids are influenced by the randomness and uncertainty of natural condition, which cause larger fluctuation of the output power, and also cause the micro source inverter DC side voltage changed, making the series micro-grids in the asymmetric state. In addition, unbalanced load, micro source inverter fault or changes in the external environment condition in the combined three-phase will make the system become asymmetric structure. Under this asymmetric structure, the operation of the system shows different performance. Therefore, this thesis analyzes operation characteristics of the micro source inverter series micro-grids under the condition of asymmetric.First, the structure of the single-phase and three-phase micro source inverter series micro-grids are introduced, as well as the main micro source in the system. On the basis of theory, Fourier expression of system output voltage is derived; the fundamental and harmonic voltage characteristic of the system when micro source inverter DC side voltage is changed is analyzed; the influence of system output voltage harmonic amplitude by the micro source number is studied; the effects of different number of micro source inverters on the output voltage characteristics of DC side voltage variation is analyzed through the output voltage wave of a simulation model.Then, the single-phase equivalent circuit model of the system is established, the running characteristics of the system when the micro source inverter DC side voltage changes is analyzed, as well as the distribution characteristics between the micro source inverter. Due to the output of the inverter unit is connected in series, the current of the various micro source inverter is equal and the distribution characteristics of total power system have the relevant with the micro source inverter DC voltage and the modulation ratio. Under the condition of total modulation ratio is constant, we reduce the effect of the system running characteristics when the micro source inverter DC side voltage change through the carrier amplitude variation and phase shift method.Finally, the calculation scheme on output voltage equation and its phase/ line voltage unbalance degree of the combined three-phase system is given by using the symmetrical component method, and the stable operation condition of the system is studied after the solution of a micro source. The effect of three-phase system load inequality, three-phase load missing phase operation, three phase load switching and the splitting of a micro source on the operation characteristics of the system is analyzed.