Change to direct current?
The world runs on alternating voltage. Its ability to be transformed was a
crucial advantage at the outset of electrification. Yet we could be facing a
change in paradigms given the advances being made in power electronics and
the changes occuring in the energy landscape. ABB is helping to bring about
the rise of direct current supply systems – at all voltage levels.
AC versus DC. Or Westinghouse versus Edison. The late 19th century duel that decided
which was the more suitable technology for electric power supply systems was electrifying in
the truest sense of the word. There were already 121 DC power stations in operation in the
eastern United States by 1887 by order of the famous inventor Thomas Alva Edison. His
rival Westinghouse ended up prevailing with alternating current, the type of current he
advocated. One reason for his success was an experiment conducted in 1891 that caused a
big stir. Charles Brown, one of the founding fathers of ABB, played a major role in its
success (see the box “War of Currents”).
Alternating current is actually not so well suited for power transmission, at least not over
really long distances. The longer the power line, the greater the capacitive and inductive
resistance and thus the reactive power; the latter also requires compensation. Direct current,
by contrast, entails no problems with reactive power. Unavoidable ohm resistance is the only
issue that really comes into play with DC – and with AC. The losses and the capacity of
transmission lines depend on various factors. Direct current can generally be expected to
involve 25 percent lower losses and two to five times greater transmission capacity for the
same voltage.
Transformation with power semiconductors
In addition, alternating current is accompanied by what is known as the skin effect. Charges
are transported almost exclusively on the surface of the conductor whereas with direct
current, they flow across the entire cross section of the conductor. That is why more current
can be transported for a given wire diameter with direct current and why AC cable
conductors have to be produced with a special substructure not required to the same extent
for DC cables.
With power semiconductors like the ones produced at ABB Semiconductors in Lenzburg,
each of the two types of current can be converted into the other with great efficiency at
converter stations. The efficiency of an individual converter station can be as high as 99
percent. The obvious conclusion from all these facts is that electric energy should be
transmitted over long distances as direct current. Given the costs of converter stations, DC
transmission is not economical until after a certain minimum length of overhead lines is
reached, on the order of 500 kilometers or so.
ABB is considered a pioneer and technology leader in high-voltage, direct-current
transmission (referred to as HVDC transmission). It is also the top supplier with about 70
systems installed worldwide. Back in 1939, the company (then called BBC) installed an
experimental HVDC line between Wettingen and Zurich with a voltage of 50 kilovolts and a
transmission capacity of about 500 kilowatts. Today, ABB can provide systems capable of
transmitting up to 7.2 gigawatts over a distance of 2000 kilometers at a voltage of 800
kilovolts. These are the key data for the HVDC line between Xiangjiaba and Shanghai in
China, which went into operation in July 2010. By comparison, a modern power station such
as Leibstadt Nuclear Power Plant has a capacity of 1.2 gigawatts.
Capacitive resistance for alternating current is substantially greater in power transmission
cables laid underground or in water than in overhead lines. This is because of the greater
capacitor effect for cables from the grounded outside conductor that follows the insulation
layer. That is why HVDC transmission is worthwhile in cable transmission for distances as
short as 100 kilometers. HVDC is therefore practically imperative for long undersea cables.
DC connection for wind farms
For this purpose among others, ABB developed HVDC Light in the 1990s and fitted it with a
somewhat different, more compact cable and power semiconductor technology geared to a
power range of 50 to 1100 megawatts. This product can be used to connect wind farms
located far off shore to the power grid. For example, BARD Offshore 1, a 400 megawatt
installation situated 125 kilometers off the German coast in the North Sea, is connected to
the grid via an HVDC Light from ABB. The cable insulation technology faces new challenges
because the electrical behavior of insulation materials for DC is completely different than for
AC.
The rapid increase in the use of renewable energies is a key driver in the renaissance of
direct current. Solar cells always produce direct current. It must be converted with inverters
into alternating current before it can be fed into the supply grid. The ABB product line of solar
inverters ranges from small, single-phase devices for residential buildings to large central
inverters for huge solar parks.
“Power converters for converting each of the two types of current into the other type
represent an important line of business at ABB,” Remo Lütolf, Head of Global Business Unit
Power Electronics & MV Drives, noted. “Further advances in power electronics are
perfecting the efficiency of these conversions to an even greater extent.” But in many areas,
a fundamental question inevitably arises: “Why convert? Couldn’t we develop simpler, more
elegant and efficient systems that make do with fewer converters?” Lütolf echoed this
question rhetorically.
DC electric circuit on ships
Isolated solutions not closely linked to the existing AC grid are particularly suitable for this
purpose; for instance, in ships with diesel electric drive. Until now they were powered mostly
with alternating current but the drives for the propeller or the bow thruster are based
internally on direct current supply. A DC on-board system developed by ABB distributes the
energy via DC circuits, eliminating the need for transformers in particular and also for
switches in some cases. Space requirements and the weight of electrical components are
decreased by up to 30 percent.
New prospects even open up in the low-voltage range. As mentioned before, solar cells
generate direct current and are installed in increasing numbers on roofs. And much of the
household equipment consuming the power uses this type of current: TVs, consumer
electronics, computers, chargers for cell phones, etc. All convert alternating current into
direct current with their own power supply unit.
Even increasingly common energy-conserving LED lights use direct current. These lightemitting diodes are semiconductors and function inversely to solar cells. Most photovoltaic
cells are made of silicon and transform light into direct current whereas LEDs are gallium
based and emit light when direct current flows through them. The innovative DC power
distribution system from ABB for data centers is also based on the idea of a direct DC
supply. Servers are generally run on twelve or five volt DC, just as other computers are.
