Chapter
AMERICAN RAILWAY ENGINEERING AND
MAINTENANCE OF WAY ASSOCIATION
Practical Guide To Railway Engineering
RailwayElectrification
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©2003 AREMA®
AREMA COMMITTEE 24 - EDUCATION & TRAINING
Railway Electrification
Andrew J. Gillespie P.E.
LTK Engineering Services
Denver, CO
agillespie@ltk.com
H. Ian Hayes P.E.
LTK Engineering Services
Denver, CO
hhayes@ltk.com
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Chapter
RAILWAY ELECTRIFICATION
9.1 Introduction
D
espite the competition
of airplanes, buses,
trucks and cars, trains
still play a major transportation
role in society, filling specific
markets such as high-speed and
non-high-speed
intercity
passenger service, heavy haul of
minerals and freight, urban light
rail systems and commuter rail.
This chapter presents an Figure 9-1 Overhead High Speed Catenary - Courtesy of LTK, Inc.
introduction to electrification of
rail systems. It is intended to provide a historical perspective and an overview of
typical design principles, construction practice and maintenance considerations. Those
interested in learning more are invited to review AREMA’s Manual for Railway
Engineering, Chapter 33, Electrical Energy Utilization, and Chapter 17, High Speed
Rail Systems, which contain sections devoted to electrification power supplies, traction
power systems studies and guidelines for the design of overhead contact systems.
9.2 Development of Motive Power
for Railways
The earliest recorded tramway served a mine in Germany, beginning in about 1550.
The tramway was developed because the rolling resistance of wheels on rails was much
less than on the roads of the time. This allowed heavier loads to be pulled for the same
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power. Horses did the work, and it was not until the steam engine was developed, first
as a stationary engine and then in the early 1800's as a locomotive, that the horse began
to be replaced. Surprisingly, this transition took another 100 years, with the last horsetramways being phased out by retrofitting electric traction equipment into tramcars and
streetcars in the early 1900's.
In both North America and Europe, the period from 1800 to 1900 was truly a
developmental age for railways. From its beginning in the 1800’s, steam traction
expanded without serious competition. Locomotives became larger, faster and more
powerful for the next 125 years, culminating with massive machines weighing over 500
tons and capable of speed of 120 mph. However, the problem with all steam engines,
irrespective of the fuel that was used (wood, coal and later oil), is the smoke, coupled
with high maintenance costs, the frequent fueling and the need for large quantities of
water. These problems led to the eventual demise of the steam locomotive.
In the late 19th century, some early steam power was replaced by electric traction
equipment that had finally become commercially viable through the early efforts of
Werner von Siemens, Thomas Edison and others. First using batteries, but later using
stationary electric generators, electric streetcars demonstrated the practicality of electric
traction. Based on these demonstrations, mainline railways, which up until that time
were 100% reliant on steam traction, began taking an interest in electric traction. In the
period 1895 to 1900, several sections of mainline track were electrified at various
voltages from 550 volts DC to 660 volts DC. The slow development of electric
traction resulted partly from the lack of available utility power. This situation began to
be rectified when the demand for electric lighting drove a need for a public electricity
supply, necessitating the development of sizeable electricity generation plants.
Compared to steam power, electric propulsion offered higher performance and
avoided smoke problems. Railways readily accepted electric propulsion on steep
grades and long tunnel territories, where significant advantage was obtained. With
respect to railway passenger operations, eastern railway trunk lines established extensive
electric commuter rail systems, some with long-distance intercity services as well.
Certain western trunk lines operated intercity passenger services over electrified
territory, built primarily to conquer grades and long, difficult-to-ventilate tunnels. In
the Midwest, a demonstration electric traction line was built to shuttle visitors to the
Colombian World Exposition in Chicago in 1893. This was followed by the operation
of numerous electric trolley lines within Chicago by the turn of the century. It was not
until 1926, that the first Illinois Central commuter trains (predecessor of Metra
Electric) began electric operation.
Throughout North America, passenger rail systems began to adopt traction
electrification systems in the late 19th century. The first successful and sustained use of
electric traction in transit revenue service occurred with Frank Julian Sprague’s streetcar
line in Richmond, VA in 1888. The first application of electricity to mainline railway
operations was the Baltimore & Ohio’s electrification of Baltimore’s Howard Street
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Tunnel in the 1890’s. The technology greatly improved with time and was rapidly
accepted by both railway and transit operators through the early part of the 20th
century. By 1920, there were thousands of electrified track miles on both railway and
transit systems.
Starting in about 1905 there was another surge in railway electrification, using AC
power with voltages up to 11,000 volts and with conversion of some DC lines to AC.
A steady stream of electrification followed for about 25 years, by which time 38
systems existed in North America, aggregating about 7000 electrified route miles.
Although creditable, this was a relatively insignificant amount of the total United States
track mileage. Clearly the steam engine was still in supremacy. However, a new type of
locomotive using a diesel engine, that had been in development for nearly 50 years,
started to come into service.
Patented in the mid-1880s, the “straight” diesel engine faced a major problem that
prevented its early adoption by railways. The problem was the lack of a reliable
mechanical transmission able to handle the horsepower required for practical mainline
operations. The first diesel locomotive was a direct drive 1,000-hp built in Germany in
1913, but it only ran experimentally for a few months before being withdrawn. Also in
1913, a 75-hp diesel-electric railcar was a built in Sweden. It ran until 1939. Other
experimental diesel-electric locomotives with different horsepower ratings followed,
but it was not until 1925 when a 1200-hp ‘mainline’ diesel-electric locomotive began
regular service on the German State Railways. Some of the first mainline diesel
locomotives to enter service in the United States were the Chicago, Burlington and
Quincy Railroad’s Pioneer Zephyr and the Union Pacific Railroad’s M10000 in 1934.
Subsequent developments with diesel power in the 1960s and 1970s included
locomotives that used Voith hydraulic mechanical transmissions, and diesel railcars that
used gas turbines paired with Voith hydraulic transmissions. Both of these
developments eventually reached an acceptable level of reliability, but have not been
able to supplant the electric traction motor powered from an overhead line (with or
without a transformer) or from an on-board diesel-electric generator.
The diesel-electric locomotive has advantages over both steam and ‘straight’ electric
locomotives, namely lower maintenance than steam and lower capital and
maintenance-of-way costs than compared to the overhead or third rail electric
distribution systems needed by electrics. However, the diesel-electric locomotive has,
to this day, not been able to match the acceleration, high-speed, or zero-emissions
capabilities of the straight electric locomotive. But for the type of service needed, the
diesel-electric locomotive remains the preeminent player on North American railways.
This is not the case in Europe and Japan.
Many railways that adopted DC traction power stayed with it until it wore out or was
upgraded to 1500 VDC or 3000 VDC, while others changed to AC at 11,000 volts or
more. These high-voltage systems all had or have overhead contact systems.
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Electrification of the main lines grew steadily into the 1920s, but when reliable dieselelectric locomotives became commercially available, the pace of electrification slowed
down. In many places in the United States, the electric locomotives and the associated
substations and overhead wiring were removed. Diesel-electric locomotives have their
limitations when it comes to very high-speed trains, with a typical upper speed limit of
100 – 110 mph. While this may be adequate for the needs of freight service, even
higher speeds are being sought for passenger service.
The traction horsepower of a diesel-electric locomotive is typically only 82% of the
rated horsepower of the diesel engine, and there is little overload capacity. ‘Straight'
electric locomotives, by virtue of the high power levels available at the wire, have
conquered all contenders in the race for high-speed locomotives on conventional rail
trackage.
Although electric traction and diesel-electric traction are now the preferred traction
options, there are several choices of traction system within each option. The outcome
of the foregoing developments has produced a variety of alternative sources of motive
power for any particular need, be they for a railway or a transit line.
9.2.1 Pioneers of Electric Traction Development
In 1835, Thomas Davenport, a Vermont blacksmith, built the world's first, albeit short,
electric railway. His experiments consisted of several train models powered by
batteries, which utilized a third rail conductor and a track return circuit. In 1842, the
first electric locomotive was built by Robert Davidson and operated on the Edinburgh
& Glasgow Railway. It was battery powered, had four-wheel drive, weighed 7 tons and
could haul 6 tons at a speed of 4 mph.
During the 1860's, the electric dynamo or AC generator was developed, although
electric traction motors powered by electricity would not be demonstrated until 1879,
when Werner von Siemens built the first practical electric railway for the Berlin Trades
Exhibition. The Berlin electrified line was a 600-yard long, 150V center third rail
narrow-gauge line, with a 3 horsepower (hp) locomotive. It could accommodate about
30 passengers on three cars moving at 4 mph. In 1881, the first public electric railway
in the world was opened in Lichterfelde near Berlin. The route was 1.5 miles long and
the cars ran on a 100 V supply, carrying 26 passengers at 30 mph. In 1883, the Volks
Electric Railway, a short length of track on Brighton Beach, operated as the first
electric line in England. It used a Siemens dynamo powered by a 2-hp Crossley gas
engine.
The next significant step forward in the development of electric traction was a 3-foot
gauge railway in Northern Ireland. This 6-mile line had an outside third rail to supply
the electricity, which was generated by waterpower. Electricity soon came to be seen as
a way to propel light vehicles on what, up to then, was horse-tramways. It also led to
the electrification of some lines that had been designed for cable haulage using
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stationary steam engines. These electric tramways were seen as a significantly better
choice than having steam engines in streets. The violent emissions of steam and noise
frightened the public, but more seriously, spooked horses. American tramways soon
caught the electrification bug and by 1888, 50 lines had been electrified. All the lines
used DC electric power, many using Edison's electric motors and generators.
Some of the first electric
locomotives to enter railway
service were built by the firm of
Siemens Bros. In 1890, they were
used on the underground electric
railway in London, England with
electricity supplied by a specially
built power station. Each
locomotive weighed 13½ tons,
had two 50-hp electric motors,
and operated on a third rail at 600
VDC. By 1907, 52 locomotives
Figure 9-2 Early Catenary - Courtesy of Ian Hayes, LTK, Inc.
had been supplied and they
operated until the line was absorbed into the London Underground Railway in 1924.
When they were replaced, it was not due to failure or breakdown, but to changes in
operating conditions necessitating faster speeds and heavier loads than originally
planned. In the United States, the first electric train service began on the 7-mile
Nantasket branch of the New York, New Haven and Hartford Railway in 1895.
