IEEE Electrification Magazine - September 2014 - 50

system is to provide electrical power with the agreed characteristics, the final objective of the railway system is to transport passengers and goods according to a schedule. For that
reason, the operation of a railway ESG (RESG) has to be different from conventional ESGs.
Most of the RPSs share some characteristics that make
the development of specific ESG technologies adapted to
railways particularly important.
1) Most electrical loads are trains, which are spatiotemporally varying loads. The consumption of the trains is
related to the way each train is driven, which allows it
to even become a generator for a limited amount of
time when braking. Just by stopping acceleration and
starting braking, the load can vary from 10 to −8 MW in
a few seconds. The RPS has to be able to deal with these
changes, which occur very often along a given journey.
2) Electrified railways are normally considered one of the
most energy-efficient modes of transport, especially
over economically viable operating distances. Its potential for energy savings is largely due to regenerative
braking, whose efficiency depends largely on when and
where it is carried out.
3) Railway lines normally cross wide areas and, therefore, are often interconnected to several electrical
grids, which are normally heterogeneous, as strong
grids coexist in the field with weaker grids. A smart
control that takes into account the specificities of
each network is crucial to improve the overall reliability and capacity utilization.
4) An RESG relies heavily on good bidirectional
communications between trains and the infrastructure, which is sometimes difficult to achieve, for
instance, in tunnels or in remote areas.

ESO 1

This article describes railway power systems and their
operation. In addition, the main control actions that can be
performed by an RESG are introduced, explaining how they
can improve the performance of traditional RPSs, e.g., reducing
costs, increasing energy efficiency, and enhancing reliability.

railway power system Grids
System Description
As shown in Figure 1, RPSs normally take the electricity
from other power systems, which, in turn, have their own
generation plants and electrical grids (transmission grid for
bulk power transfer, and distribution grid for retail power
supply) and whose characteristics may vary significantly
(strong grids coexist in the field with much weaker grids).
In liberalized electricity sectors, the transmission, distribution, and generation activities are typically carried
out by different companies. The electrical system operators (ESOs) are the companies in charge of balancing generation and demand and operating the transmission grid
in such a way that the reliability of the system is guaranteed. The distribution system operators are companies
that operate the distribution grids in such a way to ensure
that electricity is supplied to every customer with the
required quality. Finally, the generation companies are
responsible for producing the energy that has been programmed in each power plant. The energy produced can be
sold by means of contractual agreements or in organized
electricity markets (generally spot markets, including dayahead and intraday sessions), operated by an electricity market operator. However, final corrections to the program are
introduced by the ESO to solve technical restrictions and to
respond to the unexpected variations that occur in real time.

ESO 2

EMO 1

Power Plants

T&D Grid1
Power System
Side
Traction
Substation
Railway
Power System

Legend
EMO: Electricity Market Operator
RDG: Railway-Side Distributed Generation

Traction
Substation

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

T&D Grid 2

Traction
Substation
ESS

Figure 1. The interconnections of railway power systems to other power systems.

EMO 2

RDG

Traction
Substation

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