Design and Stability Analysis of a Three-Phase Triple-Stage Solid-State Transformer

The electrical distribution system is experiencing a profound evolution process triggered by the increasing integration of Renewable Energy Sources (RES) and Distributed Generation (DG), alongside the widespread use of Electric Vehicles (EVs), the related charging stations, and the growing adoption of Energy Storage Systems (ESSs). The behavior of such loads and sources, interfaced with the grid via an increasing number of power electronics converters and often intermittent in nature, together with a bidirectional power flow requirement, poses new challenges for the reliable and safe operation of the distribution system. In this context, the concept of Internet of Energy (IoE), or Energy Internet (EI), has emerged and is nowadays widely discussed in the literature as a new paradigm shift to address the growing demand for modernization of the current distribution network. The goal in the IoE scenario is reshaping the current distribution grid into an intelligent and flexible active network, both through a radical informatization process that involves the renewal of the grid communication infrastructure and the addition of distributed monitoring points and via the implementation of advanced energy management and control functionalities to enable the safe, robust, effective, and efficient integration of intermittent sources and loads. At the core of this future smart grid scenario, the Solid-State Transformer (SST) is envisioned as the best candidate due to its flexibility and advanced control features. This is because the SST is a power electronic-based transformer capable of providing advanced services and grid-supporting features, besides galvanic isolation and voltage adaptation, through its control system, and therefore is intended for replacing conventional Line Frequency Transformers (LFTs) at strategic nodes of the grid. Moreover, the core isolation stage of the SST operates at high frequencies and, therefore, it enables volume and weight reduction of the whole system compared to traditional and bulky LFTs. In the IoE scenario, the most suitable SST configuration is the triple-stage one, which consists of three conversion stages. Due to the large number of stages, the SST control is intrinsically complex. It has been shown in the literature how the coupling among controllers makes the design of the overall control system challenging and, additionally, multistage cascaded converters are significantly prone to instability due to interaction between converters. Moreover, even if the SST is stable as a standalone system, it may become unstable when connected to the grid because of dynamic interactions with other grid-connected converters, leading to the so-called harmonic instability phenomenon. In this context, this thesis aims to explore the SST stability issue from both the DC-link and grid-connection perspectives. To do so, in the first part of this work, the SST suitable topologies and their conversion stages are reviewed. Once the SST architecture is selected, the main ratings and parameters are designed according to the presented IoE application requirements. An average model of the converter, that enables faster simulations and physical insights into the SST dynamics, is then derived. Through it, the small-signal model of the SST can be obtained. Based on that, the SST control system is presented and designed and the related impedance model is derived. The latter is selected as assessment tool to evaluate the DC-link and grid-connection stability of the SST under investigation. The results obtained provide support during the design phase of the SST and its control strategy, with the aim to achieve a stable grid-connected operating system.