As more powerful electric motors were developed, major railways in the United States
began taking an interest in electric traction. Based on the proven technology of the
tramways, railways entered into mainline electrification. Typically, power was supplied
at 550 to 675 VDC, usually from railway-owned generation plants, since electric utilities
had not yet been developed. However, with changing times, electricity was in demand
and a hydroelectric power plant was constructed at Niagara Falls, to be followed in
quick succession by electric generating plants in other places.
Based on work by Nickeli Tesla, who showed the practicality and demonstrated the
advantages of AC electric power generation and the development work carried out by
George Westinghouse, AC traction power was introduced. The practicality of mainline
electrification, which required more powerful locomotives and higher speeds, now
became evident. Much higher voltages could be used, thereby resulting in lower
electrical current demands. These lower currents allowed reduction in the required size
and number of electrical conductors, thus reducing the overall cost of the
electrification infrastructure.
Ultimately, some United States railways, after conducting feasibility studies using both
DC and AC scenarios, opted for the AC alternative even though the equipment had
not been entirely proven. The New Haven Railroad was one such railway.
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Commercial service commenced in 1907, but not without many serious technical
problems in every area of the power supply and the locomotives. The resulting poor
service was recognized with apologies to the public, while claiming that the steam
service that had been replaced had not been any better.
In 1904, the world's first single-phase 15,000 V, 15 Hz AC locomotive went into
service in Switzerland. Weighing 44 tons, its 400-hp AC motor was only capable of a
maximum speed of 47 mph and ran until 1958. Since the early 1950s, many of the
earlier 15, 16 or 25 Hz power supplies have been replaced by 50 or 60 Hz systems.
These are the ‘commercial’ or 'industrial' frequencies at which utilities generate their
power, depending on the country. The most famous system to retain it's original
configuration of 12.5 kV 25 Hz is the former Pennsylvania Railroad portion of the
Northeast corridor from New York to Washington DC, now operated by Amtrak.
Transit operators rapidly changed from horse, cable and steam propulsion to electric
traction. By 1920, virtually every large city and many small cities boasted electrically
powered transit lines in the form of streetcar, interurban, subway and elevated railway
operations.
The balance of the 20th century was not kind to the electrified mileage in North
America. The Great Depression killed most of the planned railway electrification
extensions, with the exception of the Pennsylvania Railroad’s electrification from
Philadelphia to Washington, and the Reading Company’s commuter electrification in
the Philadelphia area, both undertaken in the 1930's. These were the last major United
States passenger electrification projects until the 1990s, when Amtrak finally bridged
the gap between New Haven and Boston in 1999.
The French Railways started pushing the limits
of high-speed rail further by developing trains
that were eventually capable of operating at
speeds up to 200 mph, and based on testing of
an upgraded TGV unit, speeds of up to of 320
mph. In Japan, the bullet train speed was
recorded at 277 mph with trains operating
commercially at 168 mph. Amtrak’s highspeed trainsets (shown right), operating on the
9-3 Amtrak ACELA in NEC - Courtesy of
Northeast Corridor, are eight-car, 12,500 hp Figure
LTK, Inc.
units capable of speeds up to 150 mph. These
trainsets also tilt, allowing trains to travel through curves at higher than normal
operating speeds without affecting passenger comfort.
Since the 1970's, many countries, most notably France, Germany, Italy, Spain and
Japan (and more recently Britain, South Korea, China and Taiwan), have started to
implement passenger train operations at speeds in excess of 160 mph, using new track
alignments. In these trains, the locomotives are integrated into the design of
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permanently coupled trainsets of six to fourteen cars with driving cabs at each end.
Propulsion systems may be distributed throughout the train (so that the train is a mix
of motor and trailer cars) or located in dedicated power cars at each end of the trainset.
These types of trainsets are invariably ‘straight’ electrics, which operate under highvoltage overhead contact wire systems. The alternative diesel-electric technology has
been successfully developed for train speeds up to 140 mph outside the United States
where unit loads are not as great.
9.3 Rail Operation Classification
The advent of the diesel-electric locomotive sounded the death knell for North
American electrified railways, built solely to replace steam locomotive issues addressing
steep grades and tunnel ventilation. Most of these installations were taken out of
service in the 1950’s. The former Milwaukee Road electrified territory lasted until the
1970’s, while Conrail abandoned use of the ex-PRR electrified territory for freight
purposes in the early 1980's. As a result, with the exception of a few specialized
industrial lines, all surviving electrified railways owe their continued existence to
passenger operations.
Rail operations in North America serve a wide variety of transportation modes, from
freight to long distance intercity travel, to daily commuter trips, to local urban transit
services. These operations encompass a diversity of vehicle types, operating speeds,
right-of-way requirements and service frequencies. For purposes of this chapter, the
various types of rail operations will be divided into the following categories:
•
•
•
•
•
•
•
•
•
•
Mainline and independent Short Lines
Freight – general and single product mineral heavy-haul (unit trains)
High-Speed Rail (“HSR”)
Heavy Haul – privately owned (captive fleets)
Urban
Metro
Commuter Rail
Rapid Transit (“RT” or “heavy-rail” transit)
Light Rail Transit (“LRT”)
Street Car
The general characteristics of electrification of each of these rail modes are described
below:
Mainline railway operations (high speed rail, intercity and commuter rail) tend towards
higher speeds and longer routes, utilizing equipment that is generally compatible with
freight equipment. Diesel-electric propulsion is the most typical power for passenger
rail services. There are some notable exceptions:
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Freight railways are almost all diesel-electric, with the exception of some mineral unit
trains, and heavy-haul dedicated short lines electrified at 25 kV, 60 Hz and 50 kV, 60
Hz.
High-Speed and most Intercity services on the Northeast Corridor (NEC) are
electrified with 12.5 kV, 25 Hz; 12.5 kV, 60 Hz; 25 kV, 60 Hz traction power systems,
using a catenary-type overhead contact system (OCS). At the time of this writing, the
Northeast Corridor (NEC) is the only North American rail line with high-speed
operations.
Heavy Haul railways are lines are typically less than 1000 miles in length and operated
by multiple electric or diesel-electric locomotives with up to 200 cars. Loads can range
up to 20,000 tons for iron ore or coal trains. Fifteen hundred VDC, 25 kV AC and 50
kV AC systems are used and examples exist in the United States, South Africa,
Australia and New Zealand.
Most Commuter Rail operations in the Northeast are electrified. Examples include:
§
Metro-North: 650 VDC under-running third rail (ex-New York Central) and 12
kV, 60 Hz (New Haven Line);
§
Long Island Rail Road: 600 VDC over-running third rail;
§
New Jersey Transit: 12 kV, 25 Hz OCS and 25 kV, 60 Hz OCS (ex-Lackawanna
3000 VDC lines);
§
SEPTA: 12 kV, 25 Hz OCS; and
§
MARC: 12 kV, 25 Hz.
Electrified commuter services in the Chicago area include:
§
South Shore/NICTD (former CSS&SB): 1500 VDC, OCS and
§
Metra Electric (ex-Illinois Central): 1500 VDC, OCS.
Transit systems (heavy and light-rail) typically have speeds of 50 to 80 mph and shorter
routes, but generally higher service frequencies. For that reason, transit operations
utilize smaller and lighter vehicles. The vast majority of transit operations are
electrified. All Rapid Transit and all but a few LRT systems utilize self-propelled
EMUs. North American electric transit systems are almost exclusively DC systems.
Power may be distributed to trains by an overhead contact system (OCS) or by third
rail (over-running or under-running):
§
Systems built prior to 1980 are typically 600 VDC.
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§
Systems built after 1980 typically utilize a 750 VDC standard.
§
San Francisco’s Bay Area’s BART system represents a unique case, using a 1000
VDC system.
§
Seattle Sound Transit plans to use a 1500 VDC LRT system with OCS (proposed
at this writing).
§
Vintage trolley operations are typically electrified at 600 VDC.
9.4 Mainline Railways and
Independent Short Lines
Existing mainline railways may be characterized as being for freight or for high-speed,
and all rely upon electric or diesel-electric locomotives. Each type has its advantages,
however electric locomotives require an additional large investment in fixed equipment,
comprising an electrical infrastructure that includes substations and a traction power
distribution system.
Independent heavy haul railways are frequently privately owned and operated for the
sole purpose of moving bulk commodities, such as coal or iron ore from a mine to a
power station, plant or a harbor, using unit heavy haul trains. Several of these are
electrified operations including: Sisher Saldenha in South Africa, Coccle Hampton lines
in Australia, and Black Mesa and Deseret Western lines in the United States.
Electrification of a railway can usually be justified if there is:
§
A call for very high speed (over 120 mph)1
§
A need to reduce reliance on fossil fuels
§
A high volume of traffic
§
A significant length of tunnels
§
A high level of traffic sustained throughout the day, the week, the year.
§
Need to reduce trip times
For freight railways, electrification may be justified if there is a need to:
1 Reducing track curvatures, introducing “tilt” trains, or a combination of both may accomplish the application of a
very high-speed system on an existing railway.
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§
Increase capacity without adding tracks2
§
Shorten delivery times by increasing running speeds
§
Reduce maintenance costs by phasing out obsolete units, or
§
Electrical power is available
9.4.1 Mainline Electrification Studies
Before analyzing the pros and cons of an electrified system versus a wholly dieselelectric operation, it is necessary to compare the performance characteristics of each
type of locomotive from an operations view point, while playing down relative cost.
Relative cost will inevitably appear in the cost analysis anyway. The major difference
between electric and diesel-electric motive power is that each diesel locomotive carries
its power source while the power for all electric locomotives is supplied to them at the
point of need.
Comparing electric locomotives to diesel-electric locomotives, it is found that electric
locomotives:
§
Have higher speed capabilities.
§
Have lower fuel costs because the electricity generated for traction effort has a
higher thermal efficiency when secured from large power plants as compared to
comparatively small on-board diesel power plants.
§
Are able to utilize alternate energy sources such as coal, nuclear and hydroelectric
power.
§
Have no local emissions to pollute air and have fewer fire and life-safety issues in
underground stations or tunnels.
§
Do not require fueling plants, eliminating an environmental hazard of possible fuel
leakage and spillage.
