Connecting the world's electrical grid
NVENTOR R. BUCKMINSTER FULLER first proposed the idea of connecting the world's regional electrical grids into a single global energy network in 1969. Since then, political problems, including domestic and international conflicts, have posed a formidable barrier to global energy networking. However, the development of long-range transmission systems has bolstered the technological feasibility of such plans.
In recent years a number of government agencies, utilities, and electrical equipment manufacturers around the world have set aside their geopolitical differences and joined forces to build transmission systems that can economically send thousands of megawatts across thousands of kilometers. Some of the more ambitious long-range electrical transmission systems include Russia's high-voltage alternating-current (HVAC) line, Brazil's high-voltage direct-current (HVDC) system, the Canadian-U.S. multiterminal direct-current (MTDC) system, and a recently completed thyristor-controlled series compensator (TCSC) demonstration project in Oregon.
All of these long-distance bulk power transmission systems are made possible by improvements to electrical transmission equipment itself, including thyristor valves, harmonic filters, capacitors, and inductors, augmented by computers and microprocessors. The generating equipment and power lines used in long-distance power transmission are otherwise similar to standard high-voltage power grids.
Most of the world's power-transmission networks are ac rather than dc. The former are generally simpler and less expensive to build. However, there are some advantages to HVDC that become apparent when transmitting over great distances. A dc line requires two conductors, while an ac line requires three; this means that the capital costs associated with direct current are lower. Direct-current systems are more expensive to build than their ac counterparts due to the additional cost of building the converter stations, which change the ac to dc and vice versa, in addition to the fact that dc is a more sophisticated technology. The savings represented by the two-conductor dc line can pay for the difference in long-range transmission.
Another advantage of HVDC is that dc transmits no reactive power. This keeps power losses in the dc line lower than in ac lines. An HVDC system can connect two asynchronous ac networks. By using microprocessors and thyristors—solid-state devices made of silicon crystals—utilities can exert greater control over the flow of electricity than in ac networks. Additionally, research has shown that HVDC lines can extend to 6400 kilometers, compared to 4800 for HVAC.
BRINGING AC ACROSS THE STEPPES
High-voltage technologies, built to transmit both alternating and direct current, are based on the fact that the higher the voltage a transmission line can accept, the greater its capacity and efficiency and thus the farther it can be extended. The vastness of the former SOviet Union made the development of HVAC systems there a necessity. In fact, the former Soviet Union was a pioneer of such systems.
The most ambitious Soviet high-voltage project involved transmitting 1200 kilovolts from Ekibastuz in Kazakhstan to European Russia, 1211 kilometers away. The first phase of this project in 1982 involved transmitting electricity generated in Ekibastuz to Kokchetav, a distance of 494 kilometers. It went on-line at 500 kilovolts. In 1986 it was converted to 1200 kilovolts, said Viktor Rashkes, an electrical engineer who worked at the All Union Electric Power Research Institute in Moscow. Rashkes helped commission high-voltage projects in the U.S.S.R. beginning in 1965.
The second leg of the project was to connect Kokchetav to Kustanaj, 396 kilometers away. In 1985 this line began transmitting power at 500 kilovolts and was converted to 1200 kilovolts in 1987. The final 321 kilometer line, linking Kustanaj to Chelyabinsk in the Urals, was completed in 1987; this line continues to transmit 500 kilovolts, said Rashkes. Projects to further extend the 1200-kilovolt network have been postponed due to the collapse of the post-Soviet economy, he said.
HIGH VOLTAGE ON THE PARANA
Although the Soviets shunned HVDC systems due to their troubled economy and a paucity of generating capacity, the Brazilians have built the largest HVDC link in the world. The Brazilian system, built in the early 1980s, takes power from the world's largest hydroelectric station at Itaipu, a site on the Parana river where the borders of Argentina, Brazil, and Paraguay meet. The Itaipu station generates 12,600 megawatts of power used by the power networks of Paraguay and Brazil. About 97 percent of the energy is used in Brazil, representing 25 percent of that country's electrical needs. The remainder of Brazil's electrical needs are met by hydroelectric, nuclear, and fossil-fuel plants.
Brazil's system was built by a consortium comprising ABB Power Systems of Ludvika, Sweden; Asea Ltda. of Osasco, Brazil; and Promon Engenharia S.A. of Rio de Janeiro. Two 6300-megawatt HVDC converter stations were built, one in Foz do Iguacu, located on a tributary of the Parana in Brazil, and the other at Ibiuna, near Sao Paulo, 800 kilometers away. Because the Brazilian power network has a 60-Hz frequency and the Paraguayan network is 50 Hz, the Itaipu generating station was equipped with nine 60-Hz generators and nine 50-Hz generators, each capable of generating 700 megawatts.
