Siemens is stabilizing the power grid of the Swiss railroad company Schweizerische Bundesbahnen (SBB) by providing it with the world's first mobile units that compensate for reactive power. The overhead conductor, as well as some trains, need reactive power as well as active power. The trains draw both kinds of power from the railroad electric grid. The reactive power in particular may fluctuate greatly at the local level, for example when many older trains are starting up at the same time. In such cases there is a risk that the grid will be overloaded and will break down. For this reason, the new systems are designed to make reactive power available in the specific situations where it is needed. Of the three units in total, two will go into operation at the end of 2015 on the south side of the Simplon Tunnel. The third unit will be used for the Neuhausen am Rheinfall grid district. The special feature of the new systems is their mobility. They are housed in standard containers and thus can be transported. This enables the SBB to stabilize its electric grid in exactly the places where stability is needed.
Reactive power is the electric power that is used during the creation of electric and magnetic fields. It plays a role, for example, in the operation of electric machines such as generators and electric motors, and it does not contribute to the actual work done. If large amounts of reactive power are involved, it is not efficient to transmit it via the electric grid; instead, it is generated locally in devices known as reactive power compensation systems. These systems themselves receive their auxiliary power from the railroad electric grid or the public electric mains.
The special thing about the Sitras SVC Plus units developed by Siemens is their modular multilevel power converter technology. The unit consists of many series-connected insulated-gate bipolar transistors (IGBTs) and capacitors. This kind of system generates high voltages and is connected directly to the overhead conductor by means of a throttle. The system therefore needs less room and can be installed in a railroad container together with the necessary cooling and switching technology. Each of the systems that Siemens is building for the SBB generates 15 MVAr, or 15 megawatts of reactive power.
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