Distribution systems are under constant stress due to their highly variable operating conditions, which jeopardize distribution transformers and lines, degrading the end-user service. Due to transformer regulation, variable loads can generate voltage profiles out of the acceptable bands recommended by grid codes, affecting the quality of service. At the same time, nonlinear loads, such as diode bridge rectifiers without power factor correction systems, generate nonlinear currents that affect the distribution transformer operation, reducing its lifetime. Variable loads can be commonly found at domiciliary levels due to the random operation of home appliances, but recently also due to electric vehicle charging stations, where the distribution transformer can cyclically vary between no-load, rated and overrated load. Thus, the distribution transformer can not safely operate under highly-dynamic and stressful conditions, requiring the support of alternative systems. Among the existing solutions, hybrid transformers, which are composed of a conventional transformer and a power converter, are an interesting alternative to cope with several power quality problems. This article is a review of the available literature about hybrid distribution transformers.

High voltage insulation and thermal management are the two major challenges in a high-power medium frequency transformer (MFT) design. This letter proposes novel insulation and cooling structure with potted windings in a 100 kW MFT with two FINEMET FT-3TL magnetic cores and parallel-concentric winding structure. For the first time, a three-dimensional printed bobbin with heatsink fins is demonstrated. The 100 kW MFT achieves a power density of 10.6 kW/L and a partial discharge inception voltage of 20 kV peak. The MFT is applied in a dual active bridge based medium voltage solid state transformer, achieving a peak efficiency of 98.2% at 40 kW and 97.5% at 100 kW. The MFT itself has a 99.735% efficiency with superior thermal performance at 100 kW output power.

The Hybrid Solid State Transformer (HSST) is an amalgam of a resilient yet cost effective line frequency transformer (LFT) and a fractional power rated, controllable and responsive solid-state transformer (SST). Existing works have mainly focused on Hybrid Transformers (HT) where the required isolation is provided by an auxiliary line frequency winding or transformer making the distinction between HTs and FACTs devices less evident. This article proposes and studies an HSST system through a case-study of a 20kV/500kVA Hybrid Solid State Transformer (HSST) utilizing a single stage Dual Active Bridge (DAB) based SST complemented by simulation analysis, describing the plethora of control possibilities that undoubtedly improve the operational flexibility in the grid. A hardware prototype of the 20kV HSST unit has been assembled and tested, demonstrating some of the control functionalities, and demonstrating efficiencies of up to 98.4%.