Scientific paper ID 2689 : 2025/4

Sofia has the largest and most complex urban transport system in Bulgaria, which s various types of transport - bus, tram, trolleybus and metro. The report presents the role of relay protection of the main units in the rectifier stations of ”Stolich Elektrotransport” EAD, they are the main unit for the power supply of ground urban autonomous and non-autonomous electric transport for the city of Sofia. They provide power supply to: the tram and trolleybus contact network, charging stations with input and output constant supply voltage for electric buses with fast charging. The role of relay protection is to monitor certain parameters of the relevant facility and to protect it when certain values are reached. It also has the task of automatically detecting the presence of a fault, selectively determining the damaged facility and switching off the relevant circuit breaker in order to restore normal operation of the undamaged sections. At the same time, it reacts and is activated when abnormal modes of the equipment occur.

The report presents the technical parameters and characteristics, with the common protections for all protected equipment being current section (Ito) and maximum current protection (Imtz). Maximum current protections are the most common relay protections. Current protections rely on the fact that in normal mode the current reaches values around the nominal, and at hp its value increases significantly. Usually the difference between these two currents is very large and current protections easily distinguish them. They are the most common relay protections. [1], [3].