Research Of Three-Phase Photovoltaic Grid-connected System

Solar energy is a kind of ideal energy source with advantages such as cleanness, inexhaustibility and safety. As one important part of solar energy application, the technology of photovoltaic (PV) generation is considered as one of the most potential power generation in the 21st century. The research on PV generation has significant effects on solving energy crisis, reducing environmental pollution and greenhouse effect.Firstly, this paper analyzes the influence of control variable selection of previous stage in PV grid-connected system, and gives the general control structure of previous stage. Three typical DC/DC topologies are analyzed and compared from small-signal model aspect and topology aspect in detail. It has instructive significance for the topology selection of previous stage.This dissertation theoretically analyzes and establishes the mathematic model of the three-phase inverter with LCL filter. According to the mathematical model, it is much easier to control inα-βstationary frame with no coupling than in d-q synchronous frame with strong coupling. Detailed deducing the expressions of the ripple current and its RMS, this paper combining with the concepts of total harmonic distortion (THD) and ripple attenuation factor (RAF) proposes a novel design method of LCL filter parameters, which has the advantages of simple, practical and clear physical meaning.Based on instantaneous power theory, a novel control method for PV grid-connected inverter with sinusoidal and balanced grid voltage condition is proposed inα-βstationary frame. Then the parameters of the controllers are analyzed and the design algorithm is also presented. Furthermore, this paper deduces the mathematical model of inverter working under distorted or unbalanced grid voltage; and two kinds of positive/negative-sequence fundamental extraction strategies are proposed, by which the accurate extraction of positive-sequence fundamental component of grid voltage is realized inα-βstationary frame; then sinusoidal and balanced output currents is well achieved with grid frequency offset by using the resonant control and feed-forward control of harmonic force. At last, the proposed control strategy is extended to distorted or unbalanced grid voltage condition successfully. Compared with the traditional method, unity power factors is obtained in stationaryα-βframe not only without the need of grid voltage phase detecting, rotating transformation or decoupling control, but also the disturbance of the phase detecting error and the problem of un-completely decoupled caused by un-exact parameters are reduced and the stability of system is improved.In Saber simulator, the MPPT and photovoltaic array templates are established using MAST language. Then the system simulation are carried out considering the time delay, dead time, discrete factor and other factors. And simulation results show that the proposed method has not only excellent performance but also the fast dynamic response. Finally, the RT-LAB experimental verifies the correctness of the previous stage control, the positive-sequence fundamental extraction strategies of grid voltage, the design method of LCL filter and the proposed control strategy for PV grid-connected inverter.