Power Electronic Converters for Hybrid Micro-Grids

Abstract

In this thesis, topologies and control systems of power electronic converters for hybrid micro-grids are presented. In particular, the converters presented in this thesis are single-stage AC/DC and three-port DC/DC converters. These converters are derived from a common structure, which consists of a dual inductor connected to the switch node of two switches. This structure enables high control flexibility and gives rise to providing soft-switching. A three-port nonisolated DC/DC converter and a new control system are proposed to integrate solar panels and energy storage in a DC micro-grid. The proposed control system utilizes the duty cycle as well as the switching frequency to control the power flow between the three ports. The proposed converter offers a low number of components, low cost, and high reliability. A system that comprises a totem-pole isolated, non-resonant single-stage AC/DC converter with a variable frequency control system is presented. The proposed system improves the converter's reliability by ensuring a constant dc-link voltage, which is independent of power flow. The converter uses small output capacitors to regulate the output current tightly. The converter achieves soft-switching at the turn-on of the MOSFETs. Moreover, it can achieve a high power factor and tight output current regulation. A new steady-state model was presented to describe the operation of the converter. A variable duty cycle control (VDC) system is proposed for totem-pole isolated single-stage AC/DC converters. In the proposed VDC, the switches' duty cycle is constant over a half mains period but different from each other in the negative and positive half mains periods. This feature allows high power efficiencies at low power operations. In the proposed VDC system, a new synchronization timing method and a new variable frequency modulation (FM) are introduced to mitigate the current surge issue in the transformer current at zero crossings of the mains grid. Furthermore, the time-domain analysis of the converter is presented for the VDC system.