IEEE Electrification Magazine - September 2016 - 26
resistivity and the material. To avoid inadmissible
stray-current effects for the civil structures, the longitudinal voltage U S between any two points of the
through-connected metal-reinforced tunnel structure
should be calculated.
Again, with the longitudinal length l, which is fed by
one rectifier substation, and with I being the average value
of the traction return current in the considered section
length l during the hour of highest load, the longitudinal
voltage U S is calculated as
l
Rl $ Rl
l
U S = 0.5 $ I $ l $ l R Sl $ :1 - c $ ^1 - e - l hD,
RR + RS
l
C
(3)
where l C is again the characteristic length of the system
track/structure that is described by the equivalent longitudinal resistance per the length of the interconnected
parallel running rails and interconnected parallel return
conductors (RlR), by the resistance of the structure per
length (RlS), and by the conductance per the length of the
running rails versus the earth (GlRE) . The mathematical
relationship of those four variables is given in
lC =
1
^RlR + RlS h $ GlRS
.
(4)
The maximum longitudinal voltage shall be smaller
than the permissible potential shift as per the EN 50162
standard. This is a conservative procedure that ensures
that the actual values for the structure potential with
respect to the earth will be lower than required by the
EN 50162 standard.
In the case of tunnels, viaducts, bridges, or slab tracks
with metal reinforced concrete structures, it is possible for
stray currents to flow into such structures and, from there,
influence other outside nonrailway conductive structures.
In this case, the effect of such an influence is reduced by a
means of equipotential bonding in the lower part of the
individual tunnel sections or other conductive structures
to achieve the voltage requirements. This equipotential
bonding shall be achieved by
xx
a sufficient number of reinforcing bars
xx
mats connected together
xx
other conductive structural parts
xx
additional conductors of appropriate cross-section
laid within the tunnel, if necessary.
For stray-current protection purposes only, it is
possible to achieve an adequate electrical conductivity for the reinforcing bars within a structure section
by a means of conventional steel-wire wrapping. In
the case of tunnels, the German recommendation
VDV 501/1 advises that the minimum cross section of
the metallic-conductive interconnection must be
400 mm2 on each tunnel side in addition to the reinforcement earthing conductors or earth bars of at
least 35 mm2 copper, or an equivalent cross section
has to be laid, to which the reinforcing bars lead out
26
I E E E E l e c t r i f i c ati o n M agaz ine / SEPTEMBER 2016
the expansion joints, and the metallic-conductive
units are connected. The Swiss recommendation SGK
C3 (Richtlinie zum Schutz, 2011) advises a minimum
of 800 mm2 for long tunnels in metro systems.
Testing the Continuity of the Tunnel
Although the IEC 621218-2 standard does not mention a measurement for the metallic continuity of the
structures, it is good practice to check its continuity
during each section construction. The German recommendation VDV 501/2 suggests the following circuit for continuity measurement in which the
individual tunnel sections are already bonded, as
described earlier.
According to the circuit shown on Figure 2, the resistance of the structure per length (RlS) is calculated as
1 DU S DU B m
RlS = I $ c
+
lS
lB
DU = U ON - U OFF.
(5)
The voltage drop values DU S and DU B must be measured
with the source turned ON and OFF in a cycle-for example, 10 s OFF and 5 s ON. This procedure is important
because the voltage drop may not be zero when the circuit is turned off. The fed current I is split into two currents, I S and I B , and the partial current I S flows through
the measured section while the partial current I B flows
off via adjacent tunnel sections. Since neither I S nor I B
can be measured directly, the voltage drop DU B is determined at the section block of the longer adjacent tunnel
section. It is imperative to perform the measurements
during the construction to correct possible interruptions
in the structure continuity. After the resistance measurements of the structure per length (RlS) are performed, if
the resistance is below a foreseen value according to (3),
then it can be corrected by increasing the cross section of
the additional reinforcement earthing conductor by at
least 35 mm2 copper as per VDV 501/1.
When a new rail line is built, structures (if any) such as
tunnels are constructed first, and then the tracks are
assembled (Figure 3). After acquiring measurements for
the structure continuity, resistance, and the homologation
of rail fastening systems, the next step is the measurements of the longitudinal rail resistance and the measurement of the conductance per the length of the tracks,
which should be performed at each section just after the
track assembly.
Testing the Longitudinal Rail Resistance
For the measuring arrangement in Figure 4, it is advised
that the value of the current I supplied by the source be
between 15 and 30 A. The voltage values U A and U B must
be measured with the source turned ON and OFF in a
cycle, for example, 10 s OFF and 5 s ON as previously
explained. The longitudinal resistance of the running rails
(RlR) is then calculated as indicated in (6).
Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2016
IEEE Electrification Magazine - September 2016 - Cover1
IEEE Electrification Magazine - September 2016 - Cover2
IEEE Electrification Magazine - September 2016 - 1
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