Download An Insight into the Evolution of Direct Current Systems

An Insight into the Evolution of Direct Current
Systems
Girts Stana, Peteris Apse-Apsitis
Riga Technical University (Latvia)
[email protected], [email protected]
Abstract— This paper summarizes the sequential events
related to the development of electric supply industry. Direct
Current has had its significance from the early applications of
electricity until nowadays. However, historically Direct Current
technologies had fallen into disuse as Alternating Current
technologies took the advantage upon them. This article aims to
emphasize the advantages and latest applications of Direct
Current technologies by underlining the benefits of Low Voltage
Direct Current transmission.
generate, deliver and utilize electric energy. Grounding on the
work of other inventors in some occasions and drawing on his
own vision and inventiveness, Edison and his associates
developed a circuit for using his invented high-resistance
filament lamp. Constant voltage dynamos for providing
power to the circuit were improved. After all, Edison set up
several companies to manufacture majority of other needed
equipment like switches, safety fuses, insulating materials,
junction boxes and other elements of the prospective
underground conduit system. Edison’s next task was to
demonstrate his Low-Voltage Direct Current (LVDC) system
to the public [2].
The first electric central station in the world was built on
Pearl Street in New York by Edison in 1882[3]. It was LVDC
system to supply energy to the incandescent bulbs that Edison
invented before. Steam engines fuelled with coal, wood or oil,
were used to drive a generator. The generators, supplying
electric power to the lights within Pearl Street’s area, were six
constant voltage dynamos like it is shown in Fig.1. Each
dynamo produced the power of 99 kW and supplied 1,200
lamps which were paralleled. These were 110 V highresistance carbon filament lamps [4]. Local customers were
provided with electric light.
I. INTRODUCTION
At the beginning of the coming of electric supply industry
there was a rather fierce battle between the proposals of
Direct Current (DC) and Alternating Current (AC) options for
transfer and distribution of electric energy [1]. It resulted with
AC victory, owing to the advantages of AC in comparison to
DC of that time. Therefore AC has achieved its prevalence in
almost all electric supplies to customer. For an approximate
notion of the sequence of the main events, there is written a
historical insight into the most relevant events of electric
supply advancement and so called War of Currents that took
place in USA and was fought between Thomas Alva Edison
(DC) and Nikola Tesla (AC).
Nowadays it may be said with certainty that DC is making
its retaliation. Since the end of War of Currents DC
technologies with its advantages have been applied in AC
systems. The question of renewable energy is very topical
therefore more and more DC power is being generated from
renewable energy sources (RES) both in remote areas like
power plants and in cities. Also the number of electronic
devices running on DC is increasing, some of this includes
computers, cell phones and LEDs but currently each of these
units uses its own rectifier to transfer energy from AC to DC.
Owing to the progress of power electronics and the
development of rectifiers and inverters, DC technology
ensures an economical and safe power transmission in long
distances. Thereafter the types of DC transfer and distribution
systems, that can replace AC systems, are described.
II. THE BEGINNING OF DC ELECTRICAL ENERGY
TRANSMISSION
Fig. 1. The dynamo room of the first Edison electric lighting station in New
York.
During the 19th century inventors in Europe and America
worked on developing the ideas of electrical lighting. Thomas
Alva Edison turned his attention to electric light in 1878
while other inventers advanced the development of so called
dynamos, which are planned for maintaining constant voltage,
and also lamp technology. In 1879 after thousands of
experiments with filament materials Edison drew up the first
practical incandescent light. Right from the start, Edison
realized that he would have to develop a complete electric
supply chain system, consisting of particular components, to
Eventually, Pearl Street project worked and the beginning
of the electrification era had begun. Over the next years
similar LVDC electric stations were built in central districts
of large cities throughout North America, Europe, South
America and Japan. In Europe higher voltage supply lines
were picked. Very many towns required their own generating
station because there was a huge demand for electric light. In
fact, electric light was the only purpose of delivering
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IV. EDISON VS TESLA: THE WAR OF CURRENTS
electricity. Other electric devices like TVs, washing machines
etc. came later.
The so called War of currents was the competition between
the DC of Thomas Edison and the AC of Nikola Tesla and
George Westinghouse (Fig.2). This conflict was more a
media fight. Final decisions on the type of system to be
applied ended up being based on technical reasons and AC
systems emerged victorious as it offered greater advantages
than DC systems of that time [7]. Also other inventors like
William Stanley, Michael von Dolivo-Dobrowolsky and
Elihu Thomson all contributed to AC technology.