§
Can employ regenerative braking whereby traction motors become generators,
putting power back into the contact system for another train in the circuit to utilize
2 A decision to electrify may involve selective tracks, typically mainlines and passing sidings. Branch lines,
yards and sidings along the route can remain diesel operated, typically using switchers or a special applications
locomotive, such as a 'dual-mode' locomotive or an 'electro-diesel.' The 'dual-mode' locomotive that can
handle branch lines is a mainline diesel-electric locomotive fitted with a pantograph and electrical equipment.
The 'electro-diesel' is a mainline electric locomotive fitted with a small diesel generator set, which can provide
slow speed operations off-wire in yards and sidings.
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(potentially reducing the overall system power consumption by as much as 10% or
more).
§
For equal rated horsepower, the straight electric locomotive has superior wheel-rail
adhesion through better management of available traction, making them better at
handling track grades, and allowing the use of steeper gradients with consequent
savings in track and in civil structures.
§
The straight electric locomotive has a short-term horsepower rating up to double
the nominal horsepower rating, which is ideal for accelerating trains and ascending
grades.
§
Are not limited in horsepower by the size of the on-board diesel engine. (Electric
locomotives can exceed their nominal power ratings for short periods of time,
thus, improving their acceleration and run times.)
§
Can increase line capacity without increasing infrastructure, by running a given
route much more quickly than a diesel train due to their speed and superior
acceleration and braking rates.
§
Provide better track utilization by maximizing the number of trains that can fit in a
given area at any one time.
§
Does not present the fire hazard of onboard fuel tanks that may prevent dieselelectrics from operating in tunnels and underground stations, due to local fire
codes.
§
May have lower locomotive maintenance costs than diesel-electric locomotives
over the expected service life.
On the other hand, diesel-electric locomotives have certain advantages over
electric locomotives:
§
Has a lower initial capital cost since they don’t require a power distribution system.
§
Does not require an extremely elaborate and expensive power distribution system
infrastructure, spread over the full length of the rail network, exposed to the
elements and requiring continuous surveillance and on-site maintenance.
§
Can operate independent of a power distribution system over long distances.
§
Will operate during any level of failure of the electrical supply network.
§
Does not create possible electrical safety hazards to the public due to the presence
of the bare conductors of the contact system.
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When the cost of diesel fuel was 9 cents a gallon and the supply seemed unlimited,
United States railways were not interested in alternative methods of propulsion.
Railway electrification interest peaks during times of uncertainty in the energy industry.
When fuel rose to 34 cents per gallon and the oil embargos occurred, much effort was
expended studying alternatives to hydrocarbon fuels. Studies showed that "an
estimated 34% savings in energy could be achieved by using electric power.
Electrification of just 10% of the (then) present rail trackage (in the densest traffic
corridors) could result in a 40% reduction in railway diesel fuel consumption.”
Studies made in the 1970’s also showed that approximately 6 years after electrifying a
route, the operating cost would break even when compared to the operating cost of
diesel service. At 30 years, the annual operating cost of an electrified system would be
one-third that of diesel service. In other words, over the effective life of a railway, the
cost to operate a diesel-electric system far exceeds that of an electric system. These
increased costs mainly come from the price of fuel and maintenance. Diesel
locomotives average 3 to 10 gallons or more of fuel per mile and three times the
amount of maintenance of straight electric locomotives.
The most significant aspect arising from these studies is that in order to realize the
long-term savings, a huge capital investment is needed. Even when engineering
economic studies show that an electrified system would be beneficial, raising enough
money to perform the capital upgrade is a daunting challenge. Private railways would
most likely require government assistance or financing from the utilities.
9.4.2 Mainline Infrastructure Compatibility
The electrification of a section of existing mainline cannot be undertaken without
considering the requirements that the electric locomotives, substations, overhead or
third rail power distribution systems and traction return system will place on the
existing rail infrastructure.
The more significant issues are noted below:
§
Tracks may need to be upgraded, including new track work or re-alignment. Sites
must be found and real estate acquired for substations. In rights-of-way with
restrictive width, the location of the system-wide ductbank requires coordination
with track drainage, the foundations for OCS poles and emergency walkways. In all
cases, maintenance access must be provided.
§
If DC traction is used, the effects of electrolytic corrosion due to leakage (stray)
currents must be mitigated.
§
Additional clearance may need to be provided in tunnels and at bridges. Existing
civil structures may have insufficient clearance to accommodate the proposed
electrification system. It may be necessary to lower tracks through overhead
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crossing bridges. New bridges resulting from grade-crossing elimination will need
to be built with adequate electrical clearance. Future widening of existing
overhead bridges must be considered.
§
Tunnels may be suitable for electrification, or may require costly remedial work,
enlargement or “daylighting.”
§
Integration of the electrification support structures with existing station canopies
must be considered. Station canopies that project over platform edges may need
modification.
§
Where OCS poles cannot be
installed for lack of clearance,
attachments, such as wall
brackets will need to be added
to civil structures. Pictured at
the right is an example of an
OCS cantilever attachment to
an overhead structure.
§
Signals and communication
systems will need to be replaced
or upgraded. Because electric
Figure 9-4 OCS Cantilever Attachment - Courtesy of LTK, Inc.
traction systems use the same
running rails for traction return current, it is necessary for the two electrical
systems to be electrically isolated. The signal circuits need to be “immunized”
from the traction power circuits.
§
Grounding and bonding of exposed metals is necessary to protect the public from
electrical hazards, as well as insuring that there is no interference with the signals
and communications systems.
§
A central location will be needed to supervise the power system. SCADA, pilot
wires or a relaying system must send information to a central point to insure
power is being supplied to the system when necessary.
Maintenance
More details on these and other aspects impacting the railway route are given later.
The advent of electrification increases the level of overall maintenance on the right-ofway. The traction power distribution system, comprising substations, feeder cables,
OCS or third rails, lineside disconnect switches, impedance bonds and rail bonds
requires out-of-service time to perform maintenance inspections, maintenance
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adjustments and renewal of componetry. In addition, working around electrified lines
is more difficult because clearance from the traction power system has to be
maintained or the area will require de-energizing with the loss of track occupancy time.
Existing maintenance facilities will need
to be upgraded, extended and/or adapted
to the needs of electric locomotives. New
office facilities, workshops and stores for
staff to maintain the substations and OCS
will be required. This is in addition to the
existing needs for maintenance of track
and wayside equipment such as signals
and communications. Track possession
time will need to be coordinated with
Figure 9-5 Tower Car Crew Performing Wire Maintenance
train operations schedules. Highway-rail
- Courtesy of Q&R Australia
vehicles (for line adjustments and for
OCS conductor stringing) and service
vehicles must be procured and will need to be stored. Electrical test equipment,
stagger gauges and pantograph clearance gauges, grounding equipment and special
tools will be needed.
Staff Safety
Traction power distribution systems bring additional
overhead, at-grade and underground electrical hazards, and
require extensive safeguards against damage and personnel
contact. To protect staff, safety barriers may be needed
around adjacent equipment, under low bridges and around
signal heads that are close to the contact system. Third rails
will require wooden or plastic protection boards, especially
in complicated track areas and where railway staff regularly
access. Pictured right is a glow stick that is used to test Figure 9-6 Using a Hot Stick conductors to determine if they are energized. Because of Courtesy of Q&R Australia
the proximity of bare overhead or third rail conductors,
safety-oriented work-permitting procedures must be introduced for all maintenance
personnel.
9.4.3 Impacts of Mainline Railway Electrification
on Communities
When an existing mainline railway electrifies, it will typically adopt a 25kV AC
overhead contact system. The local communities may perceive that the electrified
system will bring few advantages with it, except perhaps an absence of diesel fumes.
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Not only will local townspeople not have any use for it, but also it will likely create few
jobs and will unfortunately bring with it several distinct disadvantages to the
communities through which it passes. These include:
§
Highly visible OCS poles and wiring
§
More intensive train service, meaning more noisy periods
§
Higher train speeds creating more vibration
§
Possible electromagnetic interference (EMI) with overhead cables and
telecommunications lines
§
Electrical interference affecting TV screens
§
Electromagnetic field issues
§
Safety issues
All of these issues are addressable and must be addressed in the Environmental Impact
Study that precedes public approval of the electrification project.
9.5 Urban Railways
Urban railways comprise light rail, commuter rail and rapid transit for passengers only.
Urban railways are very different from mainline freight and high-speed railways.
Apart from a few streetcar systems that have survived from the earliest days, such as in
San Francisco, Boston and Toronto, urban railways have been reintroduced into many
cities. Since 1970, San Francisco Bay Area, Baltimore, Miami, Washington DC, Atlanta
and Los Angeles have developed rapid transit systems, and more than 10 cities have
started to develop light rail systems. Apart from BART in San Francisco, all these new
systems use standard track gauge of 4-ft 8½ inches as do mainline railways. Rapid
Transit is typified as being fully segregated, largely in tunnel in the Downtown and on
its own restricted right of way or on aerial structures elsewhere. Stations are designed
to avoid the need for passengers to cross tracks at grade. Traction power uses a third
rail and the tracks must be fully fenced against intrusion by the public. Linear motorpowered traction systems have been built in Vancouver, BC, Toronto, ON and
Detroit, MI.
Commuter rail, if recently introduced, typically provides morning ‘in’ and evening ‘out’
service for commuters and probably operates only limited service during the ‘non-rush’
hours. Unless the lines are already electrified, these commuter services are usually
diesel powered. An exception is the Peninsular Corridor, between San Francisco, San
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Jose and Gilroy, which is planned to be electrified. Electric locomotives could easily
serve commuter lines south of Boston, presently served by diesel locomotives, because
the mainline south to Providence, RI is already electrified as part of the New Haven to
Boston high-speed electrification program.
Light Rail (LRT) is exemplified throughout the United States as having short trains
running frequently up to 21 hours a day, 365 days of the year. The trains utilize the
downtown city streets, often co-existing with motor vehicles. Typically, the public can
access the entire trackage, as fencing is seldom provided. Light rail service can only be
integrated with mainline rail service by means of time separation of operations. Some
sections of track are shared, as in San Diego (after the LRT service closes each night,
the tracks can be used by diesel locomotives to move freight). Such lines must be
designed for mainline railway clearances and loadings.
