IEEE Electrification Magazine - September 2014 - 27

40
35

Effect of Feeding Distance
on Overall Voltages and
Potential Difference Between
Earth and Rails

Volts

ils

Ear

Line Constants
Same as in Figure 4

Station at End of Line

15
Shaded Areas Show
Earth Positive to Rails

Rails

Station at Middle of Line

Earth

5
Two Stations

Rails

Two Stations

Earth
Station

Station

8
10
12
Distance (Thousands of Feet)

Station
14
16

Figure 5. The effect of substation spacing on voltage [4].

away from the rails. The stray-current curves defined in
Figure 4 show the effect of the feeding distance on stray current for a defined load of 40 A/1,000 ft. This figure depicts the
total current at any point on the line, the stray current for a
grounded and bus with station at the end of the line, and a
ungrounded bus with a station in the middle of the line,
reducing the feeding distance by half. It was construed that
by providing the supply station at the middle of the line
instead of at the end, the maximum value of the stray current can be reduced from 147 to 24 A [4].
Figure 5 shows the overall voltage curves for the same
line when the station is at the end, the middle of the line,
and at one-third and three-fourths of the total distance.
These curves are based on theoretical conditions, with no
stray current, whereas the actual curves will be less since
part of the current will have leaked to the earth [4]. This was
witnessed by the electrolysis committees, and as illustrated in Figure 4, the
overall voltage is reduced by the square
of the feeding distance when the feeding distance is shortened. Considering
this marked effect on the reduction of
the stray currents and overall potential
due to the reduction in the feeding distance, further detailed studies were
conducted on this subject in the United
States. The initiation of automatic and
semiautomatic control for substations

made it economically feasible to increase the number of
feeding points. The average feeding distances in England
were around 2-3 mi (3-5 km) [4].

Three-Wire Traction Power System and Other Methods
This method was similar to the city power system, where
one trolley (wire) carried negative and the other positive,
and the tracks were a neutral conductor. With proper application, this method not only reduced the stray current to
one-half the value on some existing transit systems but
also gave a better operating voltage for the cars [4]. However,
because of the cost implications of adding a fourth rail or
running two trolley poles in parallel on a single car, this
method was not adapted by most of the transit agencies
since it required the third and fourth rail to be a positive
feed and negative return, respectively.
Along with the adaptation of the
aforementioned mitigation methods
to control the leakage of stray current,
it was recognized that further mitigation methods were required, and procedures were adopted to protect the
utility structures in the vicinity of the
transit system that could be damaged
by stray currents. These measures
included the surface coating of the
pipes, the use of conduits in cable construction, the use of insulating joints,

The process of
corrosion requires
four elements: an
electrolyte, an anode,
a cathode, and a
conductive path.

IEEE Elec trific ation Magazine / s ep t em be r 2 0 1 4

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