Master's thesis firstly focuses on an overview of electric power traction systems’ methods in Europe, described are the current Slovenian railway network, the method of connections, electrical power substations, their characteristics and the general overview (characteristics) of the electricity consumption.
Slovenia’s rail network is run by 3 kV DC power supply. DC power can cause stray currents issues which results as grounded metallic structures damages. Certain features of stray currents in the ground, such as their sources, values and distribution, has been analysed and described in the beginning of the thesis. Knowledge about the conductivity of return circuit, insulation of the return circuit towards earth, operating voltage, insulation and conductivity of the overhead line, spacing of supply substations, train current demand, regenerative braking, substation, safety earthing system and signalling requirements is inevitable if one’s effort is to restrict/limit the level of stray currents. Therefore, the level can be controlled using different systems of rail grounding and measures on infrastructure.
Freight and passenger traffic all around the World are increasingly using single-phase AC traction systems for 25 kV 50 Hz. These rail systems are mainly supplied by the transmission network of 110 kV and higher. Connection can be performed between two-phase transmission system either as a single phase 1 x 25 kV or a two-phase 2 x 25 kV using 110/25 kV transformers.
The thesis continues with description how single-phase AC system is connected to the transmission network focusing on voltage unbalance. Access to the network limits voltage unbalance if single-phase transformers are connected between two phases. Transmission system operator is obliged to check the impact which reflects on the electricity system and consequently provide guidelines and conditions to enable suitable connection to the network. Various binding of coupling transformers, shortening the distance between the substations, reduction of subways’ power supply and use of static compensators (SVC) or STATCOM's are different methods for limiting imbalances and the impact on the transmission network, all depends on the economic criteria and indicators.
Slovenia is participating in the European project, being a part of high-speed railway system (up to 250 km/h) in the corridor West-East from Barcelona to Kiev. This corridor is also included in European TEN-T project. According to the criteria set by the EU, traction has to be connected to high voltage transmission network through a single-phase transformer 110/25 kV. For further understanding is crucial to get familiar with characteristics of high-speed trains, railway systems and understanding analysis of potential location of substations where high-speed railway can get connected. All the above in terms of voltage unbalance.
The last part of thesis explains how power supply can be converted from direct current to alternating system, stating advantages and disadvantages of conversion as well. During a time period from 06.06.2016 00:00 to 12.06.2016 24:00 15 minutes long metering data of active and reactive power have been collected. Those measurements have been made for all 19 substations later used to calculate the load of substations and lastly for valuation of the voltage unbalance. In addition, the average apparent power of 15 minute interval has also been calculated based on given measurements. Voltage imbalance has been calculated according to short-circuit of power substation whereas the average apparent power has been transferred to the 110 kV substation (110/25 kV AC station). Taking into consideration the facts that the most suitable length for the 25 kV AC power feeder segment is between 15 km and 30 km (specified length represents half of the distance between the two substations) and the power station consumption on the transmission network is higher, a consumption of some electric subways in the same geographical area have been combined in a single 110/25 kV AC substation. Thesis completes with validation of voltage conditions and potential positions of the substations.