IEEE Electrification Magazine - September 2014 - 4

TECHNOLOGY LEADERS

Rail Power Supplies
Going More Power Electronic
By Stefan Östlund

N RECENT YEARS, INTERest has risen in using electrified rail operations as an
environmentally friendly and efficient
way of producing passenger and
freight services. New high-speed lines
are being designed and built throughout Europe and China, and there is
also an increasing interest in highspeed rail services in southeast Asia
and the United States. To meet this
demand, the electrification of railways
has rendered increased interest both
for new lines and for upgrading existing systems to higher power capacity.
Historically, the first rail electrifications were designed as low-voltage
dc systems, where the dc voltage was
typically generated by dc generators.
The limitations in the voltage capability of dc machines and related
equipment limited the voltage considerably. More than 100 years later,
we still have dc traction systems generated by rectifiers connected to the
three-phase grid with voltages in the
range of 650 or 750 V up to 3 kV.
Because of large voltage drops and
excessive losses, it is required that
the connecting points to the threephase system are located with a relatively short mutual distance.
A dramatic improvement was
reached when systems with high-voltage

Digital Object Identifier 10.1109/MELE.2014.2338971
Date of publication: 29 September 2014

ac were developed in the early 20th
century, permitting longer distances
between the feeding points. The only
ac motor capable of speed control was
the single-phase traction motor that,
in principle, functions like a seriesexcited dc motor. At normal frequencies, 50 or 60 Hz, the induced voltage in
the commutating coil becomes too
large, and, hence, the frequency of the
voltage has to be limited, typically to
the range of 15-25 Hz. Different frequencies were tested and, e.g., Sweden
electrified parts of the iron-ore line in
the north between Kiruna and Narvik,
Norway, with 15 Hz generated by special single-phase generators from the
Porjus hydro power station.
When the low-frequency systems
were extended, different solutions
were developed. Some countries
chose a decentralized system in
which power was delivered from the
electric power grid by means of synchronous-synchronous rotary converters connecting the three-phase
50-Hz grid with the low-frequency
single-phase system of the railways.
Since synchronous rotary converters
were used, the frequency was bound
to the frequency of the three-phase
grid, and, consequently, the frequency
162/3 Hz was chosen when the grid
frequency was 50 Hz.
Other countries designed special
single-phase power systems solely to
supply the railways. Such systems are
often referred to as "centralized" since

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

the frequency control was carried out
in the rail power supply system itself.
The low-frequency ac was generated
by single-phase generators or connected to the three-phase grid by asynchronous-synchronous rotary converters.
For a long time, rotary converters
were the only available type of converter, but in the 1970s, power electronics made its debut in rail power
systems. At that time, thyristors were
the dominating type of semiconductor
device, and the converter-based power
supply stations were built up of cycloconverters to convert a single-phase
162/3-Hz voltage from three-phase
50 Hz. When gate turn-off thyristors
(GTOs) and later insulated-gate bipolar transistors and insulated-gate
commuted thyristors were introduced,
voltage-source inverter-based
solutions were developed. Initially,
thyristor rectifiers were used on the
three-phase side, but later voltage-
source converters were introduced
also on the rectifier side. Power supply
stations with both two- and three-level converters have been constructed.
Recent developments have seen such
new converter concepts as the multilevel modular converter, M2C, being
introduced. One of their advantages is
that, because of the modular design,
they can be designed for higher voltages, thus eliminating the need for a
transformer on, e.g., the single-phase
(continued on page 60)

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