War of
Currents
In the War
of Currents era (sometimes, War of the Currents or Battle of Currents) in the
late 1880s, George Westinghouse and Thomas Edison became adversaries due to
Edison's promotion of direct current (DC) for electric power distribution over
alternating current (AC).
Edison's
direct-current system generated and distributed electric power at the same
voltage as used by the customer's lamps and motors. This meant that the current
in transmission was relatively large, and so heavy conductors were required and
transmission distances were limited, to about a mile (kilometre); otherwise
transmission losses would make the system uneconomical. At the time, no method
was practical for changing voltages of DC power. The invention of an efficient
transformer allowed high voltage to be used for AC transmission. An AC
generating plant could then serve customers at a great distance (tens to
hundreds of miles), or could serve more customers within its economical
transmission distance. The fewer much larger plants needed for AC would achieve
an economy of scale that would lower costs further. The invention of a
practical AC motor increased the usefulness of alternating current for powering
machinery.
Edison's
company had invested heavily in DC technology and was vigorously defending its
DC based patents. George Westinghouse saw AC as a way to get into the business
with his own patented competing system and set up the Westinghouse Electric
Company to design and build it. The Westinghouse company also purchased the
patents for alternating current devices from inventors in Europe and licensed
patents from Nikola Tesla. In spite of a protracted anti-AC campaign waged by
the Edison company, the economics of the alternating current system prevailed. Alternating
current was selected in 1893 for transmission of power from Niagara Falls to
Buffalo, New York - the technical and economic success of this project led the
way for the adoption of alternating current as the preferred electrical system.
The
"War of Currents" is often personified as Westinghouse vs.
Edison.[citation needed] However, the "War of Currents" was much
larger than that: It involved both American and European companies whose heavy
investments in one current type or the other led them to hope that use of the
other type would decline, such that their share of the market for
"their" current type would represent greater absolute revenue once
the decline of the other current type enabled them to expand their existing
distribution networks.[citation needed]
Direct
current remained in commercial power distribution use for about a century after
the "war of the currents", confined to high density urban areas,
where, for example, passenger
and freight
elevators ran on direct current motors. Direct current found a new application
in high voltage direct current transmission used to connect power plants to
distant customer load. Direct current remained useful in certain traction
systems, within vehicles, and in battery-operated systems. Often direct current
loads were powered from the alternating current public grid with a rectifier.
Direct current is also seeing new application in computer data centers, where
DC distribution within a building can provide useful energy savings.
Electric power transmission
DC
Figure 1 Schematic of Edision's three wire DC electrical
power distribution system
During the
initial years of electricity distribution, Edison's direct current was the
standard for the United States, and Edison did not want to lose all his patent
royalties. Direct current worked well with incandescent lamps, which were the
principal load of the day, and with motors. Direct-current systems could be
directly used with storage batteries, providing valuable load-leveling and
backup power during interruptions of generator operation. Direct-current
generators could be easily paralleled, allowing economical operation by using
smaller machines during periods of light load and improving reliability. At the
introduction of Edison's system, no practical AC motor was available. Edison
had invented a meter to allow customers to be billed for energy proportional to
consumption, but this meter worked only with direct current. The DC
distribution system consisted of generating plants feeding heavy distribution
conductors, with customer loads (lighting and motors) tapped off them. The
system operated at the same voltage level throughout; for example, 100 volt
lamps at the customer's location would be connected to a generator supplying
110 volts, to allow for some voltage drop in the wires between the generator
and load. The voltage level was chosen for convenience in lamp manufacture;
high-resistance carbon filament lamps could be constructed to withstand 100
volts, and to provide lighting performance economically competitive with gas
lighting. At the time it was felt that 100 volts was not likely to present a
severe hazard of fatal electric shock.
To save on
the cost of copper conductors, a three-wire distribution system was used. The
three wires were at +110 volts, 0 volts and −110 volts relative potential.
100-volt lamps could be operated between either the +110 or −110 volt legs of
the system and the 0-volt "neutral" conductor, which carried only the
unbalanced current between the + and − sources. The resulting three-wire system
used less copper wire for a given quantity of electric power transmitted, while
still maintaining (relatively) low voltages. However, even with this
innovation, the voltage drop due to the resistance of the system conductors was
so high that generating plants had to be located within 1.6 km or so of the
load. Higher voltages could not so easily be used with the DC system because
there was no efficient low-cost technology that would allow reduction of a high
transmission voltage to a low utilization voltage.
