Principle
Due to high transmission losses, DC systems do not reach high recuperation
rates except in very dense urban networks.
The situation may be somewhat improved by making electric substations
“reversible”, meaning that they can also operate in the reverse, feeding energy
from the catenary to the public mains. If substations are equipped with
inverters, recovered energy can be fed back into the supply grid whenever no
other train is running close enough to use the recovered energy.
Technical details
The architecture and power electronics of DC substations are almost identical
in most mass transit systems and mainly based on diode rectifiers.
If an antiparallel inverter is added to the diode rectifier, the power flow
of the substation can be reversed, i.e. energy can be fed back into the supply
grid. The inverter is only activated when recovered energy is available. The
inverter setpoints have to be controlled in such a way that a certain share of
the regenerated energy is available to be used by other trains for
acceleration.
So the priority for the use of recovered energy is given to other trains.
Only if no other train can take up the energy, it is fed by the inverter unit
into the supply grid. |