Research On A Quasi-single-stage Three-phase Multi-functional Grid-connected Inverter

In order to deal with the problems about energy shortage and environmental pollution,renewable energy generation and micro-grid have been proposed successively and achieved unparalleled development.Energy transmission with high efficiency and power quality improvement have become key points of the micro-grid technology.Multi-functional grid-connected inverters(MFGCI)are significant interface between new energy generations(such as photovoltaic,wind turbine)with AC grid.Due to the dual functions of active power conversion and power quality control,MFGCI are paid more attention recently.Since traditional single-stage MFGCI cannot adapt to wide range DC voltage,and two-stage MFGCI hardly get much higher energy transfer efficiency,a novel multi-functional grid-connected inverter with high efficiency based on the quasi-single-stage configuration is researched in this paper.Firstly,system configurations and topologies of the quasi-single-stage three-phase MFGCI are researched.A novel three-port three-phase converter(TPTPC)is constructed by introducing an additional power port into the conventional three-level DC/AC converter.This converter consists of an AC grid port,a high-voltage DC bus port and a low-voltage input port,which is connected to the DC source directly.By feat of this configuration,partial power can be transferred into grid within single power conversion stage.Secondly,the influence of zero-sequence components on voltage balance is analyzed based on the carrier-based pulse width modulation(CBPWM)strategy of traditional three-level converters.A modified CBPWM strategy based on the variable asymmetrical carriers and appropriate zero-sequence components is researched and this strategy can achieve most active power are transferred within single stage.The power transfer feature,power loss of semiconductor devices and harmonic spectrum of the quasi-single-stage MFGCI are analyzed in detail.Theoretical analysis indicates that this converter has many advantages in reducing the power rating and power loss of the front-end DC/DC converter and increasing power conversion efficiency.Thirdly,to meet the demands of power quality enhancement,the mathematical model of TPTPC is derived and the control method with multiple targets optimization is presented.By using the analysis method based on rotating coordinates,the influence mechanism of reactive currents,harmonic currents and unbalanced currents on the power transfer feature is intuitively revealed.Finally,a 3kVA experimental platform of the quasi-single-stage MFGCI is built.The experimental results of active power transfer and power quality improvement verify the effectiveness and feasibility of the topology,modified CPBWM strategy and control method.Compared with the traditional two-stage configurations,the quasi-single-stage MFGCI can combine the effective power quality enhancement and high-efficiency active power transfer,which contributes to the efficient and flexible development of the micro-grid.