Page 61 - 360.revista de Alta Velocidad - Nº 5

Current situation and prospects of electric traction systems used in High-Speed railways

section is approximately 9 kph with a total time passed since traction of roughly 22 s and
2
having run roughly 1600 m. Even so, it has been seen that the existence of neutral sections on
the line does not affect the total route time and the total amount of electric energy consumed.
Even so, and as to be expected, the ideal situation for operation would be to have as few
neutral sections as possible.

3.4 Active techniques to reduce imbalance. Introduction of Power Electronics

Furthermore power electronics applications can be used to actively reduce the voltage
unbalance in the public grid. For this purpose different technologies and applications are
introduced. Furthermore other advantages can be achieved like reduced voltage drop at the
TPS. For example static VAR compensators (SVCs) connected to the three-phase grid in parallel
to the TPS reduces the voltage unbalance imbalance but require large harmonic filters due to
the switching of the thyristors. Then synchronous static converters (STATCOMs) connected to
the three-phase or single-phase traction network were used where they also allow to filter the
harmonics produced by the traction loads.
Static frequency converters have also been used to provide the total power required by the
substation, although in a smaller number due to the lower cost of the SVC or STATCOM since
these are dimensioned for a fraction of the total power of the substation. These frequency
converters have evolved from back-to-back converters to the current modular multi-level
converters (MMC). There are two configurations for MMC converters: AC/AC direct converters
that allow to convert power from a three-phase network from 50 Hz to a single-phase network
at 16,7 Hz, and the AC/DC/AC indirect converters that allow obtaining from a 50 Hz threephase
network a single-phase 50 Hz network.

Finally, in the last years an electrification system called Cophase Power Supply has been
developed (the first 5 MW installation was installed in 2015 on the Shanxi Line in China) which
is a hybrid solution between a STATCOM and a converter of the total power of the substation.
There are different configurations but in all cases they use a single-phase converter that
generates on the three-phase side the current necessary to balance the load and compensate
for the imbalance generated by a single-phase or three-phase transformer from which the
power is supplied to the single-phase rail network. The converter provides active power to the
single-phase network and also allows filtering of the harmonics generated by the loads. As the
load to the three-phase grid is balanced, the collateral transformers can be connected to the
same phase so it is possible to eliminate the neutral sections.
The inclusion of power electronic converters implies the existence of a system of regulation of
control its operation, and to a greater or lesser extent, the behaviour of the rail network: from
the SVC that hardly influences the voltage of catenary, STATCOM and Cophase Power Supply
that can vary the voltage in catenary, and finally, the total power converter that can modify
the voltage and frequency. The last two systems have the possibility of operating without
neutral sections which allows them to distribute the load between collateral substations,
both at active and reactive power levels. These converters may have implemented control
methods to function independently of each other (such as Droop Control which is a method for
interconnecting multiple voltage sources for microgrids operating in isolation) or by methods
that exchange information, for example, to avoid recirculation by the rail network which is in
parallel with the three-phase transport network.

2 Data obtained after the tests carried out by ADIF on HSL 050 (Madrid to Barcelona) and HSL 010 (Madrid to Sevilla). An average
speed of 270 kph is considered.

International Congress on High-speed Rail: Technologies and Long Term Impacts - Ciudad Real (Spain) - 25th anniversary Madrid-Sevilla corridor 59