Research On Phase-locked-loop And Current Control Technique Of Three-phase Inverter

With the worsening of primary energy continuous consumption and environmental pollutionproblems, more and more people have focus on the development of new green renewable energy.Photovoltaic power generation is considered to be the most potential to replace primary energy aspower generation energy, in the future. Therefore, PV grid-connected inverter has become animportant research direction of domestic and foreign scholars. This paper, based on a250kWthree-phase grid-connected inverter system, focus on the control strategy of phase-locked-loop andgrid current control technology.This paper analyzes the problem of the single synchronous reference frame phase-locked loop(SSRF-PLL) under unbalanced grid condition, then, do a summary of the existing imbalance PLL andanalysis their shortcomings in dynamic performance. Therefore, this paper presents a fast phasesequence separation PLL based on method of symmetrical components to improve the dynamicperformance. For a three-phase grid-connected inverter using the normal current control strategy willcause a problem under imbalanced grid. After analyzing the mathematical and power model ofinverter during unbalanced grid, this article study two control objectives for grid-tied inverter: first,inhibition of DC voltage ripple; second, suppression of negative sequence current. Considering thecharacteristics of grid-tied PV inverter, an unbalanced control strategy based on negative sequencecomponents of voltage feed forward compensation is developed, and the corresponding controlsystem is designed.At last, built a250kW experimental prototype; the detailed hardware and software design are alsogiven in the paper. Simulation and experiment results demonstrate the correctly and feasibility of theproposed control system.