The ongoing energy transition has changed the architecture of electricity networks in ways that conventional power transformers are not able to cope with the new required functionalities. For this purpose, the solid state transformer (SST), which comprises state of the art power electronics with galvanic isolation to interconnect two separate alternating current (AC) or direct current (DC) power grids, is considered to be the dominant solution. The purpose of this paper is to provide a practical, application-oriented review of the SST. In this context, the main functionalities and possible applications of the SST are presented, including smart grids (SGs), data centres, railways, offshore wind farms, etc. Furthermore, the main developed SST prototypes are analysed with special focus on the related projects, demonstrators, stakeholders and rated values, e.g., voltage, switching frequency and power. The analysis is concluded with the future trends and challenges regarding the wider implementation of SST technology in the electrical grid.

Today, the classification and location of faults in electrical networks remains a topic of great interest. Faults are a major issue, mainly due to the time spent to detect, locate, and repair the cause of the fault. To reduce time and associated costs, automatic fault classification and location is gaining great interest. State-of-the-art techniques to classify and locate faults are mainly based on line-impedance measurements or the detection of the traveling wave produced by the event caused by the fault itself. In contrast, this paper describes the methodology for creating a database and a model for a complex distribution network. Both objectives are covered under the paradigm of the time-domain pulse reflectometry (TDR) principle. By using this technique, large distances can be monitored on a line with a single device. Thus, in this way a database is shared and created from the results of simulations of a real and complex distribution network modeled in the PSCADTM software, which have been validated with measurements from an experimental test setup. Experimental validations have shown that the combination of the TDR technique with the modeling of a real network (including the real injector and the network coupling filter from the prototype) provides high-quality signals that are very similar and reliable to the real ones. In this sense, it is intended firstly that this model and its corresponding data will serve as a basis for further processing by any of the existing state-of-the-art techniques. And secondly, to become a valid alternative to the already well-known Test Feeders but adapted to work groups not used to the electrical world but to the environment of pure data processing.

The growing emergence of Distributed Energy Resources (DER) that operate natively in DC challenges the traditional logic of unidirectional power flow and the exclusive use of AC in distribution systems due to inefficiency and control problems. DC microgrids are seen as a promising alternative to address these challenges, and Solid-State Transformers (SSTs) could be a feasible solution to enable the development of such controllable hybrid DC/AC grids. This paper presents the design, modular topology, and experimental validation of a 150 kVA SiC-based SST. This SST follows a four-module input-parallel, output-series configuration, enabling the creation of a 3 kV MV DC microgrid from the LV AC electrical grid, allowing intrinsic voltage balancing among modules, straightforward scalability in both voltage and power, and easier maintenance. Each two-stage module, in turn, consists of an active front-end and an isolated DC/DC, which is composed of a dual full-bridge driving a series CLLC resonant tank at 88-90 kHz. Laboratory and field tests demonstrate continuous DC voltage regulation in a range from 1.2 kV to 3 kV, black-start capability and support of the AC grid through reactive power. The prototype achieves a power conversion efficiency of 96.4 % under nominal conditions while transferring 151.6 kW, with line current THD remaining below 1.8 %. The SST, along with the constructed MV DC microgrids, addresses the lack of real demonstrators that can validate the benefits of DC integration.

This paper introduces a system capable of characterizing on-site digital instrument transformers from the analysis of the IEC 61850/IEC 61869-9 frames injected by the Stand Alone Merging Units (SAMU) located in digital substations. Besides, the second objective of this work is to implement a suitable computation architecture to monitor distributed Power Quality (PQ) nodes. For that aim, capacity of calculating PQ from the information sent by the SAMU will be leveraged. In a first step, this approach relies on the implementation of an IEC 61000-4-30 class-A PQ library on an industrial-based computer. Concerning the first objective, several experimental tests were driven for a solid timestamping characterization of the received data. After that, different PQ tests were carried out to verify the suitability of the proposed solution by comparing the system against one traceable generator. The calculation of normalized error indexes (En) revealed that the developed system is compatible with respect to the reference generator. In this regard, the system can be used to calibrate digital instrument transformers (via SAMUs) against one analogue reference transformer. Later, the library is migrated to a more constrained hardware to identify the maximum number of parallel nodes that the analyzer can process. On this basis, several tests were conducted to stress the low-cost platform until reaching the thermal limit on CPU. In light of the results, the maximum number of nodes was 20. This fact indicates that the proposed low-cost analyzer meets the stringent class-A requirements of IEC 61000-4-30 for real-time parallel evaluations.