The Use Of Solid-State Transformers As Part Of Smart Grids

Abstract

Currently, the existing grids are no longer able to cope with the requirements of the modern digital economy. In this regard, the role of unconventional sources of electricity, such as solar panels and wind farms, has increased. The growing role of direct current occurs due to the transition of the automotive industry from an internal combustion engine to an electric motor, as well as due to an increase in the service life and capacity of batteries. However, existing grids do not integrate well with receivers and sources using DC for the operation, which leads to a decrease in the quality, reliability of power supply and economic benefits. The way out of this situation is the transition from traditional to smart grids, which need to search for new methods, devices and solutions that meet the following rules: coordinated operation between DC and AC circuits with the ability to integrate batteries and charging stations; maintaining the quality of electricity in various operating modes, including off nominal ones; collection, conversion into digital form and transmission of information about the state and operating mode of the substation technological equipment and systems in real time operation; flexibility and reliability in the distribution and generation of electricity; high economic feasibility; self-diagnostic functions to ensure the detection of equipment and systems failure with an accuracy of a single module (block). At the same time, a classical transformer cannot cope with the assigned tasks; its disadvantages are given in this article. One of the solutions to this problem is the use of solid-state transformers - semiconductor AC power converters. The paper presents topologies and areas of application of classical AC and DC networks, their disadvantages. It is shown that with the development of direct current sources and consumers, problems arise with the integration of different types of current and voltage levels associated with the number of conversion steps and the quality of electricity. A block diagram of smart power supply networks based on a solid-state transformer is given with a description of what increases the overall efficiency of the network, the quality and flexibility in the distribution of electricity while reducing the number of conversion steps. The topologies of single-stage, two-stage and three-stage solid-state transformers are given and the areas of their application in different systems are shown. It has been established that for smart power supply networks, the most suitable is a three-stage topology of a solid-state transformer, which makes it possible to implement galvanic isolation with two DC-links, which will provide flexibility in switching power supply from DC to AC and vice versa, as well as increase the efficiency of the entire system as a whole.Mathematical modeling of the operation of a three-stage three-phase solid-state transformer for a symmetrical three-phase active-inductive load is presented. The system has output current and voltage feedback and automatic voltage regulation on the second DC-link. The tracking loop is implemented by changing the angle of block modulation in the controlled inverter and rectifier, which are part of the dual active bridge of a three-stage solid-state transformer. The controller is a proportional-integral controller. The graphs of the current and voltage changes when the load is switched on and off are given. It is shown that the voltage drop is insignificant and is almost completely compensated by the stored reactive energy in the reactive elements of a solid-state transformer.The applications of solid-state transformers, their advantages and disadvantages, as well as ways to eliminate these disadvantages are described in the article. The obtained results have shown the feasibility of using solid-state transformers in smart grids, in alternative energy, charging stations for autonomous devices of medium and high power, for energy distribution and in other areas.