Since direct
current could not easily be converted to higher or lower voltages, separate
electrical lines had to be installed to supply power to appliances that used
different voltages, for example, lighting and electric motors. This required
more wires to lay and maintain, wasting money and introducing unnecessary
hazards. These hazards, for example, proved fatal to a number of people in the
Great Blizzard of 1888, with their deaths being attributed to collapsing overhead
power lines in New York City.
Edison
considered the need for many local power plants in the direct current system
more democratic. Each locale could build electrical plants to suit its need and
would not have to rely on a large monopoly to supply electricity. The
proponents of AC counter-argued that building a local plant would be too costly
for rural areas, leaving them with no electrical supply at all.
A bipolar
open-core power transformer developed by Lucien Gaulard and John Dixon Gibbs
was demonstrated in London in 1881, and attracted the interest of Westinghouse.
They also exhibited the invention in Turin in 1884. However these early
induction coils with open magnetic circuits are inefficient at transferring
power to loads. Until about 1880, the paradigm for AC power transmission from a
high voltage supply to a low voltage load was a series circuit. Open-core
transformers with a ratio near 1:1 were connected with their primaries in
series to allow use of a high voltage for transmission while presenting a low
voltage to the lamps. The inherent flaw in this method was that turning off a
single lamp (or other electric device) affected the voltage supplied to all
others on the same circuit. Many adjustable transformer designs were introduced
to compensate for this problematic characteristic of the series circuit,
including those employing methods of adjusting the core or bypassing the
magnetic flux around part of a coil.
The direct
current systems did not have these drawbacks, giving it significant advantages
over early AC systems.
AC
Figure 2 Westinghouse Early AC System 1887 (U.S.
Patent 373,035)
In the
alternating current distribution system power could be transmitted more
efficiently over long distances at high voltages, around ten times that of the
loads, using lower current. For a given quantity of power transmitted via DC or
AC, the wire cross-sectional area is inversely proportional to the voltage
used. Alternatively, the allowable length of a circuit, given a wire size and
allowable voltage drop, increases approximately with the square of the
distribution voltage. With AC current, a transformer is used to down step the
(relatively) high voltage to low voltages for use in homes and factories. This
had—and still has—the practical significance that fewer, larger generating
plants can serve the load in a given area. Large loads, such as industrial
motors or converters for electric railway power, can be served by the same
distribution network that fed lighting, by using a transformer with a suitable
secondary voltage.
Transmission loss
Figure 3 Tesla's US390721 Patent for a "Dynamo
Electric Machine"
The
advantage of AC for distributing power over a distance is due to the ease of
changing voltages using a transformer. Available electric power is the product
of current × voltage at the load. For a given amount of power, a low voltage
requires a higher current and a higher voltage requires a lower current. Since
metal conducting wires have an almost fixed electrical resistance, some power
will be wasted as heat in the wires. This power loss is given by Joule's first
law and is proportional to the square of the current. Thus, if the overall
transmitted power is the same, and given the constraints of practical conductor
sizes, high-current, low-voltage transmissions will suffer a much greater power
loss than low-current, high-voltage ones. This holds whether DC or AC is used.
Converting
DC power from one voltage to another required a large spinning rotary converter
or motor-generator set, which was difficult, expensive, inefficient, and
required maintenance, whereas with AC the voltage can be changed with simple
and efficient transformers that have no moving parts and require very little
maintenance. This was the key to the success of the AC system. Modern
transmission grids regularly use AC voltages up to 765,000 volts.[6] Power
electronic devices such as the mercury-arc valve and thyristor made
high-voltage direct current transmission practical by improving the reliability
and efficiency of conversion between alternating and direct current, but such
technology only became possible on an industrial scale starting in the 1960s.