The Itaipu HVDC link consists of four poles working at ±600 kilovolts. The Foz do Iguacu station converts the ac from Itaipu to dc, which is then transmitted across the two bipolar lines to Ibiuna. The 60-Hz generators at Itaipu feed a 765-kilovolt ac transmission system that transmits this power to the Sao Paulo metropolitan area over two parallel 765-kilovolt lines.
Both the Foz do Iguacu and Ibiuna stations house 8-pulse converters operating at 300 kilovolts. Each converter comprises two 6-pulse converters operating at 150 kilovolts. The 12-pulse conversion reduces the generation of harmonic currents caused by the ac-to-dc and dc-to-ac conversion process.
Each of the two stations is equipped with a total of 96 thyristor valves assembled in groups of four to form a quadruple value. Three quadruple valves make up one 12-pulse converter. Each thyristor valve contains 96 water-cooled thyristors connected in series. Thyristors normally block the flow of current, but when given a signal they are activated so that they conduct it. The thysitors at Foz do Iguacu and Ibiuna are activated by means of fiber-optic cables.
Some innovations designed by ABB for the Brazilian HVDC link include the filters that reduced unwanted harmonics on the ac and dc sides of the converter stations. The process of converting dc to ac and vice versa generates unwanted harmonics at each terminal because it distorts the quality of the ac waveform. This distortion will cause interference on electrical devices and reduce the effectiveness of electromagnetic devices such as motors. Unwanted harmonics can also interfere with nearby telecommunications.
The filters consist of a combination of inductors, capacitors, and resistors. Series of inductors and capacitors are connected to the ground between each pole of the dc line. The inductors and capacitors are chosen to resonate at unwanted harmonic frequencies. On the ac network, the inductors, capacitors, and filters eliminate unwanted harmonies by being connected between the ac phases and ground and they provide reactive power to compensate for the power used by the converter. This is necessary because the converter can consume up to 50 percent of the power that is being transmitted. The harmonic filters on the dc network remove unwanted harmonics without playing a reactive role. ABB engineers designed the harmonic filters for the Itaipu project to operate at 500-kilovolt ac and 600-kilovolt dc voltages, higher voltages than any previously built ABB units.
Four synchronous compensators cooled by liquid hydrogen were also installed at Ibiuna to stabilize the ac system and to generate reactive power. Both of the converter stations are linked to the central dispatch center of the Brazilian utility Furnas Centrais Electricas S.A. in Rio de Janeiro by a microprocessor-based SINDAC computerized supervisory system supplied by ABB. Furnas and ABB used HVDC simulators to study the Iguacu system before and during its construction.
THE MEDITERRANEAN CONNECTION
Most dc power-transmission lines, including HVDCs, connect two generating sites to a load site or interconnect power networks to exchange power in either direction, taking advantage of load cycles in the different systems. A multiterminal direct-current system can connect several generating sites to several load points, ensuring the flow of power even in the event of a terminal going out of service due to maintenance or accident.
The first MTDC system was commissioned in 1987 by tapping an existing point-to-point link called SACOI, connecting Sardinia to mainland Italy through the French island of Corsica. SACOI was originally commissioned in 1966 by ENEL, the Italian electricity board, to transmit electricity produced by coal-fired power plants in Sardinia to Italy: ENEL chose direct current because of the line's long underwater crossing.
The SACOI link consists of an 85-kilometer-long overhead line starting at the converter station in Codrongianus, Sardinia, connected to a 16-kilometer underwater cable that crosses the Bonifacio Straits between Sardinia and Corsica. This is connected to an overhead line running 156 kilometers across Corsica, which, in turn, is connected to an underwater cable measuring 105 kilometers from Corsica to the converter station in San Dalmazio in Tuscany.
Growing environmental concern over the burning of high-sulfur-content Sardinian coal led ENEL to change the function of the SACOI link from transmission to interconnection, regulating the frequency of electricity sent to Sardinia.
In 1982, Electricité de France headquartered in Paris, the French electricity board, requested a tap to the existing point-to-point link to use some of SACOI's power in Corsica. The regulation and control systems of the link were changed, and vacuum valves were replaced with thyristors. The SACOI link is related at 300 megawatts, and the Corsican tap absorbs 30 megawatts.
A QUEBEC-NEW ENGLAND PARTNERSHIP
Building on the success of the Franco-Italian MTDC, Hydro-Quebec of Montreal and the New England Power Pool, which comprises nearly 100 publicly and privately owned utility companies in the New England states, joined forces to construct what is believed to be the world's largest MTDC, stretching 1500 kilometers. The North American MTDC began with the 170-kilometer connection of the Canadian utility's Des Cantons, Quebec, substation to the New England Power Pool's substation in Comerford, N.H.
