Page 59 - 360.revista de Alta Velocidad - Nº 5

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

3.3.1 Special connections

It is also possible to reduce the voltage unbalance by use of different special types of
transformers. The remaining voltage unbalance depends on the type of special transformer
used as well as on the distribution of power on the transformer’s secondary side terminals.

The connections seen in Figures 5B and 5C are usually those used when there is sufficient
short-circuit power in the three-phase substation power supply network. It is observed that
the power transformer of the railway substation does not connect to the three phases.
However, if the three-phase supply network does not have sufficient short-circuit power, a
single-phase pure connection can not be used. There are other special connections (Scott
connection, V-V connection, Le Blanc connection, etc.) that partially solve the presented
problem. This would be the case of the Japanese High-Speed network where such connections
are often implemented because sometimes the power to High-Speed trains (Shinkansen) is
supplied from low-power networks.

Figure 6. Scott connection (special). (Source: Author).

3.3.2 Neutral section

As seen in Figure 5 neutral sections are installations that are necessary in High-Speed lines
as they prevent electrical phase unbalance in the three phase grids that power them. The
existence of neutral sections may entail some occasional operating problems and therefore
an attempt is made to minimize how many of them there are and improve operation.
Considering the case of the Spanish High-speed network, a neutral section is built with two
insulated overlaps between which the no-voltage catenary is installed (Figure 7). According
to the indicated diagram, catenary 1 is powered from the electrical phase 2 (voltage V1)
while catenary 2 is powered from phase 2 (voltage 2). The de-energised catenary is installed
between the two (catenary 3).

The train that is coming along catenary 1 enters into contact with catenary 3 via the first
sectioning. Therefore, when the pantograph has lost contact with catenary 1, it only rubs
catenary 3 until it reaches the second sectioning, when it starts to come into contact with
catenary 3. It should be pointed out that the train enters the neutral section moving by
mechanical inertia as its traction switch has been disconnected. It will reconnect when it
leaves to be able to be powered from the new stretch again.

International Congress on High-speed Rail: Technologies and Long Term Impacts - Ciudad Real (Spain) - 25th anniversary Madrid-Sevilla corridor 57