The Ganz AC system
Figure 4 The prototype transformer is
on display at the Széchenyi István Memorial Exhibition, Nagycenk, Hungary
In the
autumn of 1884, Károly Zipernowsky, Ottó Bláthy and Miksa Déri (ZBD), three
engineers associated with the Ganz factory, had determined that open-core
devices were impracticable, as they were incapable of reliably regulating
voltage. In their joint 1885 patent applications for novel transformers (later
called ZBD transformers), they described two designs with closed magnetic
circuits where copper windings were either a) wound around iron wire ring core
or b) surrounded by iron wire core. The two designs were the first application
of the two basic transformer constructions in common use to this day, which can
as a class all be termed as either core form or shell form (or alternatively,
core type or shell type), as in a) or b), respectively (see images). The Ganz
factory had also in the autumn of 1884 made delivery of the world's first five
high-efficiency AC transformers, the first of these units having been shipped on
September 16, 1884. This first unit had been manufactured to the following
specifications: 1,400 W, 40 Hz, 120:72 V, 11.6:19.4 A , ratio 1.67:1,
one-phase, shell form. In both designs, the magnetic flux linking the primary
and secondary windings traveled almost entirely within the confines of the iron
core, with no intentional path through air (see Toroidal cores below). The new
transformers were 3.4 times more efficient than the open-core bipolar devices
of Gaulard and Gibbs.
The ZBD
patents included two other major interrelated innovations: one concerning the
use of parallel connected, instead of series connected, utilization loads, the
other concerning the ability to have high turns ratio transformers such that
the supply network voltage could be much higher (initially 1,400 to 2,000 V)
than the voltage of utilization loads (100 V initially preferred). When
employed in parallel connected electric distribution systems, closed-core
transformers finally made it technically and economically feasible to provide
electric power for lighting in homes, businesses and public spaces. Bláthy had
suggested the use of closed cores, Zipernowsky had suggested the use of
parallel shunt connections, and Déri had performed the experiments; The other
essential milestone was the introduction of 'voltage source, voltage intensive'
(VSVI) systems' by the invention of constant voltage generators in 1885. Ottó
Bláthy also invented the AC electricity meter to complete the competition of AC
and DC technology. Transformers today are designed on the principles discovered
by the three engineers. They also popularized the word 'transformer' to
describe a device for altering the emf of an electric current, although the
term had already been in use by 1882. In 1886, the ZBD engineers designed, and
the Ganz factory supplied electrical equipment for, the world's first power
station that used AC generators to power a parallel connected common electrical
network, the steam-powered Rome-Cerchi power plant. The reliability of the AC
technology received impetus after the Ganz Works electrified a large European
metropolis: Rome in 1886.
In North
America one of the believers in the new technology was George Westinghouse.
Westinghouse was willing to invest in the technology and hired William Stanley,
Jr. to work on an AC distribution system using step up and step down
transformers of a new design in March 1886 at Great Barrington. Stanley's
alternating current transformer, central-station system for public service was
the "very first in America beyond all dispute." Westinghouse tested
it more during summer 1886 in Pittsburgh, over a distance of 3 miles. This
system used an alternator designed by Stanley to replace the Siemens model,
which regulated voltage poorly. Satisfied with the pilot system, Westinghouse
began commercial production and shipped his company's first commercial plant to
Buffalo NY, where a local utility placed it in service. Orders for 25
alternating-current plants followed within months.With Stanley leaving Westinghouse,
Oliver Shallenberger took control of the AC project. In July 1888, George
Westinghouse licensed Nikola Tesla's US patents for a polyphase AC induction
motor and transformer designs and hired Tesla for one year to be a consultant
at the Westinghouse Electric & Manufacturing Company's Pittsburgh labs.
Westinghouse purchased a US patent option on induction motors from Galileo
Ferraris in an attempt to own a patent that would supersede Tesla's. But with
Tesla's backers getting offers from another capitalist to license Tesla's US
patents, Westinghouse concluded that he had to pay the rather substantial
amount of money being asked to secure the Tesla license. Westinghouse also
acquired other patents for AC transformers from Lucien Gaulard and John Dixon
Gibbs.
Commercial rivalry
Edison's
publicity campaign
Edison
carried out a campaign to discourage the use of alternating current, including
spreading disinformation on fatal AC accidents, publicly electrocuting animals,
and lobbying against the use of AC in state legislatures. Edison directed his
technicians, primarily Arthur Kennelly and Harold P. Brown, to preside over
several AC-driven killings of animals, primarily stray cats and dogs but also unwanted
cattle and horses. Acting on these directives, they were to demonstrate to the
press that alternating current was more dangerous than Edison's system of
direct current. He also tried to popularize the term for being electrocuted as
being "Westinghoused". Years after DC had lost the "war of the
currents," in 1903, his film crew made a movie of the electrocution with
high voltage AC, supervised by Edison employees, of Topsy, a Coney Island
circus elephant which had recently killed three men.
