AUSZUG | eb - Elektrische Bahnen INT 2/2015

21 113 (2015) INT 2 Rail Power Supply sequently, the switchgear is significantly cheaper and the variety of possible connection points is much higher (especially in the urban areas this may be the only option). In comparison of the cost, despite of the higher cost of the SFC system the savings on the switchgear and on the connection to a high-voltage grid could already mean that the SFC solution is a better economical choice. Additional savings can be achieved in terms of operating costs. This is because utilities are increasingly favoring symmetrical loads through lower energy prices whilst penalizing asym- metrical consumption. Thanks to these savings du­ ring operation, the investment in a traction power converter can pay off within a short time. In addition the SFC technology enables a con- trolled recuperation back to the three-phase network as well as improves the consumption of the regene­ rated energy within the rail network itself. It appears that using the regenerative braking could save be- tween 10% and 20% of the traction energy used [2]. At the railway grid end, the advantages of an SFC solution apply to both, the design of the overhead line as well as to the train operation. As the single- phase rail voltage is decoupled it can be synchro- nized to any frequency and voltage level. This means that comparing to standard transformer solution no phase change is needed. As a result the SFC solution creates consistent energy supply network without disconnection points and neutral sections. With the SFC system the overhead line system can also be fed from both sides between feeder stations. Feeding an overhead line section from both sides reduces the voltage drop thus allowing for fewer feeders station further apart. When in operation, overhead line with sectioning points requires more intensive inspection than con- tinuous system. Furthermore, neutral sections perma- nently interrupt the power supply of a train. These interruptions cause stress to the vehicle’s propulsion and onboard power equipment and, depending on the technology of the drive equipment, can adversely affect passenger comfort, for instance, in terms of lighting and air conditioning. Especially during high- speed operation, the reduction in average traction performance due to the short-time interruptions in power supply is a factor that adversely affects vehicle performance. A continuous overhead line network (all section breakers normally closed) enables very fast reconfiguration in case of a feeder station outage by only disconnecting the affected station. One of the other major advantages of the con- verter solution is that the converter can provide the single-phase grid with reactive power in order to boost and stabilize the voltage. This is even possible, when the three-phase grid is not available. For power upgrade of existing lines with SFC technology the fact that the short-circuit current is limited to nominal values can significantly reduce the cost of the earthing and bonding scheme already in place. Table 1 shows a selection of general advantages of the SFC solution. More information on compari- son of a SFC solution and standard transformer sys- tem can be found in [3]. TABLE 1 Selected general advantages of an SFC solution. Selected issues 50 Hz Transformer 50 Hz SFC Stations Utility grid Balanced three-phase grid load no yes Reactive power given by train load controllable; an SFC can independently provide any reactive power in utility grid Ratio utility grid voltage / rail grid voltage given by fixed transformer ratio, changeable only if mechanical tap changer is used wider range, can be adjusted electronically Harmonics given by train load known and relatively stable values Required grid short-circuit power high; often high voltage grids must be used lower values are acceptable, medium voltage grids even with low short circuit power might be sufficient Recuperation not controllable by feeder station controllable; recuperation can be adjusted or blocked remotely if required by grid status Rail grid Reactive power given by train load controllable; an SFC can independently provide any needed reactive power in rail grid in order to support grid voltage or to optimize load sharing between feeder stations Recuperation only within a feeder section (2 … 3 trains) across entire railway grid Feeder stations density X ~ half Continuous train power supply no yes Surge at section breaks yes no Protection function no limited short-circuit current Earthing & bonding higher short-circuit current endurance required reduced sizing due to lower short-circuit current level