III. COMING OF AC ELECTRICAL ENERGY TRANSMISSION
Edison proved that electrical energy was able to be
supplied efficiently and economically from a central
generating station through a conformable transmission and
distribution system to consumers [5].
However there was a relevant technical problem known as
line loss. The electricity lost much energy by travelling
through the wires. Copper conductors were also very
expensive. For instance, previously mentioned electric central
station on Pearl Street supplied DC to an area of 1.6 km in
radius. Once the wires from the generator exceeded two
kilometers long there was not enough energy left to turn on
the light bulb. Hence the customers receiving electric service
had to be within a radius of these given borders of the
generating station.
Probable advantages of AC were observed already in 1821
when Michael Faraday carried out the experiments and
realized that the change of a magnetic field within time
induces a current in a nearby locating wire. This discovery
was the basic concept for transformers which are being
applied to increase electric transmission voltage thus reducing
line current and resulting with lower line losses. Nearby
consumers’ locations step-down transformers can be mounted
in order to gain a voltage needed for utilization.
The very first AC power system was demonstrated in
London in 1881 by electrical engineers Lucien Gaulard and
John Gibbs. The system used “secondary generators” or
transformers to step voltages up and down. George
Westinghouse, the inventor of the rail-way air brake, took an
interest in AC. By 1886 the Westinghouse Electric Company
had designed the equipment needed to commercialize AC
electric system. In 1886 the first AC lighting system was
developed and built in Buffalo, New York. By 1887 more
than 30 Westinghouse AC systems were in operation.
In 1870s Nikola Tesla perceived quite revolutionary notion
of the operation of rotating magnetic fields, AC induction
motor and the fundaments of AC generating and distribution
[6]. Though working in Europe Tesla failed to find financial
backing in France and Germany for the further development
which is the main reason he emigrated to America with the
purpose of meeting Edison. In 1884 he was hired by Edison
to make improvements in the DC generating plants, but
Edison had no interest in Tesla’s ideas about AC. Therefore
Tesla left Edison’s employ and worked for a group of
investors interested in developing an improved method for
lighting systems. Afterwards Tesla received backing and
developed the principal components needed for the system of
AC electric power generation and transmission and in the
mean time Tesla made several patents. In 1888 Westinghouse
hearing of Tesla’s accomplishments and realizing their
importance to the full-scale development of AC systems,
purchased Tesla’s AC patents and employed Tesla to work on
their further development. Tesla had an opportunity to realize
his ideas about AC technologies in reality which turned out to
be a threat to Edison’s DC systems. The competition between
Edison’s DC and Tesla’s AC was also the part of the so called
Second Industrial Revolution.
Fig. 2. The three man faced in the War of Currents. From left to right: T.
Edison, N. Tesla and G. Westhinghouse.
The first long distance transmission of DC electricity was
switched on at Willamette Falls Station in Oregon 1889.
However in 1890 Willamette Falls DC Power Station was
destroyed by flood and this unfortunate event cleared the way
for the first long distance transmission of AC electricity in the
world when experimental AC generators from Westinghouse
were installed by Willamette Falls Electric Company in 1890.
In 1896, the first AC generation and transmission system was
finished in the Niagara Falls using Westinghouse equipment
(Fig.3). The advantages of AC electric utility service became
obvious and proved the superiority of AC in transmitting
hydropower at long distances for universal uses [8].
Fig.3. Power station with three Tesla AC generators at Niagara Falls,
November 1896.
Also the invention of the AC transformer had insured an
economical and efficient long-distance transmission of
electric power at high voltages thus preventing the major
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disadvantage of Edison’s LVDC. This way the victory of AC
over DC was almost complete. By the end of the 19th century
DC systems began an inevitable decline but did not come to
an absolute end. Some cities continued to use DC well into
the 20th century. For example, in Europe, Helsinki had a DC
network until the late 1940s, Stockholm lost its dwindling DC
network as late as the 1970s, and London had some loads on
DC as late as 1981. In USA, certain locations in Boston still
used 110 V DC in the 1960s [7]. The last DC circuit, a feature
of 19th century of DC system was shut down in November
2007 and that was the end of DC electric service in New York
City.
Actually the war of currents was lost by LVDC systems but
long-distance DC transmission systems in Europe were not
left behind and during the next century High Voltage Direct
Current (HVDC) systems were developed and improved.
