Coordinator: Domenico Giordano

A significant amount of energy can be saved through recovering braking energy within railway networks, underground, and metro systems, where normal working conditions involve continuous accelerations and decelerations. However, traditional supply systems: uni-directional substations (USS), do not allow a complete energy recovery and some of the potentially recoverable energy has to be wasted in resistor dumps. After measurement along the Rimini – Bologna route (120 km) the project consortium estimated that the energy wasted by the braking rheostat of 15 trains per day, with a mass of 316 Mg in about 1 GWh, corresponds to the annual consumption of about 365 families.

A complete energy recovery is possible by installing reversible substations (RSSs) which allow bidirectional power flow between the AC network ‘supply’ and the DC distribution system. Even though new RSSs are considered expensive it has been estimated that the installation cost can be paid back, by the saved energy, within 3 years. Clearly, for a specific case, an accurate analysis of potential energy saving is essential.

In addition, as a result of the installation of RSSs, some additional conducted disturbances (power quality issues) can be generated in the supply power system, both on DC and AC side. The level of these phenomena depends on working conditions and power network characteristics. An accurate analysis of the power quality impact related with RSS installation should be done to properly perform a cost/benefit analysis.

So, measurements are required to quantify the impact of the installation of new reversible substations in terms of energy saving but also in terms of power quality issues potentially generated. This requires measurement equipment and techniques to analyse the existing energy-use profiles of rolling-stock and USS, including energy regenerated by rolling-stock but dumped in chopped resistors. These capabilities can be used by operators to determine the financial viability of RSSs. Second, after RSSs are installed, the monitoring equipment can be used to verify that the expected efficiency gains are achieved. The third main function is to assess the wideband frequency and power quality effects of DC rails when RSSs are installed and the impact of these disturbances on the AC supply grid.

The measurement campaign performed along the line 10 B of Metro de Madrid, characterized by a reversible substation installed in the middle of the line provide interesting results for different operating conditions. In the best condition the RSS provides a reduction of about 30% of the energy dissipated by a braking rheostat under low traffic conditions.

For the described purposes an uncertainty lower than 1 % in the energy saving measurement becomes a fundamental goal. The metrological difficulties in reaching this value include the fact that the existing dump resistor chopper circuits operate in a discrete manner with high commutation frequency, making the use of standard current sensors problematic. Other difficulties relate to the high switching frequencies (>1 kHz) and potential control loop interactions/resonances (5 Hz – 1 kHz) which can be introduced by converters. Accurate and approximated methodology have been developed and published providing, moreover, valuable information for the determination of the uncertainty associated to the measured dissipated energy.

For more information, see the project webpage >>