Research On Circulating Current Analysis And Accurate Power Sharing Of Parallel Inverters In Microgrid

The application of distributed generation such as photovoltaic and wind, fuel cell is increasing, meanwhile the proportion of which in the whole power system is increasing as well. Micro grid, on the basis of the distributed generation, is a cutting-edge technology which can interact with large power grid to give full play to the potential of distributed generation. Mostly, distributed generation and energy storage device connect to the grid by inverter, and inverters in parallel is a ubiquitous trend. Parameter differences of parallel inverters lead to circulating current, which not only increases the power loss of system, but also deforms the output current.The design of the main parameter and hardware circuit of. photovoltaic gird-connected inverter finishes in this thesis; aimed at harmonic attenuation and improving the efficiency of photovoltaic grid-connected inverter, several practical algorithms are proposed; protection strategies mainly used in photovoltaic grid-connected unit are introduced.; the operation technology of three-phase inverter commonly used in microgrid is researched, and setting up the general mathematical models of a single three-phase inverter and three-phase inverter parallel system, meanwhile the mathematical model of the synchronous rotating dq coordinate system is introduced; the realization method of SVPWM is proposed, and on the basis of conventional 5-phase SVPWM, a kind of modified 5-phase SVPWM, is presented; according to the mathematical model of the inverter parallel system, the causes of circulation current of three-phase inverter parallel system are explained in detail; targeted restraining method of circulating current is put forward, including the method of zero vector distribution and the method of zero sequence voltage difference compensation, detailed description of their principle and control algorithm is presented as well, and building the simulation platform and experiment platform for validation, the operation experiments of a single inverter and the two inverters parallel system have been carried out.Verification results verify the effectiveness and feasibility of the method of zero vector distribution and the method of zero sequence voltage difference compensation mentioned in this paper. In the method of zero vector distribution, circulating current can be restrained by reasonable distribution of the effect of zero vector, and a good fast dynamic response and circulating current inhibition effect can be realized, but zero vector adjustment of each inverter need sampling zero sequence current of another inverter. Zero sequence voltage difference compensation dosage is gained through the control algorithm to a single inverter in the method of zero sequence voltage difference compensation, and the zero sequence voltage difference was close to zero, so as to realize circulating current suppression, this method has a good fast dynamic response, the controller design is simple, easy to implement and control effect is excellent.