When the motor turns to generator’s mode and performs
braking then a dynamic or regenerative braking is possible. In
case of the dynamic braking, the energy is transferred to the
braking resistor which is enclosed in DC link. In order to
ensure the regenerative braking DC link should be connected
with energy storage system (ESS) consisting of two main
parts: storage element for example supercapacitor and a
DC/DC converter.
V. TODAY’S TECHNOLOGY: DC APPLICATION IN AC
SYSTEMS
A. DC Links
Even though AC was supposed to be the main standard in
electric energy exploitation systems the advantages of DC
were not forgotten. It brought the idea to develop a kind of
combination of AC and DC. One of the most significant
examples is AC-AC frequency converters with DC link which
are applied in electric drive systems for AC motors’ speed
and torque regulation by changing voltage and frequency in
the input of the motor with the contribution of pulse-width
modulation techniques. A typical frequency converter
consists of bridge rectifier, DC link and inverter. There are
two main types of inverter: voltage-source (VSI) and currentsource (CSI). In VSI drive DC link consists of a capacitor
which smoothes the pulsations of voltage. In CSI drive DC
link consists of one or several sequentially connected coils
which smooth the pulsations of current.
Fig. 5. AC electric drive system with ESS.
B. HVDC Transmission Systems
Coming with the formation of the DC link in AC systems
there were developed and applied HVDC systems. High
voltage is used in electrical energy transferring in order to
reduce the energy transfer loss in wires or line loss. A brief
scheme of HVDC system is shown below.
Fig. 6. General scheme of a HVDC transmission system.
A DC line interconnects two separate AC networks.
Transformers ensure the AC system’s voltage to be
appropriate for AC/DC converter or rectifier. During the
process of rectification electrical power is converted to DC.
Then this converted energy is transferred along the
transmission line about several hundred km long and
afterwards it is converted back to AC. Further the electric
energy is distributed and fed to local consumers which are
connected to a common 3-phase Low Voltage Alternating
Current (LVAC) network.
Fig. 4. a) Frequency converter with VSI; b) Frequency converter with CSI.
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In fact HVDC and LVDC systems in general have several
advantages in comparison to AC transfer systems. There are
lower capital costs and energy transfer loses are reduced.
Constructions of transmission lines are simpler as well and
there is neither Skin effect nor Ferranti effect. There are no
problems with voltage regulation. However there are also
some disadvantages for example converters generate
harmonics and reactive power compensation is needed.
Converter stations carry a low overload capacity and its
constructions are expensive.
There are two fundamental implementations of LVDC
distribution system; unipolar and bipolar. In the unipolar
system energy is transmitted through one voltage level which
is connected to all customers involved.
C. LVDC Distribution Systems
In relation to its advantages HVDC technology is very
economical and environmentally friendly and the application
of RES can be included. Besides, recent technological
advancements guarantee the possibility to apply power
electronic devices in low voltage supply networks. PCs,
portable electronic devices and LEDs are low power devices.
It appears that low power devices are very common and that
points out the significance of LVDC systems which currently
are not applied very commonly.
LVDC systems are widely used in power plants,
substations and DC drive transport systems. Because of the
development of modern power electronics and smart systems
it is very believable that LVDC systems could be a potential
component of so called smart grids. LVDC systems facilitate
the connection with RES and ESS. For example low power
photo voltaic generators are ordinarily placed on the roofs
over houses. Majority of RES and energy storage devices
generate DC power. RES can be connected to DC/DC
converters or directly to LVDC networks by reducing energy
loses. For instance, by connecting a solar panel to LVDC
network the number of conversion stages is lesser. As there is
no need for frequency synchronization the connection of
several parallel sources is easier to apply for DC systems.
LVDC distribution systems as like as HVDC systems
consist of AC/DC and DC/AC converters with a DC link
between them. Usually AC/DC conversion is performed close
to average voltage line but DC/AC conversion can be
performed in any place according to the location of consumer.
Depending on situation, LVDC system can be equated to a
HVDC example scheme (Fig. 5.) or it can be considered as a
wide LVDC distribution system in which DC/AC conversion
is performed before every consumer (Fig.7.). Actually wide
LVDC distribution system is rather similar to LVAC network
with several branches but DC lines replace AC lines and there
are no additional AC branches [9].
Fig. 8. A unipolar LVDC distribution system.
The bipolar system is formed from two unipolar systems
connected in series and in that case customers are connected
between voltage levels in various ways as shown in Fig.9: 1.between a positive pole; 2. – between a negative pole; 3. –
between a positive and negative pole; 4. – between positive
and negative poles with neutral connection [10].
