Optimizing the train power system in Zagreb

To modernize it´s rolling stock and increase quality of the municipal public transport service, city of Zagreb, through its public transport company (ZET) made an investment into 140 new NT2200 trains. These trains incorporate asynchronous motors and AC/DC converters, as the tram electrical system is supplied by 660 V DC power. Coincidentally, the occurance of outages in the network increased 60%. This problem doesn´t just present financial losses for the public transport company, but also decreases quality of service. In a project study, conducted by the Faculty of electrical engineering and computing, the entire physical and electrical network was modeled and dynamically simulated to discover voltage drops, power losses and peak currents, which have so far been unknown on a system-level. It has been determined that the existing power supply facilities are sufficient to fulfill demands of the existing transportations trains, however with multiple changes to the distribution cables and section separators to enable continuous system operation. Using an optimization procedure, new connecting places for supply lines and cables were determined. The results were also checked by a steady-state model. With projected reinforcements, voltage drops and currents are reduced, and thus the protection devices can be set to lower breaking currents. As a result of electrical network performance indicators improvements, the company will save over 6 million EUR annually.