Abstract
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.
Highlights
Published: 24 February 2021Distribution transformers are one of the pillars of the distribution grid
Hybrid distribution transformers (HDTs) configurations are classified according to the energy source of the power converter unit, i.e., if the energy is obtained from a capacitor, from the Low-Frequency Transformers (LFTs) primary or secondary-side winding, or an auxiliary winding (AW)
New kinds of loads are connected to the distribution network by means of power converters that are supplied by conventional transformers
Summary
Distribution transformers are one of the pillars of the distribution grid. They are the interface between the medium and low-voltage grid, stepping-down the voltages to a safe and usable range for the end-users. As low voltage distribution grids are supplied by distribution transformers, these can be highly affected by the low power quality presented in these systems When operating under these conditions, the transformer suffers higher core losses, which increase its hotspot temperature. Unless storage systems are included, CPDs can not mitigate inrush currents generated by the transformer startup process or by sudden voltage variations These systems are capable of mitigating a variety of power quality issues, but the distribution transformer is not fully protected from the unpredictable distribution grid operating conditions. Solid-State Transformers (SSTs) are an attractive solution to cope with power quality problems, replacing conventional distribution transformers They are based on power electronics, which allows for fast and precise voltage and current control [21]. This work is a literature review of HDTs, focusing on the different configurations and topologies available, in conjunction with the available applications or auxiliary services that it can provide
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