Fig. 9. A bipolar LVDC distribution system.
D. LVDC Application Example
The energy efficiency in automation industry and robotics
has become in focus only in the recent years. Low voltage
600 V DC link is applied in robot drives. In most robot drive
systems regenerative braking is allowed. Braking energy is
buffered in capacitor locating in DC link and afterwards can
be reused. When one component performs braking, other that
accelerates can use that energy. There can be situations when
several components perform braking and due to the limited
capacitance not all energy can be buffered. In order to obviate
the overvoltage the braking energy is dissipated in braking
resistor connected to DC link (Fig.10.). When all components
perform acceleration all together more power from the
network is provided [11].
Fig. 7. General scheme of a wide LVDC distribution system.
Fig. 10. Use of braking resistor.
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The aim undoubtedly is to reduce wasted energy and it can
be done by increasing the capacitance of the DC link or using
a reversible rectifier. Separate industrial robots can be
coupled for optimal energy utilization. If several industrial
robots are applied, advantageous method is to share a
common DC link among several robots like it is shown
below.
Thereby, DC systems are rather frequently applied in AC
systems. In most occasions, AC supply lines’ energy is
rectified and transferred via HVDC system lines for longer
distances and then inverted again and distributed for
consumers. Besides, more and more electronic devices are
operated with DC consumer electronics. For its part, DC
operated electronic devices use their rectifiers to achieve DC
from AC. Today’s LVDC systems are rather innovative
approach of electrical energy distribution and undoubtedly
demands lot of researches. Small scale RESs generate LVDC
power and a possible idea is to link DC electronic devices to
DC power supplies but such circuits have not originated due
to the high electrical losses related with transferring a fixed
power.
Hence most RES supply power to AC networks and that
demands appurtenant power inverters. At the same time, the
working on the solutions of power inverter and converter
topologies and control takes place, in order to achieve a better
energy efficiency by applying RES and ESS for energy
saving. Further improvements of LVDC systems may
contribute in microgrid development.
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REFERENCES
Robot
[1]
O. Peak: The History of High Voltage Direct Current Transmission, the
3rd Australian Engineering Heritage Conference 2009.
[2] Robert W. Lobenstein and Carl Sulzberger: eyewitness to dc history, the
first and last days of dc service in New York City, 2008.
[3] O. Onederra, H. Odriozola, E Planas, I. Lopez and V. Lopez: Overview
of DC technology – Energy conversion, Renewable Energy and Power
Quality Journal, ISSN 2172-038 X, No.11, March 2013,
[4] David. P. Billington, David P. Billington Jr.: Power, Speed, and Form:
Engineers and the Making of the Twentieth Century, 2006.
[5] Carl L. Sulzberger: Triumph of AC, From Pearl Street to Niagara,
IEEE power & energy magazine 2003.
[6] Carl L. Sulzberger: Triumph of AC, part 2, The Battle of the Currents,
IEEE power & energy magazine 2003.
[7] L. de Andrade and T. Ponce de Leap: A Brief History of Direct Current
in Electrical Power Systems, Portugal
[8] Massimo Guarneri: The Alternating Evolution of DC Power
Transmission, 2013
[9] Jahangir Hossain, Apel Mahmud: Renewable Energy Integration:
Challenges and Solutions, 2014
[10] P.Salonen, T. Kaipia, P. Nuutinen, P. Peltoniemi, J. Portanen: An LVDC
Distribution System Concept, 2014
[11] D. Meike, L. Ribickis: Analysis of the Energy Efficient Usage Methods
of Medium and High Payload Industrial Robots in the Automobile
Industry, 10th International Symposium “Topical Problemms in the
Field of Electrical and Power Engineering”, Parnu, Estonia, January 1015, 2011
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Fig. 11. Common DC line.
All robots can also be supplied by one, more powerful
rectifier. By applying an appropriate method, approximate
energy savings could be put on almost the same level as the
amount of energy dissipated in braking resistors. But the
possibility of simultaneous accelerations of many robots, thus
creating power peaks, does exist and assumes a certain risk.
Another alternative to save a large part of braking energy is
the application of capacitor buffers.
CONCLUSION
Comprehendible, since the beginning of electrical energy
supply industry AC systems have been prevailed and applied
more than DC systems. But due to the technological progress
and the economical advantages DC systems have not been
gone unnoticed.
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