Each of them has its own power unit to convert the usual 230 volts of alternating current.
Losses occur in the form of heat and must be offset with elaborate cooling. In this case, it
makes sense to do conversion centrally and supply the servers directly via a separate DC
system. This direct current technology uses ten to twenty percent less energy for power
distribution than a conventional AC supply. It also requires less space, thus reducing the
costs of equipment, installation, maintenance and real estate. ABB is currently installing this
potentially revolutionary DC power distribution solution with an output of one megawatt in an
expansion of the existing Green.ch data center in Lupfig in the Swiss canton of Aargau.
Rapid charger with direct current
Electric cars are initiating further movement in DC applications, too. Like all batteries, the
lithium-ion rechargeable batteries in these cars run on direct current. The cars are equipped
with rectifiers so they can be charged with AC from regular household sockets. The batteries
are regenerated much faster directly with high-voltage DC. For this purpose, ABB now has a
DC rapid charging station in its range capable of charging the batter to 80 percent of its
capacity within 15 minutes. Estonia recently ordered 200 of these charging stations.
Remo Lütolf: “I am convinced that these DC applications and transmission solutions have
tremendous growth potential. The advantages are obvious and the trend is driven as much
by producers as by consumers. “Direct current offers substantial ways of increasing
efficiency. For instance, look at the drives we manufacture here in Turgi. You need direct
current to generate variable-frequency output voltage so each drive has a rectifier integrated
into it. The need for this unit could be eliminated with a direct DC supply.”
The renaissance of DC technology is unmistakable, from current transmission, data centers
and batteries to photovoltaic and LEDs. It is one of the eleven priorities ABB has recognized
as an area with substantial growth potential in its new strategy.
For further information contact:
ABB Switzerland
Lukas Inderfurth
Melanie Nyfeler
Press Office
5400 Baden
+ 41 58 585 00 00
www.abb.ch/dcpower
Two ABB specialists from the Local Business Unit Low Voltage Systems are working at the
DC supply of the new green.ch data center in Lupfig.
The DC rapid charging station of ABB.
(Auxiliary article)
Why high voltage?
The higher the amperage, the bigger the transmission losses in conductors. This follows
from Joule’s first law, which says the heat generated by a constant current flowing through a
conductor with a certain resistance in ohms is equal to the resistance times the square of the
constant current (measured in amperes). As the strength of the current decreases, the
losses from the heating of the conductor therefore decline rapidly. For use of electrical
energy, the crucial factor is the power transmitted not the level of current. This power is the
product of voltage (measured in volts) and current. The same power can therefore be
generated with increased voltage and lower current and thus with lower losses.
The War of Currents
In 1880 Thomas Alva Edison finally overcame a problem that had long been unresolved and
developed the first light bulb incorporating a high-resistance carbon filament. It was able to
compete with the prevailing gas lights in terms of durability and luminous efficiency. The
long-lasting, odorless light from the light bulb found quick acceptance, just as Edison had
anticipated. He did not want to earn his money primarily from the sale of light bulbs,
however. He wanted to earn it by providing infrastructure – the grid, including power plants,
fuses and switches.
The newly developed light bulbs could be lit up with both direct current and alternating
current. Edison opted for direct current. His intention was to use his grid to power electric
engines, which ran only on direct current at the time. Even transformers had not yet been
invented. Edison set the DC voltage to be generated and transmitted at 110 volts, about the
maximum level at which incandescent lamps could still be operated safely. Transmission
losses were substantial at this low voltage level, however. For this reason, the power plants
had to be put close to the consumers, no further than two or three kilometers. They were
equipped with steam engine generators incorporating direct voltage technology with an
output of about 100 kW each. This sufficed to power a good 1000 incandescent lamps. This
power plant could be controlled easily by merely switching the generators on and off.
Another well-known inventor, George Westinghouse, also realized the enormous business
potential of power generation and transmission. Westinghouse recognized that the limited
range of the low-voltage direct current that Edison favored was the Achilles' heel of his
competitor’s approach. It was in London that he found the decisive key to an alternative
technology. The first technically usable transformer was unveiled there in 1883.
This invention allowed Westinghouse to link an alternating current generator to a water mill
in 1886, to step up the voltage of the current and thus conduct it to the next city a few
kilometers away with relatively low losses. This was the first time electricity was used a
substantial distance away from the power plant. In 1888 Nikola Tesla presented the first AC
engine, eliminating yet a further sound argument for building up a direct current grid. This
dispute that later came to be known as the “War of Currents” was finally decided in Germany
in 1891. Charles Brown, a founding father of ABB, had a central role to play: He supplied
generators and transformers for the high-voltage alternating current transmission system
between Lauffen am Neckar and Frankfurt am Main.
This transmission line through the Forest of Odes was a fantastic 176 kilometers long. The
alternating current produced at 55 volts by the generator was stepped up to 15 000 volts.
The loss during transmission was just 25 percent. For low-voltage direct current, the losses
across this same distance would have amounted to about 98 percent. This accomplishment
settled the dispute about systems once and for all. Edison had launched an invidious
campaign to show the public the danger of high-voltage alternating current but even that did
not help any more. At his initiative, the electric chair designed for alternating current was
developed as an alternative means of execution to hanging. Edison even suggested coining
a new verb for this type of execution, namely “to westinghouse someone”. But he did not
prevail on this anymore than he had with direct current distribution.