Edison
opposed capital punishment, but his desire to disparage the use of alternating
current led to the invention of the electric chair. Harold P. Brown, who was
being secretly paid by Edison, built the first electric chair for the state of
New York to promote the idea that alternating current was deadlier than DC.
When the
chair was first used, on August 6, 1890, the technicians on hand misjudged the
voltage needed to kill the condemned prisoner, William Kemmler. The first jolt
of electricity was not enough to kill Kemmler, and only left him badly injured.
The procedure had to be repeated and a reporter on hand described it as
"an awful spectacle, far worse than hanging." George Westinghouse
commented: "They would have done better using an axe."
Willamette Falls to Niagara Falls
In 1889, the
first long distance transmission of DC electricity in the United States was
switched on at Willamette Falls Station, in Oregon City, Oregon. In 1890 a
flood destroyed the Willamette Falls DC power station. This unfortunate event
paved the way for the first long distance transmission of AC electricity in the
world when Willamette Falls Electric company installed experimental AC
generators from Westinghouse in 1890. That same year, the Niagara Falls Power
Company (NFPC) and its subsidiary Cataract Company formed the International
Niagara Commission composed of experts, to analyze proposals to harness Niagara
Falls to generate electricity. The commission was led by Sir William Thomson
(later Lord Kelvin) and included Eleuthère Mascart from France, William Unwin
from England, Coleman Sellers from the US, and Théodore Turrettini from
Switzerland. It was backed by entrepreneurs such as J. P. Morgan, Lord
Rothschild, and John Jacob Astor IV. Among 19 proposals, they even briefly
considered compressed air as a power transmission medium, but preferred
electricity. But they could not decide which method would be best overall.
International Electro-Technical
Exhibition
The
International Electro-Technical Exhibition of 1891 featured the long distance
transmission of high-power, three-phase electric current. It was held between
16 May and 19 October on the disused site of the three former “Westbahnhöfe”
(Western Railway Stations) in Frankfurt am Main. The exhibition featured the
first long distance transmission of high-power, three-phase electric current,
which was generated 175 km away at Lauffen am Neckar. It successfully operated
motors and lights at the fair.
When the
exhibition closed, the power station at Lauffen continued in operation,
providing electricity for the administrative capital, Heilbronn, making it the
first place to be equipped with three-phase AC power.
Many corporate
technical representatives (including E.W. Rice of Thomson-Houston Electric
Company, what became General Electric) attended. The technical advisors and
representatives were impressed.
AC deployment at Niagara
In 1893,
NFPC was finally convinced by George Forbes to award the contract to
Westinghouse, and to reject General Electric and Edison's proposal. Work began
in 1893 on the Niagara Falls generation project: power was to be generated and
transmitted as alternating current, at a frequency of 25 Hz to minimize
impedance losses in transmission (changed to 60 Hz in the 1950s).
Some doubted
that the system would generate enough electricity to power industry in Buffalo.
Tesla was sure it would work, saying that Niagara Falls could power the entire
eastern United States. When finished, the first Niagara River hydraulic tunnel
would have a capacity to develop 75 MW. None of the previous polyphase
alternating current transmission demonstration projects were on that scale of
power:
·
The Lauffen-Neckar demonstration in 1891 had the capacity of 225
kW
·
Westinghouse successfully used AC in the commercial Ames
Hydroelectric Generating Plant in 1891 at 75 kW (Single phase)
·
The Chicago World's Fair in 1893 exhibited a complete 11,000 kW
polyphase generation and distribution system with multiple generators,
installed by Westinghouse
·
Almirian Decker designed a three-phase 250 kW AC system at Mill
Creek California in 1893.
On November
16, 1896, electrical power was transmitted to industries in Buffalo from the
hydroelectric generators at the Edward Dean Adams Station at Niagara Falls. The
generators were built by Westinghouse Electric Corporation using Tesla's AC
system patent. The nameplates on the generators bore Tesla's name. To appease
the interests of General Electric, they were awarded the contract to construct
the transmission lines to Buffalo using the Tesla patents
Competition outcome
As a result
of the successful field trial in the International Electro-Technical Exhibition
of 1891, three-phase current, as far as Germany was concerned, became the most
economical means of transmitting electrical energy.
In 1892,
General Electric formed and immediately invested heavily in AC power (at this
time Thomas Edison's opinions on company direction were muted by President Coffin
and the GE board of directors). Westinghouse was already ahead in AC, but it
only took a few years for General Electric to catch up, mainly thanks to
Charles Proteus Steinmetz, a Prussian mathematician who was the first person to
fully understand AC power from a solid mathematical standpoint. General
Electric hired many talented new engineers to improve its design of
transformers, generators, motors and other apparatus.
