IEEE Electrification Magazine - September 2014 - 36

Utility Grid

Reversible
Converter

Classical
One-Quadrant
Substation

ESS
Between
Substations

ESS
at Substation

Feeder
Paralleling

Rail
Paralleling

Figure 3. Energy-saving techniques in dc-electrified railway systems.

combines a flexible traffic scenario generation tool and a
specific algorithm for the resolution of load flows both in
ac and dc railway systems.
A metro line in Spain with 18 passenger stations distributed along 14 km has been simulated. In this line, six onequadrant (diode) substations provide power to the trains.
The simulation results for several different traffic densities
show that, among other traffic situations, there are receptivity problems for five and a half minute headway. Thus,
the application of devices oriented to increase receptivity,
such as reversible substations or ESSs, will lead to a certain
energy savings at the substations.
Figure 4 shows the most relevant energy figures for the
traffic scenario under study (see Figure 2 for a better
understanding of these magnitudes). Regenerative energy

Energy Figures in the Simulation

Energy (MWh/h)

5
4
3
2
1

at
os
t
he

en
e

ra
te
d

l
se
fu
U

ec
t

ifi

Figure 4. The total energy figures in the simulated traffic scenario.

I E E E E l e c t r i f i c ati o n M agaz ine / september 2014

is the energy fed back into the line-the energy subtracted
from the useful energy (traction plus auxiliary) consumed
by the trains. However, it may be observed that there is
still room to improve the system from an energy point of
view by reducing the rheostat losses, which account for
approximately 4% of the total rectified energy. It must be
remarked that the system under study is a very welldimensioned system with good receptivity ratios. In other
systems with different features, the rheostat losses may
be greater in both absolute and relative terms.
Figure 5 shows the voltage and power profiles in the first
substation of the line in 1) the case where no devices have
been included in the infrastructure, 2) the case in which all
the substations have been made reversible, and 3) the case
where ESSs have been placed at all the substations.
Figure 5(a) shows the case with no devices. The most
interesting result may be observed in the voltage plot, where
the voltage in the dc side of the substation soars when there
is a strong local regeneration with nearly no inertia (especially between 170-180 s and 275-305 s). During these periods,
the substation power equals zero, and the surplus power will
be sent to rheostats. In Figure 5(b), the reversible converter
included in the substation makes it possible to have negative
power, which means that the rail system is returning regenerated power back to the utility grid. It is interesting to note
that, because of its voltage-current curve, the reversible
converter prevents the dc-side voltage from rising in an
uncontrolled way, increasing the voltage gap for trains to
regenerate. Finally, in Figure 5(c), the ESS charges in the periods of strong regeneration. The substation power equals zero
like in the first case, but the differences are that the dc-side
voltage does not rise uncontrolled due to the charging voltage-current curve of the ESS, and after the net-regeneration
periods, when the network is again a consumer, the ESS

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2014