The Des Cantons-to-Comerford dc link, commissioned in 1986, was rated at 690 megawatts. For the second phase of the interconnection, the Canadian utility considered converting Des Cantons to a 2000-megawatt substation to meet New England's increasing demand for electricity. However, at such a high power level, it was necessary to be able to isolate Des Cantons' generation from the rest of the Hydro-Quebec network in order to minimize the loss of power supplied to New England in case of a major outage in the Hydro-Quebec network. To that effect, the dc line was extended northward from Des Cantons to the James Bay area where the 2000-megawatt Radisson rectifier station supplies the dc line from generators that can be isolated when necessary.
In order for Hydro-Quebec to use the energy generated at Des Cantons but not sold to New England a third converter station, the Nicolet 2000-megawatt station, was built about halfway between the load centers of Montreal and Quebec City. This station normally inverts the direct current into the Hydro-Quebec ac network. It can also be used to supply power to New England directly from the Hydro-Quebec main ac transmission network in case of difficulties with the 100-kilometer-long dc line from the northern wilderness. Nicolet is located 1000 kilometers from Radisson and was commissioned in September 1992.
In the United States, New England Power Service Co. of Westborough, Mass., built the Sandy Pond load station between the Massachusetts towns of Ayer and Groton. Since Sandy Pond is 500 kilometers from Nicolet, the MTDC system can either transmit up to 2000 megawatts 1500 kilometers to Sandy Pond or share that power between Hydro-Quebec and its neighboring utilities, said Jacques Lemay, a senior electrical engineer and technical coordinator at Hydro-Quebec.
The Radisson-to-Sandy Pond line was commissioned in November 1990. In 1992 the inclusion of the Nicolet substation made it a true MTDC system.
The assurance of alternative sources of electricity is a major benefit of the MTDC system. "If Radisson went off line due to maintenance or equipment failure, the New England Power Pool could continue to receive power from Nicolet," said Doug Fisher, an electrical engineer and manager of transmission system engineering at the New England Power Service Co.
The MTDC system takes advantage of several characteristics inherent in dc transmission systems. One is the ability of dc to interconnect asynchronous systems. "Over time, there is an arbitrary phase displacement with Hydro-Quebec and the New England Power Pool," explained Lewis Vaughan, senior researcher at Hydro-Quebec's power system simulation laboratory in Varennes, Quebec. An ac transmission system would require synchronous systems because the phase displacement would interfere with the flow and direction of power. Using dc allows the North American MTDC network to convert power from dc to ac and vice versa, eliminating the need for synchronous systems.
Another advantage of dc terminals is that they can be brought back on-line in as little as one hour, said New England Power Service's Fisher. "You have a much bigger job putting a coal-fired or nuclear power plant back on-line if it should go out of service," he said.
Over long distances, dc transmission systems can be more economical than ac transmission, although dc terminals generally cost more to build than their ac counterparts. Economies of scale come into play with larger-scale power transmission. "If you are moving more than 1000 megawatts, you can start to think about the investment needed for building a multiterminal direct-current transmission system," Fisher said.
A large portion of the capital costs of the Quebec-New England MTDC terminals was incurred by the converters, controls, and filters that give system operators greater control over the power flow and voltage. This is necessary in long-range transmission systems that send electricity to regions that have differing peak-demand periods. ABB Power Systems Inc. of Raleigh, N.C., supplied this equipment for the Hydro-Quebec-New England Power Pool MTDC project.
There are water-cooled thyristor valves suspended from the ceilings of the Radisson, Nicolet, and Sandy Pond terminals that control the flow of electricity when they are activated. Each valve contains a number of connected thyristors. "We connected 90 thyristors in series at Radisson because of its higher voltage and 84 in series for the valves at Nicolet and Sandy Pond," said Lars Bergstrom, an electrical engineer at ABB Power Systems who worked on the North American MTDC project. The thyristors contain 100-millimeter-diameter silicon wafers.
The activation, or firing of the valves at the MTDC terminals is accomplished by the dedicated microprocessor-based single-board computers that ABB Power Systems engineers have developed for HVDC control and protection.
MORE CONTROLLABLE AC
Both HVDC and MTDC power systems rely on direct current. Improvements in thyristors could make long-range HVAC systems more feasible. The Electric Power Research Institute (EPRI) in Palo Alto, Calif., has joined forces with the Bonneville Power Administration of Portland, Ore, and General Electric Industrial and Power Systems of Schenectady, N.Y., to build a thyristor-controlled series compensator demonstration project at Bonneville's Slatt substation in northern Oregon. The TCSC uses thyristors to direct the flow of electricity precisely along specific transmission lines, to stabilize the power swings caused by short circuits or other disturbances, and to substantially expand the usefulness of key lines.
