Split DC bus converters for power electronic and AC-DC Microgrid applications

Power electronic converters are used extensively for electrical power conversion in applications such as renewable energy systems, utility applications, and electric vehicles. Such converters are needed as it is rare for a source voltage to fit the needs of a load or a set of loads for any particular application. They consist of active semiconductor switches and passive components that are combined in circuit structures (topologies) that are operated with a control strategy. The focus of this thesis is on AC-DC and DC-DC converters and their applications in AC-DC microgrids. AC-DC converters are typically two-stage converters that consist of a front-end AC-DC converter followed by a DC-DC back-end converter. The AC-DC front-end converter converts AC voltage from an AC source such as the grid to a DC bus voltage that has been filtered by an intermediate DC bus capacitor; the DC-DC converter then converts this DC voltage into the desired output voltage. A less expensive alternative to this two-stage approach is to have just one converter perform AC-DC and DC-DC conversion. This thesis examines isolated single-stage AC-DC converters and back-end DC-DC converters for two-stage converters that have a split DC bus, with either two capacitors in series across the bus to split the voltage or with two parallel current paths to split the bus current. These converters have fewer components or fewer light-load losses than converters with conventional topologies. Four new power converters with a split DC bus are proposed in this thesis: a reduced-switch three-phase AC-DC converter, two lower power DC-DC converters, and an AC-DC converter that can be used to simplify the architecture and control of AC-DC hybrid microgrids. The proposed converters increase efficiency and reduce the control complexity of hybrid microgrids. The operation of each converter is explained, the steady-state characteristics and the dynamic model of each converter are determined by mathematical analysis, and a procedure that can be used for their power and control stages design is developed. Experimental and simulation results are used to confirm the feasibility of the converters and simplified AC-DC hybrid microgrid, and conclusions that resulted from the thesis work are stated

Analysing integrated renewable energy and smart-grid systems to improve voltage quality and harmonic distortion losses at electric-vehicle charging stations

Due to environmental impacts of fossil fuels, a move towards using Electric-Vehicles (EV) to reduce carbon emissions and fossil fuels is regarded as a good solution to the climate change problem. In recent years, a dramatic increase of EV and charging stations has raised voltage quality and harmonic distortion issues that are affecting the electrical grid network. To address these issues there is a need to redesign the integrated renewable energy and smart grid network by applying new methodologies. The aim of this work is to propose an isolated smart micro grid, which connects renewable energy generation units to the electric vehicles charging station without degrading voltage quality or causing harmonic distortion losses. A topology has been identified for the smart grid that is simulated with the intention of implementing it with the integration of modern communication technologies that enables the components to produce and reflect data in an efficient way to assist better regulation in the power flow. The power flow is investigated by simulating unpredictable renewable energy and by using car batteries at the electric vehicle charging station. It is investigated how micro grid parameters are affected in the presence of super capacitors, car batteries and the use of larger power electronic converters. In the simulations, an electrical power control system is implemented at power conversion units which generates the correct duty cycle of the converter switches and controls the power flow operation at the smart grid. Then the proposed electrical power control system is compared with other systems such as maximum power point tracking (MPPT) algorithm and space vector pulse width modulation (SVPWM). A smart sensor system and smart protection are connected to protect the grid and to maintain system stability over a long time. The research focuses on developing a smart grid that performs the communication among the converters, performs power sharing, and does preventive management. It also monitors the energy efficiently and balances the energy in the grid irrespective of load or power generation variations. A mathematical model is developed to predict grid behaviour and is validated via MATLAB simulation of the grid. It is noticed that an improvement is made in the efficiency of renewable energy transmission to the electric-vehicle charging station