Power Converters and Control Systems for DC Smart Grids and Smart Transformers Applications

The recent rapid growth of renewable energy technologies, such as solar photovoltaic (PV) and wind turbines, are changing the nature of transmission, distribution and utilization of electrical energy. Most of the electrical loads-lighting, adjustable speed motors, brushless DC motors, computing and communication equipment are more compatible with DC power. Most distributed renewable energy generator including solar PV, wind, fuel cell, produce DC voltage. Thus DC power has great potential for increased compatibility with high-penetration, distribution-connected solar PV and other Distributed Renewable Energy Generators (DREGs). Trying to foresee the possible future scenarios of the power systems, it can be noticed that DC Smart Grids with its structure enables interconnection of different kind of sources requiring different voltage levels due to integration of different renewable sources, loads and energy storage devices, hence the DC Multibus provide a viable solution. A DC Multibus operating at different voltage levels can provide flexibility and redundancy to a DC Smart Grid. The role of a Single-Star-Bridge-Cell Modular Multilevel Cascaded Converter (SSBC MMCC) for the creation of a DC Multibus has been investigated and its model, from the switching function to the small signal model, has been studied and verified in detail. A proper control system has been designed in order to manage voltage regulation and power balance. The performances of the system have been verified in simulation and experimental results in the particular case of a 5-level SSBC MMCC confirming the validity of the proposed solution and the robustness of the control system in a wide range of loads variations (up to 50% of power unbalance operating condition). The SSBC MMCC is combined with DC/DC converters in order to extent the operating voltage range. Looking at the DC Smart Grids, the Dual Active Bridge (DAB) converters provide flexibility to the DC Multibus thanks to the possibility to extend the voltage range operation and to ensure galvanic isolation avoiding faults propagation. The DAB converter has been investigated and its model, from the switching function to the small signal model, has been studied and verified with simulation and experimental results. A proper control system has been designed in order to manage its output voltage. The use of the DAB converter becomes necessary in other examples of DC Smart Grids such in a More Electric Aircraft (MEA) where volume and weight optimization is a critical issue. An advanced active rectifier for a MEA has been proposed in this thesis. It is based on a SSBC MMCC consisting of four single-phase H-Bridge cells for each phase. The system is coupled to four DAB converters providing output voltage equal to 270V in compliance with the aircraft standards. The proposed topology exhibits high power quality performances with a THD(%) of injected current equal to 3.5%, since the operation of the power conversion stage is characterized by a virtual switching frequency of 80 kHz in case of rated frequency of 400 Hz. High performances are guaranteed also in case of load changes and frequency variations in the range 360-800 Hz. Besides high efficiency, high power density is another of the main goals related to the power generation and distribution in MEA. As a consequence, the number of the power conversion stages has to be reduced at the least in order to reach this target. Isolated DC/DC converters topologies can provide the required high voltage gain: 270V/28V. A 270/28 V SiC MOSFET DAB converter has been proposed for MEA applications. An investigation about the thermal behaviour of the converter in case of high ambient temperatures has been provided in order to verify compliance with harsh environments conditions. Feasible operation of the DAB converter can be guaranteed for ambient temperatures over 100°C thanks to the superior performances of the SIC MOSFETs. Further improvements have been obtained in case of optimization of the power stage or using more advanced modulation techniques. A significant efficiency improvement of the 2% at light load is obtained through the Trapezoidal Modulation technique instead of the traditional Phase Shift Modulation. A further improvement is achieved in case of a hybrid realization based on SiC MOSFETs plus Si automotive qualified MOSFETs instead of an all-SiC DAB converter. However, when specifications require high switching frequencies together with reduced size of the cooling system and harsh operation, the all-SiC DAB converter is the preferable choice. Finally, a possible three stages Smart Transformer (ST) is developed based on a SSBC MMCC converter in the Medium Voltage (MV) side and several DABs converters in the isolation stage. On the basis of the thermal monitoring information of the power conversion cells, a power routing techniques can be used in order to increase the reliability of the ST. The overall small signal model for such ST is derived and a new voltage balance control operating in the isolation stage has been compared with the classical MV voltage balance control. It has been demonstrated that the balancing in the isolation stage can achieve a significantly higher bandwidth than in the commonly applied MV stage. The superior dynamic performances are verified by simulation and experimental results on a ST with a 5-levels SSBC MMCC. The validity of the proposed voltage balance control has been remarked in a new ST start-up procedure, where the balance voltage condition is verified during all the start-up (also when the SSBC MMCC is not already turned on) since the control is demanded to the DAB converters.