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Engineers must deal with a number of factors when
designing a power system for transportation facilities. One of the
first steps is to analyze the power demand of the system. For a
highway tunnel such as the H-3 project in Hawaii, the safety of
the tunnel operations requires a highly reliable and redundant source
of power, and backup emergency power is required in order to keep
the tunnel operating in a safe mode as long as needed, as Keene
Matsuda illustrates in the following article.
When designing transit and rail systems, engineers must consider
all the factors that affect power demands, including train weight,
acceleration and speed required to meet the train schedule, headway
between trains, locations of stations and other stops, and signal
systems. Power engineers utilize software programs that take these
factors into account when performing piecewise linear calculations
to identify mechanical demand. Then, using the electrical characteristics
of the drive system, these programs calculate the electrical demand
for the train. In Daren Stinton and Allan Ponsonby's article, you'll
read how a probabilistic simulation technique (of 1000 iterations)
was applied to consider the total demand of a London Underground
system with some 300 trains accelerating and braking at random.
The traction power distribution system needed is usually determined
by the type of rail system being designed. The proper operating
voltage allows substations to be located farther apart, resulting
in cost savings. On light rail and short length systems such as
the London Underground and the Bangkok Transit System, which have
a relatively low demand for power, the voltage is low, normally
600 to 750 volts DC. In some applications where electrical demand
is higher, as in the Tseung Kwan Extension project in Hong Kong,
the design can be 1500 or 3000 volts DC.
On heavy gauge and longer haul systems, typical applications would
be at 11,000 volts AC to 25,000 volts AC and higher. The design
of an AC system is more varied, with stub supply being the simplest
design. The transmission is via a simple catenary configuration.
A boosting transformer may be used on long distances to compensate
for voltage drop. Bassam Mansour's article, which traces the history
and development of switching station design for the 25 kV AC power
supply delivery to the overhead line equipment on the UK railway
network, provides additional information on these design issues.
Third rail can be used on dedicated rights of way, where the system
is closed to pedestrians. Overhead catenary can be used in mixed-use
situations.
The design of the power circuit return is another concern that is
usually handled by using the running rail as the return. Stray currents
on this type of system can cause corrosion problems, however, and
in some systems, there are separate third and fourth rails for supply
and return, keeping stray currents to a minimum. Ray Leach's article
and the article by James Pang and Sam Ng discuss these important
engineering issues-traction power distribution, return current and
stray current-extensively.
Overall, the engineer designs the distribution system to meet the
electrical demand. The distribution system can be further modified
to enhance reliability and to meet the contingency operating conditions
under which the system is expected to maintain service. The system
design can be optimized to reduce the number of substations required,
while maintaining a given level of reliability.
Jalal Gohari
Senior Professional Associate, PB Transit & Rail Systems, Newark
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