High Efficiency Interfacing Converters for Distributed Energy Systems

The ever-growing amount of renewable energy sources connected to the grid call for a revision of the traditional concept of energy distribution network. In addition, the world climate change issue is becoming a major task for the green energy technology development in the last decades. In the last years, the smart grids are the promising paradigm shift for the low- and medium-voltage grids. The term smart grid indicates the symbiosis of the traditional distribution network with a deeply branched ICT infrastructure. The fundamental benefits of this cooperation is an increase in efficiency, reliability and stability of the whole electrical system, and a minimization of the costs and environmental impacts. The main differences with respect to the traditional distribution grid are the growing arrays of customer-sited distributed energy resources (DERs), including renewable energy sources and storage systems, instead of the large and centralized power plants present in the tra- ditional grid. To control and monitor the overall grid system, the ICT infrastructure must be able to manage the present agents, i.e. DERs, and the latter need to be interfaced with the distribution network through Electronic Power Converters (EPCs) forming a micro-, nano-, pico-grid according to the geographical extension. A typical example of pico-grid is a smart building: renewable energy sources, e.g. photovoltaic (PV) panel or wind generator, and energy storage systems, e.g. battery and/or supercapacitor, are connected together through a common DC Link bus. The interface between DERs and DC Link is demanded to EPCs that manage the power flow, optimize the source utilization or the time-profile generation via the energy storage. The power transfer, besides taking place among the connected DERs, can occur between the DC link and the grid through an AC/DC converter (inverter). The main aim of the presented work is to look into the EPC design procedure to achieve a deep integration of DERs in the future smart grid. To this end, the efficiency and the power density are considered the main performance indices to achieve a more effective EPC design. Since the design process involves the optimization of many performance indices, the first part of this work is dedicated to a general overview of Multi-Objective Optimization problem is discussed to provide the sufficient theoretical background to handle with new methodology. Initially, an example of application of MOO is presented to introduce the notation and show the methodology in a simple case, then the EPC optimization is included. The central part of the dissertation focuses on the needed background to allow a proper converters modelling. The considered objectives, i.e. efficiency and power density of the considered EPC, are related to the losses, that occurs during the power transfer in the EPC, and the final volume of the converter. Because of this, the estimation of the losses and volume for each electronic components of EPC becomes mandatory. However, the choice of the accuracy of the models used to predict losses and volume is a degree of freedom, since it greatly impacts on computational time. Since this approach is considered in this dissertation a first valuation to make comparison between innovative solutions and the state of the art, the models consider only the main contribution of each loss phenomenon and the main volume contributions. In this way, the MOO analysis doesn’t become a high time- and data-consumption tool. The series resonant converters will be treated in detail because of their many advantages: inherent short circuit protection, higher operation frequency, lower electromagnetic interferences and soft-switching modulation. An innovative mathematical framework is proposed for their steady-state analysis, providing the closed-form solutions of the sampled resonant impedance state, the voltage conversion ratio and the transferred power in a single matrix formulation. This tool has been validated by means of a high efficiency DC/DC topology for renewable source interfacing: the Interleaved Boost with Coupled Inductors (IBCI). The proposed framework has allowed to identify six different operating modes providing the closed-form expressions of the voltage conversion ratios and the operating boundaries. Furthermore, the sampled state variable trajectories lays out some soft switching considerations. The testcase demonstrates the effectiveness of the proposed method also with complex topologies avoiding simulation-based or approximated analysis. In the latter part of the work, the input stage of an AC/DC converter is presented to validate the usage of the MOO approach: the Power Factor Correction (PFC) Boost stage design in a Medium Voltage Solid State Transformer . The analysis is carried out to obtain the concrete benefits introduced by the implementation of a proposed solution to reduce the losses in the switches.