The Research On DC Component For Non-isolated PV Grid-connect Inverters

This thesis focuses on DC component for the non-isolated transformerless PVgrid-connected generate systems, the purpose of the study for this thesis is to understand themechanism and the hazards of the DC component, the more important to study the methods ofsuppress or eliminate DC component. Thus the security and reliability has been improve forthe non-isolated transformerless PV grid-connected generate systems, the modified topologyand the control model based on virtual capacitor has been proposed by studying, and validatedby simulation.With the rapid economic development and increasing energy crisis, changing the energystructure is imminent. More and more people put their focus on solar energy because of itsrenewable, non-polluting, ease of use and so on; PV grid-connected power generation is themain direction of development of the use of solar energy. Because of the light widedistribution, intermittent, and the complexity of the terrain and other factors, the distributedgeneration system is becoming more and more important. In order to reduce the cost of thePV system, usually the distributed generation system without power frequency isolationtransformer, this will lead to generate common-mode currents and the DC component of thesystem.For the generation mechanism of the DC component, this thesis explores the methods ofsuppressing or eliminating the DC component from topologies and control strategies. Thisarticle analysis the working principle of single-phase full-bridge grid inverters and with ACbypasses grid inverters, the structure of single-phase full-bridge grid inverters is simple, but ithas common-mode currents and pose a great risk to the system; with AC bypasses gridinverters because of its more power devices, the larger the loss of the system. This paperproposes a modified topology, and makes a detailed analysis of the work process andstructural characteristics, this topology has the characteristics of electrical isolation, and betterable to suppress the DC component.Established a improved control model which based on virtual control strategy. In spite oftraditional PI control model control algorithm is simple but the control is not precise enough,the system exist steady-state error; Although the PR control model can eliminate the steadystate error, can not suppress the DC component, and this don’t match PV grid-connectedpower generation requirements. The DC component of the grid voltage and the referencecurrent can be analyzed separately for the modified topology, this model can completelyeliminate the DC component in the grid voltage by improving the PI feedback signal; andisolate the DC component of the reference current and prevent DC injected into the grid with DC blocking action of the virtual capacitor.At the end of this paper we have established a400V/10A PV grid-connected powergeneration system. Respectively for single-phase full-bridge grid inverters, with AC bypassesgrid inverters and the modified topology inverters for a simulation study, by using oftraditional control strategies and improved control strategy simulation and the simulationresults verify feasibility of the control model.