Research On Filtering And Phase Lock Method Based On Three Phase Grid-connected Inverter

With the shortage of fossil fuels, increasingly serious environmental pollution, and increaseddemand for energy consumption of human, distributed power generation systems are becoming hotspots in every country. At present, grid-connected inverters play an important role in the interfacebetween renewable energy and the grid, and their performances determine the development andapplication prospects. Phase locked loop is closely related with the performance of three-phasegrid-connected inverters, and filtering methods determine harmonics of grid current, so the phaselocked loop technologies and filtering methods are the hot spots in the study of three-phasegrid-connected inverters.Firstly, this paper introduces the research status, applications of grid-connected inverters, andsorts the inverters later. Then it brings the brief introduction to the control strategies, filtering methodsand phase lock loop technologies.According to the control strategy based on grid voltage oriented control with common spacevector modulation algorithm, this paper establishes the mathematical models of three-phasegrid-connected inverters in abc coordinate, αβ coordinate and dq coordinate respectively and achievesthe goals of dq current decouple.With the mathematical model of LCL filter, a design process based on the standard of harmonicsof grid current and harmonic spectrum of bridge voltage is presented. And the stable region can beachieved by analyzing discrete domain model of three-phase grid-connected inverters with undampedLCL filter. At the same time, the efficiency influence and filtering effect of damping resistor areanalyzed.After analyzing the lock error of single synchronous reference frame for phase locked loop(SSRFSPLL), a second order generalized filer(SOGI) is used for SSRF SPLL, and some modules such asadaptive frequency part, DC offset cancellation and improvement of start-up are added to quickly andaccurately lock the grid phase.Finally, a30kVA three-phase grid-connected inverter based on DSP digital control is built andthe hardware and software designs are introduced. Then this paper compares the efficiency andharmonic distortion of systems respectively with L filer, passive damping LCL filter and undampedLCL filter on the basis of the prototype which is regarded as the experimental platform. In themeanwhile, the experimental results verify that accurate lock of grid phase can be obtained quickly after grid faults occur. The experimental results correspond with the theoretical analysis andsimulation results, and also verify the correctness of the theoretical analysis.