9.5.1 Impacts of an Urban Electrified Light Rail or
Commuter Rail System on the Community
The impact on the community of light rail and commuter rail electrification is quite
different to that of freight railway electrification.
In the first place, the implementation of a light rail system is typically with voter
approval. Light rail:
§
Encourages commuters to leave their car outside of the city center.
§
Reduces travel times for car owners and bus and LRT passengers.
§
Reduces overall vehicle emissions.
§
Typically leads to an increase in property values within walking distance of
stations.
However, there are some negative issues. Light rail:
§
Requires large parking facilities.
§
Occupies downtown streets, thereby reducing automobile traffic flow in the city
center.
§
Overhead contact system (OCS) wiring is considered visually intrusive.
§
Creates electrical safety issues.
§
OCS safety screens on bridges may be visually intrusive.
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Size of the LRV belies their speed, creating hazards for people and road vehicles.
9.6 Existing Electrification
Systems
A variety of different designs of traction power systems exist and many have been in
place for years. However, they can all be grouped into one of two categories, AC or
DC. The most common are 11/12.5kV AC or 25kV AC overhead contact systems,
1500 VDC overhead contact systems, 600-750 VDC overhead systems and 600-750
VDC third rail systems.
In the early days of mainline electrification, the only type of traction motors available
were DC drive motors derived from early tramway motors on streetcars. This resulted
in DC being selected for mainline electrification projects. At the turn of the century,
the Swiss and Italians experimented with using three-phase 600 VAC propulsion by
using two overhead conductors and the running rail. This early three-phase system
was very complex to build. In addition, there was no flexibility for operations as the
traction motors were essentially constant speed, only allowing one or two operating
speeds.
By the 1950’s traction power technology had improved to allow the direct distribution
of electric power at commercial frequencies, either 50 or 60 Hz. By 1960, studies
determined that 25 kV systems would, for most railways, produce the most cost
effective design by reducing the number of supply stations needed to connect to the
commercial supply grid as compared to 1500 volt or 3000 volt systems. In addition,
conductor sizing could be reduced, which in turn reduces required conductor tension
and allows use of lighter supporting structures. In the early 1970's, the US pioneered
50kV systems for railways in regions where there were few, if any, transmission lines.
Four such lines were built in North America: The Black Mesa & Lake Powel Railway
near Flagstaff Arizona; the Deseret Western railway from Vernal, Utah to Rangeley,
Colorado; the high speed test track in Pueblo, Colorado; and the Tumbler Ridge
Branch Line in British Columbia (electric operations on this last line recently ceased
due to business conditions).
Existing electric railways can be typically identified as one of six operating scenarios.
The six scenarios are:
• Inter-City, TGV, Shinkansen, ICE, Amtrak Acela Express are examples of InterCity operations. These systems use 11/12.5 kV or 25kV AC, with or without
autotransformers. Twenty-five kV AC is well proven and is the most economic
electrification system under normal conditions. Older 3000 VDC systems are still
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prevalent in Russia, South America and other areas around the world that chose not to
change their systems.
• Heavy Haul, including Sishen-Saldhana; Black Mesa and Lake Powell; and Tumbler
Ridge (British Columbia). These railways use 50kV AC, because of the limited
availability of connection points to public utilities, because they generate their own
power at one end of the line, or for other economies. Substation spacing is typically 40
miles apart on 50 kV systems. However, substations on the Sishen–Saldahna iron-ore
line are over 80 miles apart, in order to reduce the number of long spur distribution
lines from the main power network. Voltage in the catenaries can drop as far down as
25kV and the electric locomotives still operate satisfactorily. The Sishen–Saldahna
iron-ore line is a case where unique needs have driven the creation of a special type of
traction power system.
• Commuter Rail, typified by older suburban lines that do not use streets to penetrate
into the city they serve, such as around New York, Chicago, Baltimore; London,
Birmingham and Newcastle in England and around Sydney and Brisbane, Australia.
These lines operate in segregated right-of-way with no authorized public access. Some
are third rail systems; some are 25kV AC. The lines that formed the old Southern
Railway System in England (which today cover at least 1000 miles of route, including
about 150 miles of four-track route, the rest being primarily two-track) form one of the
world’s largest 660 volt third rail distribution systems. New York, which banned steam
locomotives in the early 1900s, quickly developed third-rail subway service that
operates at 625 volts. Sydney and Melbourne in Australia have extensive 1500 volt DC
systems. Suburban lines to the north of London were originally converted from steam
using the 1500 volt overhead DC system; but in 1956, British Railways selected 25kV
as standard and all lines were converted from 1500 volts DC to 25kV AC. The
London Underground System, although using sub-surface tracks and deep ‘Tube”
tunnels, radiates well out of the city to the north, having taken over some of the old
steam tracks of earlier railways. It uses two “third-rails,” one located in the customary
position outside of the running rails and the other midway between the running rails.
Extensions to existing third rail systems might also be third rail for uniformity, but dual
voltage AC/DC systems operate very successfully in London. All commuter rail
systems are “ heavy rail.”
• Metrorail, (actually METRO) is the name given to heavy rail systems built since
1970 and include WMATA in Washington DC, MARTA in Atlanta; BART in the San
Francisco Bay Area; and the Los Angeles Red Line. Metrorail systems may have
extensive lengths of tunnels and/or elevated sections where an overhead contact
system would be considered unaesthetic or impractical. Consequently, all use DC third
rail systems with voltages of 750 volts, except BART, which uses 1000 volts.
• Light Rail, including systems in MTDB, San Diego; RT Sacramento; VTA, San Jose;
Tri-Met, Portland; RTD, Denver; Metro, Buffalo; MTA, Baltimore; RTA, Cleveland;
PAT, Pittsburgh; MUNI, San Francisco, DART, Dallas; UTA, Salt Lake City; Metro,
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Houston; Hiawatha, Minneapolis; Sound Transit Seattle; and Calgary. All of these
agencies use DC overhead contact systems, and much is installed in city streets.
Downtown, a single contact wire may be installed for aesthetic reasons, but
underground parallel feeder cables must supplement it, which makes for a relatively
expensive installation. Low-profile catenary systems with low visual impact can be
considered as a more economical alternative in some cases.
MTDB, RTA and MUNI use 600 volts; Metro Buffalo and PAT, use 650 volts; and
the rest, except Seattle (1500volts), use 750 volts.
• Street Car, typified by TTC, Toronto, which still uses trolley poles on their 240
streetcars and operates at 580 volts, even though nearly every other agency which
operated with trolley poles has converted to pantographs. Earlier users of trolley poles
included San Francisco, Boston, Newark, NJ and the Chicago South Shore Line and
every former tramway/streetcar system has since been shut down. This system is now
considered out-of-date, both from an operator’s standpoint and in terms of aesthetics.
Operationally, any time that the trolley pole dewires, the driver must stop to reattach
the pole to the trolley wire. To do this, the operator must exit the vehicle into possibly
street traffic, which is dangerous, more so at night. Second, the trolley wire must be
held close to the optimum operational path of the trolley pole, which means
registration guy-wires every four feet along the trolley wire at street intersections and
on some sharp curves.
• ALRT, An alternative suburban electrified rail system is known as ALRT. This
acronym for ‘Advanced Light Rapid Transit’ System uses linear motor technology.
Toronto has a 4-mile elevated double track system. Vancouver, British Columbia has
over 20 miles of route, and Detroit uses this technology on a downtown circulator
people-mover system. These standard gauge systems operate on 600 volts DC, which
is collected from two side rails and fed into linear induction motors through variablevoltage variable-frequency converters.
9.7 New Electrification Systems
Today, new electrification systems need to serve a wide variety of rail applications. The
requirements of light rail, commuter rail, rapid transit, heavy rail, intercity, high-speed
passenger service, mixed and heavy-haul freight are quite different. These different
requirements result in a variety of potential electric traction system solutions.
In urban settings using city streets and malls, safety and insulation requirements (due to
the close proximity to buildings, and integration with motor vehicle and pedestrian
traffic) requires that light rail systems use overhead contact systems at voltages of 1500
volts DC or less. On the other hand, rapid transit and commuter rail systems, by virtue
of precluding public access to the tracks, are able to use a third-rail power distribution
system of 600 volts or more if the conductor rail is 'protected' and access to the right-
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of-way is restricted. In these two examples, safety considerations limit the voltages.
Use of lower voltage DC systems is not as efficient as high voltage AC systems. The
high direct currents required at these voltages require heavy conductors, require closely
spaced substations, suffer relatively high line resistance losses, and require mitigation of
stray currents. However, in the case of light rail, there is no safe alternative.
For rapid transit and commuter rail, the use of third rail lowers construction cost of
tunnels by reducing the tunnel bore diameter, as compared to the diameter required for
pantograph operations. At one point, 600 volts DC was the preferred traction voltage
because this allowed the carborne electrical gear to be simpler. However, with modern
equipment this is no longer an issue.
Generally, the economic selection in terms of the cost of traction power for new
mainline electrification and other systems using segregated and restricted right-of-way,
will lead one to use one of the high voltage AC systems such as the 12 kV, 25kV or
25kV/25kV auto-transformer, or a 50kV system. However, every scenario will require
a detailed examination to determine the feasibility of electrifying and the type of
traction power system that will best serve site-specific requirements.
There are four main parts to a traction power system:
•
Sources of primary power
•
Substations to transform the power into a form suitable for train operations.
•
A power distribution system and
•
Current collectors (on the locomotives or power-cars) to draw on the power
9.7.1 Sources of Primary Power
Railways usually receive electrical power from utility companies. Power enters a
traction power system at supply substations. Rarely, railways may opt to generate their
own power by any method economically available. Often, there will actually be several
sources of primary power for each substation. Thus, each substation will be fed from
two or more separate supplies in order to provide an alternative feed in the event of
failure of one of the primary supply(s).
On an AC system, commercial three-phase power comes into the supply stations with
one, two or three main transformers. In order to balance loads evenly between the
phases, substations with two or three transformers will be fed from different phases.
For example, the A-B phase will feed the first transformer, while the second is fed
from the B-C phase. The two transformers will supply two different power sections.
This phase balancing provides one level of redundancy, so that if one transformer is
taken off-line, the remaining power section could be fed by means of circuit breakers
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and disconnect switches. Systems that use frequencies other than that supplied by the
utility will require frequency converters to convert to the operating frequency.