Both EPRI and the Bonneville Power Administration hope that by replacing mechanical switches with TCSCs they will create a flexible ac transmission system to facilitate the transfer of bulk power between utilities. Such system could also help the transmission of bulk power over the long distances envisioned by global energy networking, said Ben Damsky, manager of electronic systems at EPRI, the research and development arm of the electric power industry.
If the TCSCs prove successful, they may be able to give ac networks the same degree of control that dc systems have. "Most of the power-transmission systems in the world are ac because they are less expensive to build than dc, which is only used when more control is needed, as in the connection of asynchronous power systems," said Joseph Urbanek, an electrical engineer and manager of advanced systems in GE's Capacitor and Power Protection Operation.
At present, ac cannot be controlled well enough to travel thousands of kilometers away, but that could change with the use of TCSCs. "A utility equipped with TCSCs on the west coast could make deals with three or four other power networks to send power to the east coast," Urbanek explained. This arrangement could cross national frontiers just as easily.
THYRISTORS OF TOMORROW
GE engineers constructed the TCSC demonstration substation and manufactured the special thyristors it required. The demonstration facility at Bonneville's Slatt substation will use the T CSC to control a 500-kilovolt transmission line with a 2500-megawatt capacity, sufficient to provide power to all the residents of Portland, OR.
The Slatt substation was chosen because it has the high current and voltage needed to demonstrate the TCSC system, but it is not a key line. The last consideration is important because Bonneville intends to short-circuit the line during its testing of the TCSC system without interfering with its supply of power to the Portland area.
Each of the three phases in the series-compensation system comprises six thyristor-controlled capacitor modules. Each TCSC module contains a series capacitor with a thyristor switch and current-limiting reactor in parallel. Engineers in GE's Capacitor and Power Protection Operation included a metal oxide protective device within the module to limit overvoltages. During faults, the thyristor switches bypass the excessive fault currents to protect the system. A bypass breaker outside the module provides an extra measure of protection and can remove the TCSC from the circuit when it is not needed. GE engineers designed and constructed an electrically and thermally insulated deionized cooling system that removes heat from the thyristor valves.
The engineering team at the TCSC project installed General Electric capacitors in the TCSC line to balance capacitive and reactive impedance. When these impedances are out of balance, they create a plugging effect that blocks power. The capacitors themselves are steel and/or aluminum cans with porcelain bushings. Inside of the capacitors are two sheets of aluminum separated by plastic film. An electrode is attached to each piece of aluminum foil. Oil in the capacitor assembly improves each unit's dielectric strength.
GE engineers made the thyristors for the TCSC modules large enough — 100 millimeters in diameter — to handle 60,000 amperes; typical units have diameters of 52 or 77 millimeters. "The silicon layer within the thyristors is 0.5 millimeter, half the thickness of conventional thyristors, to reduce their electrical resistance," said Urbanek. The Slatt thyristors are also equipped with a special gate structure that can open the switch three to five times faster than the average thyristor, providing a faster response to power demand, he said.
Within the silicon layer in each thyristor are different layers of doping. The silicon is alloyed to tungsten plate to provide mechanical and thermal stability. A molybdenum strain buffer contacts the copper posts that carry electricity into the unit. The thyristors are shielded from corrosion by a nitrogen blanket and hermetically sealed within porcelain housings whose surface length convolutions improve their dielectrical strength. The Slatt thyristors are cooled by a deionized water system.
During the testing of the TCSC project, line current from the platform is detected by current transformers, digitized, and sent via fiber optics to the control room. The information is fed to seven modified General Electric DC 2000 digital drive controls that process the data and signal the thyristor switches to fire over a separate fiber-optic link. This enables the thyristors to either bypass or insert a capacitor as needed. When the capacitor is inserted, the liquid-cooled thyristor valves can be phase-controlled to vary the effective fundamental frequency impedance of the capacitor.
SHARING POWER ACROSS BORDERS
As the technological barriers to the long-range transmission of bulk power fall, engineering societies and governmental agencies are taking another look at the possibility of global energy networking. One of the ventures proposed by the European Community to promote greater cooperation among Mideast economies is an 800-megawatt hydropower project involving Israel, Jordan, and the occupied territories. The project would exploit the 400-meter height differential between the Mediterranean and Dead seas.
Joseph Falcon, a former president of ASME and now a senior partner at the power engineering consulting firm J.A. Falcon and Associates in Huntington Beach, Calif., said, "Energy forms the basis of an economy. Sending hydroelectricity long range will generate much-needed revenues for developing countries while reducing the need for the construction of new power plants in the industrial nations."