Making light of HVDC transmission in Gotland21 May 1998
HVDC Light is a new transmission technology that promises to make HVDC (high voltage direct current) – hitherto used almost exclusively for transmitting very high powers over long distances – economic down to just a few MW. It has been made possible by the rapid developments in power electronics of recent years, notably in Insulated Gate Bipolar Transistor (IGBT) technology. A prototype link, between Hellsjön and Grängesberg, has been successfully tested. A commercial-scale development and demonstration installation, supported by the Swedish government, is being installed on the island of Gotland in Sweden.
The island of Gotland, in the Baltic Sea, some 90 km east of the Swedish coast, has long been associated with historic innovations in HVDC transmission technology. The world's first large scale HVDC transmission system, using mercury arc converters, was taken into service there in 1954, linking the island of Gotland (current population 56 000) to the Swedish mainland. In 1970 the world's first HVDC transmission system using power semiconductors (thyristors) entered service on the island, with the completion of a second submarine link to the mainland.
The GotLite project
Gotland's pioneering role in electrical transmission technology is set to continue with installation of the world's first commercial-scale HVDC Light transmission system, which is due to be in operation in May 1999.
A project agreement to develop, demonstrate and commission this system was signed in December 1997, by GEAB, the Gotland electricity supply company (a subsidiary of Vattenfall), and ABB. This 70 km demonstration installation, rated at 50 MW, will run from Näs in the south, where a wind park is situated, to the town of Visby, which is the main load centre on the island. The project is being supported by the Swedish government, through the Swedish National Energy Administration, which sees a technology such as HVDC Light as a prerequisite for development of small-scale electricity generation technologies, for example wind power.
Gotland consumes about 900 GWh of electricity per year, most of which is supplied from the mainland through the HVDC submarine links. But in recent years, with the push towards renewable energy sources, there has been increasing interest in expanding wind power generation on Gotland, whose coastline is frequently exposed to strong winds from the Baltic sea. Some 48 MW has been added since the beginning of the 1990s, of which 35 MW is located in the southern tip of the island. An additional 50 MW of wind power capacity is due to be started up over the next few years.
Not surprisingly this rapid development has impacted on Gotland's electricity grid and there is a pressing need for additional transmission capacity, as well as for better means to maintain good power quality. The variable operating conditions that wind power systems are subjected to result in flicker and in variations in reactive power. In the longer term, the grid will also have to cope with varying active power flows and there is limited generating capacity on the island so that very low short circuit levels are encountered.
Another factor is that traditional overhead transmission lines are particularly unwelcome on the island, which is a sensitive wildlife environment and a popular holiday location.
Initially four possible solutions to the island's power transmission problems were considered: two based on traditional high voltage AC technology and two based on HVDC Light technology.
The AC approach featured either an overhead transmission line and a synchronous condenser and/or a static var compensator, or an alternative solution based on underground AC cables, a synchronous condenser and a static var compensator.
The HVDC Light approach featured either overhead lines or underground DC cables.
After a careful review of the four alternative solutions, the HVDC Light solution with underground cable, using a recently developed extruded DC cable technology, was selected as the best option. The criteria on which this choice was based included technical viability, economics (in terms of both investment and operation) and environmental compatibility.
It was found that the AC alternatives were penalised by the low short circuit power levels experienced in the Gotland grid, and by the ensuing voltage control problems. The AC options did not look viable in these extreme conditions, but study of them helped establish a reference cost for the transmission link.
The HVDC Light alternative featuring an overhead DC transmission line was discarded as a result of the stringent environmental considerations. The alternative, featuring underground DC cables, was deemed technically superior. Its environmental impact is minimal, it is virtually free of magnetic fields since two cables are used with balanced currents, and both active and reactive power can be rapidly controlled. This last feature is key when it comes to maintaining a high level of power quality in the supply of electricity to end-users. The HVDC Light system's ability to separately control active and reactive power was perhaps the most important point from the utility's perspective, who were also attracted by the potential of HVDC Light to enable them to make better use of the parallel AC system.
Another attractive feature is that the compactness of HVDC Light means that all equipment for the GotLite installation will be installed in containers in the factory, and extensively factory tested – reducing the amount of civil, installation and on-site commissioning work.
The Hellsjön project
The GotLite project will be instrumental in confirming the operational performance of the HVDC Light and extruded DC underground cable technologies, particularly in transmission applications where wind power parks are to be connected to the main grid. But it is not the first HVDC Light installation. This distinction falls to the Hellsjön-Grängesberg link in central Sweden, rated at 3 MW and ± 10 kV DC, which is making use of a 10 km long temporarily decommissioned AC line (following, interestingly, the route of another pioneering link, Sweden's first three-phase power transmission line, built in 1893 to supply the Grängesberg mines from the Hellsjön hydro station).
The container-based converters for the Hellsjön-Grängesberg HVDC Light system were factory tested in the latter part of 1996 and the link has been in operation since March 1997.
In the Hellsjön project, the HVDC Light transmission system serves either as a feeder into the Grängesberg AC grid or into an islanded part of that grid. In the latter case, the DC system feeds into a passive load with no other source of power. The HVDC Light then has sole control of voltage level and frequency.
The Hellsjön trial has produced generally positive results. Transmission has proved stable, performing as predicted during both steady state and transient conditions, with measurements showing that the converters will meet expected requirements in terms of sound levels, harmonic distortions, telephone disturbances and electromagnetic fields.
Advantages and future applications
As well as making HVDC economic down to low powers, ABB believes the new technology "opens up new, hitherto unseen possibilities for power quality improvement in AC power networks."
The company also believes that when HVDC Light is combined with new underground DC cable technologies it will "become a very advantageous alternative to overhead transmission in many cases". Among the factors helping to make HVDC Light economically attractive relative to conventional HVDC are its simplicity and compactness (a 20 MW installation would occupy less than 250 m2) and its high level of modularization and factory-build.
Possible niches envisaged for the new technology include:Remote, small-scale power plants, such as low head hydro and wind (as in the Gotland project). Delivery of electricity to islands. Electricity from a remote power system could successfully replace diesel generating sets, especially in view of the relatively high operating costs of transporting diesel fuel and the relatively low energy conversion efficiency of small diesel generator units. Other possibilities are in the supply of electricity to offshore oil and gas platforms, where space and equipment weight are at a premium, and in enabling excess gas from oil platforms to be used to generate and transmit electric power to the mainland. Feeding power to large and rapidly growing cities. Land space being scarce and expensive, substantial difficulties arise whenever new rights-of-way are needed. Also, with increasing power levels, the risk of exceeding the short-circuit capability of switchgear equipment and other network components becomes a real threat to further expansion. HVDC Light opens up the possibility of using underground DC cables, while the converter stations themselves are compact and do not contribute to short-circuit levels. Feeding power to remotely located loads. Small cities, mining districts, villages and other places that are located far from any electrical network could now be economically fed from larger networks via an HVDC Light link. Thus the advantages afforded by large electricity networks can be extended to just about anywhere. In the past, for loads below about 100 MW, local generation has been the only option if the distance between the existing grid and the remote load was greater than could be economically served by traditional AC technology.
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