On DC systems, incoming power is both stepped down and converted from AC to
DC by the use of rectifier transformers. DC systems, like their AC counterparts, also
have built-in redundancy by designing the traction electrification system to be able to
supply enough power with one or more substations off line. In addition, steps are
often taken to ensure that neighboring substations are fed off of different power
sources, whether it is different power grids, different phases or different breakers
coming from the same utility substation.
9.7.2 Substations
On almost every DC traction power
system, AC power is supplied to
substations
equipped
with
rectifier/transformers to convert the
power to DC at the required
distribution voltage.
Rarely, DC
power may be supplied directly from
the utility. AC systems usually receive
power at commercial frequency and
will transform it to the traction
Figure 9-7 Motor Generator Set - Courtesy of LTK, Inc.
voltage. Those systems that use a
frequency other than commercial
frequency will convert the power using either motor generator sets or frequency
converters, which may be located separately from the substation. In addition to
supplying power for train propulsion, signal and ‘house’ power may also be supplied
from the substations.
Supply
substations
with
autotransformer systems have
two busbars, a ‘line’ or OCS
busbar, similar to a busbar on a
single-phase system, and an
autotransformer feeder busbar.
There may be four or more
autotransformer
substations
located
between
supply
substations and are connected
by the autotransformer parallel
feeder cables and the OCS.
Figure 9-8 Supply Substation - Courtesy of LTK, Inc.
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Each autotransformer substation has a unique autotransformer for each traction power
section required.
Because components of the traction power system constitute a large capital investment,
different types of protection are incorporated into the design. The cost of the
substation as compared to the cost of protection is a small fraction of the capital cost.
To insure that components last for their intended service life, system sectioning,
primary breakers, secondary breakers, relays, PLC controls, SCADA systems and other
protecting type devices are used.
A great deal of consideration is given to providing redundancy or contingency
operation when designing a traction electrification system. Contingency operation
plans are made at many levels. For example, the design of a typical traction distribution
system allows for normal train operations even when one or more substations is offline.
This means that the substations must be spaced and have reserve capacity to handle
the normal load if any one or more is off-line. Further information on traction power
system dependability is given later in this chapter.
A transmission system is sometimes used with AC systems. The use of a transmission
system permits power to be moved around a system at more efficient higher voltages.
In addition, if a property is large enough, a transmission system will allow multiple
power sources to be utilized, providing redundancy and competitive pricing from utility
companies. Multiple supplies, whether from the same utility or not, may require
isolation since it is unlikely that the phases will be synchronized. Similar to phase
balancing, the different sections must be separated by a phase gap. A transmissions
system will require a fault detection system and load balancing. Having a trackside
transmission system as a means to improve contingency operations is bought for the
cost of the additional conductors and supports, although use is usually made of the
OCS poles. In addition, communities may oppose such systems as they may be
considered as visibly obtrusive.
9.7.3 Power Distribution Systems
Traction power distribution systems comprise three sub systems:
§
Feeder cables (from the substation to the bare conductors).
§
Negative return cables, connecting to the rails or the return conductor back to the
substation.
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§
A contact system comprising bare conductors (overhead or third rail) located
along the track, from which the trains draw power through some form of sliding
contact. Parallel feeder cables or auto-transformer feeder cables may electrically
support the contact system.
§
Vehicles collection equipment to pick up the electric current.
Feeder Cable Sub Systems
Traction power is fed to the distribution system;
whether it is an OCS or third rail, via traction power
feeder cables, which, if underground, will be
insulated cables, but if aerial may be bare conductors.
Underground cables will normally be installed in
ducts or in surface mounted troughs and will be
routed to the OCS or third rail as specified by the
traction power sectionalizing plans. Each cable may
have a disconnect switch to facilitate isolation of the
substation.
Parallel, or along track feeders, are conductors that Figure 9-9 Feeder Cables - Courtesy of
are parallel to the contact system and provide LTK, Inc.
additional power. On single wire systems and third
rail systems, the parallel feeder is “tapped” every 300 to 500 feet with a connection to
the contact system.
Negative Feeder Cable Sub Systems
All railway electrification systems
utilize rails for the return current.
Normally the running rails are used
for the negative return, but the
London Underground employs a
second conductor rail located
between the running rails, thus
avoiding stray currents, which could
cause serious electrolytic damage.
Figure 9-10 Negative Return Via the Running Rail - Courtesy of
LTK, Inc.
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Contact System Sub Systems
Contact sub systems include:
§
Third rail systems
§
Overhead contact systems
When selecting the appropriate type of contact
system for a new urban electrification, safety and
reliability will be the first issue, aesthetics second,
followed lastly by economics. For instance, safety
precludes using a third rail system wherever the
public has direct access to tracks, such as in streets,
although a third rail is far less visually intrusive than
an overhead system.
Contact systems, by their nature, are required to
cover almost every inch of electrified track. This
results in a network of very many miles of conductor
wire or conductor rail. A feature of all contact
systems, except for very small installations, is the
provision of sectionalizing to enable segments of the
traction power distribution system to be deenergized for maintenance or in an emergency. This Figure 9-11 Third Rail Contact System is accomplished by the provision of disconnect and Courtesy of LTK, Inc.
sectioning switches, enabling sections of the system
to be isolated from each other, or alternatively, “tied” together for contingency
operations.
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Third Rail Systems
As the name suggests, a third rail provides the positive supply in a DC powered
traction system from a traction rail that parallels the track. The third rail typically rests
on insulators on the field side of either side of the track. One or both running rails are
used for the negative return, however a fourth rail, also on insulators, may be used.
Overhead Contact Systems
Overhead contact systems comprise a support system (poles, wall and soffit
attachments, cantilevers, cross-spans, etc.), conductors (which may be arranged in one
of a variety of configurations) and anchorages to tension the conductors.
The conductor will be a continuous, energized, un-insulated contact surface suspended
above the railway tracks from which electric locomotives can draw power by means of
current collectors. This conductor is typically about 20 feet above the track but can
range from 12 to 24 feet above top of rail.
9.7.4 Current Collectors
The current collectors that draw power from the third rail or from the OCS contact
wire are normally:
For third rail:
§
Contact ‘shoe’
For OCS:
§
Trolley pole or
§
Pantograph
Contact Shoe
The contact shoe of a third rail
system can be of several different
types. The overrunning system is
most common as it allows for
simple attachment of the conductor
rail to the ties. A contact shoe
slides over the top of the conductor
rail, hence its name. The under
running contact shoe, pictured
Figure 9-12 Third Rail Contact Shoe - Courtesy of LTK, Inc.
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right, runs along the bottom of a conductor rail that is suspended over the end of the
tie. A third type is the side running contact shoe. Here, the contact shoe extends out
horizontally from the vehicle and slides along the conductor rail that is again supported
off the end of the tie.
Trolley Poles
Trolley poles, whose length is typically
14 to 16 feet, are fitted with a trolley
shoe (also called ‘harps’ or wheels) at
the upper end, which are grooved to
form a channel for the contact
(trolley) wire to slide within. A
constant upward force keeps the
trolley shoe or wheel in contact with
the wire as the wire elevation changes.
Although very commonly used in
Figure 9-13 Trolley Pole - Courtesy of LTK, Inc.
the past, trolley poles suffer certain
disadvantages over pantographs. A primary disadvantage is frequent dewirement,
which requires the vehicle operator to exit the vehicle and replace the shoe on the wire
while exposed to both traffic and weather. Second, with the increased power
consumption of light rail vehicles due to improved performance and air-conditioning,
the carbon insert is often electrically overstressed and may need frequent replacement.
Third, trolley harps require many special pieces of hardware to be installed in the trolley
wires for turnouts, wire crossings and on tight curves. Fourth, the alignment of the
trolley wire relative to the track is much more critical than a conventional single wire
simple pantograph system. The trolley requires more overhead equipment and more
maintenance of that equipment. Fifth, depending upon complexity of the track layout
(especially at street intersection in city centers), the quantity of special work in the
overhead trolley system may itself be visually intrusive.
Pantographs
As can be seen in the picture to the right,
pantographs have a wide rubbing strip on
the pantograph head and collect power
through their carbon strips at any point.
Most pantographs for modern light rail
systems are about 6ft.-6in. over the horns
with a 4 foot-4 inch rubbing strip. These
dimensions typically permit 240-foot spans
on tangent track, which is considered ideal.
Narrower pantographs will require more
Figure 9-14 Pantograph System - Courtesy of Q&R
Australia
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supports for the overhead contact system, which means a more costly installation, and
more support cantilevers and cross-spans to maintain. There are real short and longterm benefits to using wide pantograph heads.
9.7.5 Characteristics of Third Rail System
Typically, the third rail is
mounted on an insulator at the
ends of the ties on either side of
the track and physically parallel to
the running rails. The third rail is
supported on the ties and is
relatively rigid when compared to
a contact wire. Obviously, the
third rail does not sag and
therefore does not need to be
tensioned.
Figure 9-15 Turnouts in Third Rail System - Courtesy of LTK, Inc.
At track turnouts and diamonds, the third-rail is interrupted since it cannot pass over
the running rails. The conductor rail is also interrupted at highway grade crossings,
which for the most part, have been eliminated from modern metro systems. If the gap
between the sections of rail exceeds the span between the contact shoes on the power
car, the power car loses power.
The DC third rail system is accompanied by the limitations of voltage, the need for
close substation spacing, complicated feeding arrangements at turnouts, icing of the
third rail, the public safety hazard and the difficult issue of mitigating the effects of
stray return currents, which leak into the ground and could cause electrolytic damage to
underground utilities and civil structures. If not safeguarded, electrolysis causes
corrosion and possible failure of metallic water mains, cable sheaths, gas pipes, steel
ducts, steel bridges and various other metal paths, including reinforced concrete
through which currents flows on its way back to the substations. The problem can be
mitigated by insulating the running rails from ground by placing them on an insulating
plastic pad in open line or surrounding them with a rubber ‘boot’ if they are embedded
in the street. Electrolysis can also be mitigated by the use of negative feeders or by
reducing sub-station spacing. To minimize this particular problem, the London
Underground uses two conductor rails throughout its system, so that there is no
current in the running rails.
Conductor Rail Supports
Third rail systems require special ties or tie extension brackets to support the third rail.
