IEEE Electrification Magazine - September 2016 - 18

or impossible in practice and whose measurements cannot be understood on the real system. Sensitivity analysis
and worst-case conditions are the two factors in favor of
modeling and simulation tools, in lieu of extensive experimental activities that are much more expensive and time
consuming and that require system availability for the
duration of the tests.
Replacing experimental with simulation evidence
requires that the latter is based on validated, reliable, and
robust simulation tools, where not only the calculation
itself matters but also the modeling approach, user interaction (for defining parameters and configurations), and
result analysis. The three coupling phenomena have
somewhat different requisites that are better illustrated in
the following section.

Magnetic Field Emissions
Defining suitable magnetic-field interference limits at low
frequency requires a thorough analysis of the victim
equipment that may fall inside the area of influence of the
electric transportation system, which may be extensive
because of the significant susceptibility of some scientific
and medical equipment (for nuclear magnetic resonance
and scanning electron microscope products, external field
values of below 0.1 nT are usually required, down to a few
nT for some performing equipment, as described by
A. Mariscotti) and the first-order attenuation with distance
(of the 1/r type). A well-known example described by Bernardi et al. is the San Francisco Bay Area Rapid Transit
negatively impacting geomagnetic instruments located at
the University of California at Berkeley, about 70 km away.
Because of the physical operation principles of the
employed sensors and the time duration of the single
diagnostic or experimental phase and of the overall test,
susceptibility is critical for dc and low-frequency components, up to about the mains frequency or slightly above.
Traction circuit design and implementation for ac systems
favors reducing magnetic-field emissions and induction
by placing one or a few return conductors close to the catenary and attracting the return current with purposely

TABLE 1. Amplitude-time limits curves

for electrical safety.

Clearing Time (ms)

EN 50122-1 (V)

ITU-T K.33 (V)

# 20

940

# 50

935

# 100

842

2,000

# 200

670

1,500

# 300

497

# 350

1,000

# 400

305

# 500

225

650

# 1,000

430

I E E E E l e c t r i f i c ati o n M agaz ine / SEPTEMBER 2016

arranged transformers (autotransformers and booster
transformers), artificially increasing the mutual inductance with the hot path. For dc systems, transformers are
ineffective, and return current may be shifted toward the
catenary only by virtue of very large (and expensive) crosssection values, comparable, in terms of equivalent resistance, to those of running rails. The cost of copper and the
feasibility of a fastening and suspension system for bulky
conductors should be considered during design. Because
there are many dc transportation systems inside cities
and closer to potential victims of magnetic field emissions, this problem has attracted a lot of attention.

Induction
Using the same methods described in the previous section, it is also possible to address the problem of induction
between cables, such as when the source circuit is a medium-voltage or low-voltage line, or between feeders carrying traction current, thus supporting cable segregation
policy in special circumstances (arguable use of available
cableways, violation of prescribed separation due to contingency, etc.). In this case, the separation distance is
reduced further between source and victim and the simplified CCITT method is unreliable.
As anticipated, induction must be analyzed addressing
both electrical safety and interference. Electrical safety
should be evaluated at the supply frequency and a few
other characteristic frequencies during normal conditions,
including overloading, and in case of faults. Whereas dc
systems might be, in principle, exempt, a few ac components characterize normal (substation rectifier harmonics)
and transient (ripple between peak and regime of short
circuit) operation. Time duration is relevant to determine
hazardous exposure and damage or injury to equipment
or people in contact with the live parts and is established
by protection clearing time. When safety is considered,
worst-case assumptions should cover a first failure of fast
protections and the intervention of slower backup protections (once fuses, today mostly distance relays). Amplitude-duration curves are established for human electrical
safety and some standards exist for railway applications
(EN 50122-1) and telecommunications (ITU-T K.33), as
shown in Table 1. For equipment safety, overvoltage withstanding capability should be considered.
When considering fault current intensity and current
flow during fault, phase-to-phase and phase-to-ground
faults are generally relevant for both utility and traction
lines. With isolated neutral, the phase-to-ground fault
current is smaller, although the inducing circuit area is
larger. The phase-to-phase fault always features the largest current. The latter is a more complex problem, where
the susceptibility of victim equipment connected to
induced cables should be clarified. Analog track circuits
send power and audio frequency over connecting cables.
Digital track circuits, axle counters (which sense and
count passing wheels by detecting mechanical

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2016