From Staff Reports
Since its commercial introduction in the 1950s, high-voltage direct current (HVDC) systems have proved themselves as a viable alternative to alternating current systems for the bulk transmission of electric power. In some instances, HVDC can be the preferred mode of electric power transport when compared to AC transmission. Some applications that are particularly well-suited to the use of HVDC include: long undersea cable links, overhead lines greater than 600 km, and connecting asynchronous grids or networks. The virtues of HVDC and some of its more notable installations are outlined in this issue’s previous feature (pages 20-24).
Due to technological innovation from companies such as ABB and Siemens, this 50-plus-year-old technology has gotten a facelift in recent years. ABB’s HVDC Light and Siemens HVDC Plus make up the “new breed” of HVDC, and these relatively new offerings are proving to be quite effective in filling needs where neither AC power transmission or traditional HVDC are feasible.
Traditional HVDC vs. the New Breeds
While based on tried-and-true traditional HVDC technology, both HVDC Plus and HVDC Light have some fundamental differences that may make them the preferred choice in certain applications. The differences between traditional HVDC and Siemens’ HVDC Plus are outlined in Table 1 on page 28.
Some of the differences between traditional HVDC and ABB’s HVDC Light include:
Power range: Traditional HVDC is most cost-effective in the high power range, above approximately 250 MW. HVDC Light, on the other hand, comes in unit sizes ranging from a few tens of MW at the low end to an upper range 1,200 MW and ±320 kV.
DC transmission circuit: The DC transmission circuit for traditional HVDC can be an overhead line or a DC cable. The cables are normally mass-impregnated (MIND) cables suitable for HVDC with a copper conductor and insulation made of oil-impregnated paper. The vast majority of classical DC cables are submarine cables.
The DC transmission circuit for HVDC Light, on the other hand, is made by extruded polymer cables both for land transmissions (underground) and across water (submarine). HVDC Light is by nature bipolar. The DC circuit is not connected to ground. Therefore two cables are needed. HVDC Light can also be built with overhead lines.
Modular: HVDC Light is based on a modular concept with a number of standardized sizes for the converter stations. Most of the equipment is installed in enclosures at the factory. Conventional HVDC is always tailor-made to suit a specific application.
Independence of AC network: HVDC Light does not rely on the AC network’s ability to keep the voltage and frequency stable. Unlike conventional HVDC, the short circuit capacity is not important. HVDC Light can feed load into a passive network.
Active and reactive power control: Traditional HVDC terminals can provide limited control of reactive power by means of switching of filters and shunt banks and to some level by firing angle control. But this control requires additional equipment and therefore extra cost.
The HVDC Light control makes it possible to create any phase angle or amplitude, which can be done almost instantly. This offers the possibility to control both active and reactive power independently.
Mike Bahrman, ABB’s marketing manager for HVDC products, noted that traditional HVDC will remain the workhorse for long-distance bulk power transmission and large asynchronous interconnections because traditional HVDC operates at higher voltages.
“But HVDC Light is getting heavier now,” Bahrman said. “We’re up to ±320 kV. For overhead we can go higher; we can go up to ±640 kV.”
Market for HVDC’s New Breeds
ABB’s Bahrman noted that a number of HDVC Light systems are already in operation around the world. He talked about two HVDC Light implementations in Australia:
The Terranora interconnector (previously called Directlink) was commissioned in 2000. It is a 180-MW underground HVDC Light system, which connects the New South Wales and Queensland electricity grids in Australia, allowing power to be traded between the two states. The link was built by Transàƒâ€°nergie Australia, a subsidiary of the Canadian utility Hydro Quebec and Country Energy, an Australian energy provider. The Terranora interconnector consists of three HVDC Light independent links of 60 MVA each operating at 80 kV. The six underground cables are 59 km each.
The second HVDC Light project in Australia is the Murraylink 220-MW interconnector between the Riverland in South Australia and Sunraysia in Victoria. Murraylink was commissioned in 2002 and is a 180-km underground high-voltage power link. Murraylink benefits both South Australia and Victoria by enabling electricity trading in Australia’s deregulating power market. Murraylink has used existing corridors and required no private easements, nor use of private land.
Bahrman said the most recent HDVC Light installation is the Estlink HVDC Light system, which links Estonia and Finland. It was commissioned in 2006 and runs both undersea and underground for its 130-km length. HVDC Light was chosen for Estlink due to the need for long underground and undersea spans, as well as its ability to connect non-synchronous AC systems.
ABB’s HDVC Light has two installations in the U.S. One is the 36 MVA back-to-back HVDC Light link at Eagle Pass, Texas, which was commissioned in 2000; the other is Cross Sound Cable, an HVDC Light underwater cable link between Connecticut and Long Island, N.Y., which was commissioned in 2002.
According to Cristen Schimpf of Siemens Power T&D, the first commercial installation of that company’s HVDC Plus will be the Trans Bay Cable Project, which seeks to connect the City of Pittsburg, Calif., with downtown San Francisco via a submarine cable along the San Francisco Bay. Trans Bay is currently under construction, and the commercial operation will commence in early 2010, Schimpf said.
“The market (for HVDC Plus) in the U.S. as well as internationally is growing rapidly,” Schimpf said. “HVDC Plus applications come into play more and more where there is a need is for bringing bulk power into load centers (such as big cities) where space is a constraint.”
Bahrman agreed, noting that the footprint for his company’s HVDC Light offering is around a quarter that of traditional HVDC, making it well-suited for congested urban installations.