In addition, the alignment and elevation of the third rail must be kept in proper spatial
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relationship to the running rail. Insulators must be maintained in good condition to
avoid excess current leakage.
There are three types of third rail systems: overrunning, under running and side
running. Overrunning systems use a post type insulator to support the conductor rail
so that the contact shoe can slide along the top. An under running system suspends
the conductor rail so that the conductor shoe can slide along underneath. Lastly, a side
running system supports the conductor rail so that a shoe can slide along the side of
the rail.
9.7.6 Characteristics of an Overhead Contact
System
There are two basic types of overhead contact systems in use today:
•
Single wire system
o Pantographs
o Trolley wire only (for trolley poles)
•
Catenary system (for pantographs)
Figure 9-16 Different Styles of OCS - Courtesy of LTK, Inc.
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Single Wire System
The ‘single contact wire’ system for pantographs and the ‘trolley wire only’ system for
trolley poles are two quite distinct styles of the single wire system.
These two configurations (‘style’) of overhead contact system are initially dependent on
whether pantographs or trolley poles are to be used. Note that both types of power
collection can be accommodated with some increase in complexity of the overhead
line.
The differences in the styles are reflected in the design approach as shown below:
Current Collection Device
Single Contact Wire
Trolley Wire Only
Pantograph
Trolley Pole
Horizontal wire alignment on Wire staggered to wear Wire kept relatively straight
Tangent
current collector evenly
to minimize wear to pan
shoe
Horizontal
curves
alignment
on Wire placed to the outside Wire placed to the inside
of curves, allowed to of curve, trolley pole
sweep over entire head
pulled towards centers of
curves
Converging and diverging Crossing
wires
at
tracks
intersections that are kept
at the same elevation allow
the pantograph to pass
from one wire to the other.
The pantograph is kept
from de-wiring because
one or both wires keep the
pantograph below the
contact surface at all times.
Placement of wire crossing
is important but has a
greater degree of flexibility
when compared to manual
trolley frogs.
Special devices called
trolley switches or frogs are
used to make intersections.
Mechanical trolley switches
use a switch point to
determine which wire the
trolley
pole
follows.
Manual trolley switches rely
on the dynamics of the
vehicle to pull the trolley
pole from one side or the
other. Placement of trolley
switches is very critical,
improper placement will
lead to dewirement.
Contact surface
Trolley shoes contact both
the bottom and sides of
the trolley wire. Clips, nuts
and bolts and other
supporting hardware must
Pantograph is intended to
contact only the bottom of
the contact wire under
normal
operating
conditions. All supporting
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Single Contact Wire
Trolley Wire Only
hardware must be kept be kept out of the area.
above the contact surface. Different
types
of
hardware are used for
trolley systems because of
this.
Single Contact Wire Systems for Pantograph Operations
If a single contact wire provides adequate power for revenue service, it is an
economical choice for an overhead contact system. However, it is invariably found to
be deficient in power for mainline application, but is still economic for application in
storage yards and shops.
For mainline operations, single contact
wire is commonly selected for application
in city centers because of its low visual
impact, in which case underground
parallel feeder cables must electrically
reinforce it. This is expensive, since it
multiplies the cost by a factor close to ten.
However, since the length of route to be
given this treatment is typically fairly short,
the overall impact on OCS costs will, by
its nature, be acceptable.
Figure 9-17 Single Contact Wire - Courtesy of LTK, Inc.
Single contact wire systems have a span length similar to that used in the spacing of
downtown streetlights and opportunity therefore exists for joint-use poles, thereby
reducing pole ‘clutter.’
With pantograph operation, it is possible to use an auto-tensioned single contact wire
system, concealing the balance weights within tubular poles. However, these ‘anchor’
poles will be of larger diameter than normal, which increases their visibility. Portland,
Oregon has auto-tensioned single contact wire downtown.
Trolley Wire Only Systems for Trolley Pole Operations
‘Trolley Wire Only’ systems with trolley poles are few and far between these days since
cities such as San Francisco and Boston have, out of necessity, changed over from
trolley pole operations to pantograph operations.
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A few cities still operate trolley poles in combination with historic cars in tourist areas.
San Francisco continues to run historic trolleys from all over the world, but these
rolling museums are only run on the surface on Market Street where they use the
positive wire of the existing trolleybus overhead.
The largest surviving trolley pole
operation in North America is in
Toronto, where 240 streetcars operate
over 100 miles of wire, including over
100 complex street intersections.
Trolley wire systems are typically fixed
terminated systems.
Since most
trolley systems are used in downtown
urban areas, the structures are often
relegated to sidewalks. To help the
structures fit into the urban
environment, the structures will often Figure 9-18 Trolley Wire Contact System - Courtesy of LTK,
Inc.
serve double duty by acting as light
poles, traffic signal poles, etc. On
straight or slightly curved track, either cantilevers or cross-spans support the trolley
wire such that it is placed over the center of the track. When the track requires tight
curves, the trolley wire is held in place with cross-spans, pull-offs and back bones.
Although trolley poles pivot at the base, the trolley harp does not pivot so that the
trolley wire must be placed towards the center of the curve on sharp curves to allow
the trolley shoe to track efficiently. The trolley shoe must be drawn tangentially along
the trolley wire, thereby not rubbing against the ‘cheeks’ of the groove. Only by using
rigid harps can the trolley shoe diverge onto the correct trolley wire at turnouts, as the
pole operates passively being positioned only by the direction of the streetcar on its
tracks.
Catenary Systems
Two-wire systems are referred to as
simple catenary and utilize a contact
wire and above it, a messenger wire.
The messenger wire serves two
purposes (1) to support the contact wire
vertically between structures by use of
hangers and (2) to provide more
electrical conductivity.
Variations of simple catenary exist, such
as low profile simple catenary, which
can be considered as a three-quarter-
Figure 9-19 Two Cross-Span Wires with Full Simple Catenary Courtesy of LTK, Inc.
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scale version of the most economic simple catenary style. The low profile simple
catenary has reduced visual impact by virtue of requiring only one cross-span wire for
support between poles compared to the necessary two cross-span wires with full
simple catenary as pictured (Figure 9-19). Structure spacing is, however, reduced, thus
increasing the pole count by about 30%.
Nevertheless, it is still only about half the cost of a single contact wire system with
parallel underground feeders, which would be electrically equivalent. Twin contact
wires are also commonplace on light rail systems in Europe. Other systems using three
conductors called compound catenary are operating, but are more costly and are
generally not considered necessary for new installations. Compound catenary utilizes
three or more conductors, with a main messenger being the top conductor, the contact
wire serving as the bottom conductor, and an auxiliary messenger located between the
two. Other styles, which have been installed in the past, include stitched catenary,
triangular catenary and ‘hanging beam’ catenary, and all continue in use today.
Inclined catenary exists to the present day in the Northeastern United States and
requires the use of the messenger, and on severe curves, an auxiliary messenger to align
the contact wire around curves. This is accomplished by inclining the OCS so that the
messenger wire is moved to the outside of the curve while the contact remains close to
the track centerline. Sloping hangers support the contact wire at a carefully calculated
angle to provide the lateral restraint. Inclined catenary has fixed terminations, which
means that the contact wire moves up and down relative to the track surface as
temperatures change. Thus greater clearances are required under structures and over
grade crossings. Because of the special techniques needed to align inclined catenaries,
the trend today is to replace them with chordal (simple) catenary, where the messenger
is located directly above the contact wire.
Catenary systems are designed to allow the contact
wire to operate satisfactorily over the full extent of
the carbon-rubbing strip of the pantograph.
Careful calculations are performed to determine the
extent that the wire can be staggered at the OCS
registrations (supports) and to ensure that the
pantograph does not dewire in a combination of
adverse operating conditions, including strong
winds, maximum vehicle sway and poor quality
track.
These calculations are then used to
determine how much the contact wire can be
allowed to be placed off the centerline of the track
and still allow safe operations. On tangent tracks,
the wire is intentionally staggered from one side of
the track centerline to the other at successive poles
to prevent grooves from forming in the middle of
the pantograph carbons.
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Figure 9-20 Contact Wire Placement in a
Curve - Courtesy of LTK, Inc.
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All overhead contact systems exhibit the characteristic of increased sag between
supports and loss of tension when conductor temperatures rise due to solar gain
and/or current heating. Although small variations to sag and tension do not adversely
affect current collection, also called ‘commutation,’ large variations, say over 6 inches,
can be unacceptable. In order to control conductor sag between supports, two options
are available:
§
Limit span length (length between poles)
§
Tension compensation (described later)
Both options apply to Single Contact Wire (SCW) systems and to multiple conductor
catenary systems to be described later.
Fixed Terminated Conductors (FT)
A typical operating temperature range for an OCS in the United States is from -10°F to
130°F. Conductor tensions are selected such that at the coldest temperature, a safety
factor of 2 against breakage is available. For the contact wire, this factor of safety is
preserved with the wire in the worn (typically worn 30%) condition. With no form of
tension compensation, the contact wire tension in the hot condition may be so slack
that the pantograph head or shoe on the trolley pole could elevate high enough to
strike parts of the conductor support and registration system. To prevent this from
happening, conductor spans are typically limited. For single contact wire (SCW)
systems, the maximum span is typically 125 feet.
Tension Compensated Conductors
Conductors
Tension compensation, also termed constanttensioning, endeavors to maintain the tension in
the conductors at a very-nearly constant value
over the full range of possible conductor
temperatures. This is usually achieved by
installing balance weights at one end of each
half-mile length of the conductor(s). Two such
‘half’ tension-sections may be installed back-toback, to form a mile-long tension-section of
catenary. To maintain a continuous contact
surface for the pantograph, consecutive tensionsections are overlapped by paralleling the wires
in the last running spans at the end of the
tension-sections.
Figure 9-21 Balance Weight Tensioning Device Courtesy of LTK, Inc.
Technically there is no restriction to how long an
auto-tensioned span could be based on commutation requirements. However, a limit
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will be set by determining the maximum span-length that is secure against dewirement
from conductors being deflected by wind. Typical spans for constant tension
equipment are as follows:
For single contact wire, SCW, maximum span is typically 125 feet, but auto-tensioned
spans of 160 feet have been used very successfully.
For catenaries, maximum span is 240 feet for a pantograph width up to 6ft.-6in,
although longer spans have been used with wider pantographs, (8ft.-6in) or tighter
track construction and maintenance tolerances.