In Europe,
Siemens & Halske became the dominant force. Three phase 60 Hz at 120 volts
became the dominant system in North America while 220-240 volts at 50 Hz became
the standard in Europe.
Alternating
current power transmission networks today provide redundant paths and lines for
power routing from any power plant to any load center, based on the economics
of the transmission path, the cost of power, and the importance of keeping a
particular load center powered at all times. Generators (such as hydroelectric
sites) can be located far from the loads.
Remnant DC distribution systems
1947
advertisement for the Dremel Moto-Tool. Note the "AC-DC" designation.
Some cities
continued to use DC well into the 20th century. In central Helsinki, there was
a DC network in existence up until the late 1940s, and in the 1960s,
Stockholm's dwindling DC network was eliminated. A mercury-arc valve rectifier
station could convert AC to DC where networks were still used. In 1942, the
Greenwich Village neighborhood in New York City used DC. Parts of Boston,
Massachusetts along Beacon Street and Commonwealth Avenue still used 110 volts
DC in the 1960s, causing the destruction of many small appliances (typically
hair dryers and phonographs) used by Boston University students, who ignored
warnings about the electricity supply. New York City's electric utility
company, Consolidated Edison, continued to supply direct current to customers
who had adopted it early in the twentieth century, mainly for elevators. The
New Yorker Hotel, constructed in 1929, had a large direct-current power plant
and did not convert fully to alternating-current service until well into the
1960s. This was the building in which AC pioneer Nikola Tesla spent his last
years, and where he died in 1943. In January 1998, Consolidated Edison started
to eliminate DC service. At that time there were 4,600 DC customers. By 2006,
there were only 60 customers using DC service, and on November 14, 2007, the
last direct-current distribution by Con Edison was shut down.[49][53] Customers
still using DC were provided with on-site AC-to-DC rectifiers. The city of San
Francisco, California featured a DC power grid to supply power for pre-1940s
winding-drum elevators. Around the end of 2010, the DC grid was divided into
171 separate islands with each island supplying 7 to 10 customers.
The Central
Electricity Generating Board in the UK continued to maintain a 200 volt DC
generating station at Bankside Power Station on the River Thames in London as
late as 1981. It exclusively powered DC printing machinery in Fleet Street,
then the heart of the UK's newspaper industry. It was decommissioned later in
1981 when the newspaper industry moved into the developing docklands area
farther down the river (using modern AC-powered equipment). The building was
converted into an art gallery, the Tate Modern.
Electric
railways that use a third-rail system generally employ DC power between 500 and
750 volts; railways with overhead catenary lines use a number of power schemes
including both high-voltage AC and high-current DC.
Long distance DC power transmission
High-voltage
direct current (HVDC) systems are used for bulk transmission of energy from
distant generating stations, or for interconnection of separate alternating
current systems. These HVDC systems use electronic devices like mercury-arc
valves, thyristors, or IGBTs that were unavailable during the War of Currents
era. Power is converted to and from alternating current at each side of the
HVDC link. An HVDC system can transmit more power over a given right-of-way
than an AC system, which is an advantage in overall cost. HVDC systems allow
better control of power flows in transient and emergency conditions, which help
prevent blackouts. HVDC is an alternative to AC systems for long-distance,
high-load transmission.
DC uses
DC power is
still common when distances are small, and especially when energy storage or
conversion uses batteries or fuel cells. Some of these applications include
electronic devices, Vehicle starting, lighting, and ignition systems.
"Off-grid" isolated power installations using wind or solar power may
use DC between sources and loads, over limited distances. One concept for use
in a computer data center would power individual processing units from a DC
system distributed around a computer room or building. This would eliminate
individual system rectifiers. The distributed DC system would eliminate some
energy losses associated with individual computer power supply rectifiers. By
feeding the system from batteries, the cost and unreliability of individual
uninterruptible power supplies (UPS) would be reduced. The 380 volt level is
compatible with the typical ratings of components now used in computer power
supplies.[55] Similarly, copper telephone lines can be used to transmit network
line power at 380 volts DC for long distances, which then can be converted to
lower voltages to power outdoor electronic equipment. Power and safety
standards for network line power (NLP) technology are defined in the National
Electric Code (NEC) Article 830.
