A high-voltage, direct current (HVDC) electric power transmission system (also called a power super highway or an electrical super highway) uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current (AC) systems.
HVDC allows power transmission between unsynchronized AC transmission systems. Since the power flow through an HVDC link can be controlled independently of the phase angle between source and load, it can stabilize a network against disturbances due to rapid changes in power. HVDC also allows transfer of power between grid systems running at different frequencies, such as 50 Hz and 60 Hz. This improves the stability and economy of each grid, by allowing exchange of power between incompatible networks.
The modern form of HVDC transmission uses technology developed extensively in the 1930s in Sweden (ASEA) and in Germany. Early commercial installations included one in the Soviet Union in 1951 between Moscow and Kashira, and a 100 kV, 20 MW system between Gotland and mainland Sweden in 1954.
High voltage transmission[편집]
High voltage is used for electric power transmission to reduce the energy lost in the resistance of the wires. For a given quantity of power transmitted, doubling the voltage will deliver the same power at only half the current. Since the power lost as heat in the wires is proportional to the wires' resistance as a share of the total resistance, and doubling voltage allows for the quadrupling of non-transmission resistance without losing power, doubling the voltage reduces the line losses per unit of electrical power delivered by approximately a factor of 4. While power lost in transmission can also be reduced by increasing the conductor size, larger conductors are heavier and more expensive.
Practical conversion of power between AC and DC became possible with the development of power electronics devices such as mercury-arc valves and, starting in the 1970s, semiconductor devices as thyristors, integrated gate-commutated thyristors (IGCTs), MOS-controlled thyristors (MCTs) and insulated-gate bipolar transistors (IGBT).
Advantages of HVDC over AC transmission[편집]
A long distance point to point HVDC transmission scheme generally has lower overall investment cost and lower losses than an equivalent AC transmission scheme. HVDC conversion equipment at the terminal stations is costly, but the total DC transmission line costs over long distances are lower than AC line of the same distance. HVDC requires less conductor per unit distance than an AC line, as there is no need to support three phases and there is no skin effect.
Depending on voltage level and construction details, HVDC transmission losses are quoted as about 3.5% per 1,000 km, which are 30 – 40% less than with AC lines, at the same voltage levels. This is because direct current transfers only active power and thus causes lower losses than alternating current, which transfers both active and reactive power.
HVDC transmission may also be selected for other technical benefits. HVDC can transfer power between separate AC networks. HVDC powerflow between separate AC systems can be automatically controlled to support either network during transient conditions, but without the risk that a major power system collapse in one network will lead to a collapse in the second. HVDC improves on system controllability, with at least one HVDC link embedded in an AC grid—in the deregulated environment, the controllability feature is particularly useful where control of energy trading is needed.
The combined economic and technical benefits of HVDC transmission can make it a suitable choice for connecting electricity sources that are located far away from the main users.
Specific applications where HVDC transmission technology provides benefits include:
- Undersea cables transmission schemes (e.g., the 580 km NorNed cable between Norway and the Netherlands, Italy's 420 km SAPEI cable between Sardinia and the mainland, the 290 km Basslink between the Australian mainland and Tasmania, and the 250 km Baltic Cable between Sweden and Germany).
- Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', usually to connect a remote generating plant to the main grid, for example the Nelson River DC Transmission System in Canada.
- Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install.
- Power transmission and stabilization between unsynchronised AC networks, with the extreme example being an ability to transfer power between countries that use AC at different frequencies. Since such transfer can occur in either direction, it increases the stability of both networks by allowing them to draw on each other in emergencies and failures.
- Stabilizing a predominantly AC power-grid, without increasing fault levels (prospective short circuit current).
- Integration of renewable resources such as wind into the main transmission grid. HVDC overhead lines for onshore wind integration projects and HVDC cables for offshore projects have been proposed in North America and Europe for both technical and economic reasons. DC grids with multiple voltage-source converters (VSCs) are one of the technical solutions for pooling offshore wind energy and transmitting it to load centers located far away onshore.