Other tensioning devices such as hydraulic and pneumatic tensioners have been used,
but with limited success. Spring tensioners are often used on short wire runs of less
than 500 feet, such as at crossovers.
Although the maximum span length on tangent tracks may be 240 feet, the maximum
span lengths on curves will require considerable shortening due to the chording of the
curve by the contact wire. Span length analysis is performed to determine the
span/curve radius relationship.
9.7.7 OCS Style Selection
There is an almost limitless variety of configurations of OCS, but they can be broadly
classified under four general styles:
(1)
(2)
(3)
(4)
Simple Catenary – Auto Tensioned (AT).
Simple Catenary – Fixed Tensioned (FT).
Low Profile Catenary – Fixed Tensioned (FT) (normally)
Single Contact Wire with Parallel Feeder, normally FT, but AT system exists in
Portland.
Style selection of OCS for mainline application is based on the following
considerations:
§
Location and environment
§
Equivalent copper cross-sectional area
§
Economics
§
Speed and line characteristics
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Location and Environment
Location can be typified by:
o Urban
o Suburban
o Rural
o Remote or open line
Environment can be typified by:
o Desert
o Coastal
o Polar
o Tropical
Copper Cross-sectional Area
All four styles of catenary can be designed to be equivalent electrically and can be used
for mainline service. Equivalent copper cross-sectional area can be achieved by the
proper selection of messenger and contact wire sizes.
The required cross-sectional area will be determined by traction power analysis, which
factors in the various parameters that represent the proposed electrification operation
including:
•
Track alignment
•
Track profile
•
Train weights
•
Line speeds
•
Train frequency
•
Locomotive performance characteristics
•
Substation locations
•
Assumed substation ratings
•
Climatic conditions
For light electrical duty, such as in yards and sidings, a single contact wire without
parallel feeder will be adequate and economic.
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In maintenance shops, conductor rail supported from roof trusses has application in
special circumstances and has the advantage over a single contact wire in that it is
installed untensioned, thereby avoiding horizontal tensile loads on the shop walls or
door frames.
Economics
Catenary economics is a function of the following factors:
§
Aesthetics
§
Pole types
§
Pole quantities
§
Maintainability
§
Selected design parameters controlling the type and quantities of poles and
foundations.
Aesthetics
If one includes the cost of the overhead system, paired with the cost of the substation,
duct backs and feeders, an auto-tensioned simple catenary is the most economic OCS
style. However, some find auto-tensioned OCS less appealing aesthetically than a
single contact wire style. The balance weights are often considered to be too unsightly
to be used downtown and in city centers. Consequently single contact wire styles are
often proposed on city streets, even though either of the other two options, Simple
Catenary – Fixed-Tensioned, or Low profile – Fixed-Tensioned, would be more
economic.
A concern with the single contact wire system when used downtown is that it requires
an underground parallel feeder cable, which though invisible to the public, represents a
large expense in underground ductbank, manholes, hand holes and insulated feeder
cables. Single contact wire systems also have twice the number of poles compared to
simple catenary and requires feeder-riser cables and jumpers. More poles means more
support brackets, which together with the feeder connections makes for more clutter.
In addition, there is more physical plant to maintain, especially because of the
underground feeder cables.
Single contact wire with underground parallel feeder cables is, these days, not the
preferred choice from an economic perspective. It is costly in the first place, and the
underground feeder cables could turn out to be costly to repair and disruptive to road
and rail operations if they fail. For these reasons, low-profile catenary is often used as
an aesthetically friendly and reasonably economic style for use in city streets. Low
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profile catenaries are used in San Diego and Tacoma, and are now recommended for
use on many upcoming projects.
Pole Types
Poles for railway electrification are typically made of
steel, which must be galvanized, painted or both, to
prevent corrosion. In addition, a corrosion control
system is required to protect the steel from premature
corrosion as well. Generally, the most economic
section for mainline use is the wide-flange beam, since
the predominant OCS loading is in one direction,
across-track. A down-guy is required when wide
flange-beams are used for dead-ending the conductors.
Wood poles, like those pictured above right, can be
used with success. But the expected life span of a
wood pole is generally no longer than 20 years, even
though some have been reported to last as much as 50
years. Some transit
Figure 9-23 Welded Steel Pole on Steel Pile
Foundation - Courtesy of LTK, Inc.
Figure 9-22 Timber Catenary Pole Courtesy of LTK, Inc.
Figure 9-24 Street Light Catenary Pole - Courtesy of LTK,
Inc.
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agencies use tapered tubular poles for aesthetic reasons (see below right), especially
where the poles need to be in sidewalks. Some transit agencies conceal OCS feeder
cables and/or balance weights inside tubular poles, again for aesthetics, but from a
maintenance point of view. The difficulty in access for repair and adjustment is a
drawback. For transit applications, poles are normally supplied with anchor-bolt type
base plates. However, for mainline applications, direct embedment of poles into
concrete foundations shows economy at the cost of reduced adjustability and flexibility
for replacement.
Pole Quantities
Pole count is the indicator of an efficient and economic OCS layout. The lower the
count of poles per unit length, the better for five reasons:
§
Lower cost
§
Fewer poles, cantilevers and headspans to inspect and maintain
§
Faster installation
§
Less visual intrusion
§
Less risk of being damaged or demolished
The lowest pole count is obtained by maximizing the spacing of OCS poles within the
constraints of the design criteria. Contributing factors to consider in maximizing the
length of OCS spans are:
•
Increased width of the pantograph head required in order to address issues
associated with wind blow-off
•
Chording of curves by the OCS
•
Track quality, especially cross-level
Variations in track cross level must be accounted for when calculating the sweep of the
contact wire across the pantograph head.
The location of poles is also a primary factor in pole count. Double-track transit lines
often have 14-foot track centers and a single pole placed between the two tracks is
ideal. However, center poles limit the redundancy of the OCS. If a center pole is
damaged due to an accident, both tracks of OCS will be affected. While OCS support
poles located to the field side or outside of each track require twice as many poles, a
level of redundancy is achieved by maintaining the independence of each track’s OCS
system.
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Cost Factors of OCS Styles
Style selection must clearly be based on both aesthetics and economics. The least
costly OCS style for two-track mainline is a constant tensioned simple catenary with
poles between tracks. Using this style as a basis, typical cost factors for other
configurations might be expected to be as follows:
Simple catenary with center poles – AT……………………….
Simple catenary with center poles – FT……………………….
Simple catenary with outside poles – AT……………………...
Simple catenary with outside poles – FT……………………...
Low profile simple catenary (only with outside poles) – FT….
Single contact wire - FT, with underground feeder cables…… up to
1.0
1.1
1.6
1.8
2.5
10.0
All theses styles use the same types of OCS components; only the configurations of
the support and termination assemblies, including cantilevers, cross-spans and head
spans, counter weight terminations and fixed-end terminations are different.
OCS Design Basics
Once the OCS style or styles have been selected (based on analysis of the power
demand of the trains), conductor tensions will be selected for the most economic
design of the complete installed system. The design of the OCS structures are
typically a function of two things, the loads being supported by the structures and
the clearances necessary to allow safe passage, which is based on the dynamic
envelope of the vehicle and the electrical clearance envelope.
The process of determining the loading conditions starts by examining the National
Electric Safety Code (NESC). The NESC outlines loading conditions ranging from
temperature ranges to wind loading. In addition, criteria are established for
conductor strength requirements and factors of safety. The NESC also specifies
minimum wire heights, minimum insulation levels and required clearances for
various voltages that must be maintained between the OCS and structures and
utilities. State and local agencies may supplement or supercede the NESC with more
stringent requirements, which therefore need to be examined.
An OCS requires its own structure to support the conductors over the track. These
supporting structures include:
•
Portals
•
Headspans
•
Bracket poles
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Cantilever poles
•
Cross-spans and pull-offs.
E L E C T R I F I C A T I O N
In addition, the OCS can be supported from the underside of bridges, tunnel soffits
or the underside of building roof trusses. In urban areas, cross-spans can be
connected to buildings using eye-bolts, thereby eliminating poles. OCS poles can be
jointly shared with streetlights, traffic signals and signs.
9.8 Electrification Interfaces with
Other Rail Elements
When an established railway is to be electrified, there can be significant engineering and
operational impacts on the existing infrastructure. The more significant impacts
involve:
§
Right-of-Way
§
Track Structure
§
Civil Structures
§
Signaling and Communications
9.8.1 Right-of-Way
Track Layout/Realignment
It is desirable that track alignment and modifications to track crossovers and turnouts
be completed before route electrification occurs. Additionally, track renewals and track
lowering measures, as described below, should have been finished. Future track
improvements may need to be accelerated to avoid the need for later changes. Old
redundant track should be removed before initiating electrification so that cranes are
not impeded by the presence of high voltage catenary wires, conductor rails or cables.
Substations
Typically, 25kV substations require a site area of about an acre in size, with road access
suitable for trucks delivering the largest piece of substation equipment. DC substations
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are smaller, ranging in size from 2000 to 5000 square feet, but are generally more
numerous than AC substations.
Supporting Structures for the Contact System
On existing main line routes, particularly those with more than two tracks, there will
probably not be enough room between tracks to install OCS pole foundations.
Therefore, the poles will be allocated to the outside of the line. The right-of-way needs
to be examined to insure that structures and any supporting back guys fall within the
ROW without impeding drainage. Since third rail is attached to the end of the ties,
ROW limits are not as critical for third rail systems as for overhead systems.
Systemwide Ductbanks
Ductbanks are required for power distribution cables and should be designed to
accommodate new signal or communication cables, should existing aerial signal and
communication cables need replacement. The location of parallel track and cross-track
ductbanks will need to be coordinated with drainage pipes, foundations for signals and
OCS poles, and emergency walkways.
9.8.2 Track Structure
On rail lines, the area extending from
track centerline in which no wayside items
can be placed is known as the structure
clearance envelope. On non-electrified
lines, this envelope is based on the
dynamic envelope of the vehicle along
with passing clearances. For electrified
lines, this envelope has to be increased to
allow for the electrical clearance envelope.
This increased envelope insures that no
wayside structures come close enough to
any “live” part of the vehicle to create an
electrical hazard. One way to provide the
requisite vertical clearance at overhead
bridges is to lower the tracks. However, if
significant lowering is required, the track
subbase may need to be excavated first,
which may be a prohibitive operating precondition.
