Research On The Control Strategy Of Three-level Converters In AC And DC Hybrid Microgrids

In recent years, because of the advantages of distributed energy and energy storage devices, the demand of the power supply, a AC-DC hybrid microgrids has become a key research topic. This thesis focuses on control strategies of the bi-directional DC/DC converter and DC/AC converter in hybrid microgrids. Three-level converter has some merits, such as low voltage stress, low switching loss, and low harmonic content. And it can operate in full-voltage mode or half-voltage mode, so three-level buck-boost bi-directional converter is chosen as an converter and three-level TNPC converter as an interface converter which is connected DC bus with AC bus. This paper mainly launches the research from the following three aspects.Firstly, research the control strategy of one three-level converter. It presents the working principle and researches the single-loop control and the double-loop control. Introduce a control strategy based on electric quantity ampere-hour balance and constant output voltage. The simulation model of control systems are built in Matlab, and a3kW experimental platform is built with the use of dSPACE1104. Simulation and experimental results verify the validity of the control strategy.Secondly, research the coordination control of two three-level converters in parallel. It compares droop control with other controls. Against the voltage drop which droop control causes and the influence of line impedance on the output current sharing accuracy, this paper adopts a hierarchical control with droop characteristic to compensate the voltage drop and improve the precision of current sharing. The simulation model of control system is built in Matlab, and two3kW converters experimental platform in parallel is built with the use of dSPACE1104. Simulation and experimental results verify the validity of the control strategy.Finally, research the control strategy of TNPC three-level converter which is running in grid-connected mode and stand-alone mode. It describes the working principle of the converter and compares three-level SPWM modulation algorithm of in-phase carrier with one of inverted carrier. It studies the network voltage oriented grid-connected control strategy and the off-grid control strategy which uses capacitor voltage outer loop and inductance current inner loop. The simulation model of control systems is built in Matlab, and a3kW experimental platform is built. Simulation and experimental results verify the validity of the control strategy.