IEEE Electrification Magazine - September 2014 - 37

1.5
0

100

150 200
Time (s)

250

300

ac Node
dc Node

1.7
1.6
1.5
1.4

100

150 200
Time (s)

250

300

1.6
1.5
1.4

100

150 200
Time (s)

1,000
0
-1,000

100

150 200
Time (s)

250

300

100

150 200
Time (s)

250

300

100

150 200
Time (s)

250

300

2,000
1,000
0
-1,000

(b)

ac Node
dc Node

1.7

2,000

(a)
Delivered Power (kW)

Voltage (kV)

Delivered Power (kW)

1.6

1.4

Voltage (kV)

ac Node
dc Node

1.7

without devices, as these equipments return the energy
they have previously stored to the railway system. As a
result, the energy consumed from the utility grid will be
reduced, producing a load-shaving effect, which is beneficial and may lead to economic discounts if billing includes
a peak-power term. Both alternatives lead to a reduction
in the amount of energy sent to rheostats.
Figure 6(b) shows the total reduction in substation energy
consumption. The energy savings achieved is slightly greater
in the reversible-substation case due to the power and energy rating limits included in the ESSs (700 kW maximum
power and 2.5 kWh maximum stored energy). Actually,
reversible substations have no energy limits, and the powerrating effect is less important than that of ESSs. Furthermore,
in this particular case, the energy efficiency of the ESSs is
lower than that of reversible substations. In any case, with
the two alternatives, approximately 200 kWh/h in energy
savings has been achieved, which represents more than 4%
of the total substation consumption in the case with no
additional devices.
It is important to point out that, to extract general conclusions on the feasibility of investing in any of these

Delivered Power (kW)

Voltage (kV)

sources the stored energy, having lower substation power
than in the case with no devices.
Figure 6 shows the energy results in the case-study line
after applying first reversible substations and then ESSs. In
both cases, these devices have been placed in all substations of the line.
Figure 6(a) has been included to reinforce the way
these devices work by giving aggregated line power
figures. In the first part (from instant 290 to 300), the net
power consumption in the system is negative, i.e., the set
of trains in the line regenerate more power than they consume, and, hence, there is a regenerative power surplus.
As explained earlier, reversible converters allow for negative power, which means that the rail system will return
this power to the utility grid. This energy has not been
wasted in rheostats bringing a clear energy savings to the
system. This will be an economic savings as well if local
regulations allow the railway operators to have the corresponding discount in their energy bill. Spanish laws have
recently been modified to allow operators to have this discount. In the case of ESSs, it may be observed how the
power delivered by substations is lower than in the case

250

300
(c)

2,000
1,000
0
-1,000

Figure 5. The voltage and power at substation 1 for each infrastructure topology studied: (a) no devices, (b) reversible substations, and (c) ESSs.
IEEE Elec trific ation Magazine / s ep t em be r 2 0 1 4

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