22-10-2012, 11:47 AM
CONTROLLING RAIL POTENTIAL ON DC SUPPLIED RAIL TRACTION SYSTEMS
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ABSTRACT
Most of the modern DC electrified mass transit systems use
a totally floating earth as their grounding strategy. A well
known problem related with totally floating systems is that
the touch potentials can be dangerously high. In order to
reduce the voltages on rails, several devices exist. Most of
these devices allow a direct connection to earth when a
certain voltage threshold is exceeded. In this paper, working
principals of these devices are given and the effect of certain
parameters related with these devices on the minimum
achievable touch potentials is investigated.
INTRODUCTION
In most of the older traction power systems, the rails were
not insulated from the ground, and direct connections
between the negative busses of substations and the earth
were established. Such systems are called directly
connected earth systems [1]. The return current passes
through mother earth as well as rails in such traction
systems. A particular problem that is related with this
strategy is the increase in stray currents, which results in
energy loss, and more importantly, corrosion in
surrounding utilities [2,3].
After observing the adverse effects of corrosion near mass
transit systems, most of the modern railways started to
insulate the negative return system including rails and
negative busses of substations such that the whole system
is “floating”. Such systems are called totally floating earth
systems. Actually, most of the DC traction rail systems in
continental Europe are totally floating systems [4]. The
problem with this strategy, on the other hand, is the
increase in rail voltages (sometimes to dangerous levels).
Therefore special precautions must be taken when
running rails are used as the return current conductor and
insulated from the ground. This is usually done by placing
electronic devices (usually called as rail potential control
devices (RPCD)) [1,5].
SIMULATION PROGRAM
A DC fed rail mass transit system power network solution
involves solving of numerous non-linear equations.
Iterative solution of sparse matrices whose size depend on
length of line and selected parameters for examination is
required in such calculations. This can only be achieved
by help of simulation programs.
A multi – line, multi – train simulator called SimuX [6,7]
is used to do analysis presented in this paper. SimuX
enables users to simulate DC fed rail systems in a userfriendly
environment taking into account the regenerative
braking and under-voltage behavior of the vehicles.
All kinds of details including the characteristics of trains
and transformers, gradients, curves, passenger stations,
properties of power lines and rails, section insulation
points, jumpers and depots can be entered to the
simulation program to obtain a realistic simulation.
MODELLING RAIL VOLTAGES
Usually stray currents and rail voltages are modelled
together in a traction rail simulation program. Stray
currents occur as a result of the conductance between the
running rail and the earth. For loosely insulated systems
(such as directly connected earth systems) the
conductance between the rail and the ground could be in
the range of 1 – 10 S/km, whereas this value can be as
low as 0.003 S/km for highly insulated systems (such as
floating earth and diode earth systems). For simplicity,
rail-ground conductivity is taken as 0.01 S/km throughout
this paper.
A way of representing the conductivity between the rail
and the ground is dividing the track into smaller segments
called as cells, and for each cell representing the
conductivity by a resistance connected at a single point
[10] and [11]. Figure 1 illustrates a simple model that can
be used in simulating stray currents and rail voltages for a
single train running on a single track between two
substations. Here, RRG represents the resistance between
the rail and ground, RNG is the resistance between the
negative bus of the substation and the ground, Rr is the
resistance of a rail cell, and RL1 and RL2 represent
resistance of the catenary line. The length of each cell is
given by L. The smaller values of L result in better
approximations in the simulation. We note that as the
train moves along the track the model changes
accordingly.
RAIL POTENTIAL CONTROL DEVICES
On a totally floating system, the potential difference
between the rail and the ground needs to be restricted
especially in safety critical places such as depots and
passenger stations, to ensure the safety of the personnel
and public. This is usually achieved by the help of Rail
Potential Control Devices (RPCD) [2]. An illustration of
an RPCD is shown in Figure 6. Here, a control unit
constantly monitors the potential difference and the
flowing current between the rail (or negative bus) and the
ground. The switch is open under normal operating
conditions (floating earth). When a predefined threshold
voltage (Vr) is exceeded the switch is closed to allow
current to flow through ground and limit the rail voltage.
RPCD is said to be in ON position when this happens.
The control unit opens the switch back to its normal
position only after the current flowing through the circuit
is below a given threshold (Ir) and a certain time limit
(minimum ON time, TON) passed. Usually direct
connection to earth is not possible and therefore a small
resistance (RG) is assumed to exist between RPCD and the
ground.
CONCLUSION
In this brief note we have discussed how to reduce the rail
voltages on a DC power traction power system that uses a
totally floating earth scheme. In particular, it is shown
that RPCDs can provide an efficient mechanism to control
the touch potentials. In order RPCDs to be used
efficiently, however, their settings have to be done
correctly.
Usually there are lower limits on voltage thresholds (Vr)
of RPCDs. Setting the thresholds lower than these limits
would not decrease the rail voltages and only increase the
stray currents.
Grounding of RPCDs plays an important role in
determination of rail voltage limits and therefore has to be
done very carefully.
Possible future research includes examination of stray
currents at different settings of RPCDs and the effects of
regenerative braking on rail voltages and stray currents.