The disadvantages of HVDC are in conversion, switching, control, availability and maintenance.
HVDC is less reliable and has lower availability than alternating current (AC) systems, mainly due to the extra conversion equipment. Single-pole systems have availability of about 98.5%, with about a third of the downtime unscheduled due to faults. Fault-tolerant bipole systems provide high availability for 50% of the link capacity, but availability of the full capacity is about 97% to 98%.
The required converter stations are expensive and have limited overload capacity. At smaller transmission distances, the losses in the converter stations may be bigger than in an AC transmission line for the same distance. The cost of the converters may not be offset by reductions in line construction cost and lower line loss.
Operating a HVDC scheme requires many spare parts to be kept, often exclusively for one system, as HVDC systems are less standardized than AC systems and technology changes faster.
In contrast to AC systems, realizing multiterminal systems is complex (especially with line commutated converters), as is expanding existing schemes to multiterminal systems. Controlling power flow in a multiterminal DC system requires good communication between all the terminals; power flow must be actively regulated by the converter control system instead of relying on the inherent impedance and phase angle properties of an AC transmission line. Multi-terminal systems are rare. As of 2012 only two are in service: the Hydro Québec – New England transmission between Radisson, Sandy Pond and Nicolet and the Sardinia–mainland Italy link which was modified in 1989 to also provide power to the island of Corsica.
- “ABB opens era of power superhighways”.
- “High Voltage Direct Current (HVDC) Transmission Super Highway Benefits to the Plains and Southeast”.
- “Wind Power ‘Superhighway’ Could Help Transform Panhandle Into U.S. Energy Hub”.
- “The Governance of Energy Megaprojects: Politics, Hubris and Energy Security”.
- Arrillaga, Jos; High Voltage Direct Current Transmission, second edition, Institution of Electrical Engineers, ISBN 0 85296 941 4, 1998.
- Narain G. Hingorani in IEEE Spectrum magazine, 1996. [깨진 링크]
- Jos Arrillaga; Yonghe H. Liu; Neville R. Watson; Nicholas J. Murray (2009년 10월 9일). 《Self-Commutating Converters for High Power Applications》. John Wiley and Sons. ISBN 978-0-470-74682-0. 2011년 4월 9일에 확인함.
- Siemens AG – Ultra HVDC Transmission System
- Skog, J.E., van Asten, H., Worzyk, T., Andersrød, T., Norned – World’s longest power cable, CIGRÉ session, Paris, 2010, paper reference B1-106.
- Basslink website
- ABB HVDC website
-  website
- “HVDC Classic reliability and availability”. ABB. 2014년 2월 8일에 원본 문서에서 보존된 문서. 2009년 9월 11일에 확인함.
- “Design, Modeling and Control of Modular Multilevel Converter based HVDC Systems. - NCSU Digital Repository”. 《www.lib.ncsu.edu》. 2016년 4월 17일에 확인함.
- Donald G. Fink and H. Wayne Beaty (2006년 8월 25일). 《Standard Handbook for Electrical Engineers》. McGraw-Hill Professional. 14–37 equation 14–56쪽. ISBN 978-0-07-144146-9.
- “The HVDC Transmission Québec–New England”. ABB Asea Brown Boveri. 2011년 3월 5일에 원본 문서에서 보존된 문서. 2008년 12월 12일에 확인함.
- The Corsican tapping: from design to commissioning tests of the third terminal of the Sardinia-Corsica-Italy HVDC Billon, V.C.; Taisne, J.P.; Arcidiacono, V.; Mazzoldi, F.; Power Delivery, IEEE Transactions on Volume 4, Issue 1, Jan. 1989 Page(s):794–799
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- ABB HVDC website
- GE Grid Solutions HVDC website
- HVDC PLUS from Siemens
- UHVDC challenges explained from Siemens
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