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Hybrid stepless distribution transformer with four-quadrant AC/DC/AC converter at low voltage side - simulation tests DOI:10.15199/48.2018.06.23

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The present energy distribution grid is in operation for decades and works with the same principles since its establishment [1]. This grid was designed to supply linear loads, i.e. light bulbs or heaters. For such devices the power drawn from the grid decreases together with the voltage, mitigating the negative effects of lowering the voltage. In recent years the operating conditions of the distribution grid have changed. Most of modern loads work with constant power demand. In such cases the decrease of the voltage causes an increase of the current drawn, which intensifies the problems occurring in the distribution systems. The change of grid operation conditions is also the effect of the work of distributed generation systems, with a large contribution of renewable energy sources (RES). The increasing number of dynamic loads, i.e. fast charging stations for electric vehicles (EV) are also a reason of this issue. As a result, increasing voltage fluctuations in the grid become more severe. In order to control the voltage level, conventional distribution transformers (DT) are complemented by various Automatic Voltage Regulator (AVR) devices, starting from electromechanical servomechanisms, through transformer tap changers (TC), to Solid State Transformers (SST) [1-4]. Currently, despite such disadvantages like arcing, high operation costs or poor dynamics, mechanical tap changers are the most common solution. However this AVR technology does not keep up with the present grid voltage regulation requirements, especially in terms of dynamics and accuracy, anymore. This is the fundamental reason behind the growth in the application of power electronic based AVR devices. Thyristor tap changers (DT+TC) [4-6], although operating on a similar principle as their mechanical versions, they eliminate arc problems and guarantee higher dynamics of regulation. These switches, however, still operate in a step manner, and the indicated de[...]

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