Figure 9-23 Clearance Envelope - Courtesy of LTK, Inc.
Because the running rails will be carrying the high currents of the traction return
system, it is necessary that all bolted rail joints be paralleled with traction bonding
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cables to carry the 1000 amps or more of traction current. Defective bonds can give
rise to severe arcing between the rails and cause enough damage to curtail normal train
operations.
The possible effects of electrolytic corrosion due to leakage (stray) currents from the
track rails, especially with DC power must be addressed. With AC systems, the effects
of leakage currents is considered to be minimal, but still needs to be checked. Leakage
currents can cause and/or accelerate corrosion in underground piping, steel
reinforcement in concrete structures and may damage underground utilities.
Special precautions may also need to be taken to keep the track ballast free from dirt
and fines, which could reduce its natural insulation value. Therefore, steps must be
taken to keep the track rails insulated from ground to prevent leakage currents. Wood
ties placed on good clean ballast will effectively isolate the rails from earth. These
conditions must be maintained. Special rubber ‘boots’ may be provided where the rails
would otherwise be in direct contact with asphalt or similar road materials, such as at
grade crossings.
9.8.3 Civil Structures
Typically, electrical clearances may need to be provided in tunnels and at bridges. New
bridges (such as those resulting from grade-crossing elimination) will need to be built
with adequate electrical clearance. Station canopies that project over platform edges
may need modification. Provisions may need to be made to attach components to
walls. OCS pole anchor bolts will need to be incorporated into any new flyovers.
Tunnels to be Electrified
If complete tunnel replacement or day-lighting is too costly, the existing ballasted track
may be lowered or completely replaced by direct-fixation track. Sometimes sections of
tunnel soffit may require to be ‘chased’ to provide adequate clearance for pantographs.
To reduce the space required for the OCS, cutouts or ‘pockets’ in the soffit to house
the OCS supports may be easier to provide than a more costly program of general
clearance reconstruction. Third rail systems do not require the same degree of
additional space as does an overhead system. Because of this, the diameter of a tunnel
bore can be somewhat reduced. This reduction in tunneling requirements is one of the
significant reasons why third rail is used in many of the underground “metro” systems
around the world.
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Bridges Over Electrified Track
The electrification designer should consider the possibility that an overhead bridge may
be widened in the future. Provisional designs should be prepared for OCS designs to
take into account future widening.
Where practical, tracks can be lowered, but this may require that the track sub-base be
lowered too. Such track lowering can be involved and a difficult process that should
be approached with care.
Bridges Under Electrified Track
The increased speeds associated with electric trains, may require that bridges be
replaced to accommodate the new track ballast or direct fixation track. Longer bridges
and viaducts may need to be strengthened. OCS pole anchorages may need to be
provided.
Station Canopies
Existing
stations
that
have
overhanging roofs or canopies may
need to be “cut back” to allow
clearance for pantographs. Pictured
on the right is an example of a station
that has been designed for the use of
an overhead system.
Figure 9-24 Station Canopy - Courtesy of LTK, Inc.
OCS Attachments
Where OCS poles cannot be installed
because of lack of clearance,
attachments such as wall brackets will
need to be made to civil structures.
Pictured at right is an example of an
OCS cantilever attachment to an
overhead structure. Where existing
overhead bridges and the walls of
“boat sections” are available, the
design of OCS wall brackets will need
to be coordinated with the structural
Figure 9-25 OCS Cantilever Attachment to an Overhead
Structure - Courtesy of LTK, Inc.
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designers, including the provision of wire loading values.
9.8.4 Signals and Communications
As described in Chapter 7 - Communications and Signals, the running rails are used to
detect the presence of trains. The signal system can precisely locate each train because
the track route is first divided up by track sections, separating inbound from outbound,
sidings from branches, and then dividing each track up into block sections. Each
block section represents a track length between a few hundred feet and 1- 2 miles. The
presence of a train is detected through electrical track circuits of the train control
system, which are mimicked on a display board in the train control center(s).
In order for the traction power return circuits in the rails to be maintained without
interfering with the operation of the signaling system, the two systems need to be
immunized from each other. With early DC electric traction, signal circuits were
converted to AC. When AC traction power was introduced, signal circuits were
converted to a frequency immune to the traction frequency.
However, the use of insulated track
joints in the signal system meant that
the rails could not be used for the return
of the traction current to the substation.
Thus, the return path for the traction
current was limited to through the
ground and anything in the ground
(such as pipes and cable sheathes),
which is potentially dangerous and
totally inefficient.
The technical
solution to this predicament was the
Figure 9-26 Impedance Bond - Courtesy of LTK, Inc.
development and use of impedance
bonds that are located trackside and
connect to the track rails across the insulated joint. An impedance bond is an electrical
device that allows the traction current to pass through while effectively keeping the
signal system’s track circuits separated. Various kinds of impedance bonds exist,
depending upon whether the traction power is AC or DC, the traction voltage, and the
operation frequency of the track circuits. See Table 9-1 below.
Some systems were developed that allowed one rail to be used for traction current
return while the other rail was used for the signal system. This solution has a serious
disadvantage in that broken rails could not be detected reliably.
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Table 9-1 Electrification and Signal Interface
DC Track Circuits
AC Track Circuits
DC Traction
Cannot be used
Requires impedance
bonds
Coded Track/Cab
signals
Requires impedance
bonds
AC Traction
Cannot be used
Requires impedance
bonds as well as
frequency variations
Requires impedance
bonds as well as
frequency variations
9.9 Interfaces with Project-Wide Staff
The traction power system is only one component of a proposed new electrified
mainline railway or light rail system. Detailed design work will only start once a project
is authorized and funded. Planning, project definition, investment analyses and
environmental studies involving the staff of the transit agency, Federal, State and local
agencies must be accomplished.
Traction power distribution systems, including contact rails and overhead contact
systems, are only part of the engineering that goes into an electrified railway. A
number of other railway engineering disciplines (agency technical staff and architects)
will require inclusion in the development of the traction power system design and
installation.
The following list contains interfaces that are typical of light rail systems that share
streets with motor traffic and penetrate neighborhoods. Mainline railways operate on
segregated rights-of-way, which obviously avoid many of the interface issues of light
rail.
Staff interfaces include:
§
City engineers, planners, agencies and architects
§
Agency operations and maintenance staff
§
Civil consultants
§
Station designers
§
Design team for the maintenance facility
§
Track alignment specialists and track designers
§
Signals and communications staff
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§
Corrosion control staff
§
Highway engineers
§
Street lighting consultants
§
Geotechnical engineers
§
Consultants handling underground utilities
§
Vehicle designers
Common issues for discussion with Agency staff and others, include the following:
§
City engineers, planners, agencies and architects
§
Fire safety requirements downtown and in tunnels, including emergency deenergization and safe condition detection
§
Tapered tubular poles on sidewalks
§
Historic trolley
§
Aesthetic selection of OCS styles
§
Ornamental poles
§
Paint colors
Agency operations and maintenance staff:
§
Tapered tubular poles on station platforms
§
Sectionalizing requirements
§
Maintenance tools and equipment
§
Maintenance vehicles
Civil and structural consultants:
§
Overgrade bridges, undergrade bridges, tunnels, “boat sections,” retaining walls
§
Movable bridges
§
Headroom for OCS
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OCS loadings on structures
Station designers/architects:
§
Platforms and canopies
§
Safety screening of canopy from OCS
§
Grounding and bounding
Design team for the maintenance facility:
§
Yard lighting
§
Shop OCS
§
Storage
Track alignment specialists:
§
Track layout, including turnouts, crossovers and curves
§
Track clearances
§
Space for OCS poles between tracks
§
Maintenance yard layout
Track designers:
§
Special trackwork
§
Traction rail bonding
Communication and signal staff:
§
Aerial cables on OCS poles
§
SCADA and CCTV
§
Impedance bonds
§
Bonding and grounding
Corrosion control consultants:
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Atmospheric pollution
§
Electrolytic corrosion
§
Preventive measures
E L E C T R I F I C A T I O N
Electric power utilities:
§
Primary power supplies, capacity and location
§
Redundant feeds
§
Harmonics
Highway engineers:
§
Traffic signals, height/reach of bracket arms, joint use
§
Maintenance clearance from OCS for signal maintainers
Street lighting consultants:
§
Joint-use of poles
Geotechnical engineers:
§
Soil conditions
Consultants handling underground utilities:
§
Quality of records
§
Utility location
Vehicle designers:
§
Pantograph criteria
§
Vehicle envelope
§
Electrical interface with vehicle electric equipment
Safety specialists:
§
Safe working zones for maintenance
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§
Safe working zones under bridges and in tunnels
§
Double insulation
§
Screening of OCS at over bridges
§
Screening along walls of boat-sections
§
Warning signs
§
Screening of ladders for signal posts adjacent to OCS
Bibliography
1. Journal of the International Institute of Rail Electrification Engineers, Volume 2 Issue
1, August 2001.
2. Guinness Book of Rail Facts and Feats, John Marshall, 1975.
3. “When the Steam Railroads Electrified,”
W.D. Middleton, Kalmabach Press
(1974).
4. “Electrifying the Caltrain/PCS Railroad,” Caltrain, 1992.
5. “Rail 1950,” Jack Simmonds, Metheun, 1975.
6. Design of the 50 kV Overhead Contact System for the British Columbia Railway Tumbler
Ridge Branch Line, L.C. Tait and B. Anderson, IEEE, 1984.
7. “The Electric Railway,” Fred H. Whipple, 1889.
8. “Railways: Mechanical Engineering,” J.B. Snell, 1971.
9. “The Illustrated Directory of Trains of the World,” Brian Hollingsworth, 2000.
10. “American Locomotives in Historic Photographs 1858 – 1949,” Ron Ziel, 1993.
11. “The Story of the Train,” National Railway Museum, UK, 1999.
12. “Croydons Transport Through the Ages,” UK, John Gent, 2001.
13. “Electricity,” Steve Parker, 1992.
14. “Electricity in Transport,” English Electrical, 1951.
15. “Ultimate Train,” Peter Herring, 2000.
16. “Pictorial History of America’s Railroads,” Mike Del Vecchio, 1998.
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