Although the metro is a very energy efficient option for passenger transportation compared to internal combustion vehicles, it is still a very prominent energy consumer. Further, due to the global pressure to reduce CO2 emissions, there is a worldwide demand for efficiency and sustainability improving solutions and approaches for railway traction systems. Also, in Amsterdam, the municipality sets great ambitions regarding installed PV power in the city and the emissions related to public transit. The municipality of Amsterdam has the goal to have an installed PV power capacity of 250 MWp in 2022 and to reduce CO2 emissions with 55% by 2030 compared to 1990. Also, the metro system has to become 35% more energy efficient in 2030 compared to 2013. Moreover, the municipality wants to offer emission free public transport in 2025. To reach these ambitious goals, no piece of land suitable for PV can be left unused. 

In the past, PV systems have already been installed on metro station roofs, connected to the AC-side of the metro traction network. However, no PV systems have yet been connected to the DC-side of the traction network, which would have enabled the connection of PV systems anywhere along the track. Also, there is still a lot of unused land along the metro tracks in Amsterdam, with a total available surface of 47991 m2. This land could potentially be used to install PV systems with a total installed power of 4511 kWp [1]. To increase the amount of installed PV power and to fully utilise the available land along the metro tracks, this report focuses on the connection of PV systems directly to the metro traction system, at the DC-side of the traction network, to accelerate the energy transition and to reach the sustainability goals in Amsterdam.

This report includes a comprehensive study on the challenges related to the integration of PV systems at the DC-side of the traction network, as well as possible solutions to mitigate these challenges. To further increase the energy efficiency of the traction network with PV systems, the integration of additional solutions like energy recuperating inverters in power substations and energy storage systems are discussed as well.

To study the energy saving effects as well as challenges of PV systems connected to the DC-side of the traction network, a Simulink model of the traction network is created, using measurement data of an M5 metro, the metro timetables and traction network parameters as an input. The input data for a specific pilot location, located nearby station Amsterdam RAI, is used for the simulations. Also different PV systems, with sizes of 202 to 799 kWp are simulated with PVsyst, using meteorological data for the pilot location. The simulation results are used in the Simulink model, to simulate a PV system connected to the traction network. Also inverters and storage systems are simulated using the Simulink model. 

The results show an energy saving enhancement effect, due to decreased ohmic losses in the network as a result of the injection of PV power, meaning that more energy is saved than supplied by the PV system. However, due to the intermittency of the load and regenerative braking, only 51.5-83.0% of the potential PV energy yield can be supplied to the traction network, depending on the PV system size, resulting in a waste of PV energy. A variable PV output control strategy is proposed, which can increase the energy output of the PV system with about 13-22%. When using an inverter in a substation, the amount of supplied PV energy can be increased to 83.4-97.6%, by exporting the surplus of energy to the power grid. Also, using the inverter, a significant amount of braking energy can be recuperated, which would otherwise have been wasted in the braking resistors of the metros, leading to even more energy savings. Also, an energy storage system is simulated. The results show that the storage system could theoretically prevent any waste of PV energy, while also saving braking energy and decreasing ohmic losses even more. 

Using the results for an economical feasibility study on a PV system with and without inverter shows that a PV system operating alone would have payback period of 11 years, which can be reduced to 5 years by using an inverter. Although this research focuses on integrating PV systems on the metro network of Amsterdam, the methodology can be applied to DC traction systems worldwide.