IEEE Electrification Magazine - September 2016 - 7

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network that directly supplies locomotives by pantographs or conducting shoes. Because electric trains do not
carry the energy source on board, they can be lighter
and more powerful. However, electric railways have higher
initial capital and maintenance costs and are, therefore, economically justified
only if the traffic on the line
is substantial.
Historically, tramways
were first electrified with a
low-voltage, direct-current
(dc) supply (<750 V). The
first electrified mainline
used a three-phase alternating-current (ac) supply
with two overhead wires
(the third wire was the running rails), which required a
complex pantograph system. Therefore, after World
War I, a simpler singlephase ac system, operating
at 15 kV and 16.67 Hz, was
adopted, initially in Switzerland and later in Austria,
Germany, Sweden, and
Norway. Other countries in
Europe instead used dc networks operating at 1.5 and
3 kV, whereas electrified
railways in North America
opted for a frequency of
25 Hz. With advancements
in high-power converters,
the supply type became
independent of the traction
motor type used for the
trains, and, therefore, single-phase 25 kV at 50 Hz
became a popular solution.
The main advantage of 50
Hz is the simplicity of the
feeder stations and lower
transmission losses compared to 3-kV dc systems and smaller onboard transformers compared to 15-kV, 16.67-Hz systems. The 25-kV,
50-Hz electrification systems spread across Asia and is
currently in use in China and India.
Both dc and low-frequency railway supplies in Central Europe and North America require frequency converters to get power from the public three-phase grid.
Throughout the 20th century, rotary converters were
most often used, with a three-phase motor mechanically coupled to a single-phase generator with a different number of pole pairs, as shown in Figure 1. The

rotary converters are connected in parallel to the mainrail frequency supply and automatically share the traction load. They require maintenance, as they have
moving parts, and the low frequency of the rail supply
causes mechanical oscillations of the motor-generator
pair that need damping to extend the service life.
As power electronics devices were further developed
in the 1980s and 1990s, cycloconverters with thyristors
became a viable replacement for rotary converters.
Cycloconverters can be fully controlled to regulate the
voltage of the single-phase side, reducing the regulation
effect due to train loads. However, the low switching frequency of thyristors requires large reactors to reduce
the harmonic content of the output voltage waveform.
Modern static frequency converters with insulated-gate
bipolar transistors (IGBTs) and gate turn-off thyristors
(GTOs) that switch at frequencies in the range of 1 kHz
have been successfully deployed in Germany and Switzerland as a replacement for cycloconverters. They have
active control and can strongly mitigate the effects on
the public grid of nonlinear traction loads.
Frequency converters are not necessarily required
for 25-kV, 50-Hz systems. The feeder stations are connected to high-voltage buses, generally operating at 400
or 132 kV, so the railway load is a small fraction of the
total power drawn from the bus bars, and the imbalance of the single-phase load is negligible. The techniques to reduce the imbalance can be divided into
passive and active methods. Passive methods alternate
the phase that supplies each section of the track, as
shown in Figure 2. The major drawback of this type of
method is the requirement for neutral sections that
ensure the electrical isolation between consecutive sections of the track. Special transformer connections (e.g.,
V-V, Le-Blanc, or Scott) can further reduce the imbalance, as shown in Figure 3, but they are effective only
for specific loading conditions.
Active methods use power electronics converters to
reduce the imbalance and the voltage drop at the bus bars
of the feeder stations. Static voltampere reactive (VAR)

U
V
W

Three-Phase ac 50-Hz
G
Public Grid
M G
M G Single-Phase
ac 16.67-Hz
Distribution

15-kV Single-Phase ac
16 .67- Hz Catenary

Figure 1. A typical railway supply network in Central Europe with
mixed dedicated and nondedicated power stations. M: motor;
G: generator.

IEEE Electrific ation Magazine / S EP T EM BE R 2 0 1 6

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Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2016