Development of a multi-port DC-DC converter for a magnetically-coupled residential micro-grid
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Over the last century, the global average air temperature at the earth surface has been raised for about 0.74ºC, which has generated serious concerns all around the world about the global warming and consequent environmental problems. The electricity generation as one of the major contributors to the environmental pollutions should undergo a fundamental change towards the clean energy sources. In the residential sector, one of the major electricity users, the demand for renewable energy sources is increasing significantly. This thesis presents an effort to develop a residential micro-grid, including multiple renewable energy sources, energy storage, and local loads with multiport power electronic converters capable of bidirectional power flow and intelligent algorithms for power converter and micro-grid controls. A topology of multi-port converter using a high frequency magnetic link is proposed for residential micro-grid applications. Using the magnetic link in the proposed multi-port converter can reduce the complexity and size of the entire micro-grid effectively. The micro-grid is designed to supply a 4.5 kW residential load from combined energy sources of a PV array, a fuel cell stack, and a battery bank. It is controlled by a Texas instrument DSP (C2000/TMS320F28335) at the device level and a PC system as the energy management unit (EMU) at the system level. A single phase bidirectional inverter is designed to link the proposed micro-gird to the main grid. The inverter is controlled by a second DSP at the device level and by the EMU at the system level. The proposed micro-grid is able to operate in different operation modes based on the power flow directions and energy management scenarios. The EMU defines the appropriate operation mode of the system based on the short-term and long-term predictions of PV generation, and load demand by changing the power flow directions between the sources, energy storage unit, and loads. Due to the importance of the magnetic link in the micro-grid performance and complexity of design of high-frequency multi-winding magnetic components, a major part of the research is focused on the design, development and experimental test of the magnetic link. The geometry of the magnetic link including the dimensions of magnetic core and windings are designed through numerical analysis by using the reluctance network model (RNM). The core loss and copper loss analysis of the magnetic link are carried out accurately considering the non-sinusoidal effect of voltage and current waveforms. The designed component is then evaluated for the thermal limits by using the thermal electric model. The last part of this stage is the prototyping, experimental tests, and measurement of the component parameters and performance. The second part of the research is mainly focused on the design and analysis of the converters as the device level analysis of proposed micro-grid. It contains the analysis of the three dc-dc converters in the steady and transient states, discussion on the modulation technique of each converter, power flow control techniques, small signal modelling, and closed loop control design. The converter steady state waveforms are simulated and the soft-switching operation range is discussed. The converter waveforms are experimentally measured and compared with the numerical simulation results. The third part of the research is dedicated to the system level control of the micro-grid and energy management analysis. In this section, the main operation modes of the system are defined for both grid-connected and isolated operation conditions according to the power flow directions in the system. An energy management strategy is proposed considering both the short- and long-term energy forecasts and the real-time operational data of the system. The proposed strategy is implemented in an energy management unit using MATLAB/GUI and is used to control the system operation modes considering different control objectives and scenarios.
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