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Top1. Introduction
Single-phase short circuits (SC) are the most common emergency modes of 0.4 kV electrical networks (Beaty, 2001; Popov, 2005, 2010). The value of single-phase SC depends on such parameters as the length of the power transmission line (PTL) to the point of the short circuit, cable grade and cross-section of wires used in the PTL, the power and type of transformer which PTL is powered from. In fact, the value of single-phase SC depends on the resistance of all these elements. According to (Beaty, 2001; Shapovalov I. F., 1974; T. A. Short, 2018; Thomas Allen Short, 2014), the recommended length of 0.38 km power transmission lines (PTL) should be no more than 0.5 km. In reality, they often has the PTL lengths more than recommended (Kocaman et al., 2012). For example, the average length of 0.38 kV PTL ranges from 0.9 km to 1.2 km for different areas (Murthy & Raju, 2007; A Vinogradov et al., 2020a). Therefore, the currents at the end of PTL, when a short circuit, have a small value. Hence, they sometimes can be even less than load currents and a protective switching device installed at the beginning of a transmission line cannot react to them. As a result, there can be a damage to the equipment of power lines and transformer substations as well as the occurrence of fires at consumer facilities, electric shock to people. This problem is relevant for the electrical networks of many countries such as Poland, Romania, Russia and a number of other European countries, where the accident rate of these networks is quite high (The World Bank, 2020; Alexander Vinogradov et al., 2020).
The selection of a switching device with the value of operating current, which is sufficient for sensitivity to SC, reduces the transmission capacity of PTL since it also limits the load currents in it.
The sectionalizing of power lines so as to increase the security of 0.4 kV transmission lines is proposed by installing circuit breakers and fuses in the paper (R.G., 2013). However, this method is not rational due to the fact that it does not use switching device automation. This leads to additional time and resources for switching device maintenanceю Because there is not a monitoring system for their operation, it can increase power supply outage time for PTL disconnected by this method. This is due to the fact that monitoring means installed on a transformer substation (TS) would not report the disconnection of the switching device installed in PTL and would signal the presence of voltage on them.
So as to increase the sensitivity of PTL from short circuits, and thus to increase the power supply reliability for consumers, automatic sectionalizing units (SU) is proposed in the book (A. V. Vinogradov & Vinogradova, 2016), where technical solutions for SU automation were also developed. A literature review showed that there are no analogues to these solutions for networks of the lowest voltage class. At the same time, the use of sectionalizing devices in medium voltage networks has recently been quite widespread and shows high efficiency in ensuring power supply reliability and in increasing network protection from emergency modes (Abiri-Jahromi et al., 2012; Gechanga et al., 2019). Solutions for networks of medium voltage class have their own characteristics associated with their execution, the operation modes of their neutral. Therefore, they cannot be transferred directly to networks of lowest voltage class.
Despite the solutions on the design and application of SU and SUATS highlighted in the book (A. V. Vinogradov & Vinogradova, 2016), the issues of choosing the place of their installation in 0.4 kV electrical networks according to the criterion of ensuring the sensitivity to short-circuit currents remain open. Therefore, this paper presents a methodology for selecting the installation places and number of both SU and SUATS in 0.38 kV electrical networks indicating their features.