An Insight into the Evolution of Direct Current Systems

Transcription

1 An Insight into the Evolution of Direct Current Systems Girts Stana, Peteris Apse-Apsitis Riga Technical University (Latvia) 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. 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 TRANSISSION During the 19 th 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 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. Fig. 1. The dynamo room of the first Edison electric lighting station in New York. 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 62

2 electricity. Other electric devices like TVs, washing machines etc. came later. III. COING OF AC ELECTRICAL ENERGY TRANSISSION 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 ichael 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. IV. EDISON VS TESLA: THE WAR OF CURRENTS 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, ichael von Dolivo-Dobrowolsky and Elihu Thomson all contributed to AC technology. 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 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 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 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 63

3 disadvantage of Edison s LVDC. This way the victory of AC over DC was almost complete. By the end of the 19 th century DC systems began an inevitable decline but did not come to an absolute end. Some cities continued to use DC well into the 20 th 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 In USA, certain locations in Boston still used 110 V DC in the 1960s [7]. The last DC circuit, a feature of 19 th 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 SYSTES 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. Fig. 4. a) Frequency converter with VSI; b) Frequency converter with CSI. 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. 64

4 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. 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. ajority 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]. 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. 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. 65

5 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. 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. 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. REFERENCES [1] O. Peak: The History of High Voltage Direct Current Transmission, the 3rd Australian Engineering Heritage Conference [2] Robert W. Lobenstein and Carl Sulzberger: eyewitness to dc history, the first and last days of dc service in New York City, [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 X, No.11, arch 2013, [4] David. P. Billington, David P. Billington Jr.: Power, Speed, and Form: Engineers and the aking of the Twentieth Century, [5] Carl L. Sulzberger: Triumph of AC, From Pearl Street to Niagara, IEEE power & energy magazine [6] Carl L. Sulzberger: Triumph of AC, part 2, The Battle of the Currents, IEEE power & energy magazine [7] L. de Andrade and T. Ponce de Leap: A Brief History of Direct Current in Electrical Power Systems, Portugal [8] assimo Guarneri: The Alternating Evolution of DC Power Transmission, 2013 [9] Jahangir Hossain, Apel ahmud: 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. eike, L. Ribickis: Analysis of the Energy Efficient Usage ethods of edium and High Payload Industrial s in the Automobile Industry, 10th International Symposium Topical Problemms in the Field of Electrical and Power Engineering, Parnu, Estonia, January 10-15,