Integrating PV systems directly on metro traction networks

Abstract

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.