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.

A hybrid distribution transformer (HDT) combines a power converter and a conventional low-frequency transformer (LFT) in a single device. The most recurrent configuration consists of a shunt converter fed by the auxiliary winding (AW) of the LFT and a series converter connected to the low-voltage grid. Nonetheless, load harmonics circulate through the power converter and LFT, affecting their lifetime. In this work, an HDT composed of a series converter connected to the primary side through coupling transformers and a shunt converter directly connected to the secondary side is presented. The HDT model and discrete-time state-feedback control are presented. The results show that the HDT can improve the power quality of the grid, focused on extending the LFT lifetime.

Hybrid distribution transformers (HDTs) have been proposed as an alternative to conventional transformers (low-frequency transformers) and solid-state transformers, combining the reliability of low-frequency transformers (LFTs) and the controllability of power converters. Although there are several HDT configurations, those that connect the series converter on the primary side and the parallel converter on the secondary side can improve the operating conditions of the main LFT. Nevertheless, during voltage sag compensation, there is a circulating active power flow (CAPF) between the main LFT and the power converters. This CAPF increases the total power circulating through the LFT, which finally affects the overall efficiency of the HDT. This article presents an analysis of the different operating conditions of the HDT, their relationship with the CAPF, and their impact on the efficiency of the HDT. Two methods to reduce the CAPF and their advantages and disadvantages are introduced. Finally, a minimum controller for the HDT to operate under balanced and unbalanced conditions and to carry out the experiments is presented.

Multiport smart transformers (MSTs) are transforming modern power systems as they offer advanced flexibility, improved energy efficiency, fault-tolerant operation, seamless integration of renewable energy sources (RESs), bidirectional power flow, grid support, and intelligent control. These advanced functionalities of MSTs make them a key element of next-generation grids, where adaptability and fault-tolerant operations are crucial. MSTs help balance variable generation and load demands through enabling the seamless integration of RESs. Moreover, the ability of MSTs to support bidirectional power flow plays an important role in modern applications such as electric vehicle (EV) charging, microgrids, and other energy systems. This article provides a review of MST architectures, focusing on design principles, topologies, and the essential control strategies for bidirectional power flow regulation, voltage regulation, and protection. The article also highlights real-world applications and demonstrates how MSTs enable RES integration, improve grid resiliency, and support advanced technologies like vehicle-to-grid (V2G) operation. The article also explores recent advancements in MST technology, such as efficiency improvements and thermal management, while addressing current challenges related to standardization and control complexity.

This paper compares two finite-control-set model predictive control (FCS-MPC) strategies in the context of a grid-connected single-phase quasi-Z source inverter (SP-qZSI). Both schemes use discrete-time models of the inductor current and capacitor voltage for the DC side, as well as the output current on the AC side. To enhance the converter’s performance, given the non-minimum phase characteristics of the DC side, a long prediction horizon is implemented for the predictive control. However, a horizon of this nature can be highly demanding in terms of processing load, rendering it inapplicable for some microcontrollers. To address this issue and mitigate the processing load, an alternative control strategy is presented that divides the total number of candidate solutions to be evaluated into smaller segments. The performance of the two control strategies is compared using total harmonic distortion (THD) and simulation times as evaluation metrics. The results indicate that the proposed strategy achieves significantly shorter simulation times than the compared control strategy when increasing the prediction horizon. Additionally, a reduction in the THD was observed in the proposed strategy, reaching an average of 2.8%, which is lower than the compared strategy that exhibited THD close to 3.5%.

The Hybrid Distribution Transformer (HDT) has been proposed as a solution to cope with the low short-circuit capability of solid-state transformers. Among the available HDT configurations, the one that connects a series/parallel converter on the primary/secondary side can be highlighted. This configuration improves the voltage and current waveforms on the transformer and regulates the voltage supplied to the AC microgrid. Nonetheless, this HDT suffers from a Circulating Active Power Flow (CAPF), affecting its efficiency. Moreover, during the unbalanced operation of the HDT, an additional CAPF component exists. Depending on the grid and load conditions and whether the parallel converter compensates for the load unbalances, the CAPF can either increase or decrease. Although the CAPF can be eliminated by employing the DC port of the HDT, it is not always possible to extract energy from it. This work contributes with the analysis of the operation of an HDT under an unbalanced grid voltage and load, along with an extended CAPF model that considers the losses of the HDT. The effect of the unbalanced components on the CAPF is analyzed, and the conditions in which the CAPF is minimized are obtained. Nonetheless, in most scenarios, a minimum CAPF does not coincide with the maximum efficiency of the HDT. Therefore, the conditions for achieving maximum efficiency are also determined. A simpler suboptimal condition is obtained due to the complexity of requiring precise parameters and operating conditions of the HDT. Moreover, the suboptimal condition allows for improving the power quality of the HDT. Therefore, a certain amount of CAPF is desired to operate the HDT properly.