Applications and System-Level Considerations of Hybrid Transformers Interfacing MV and LV Distribution Systems

The fast rise in variable distributed generation connected to medium-voltage and low-voltage grids is putting the distribution system under increased stress. Problems such as current and thermal overload, voltage rise, and higher presence of harmonics are expected to worsen towards meeting the goals of the 2030 Climate Target Plan of the European Union. Advanced active management techniques in distribution networks appear as promising solutions to deal with the envisaged issues. Many of the solutions currently proposed to achieve a more flexible, controllable, and stable grid rely on power-electronics-based technologies, such as active filters, HVDC, and FACTS-devices. Particularly, Power Electronic Transformers have been extensively researched as interfaces between medium-voltage and low-voltage systems, providing new active control functionalities in terms of power flow control, fast voltage regulation, and limitation of neutral and fault currents. The Hybrid Power Electronic Transformer, resulting from the combination of a conventional transformer with one or more power converters, has been proposed as a promising trade-off considering controllability levels, costs, internal losses, and reliability. While all these technologies offer high flexibility, they are still immature and considerably more expensive than the conventional passive transformer. Additionally, while the Power Electronic Transformer and its Hybrid version are being increasingly researched, the focus of this research is mainly at the converter and semiconductor level. Investigating the system-level issues and benefits from including these active devices in the distribution grid is still lacking, particularly for Hybrid Transformers.Therefore, one of the objectives of this work is to address the system-level study of Hybrid Transformers in terms of its applicability, convenience, and impact in the distribution system. Towards that goal, a system-level study is presented, quantifying the impact of Hybrid Transformers in the distribution system in terms of voltage problems, line overloads, and overall system losses. The obtained results demonstrate the ability of the Hybrid Transformer to mitigate voltage problems created by the power injections from residential PV systems. However, the extent of the mitigation presents a high variability among the different studied networks. More importantly, the system losses were found to be increased between 9% and 18% in the Hybrid Transformer case, depending on the network and amount of power injected by the PV systems. Another goal of this project is to investigate the Hybrid Transformer from a converter-level perspective, considering the provision of different ancillary services to the grid. In that regard, a set of switching models for EMT simulations along with a hardware prototype of a complete Hybrid Transformer have been built, and the converter-level aspects regarding voltage regulation, reactive power compensation, unbalanced operation, and harmonics mitigation are researched in this work. The results from a set of hardware tests are presented. The stability aspects of the employed Hybrid Transformer topology are also considered in this work at a converter level. A small-signal model of the complete Hybrid Transformer has been developed, and the stability boundaries depending on different circuit and controller parameters are studied. The analysis is focused on the interactions between different Hybrid Transformer subsystems, as well as the critical parameters and components of the Hybrid Transformer from a design point of view. The obtained results demonstrate that operation of the Hybrid Transformer is inherently stable despite its interlinked characteristic. Additionally, the most critical parameter regarding stability is the tertiary-winding leakage inductance, which create fast oscillation modes when it interacts with the filter elements of one of the power converters.