By Steven M. Brown, editor in chief
In mid-November, ABB kicked transmission system technology up a notch with the unveiling of its ultra-high voltage DC (UHVDC) test facility in Ludvika, Sweden. The facility will allow long-term testing of direct current rated at 800 kV. According to the company, a pioneer in HVDC, this marks the first major voltage increase in HVDC systems since ABB built a transmission line rated at 600 kV for Brazil’s Itaipu hydroelectric plant.
The primary application for UHVDC technologies, at least initially, will be to transmit power over extremely long distances in countries where hydropower resources are located far from major population centers. China and India are expected to be the main users. India plans to build five UHVDC lines over the next 10 years, each with a capacity of 6,000 MW. China is planning one line every year for the next decade, each with a capacity of 5,000 to 6,400 MW. Brazil and Africa are also potential users of the technology.
“In all four countries, you will find enormous hydropower resources that are a long distance from users,” said Olof Heyman, technology manager for ABB Grid Systems. “In India and China, there are more than 100 GW of hydropower to be built in Himalaya, but you have to transport it more than 2,000 km to the more densely populated areas. The same is true for Africa and Brazil.”
While there are currently no plans for the use of UHVDC in North America, ABB’s Heyman speculated on a couple of possible applications that could present themselves in the future.
“There are significant hydropower resources in the northeast part of Canada that could be used,” Heyman said. “One can also consider hydropower from Amazonas transported to North America. This technology gives the possibility to transport electricity over long distances in an efficient way.”
UHVDC in Brief
ABB’s Heyman provided a brief rundown of UHVDC technology, the component parts of which are mostly similar to traditional HVDC systems:
An HVDC converter station uses thyristor valves to perform the conversion from AC to DC and vice versa. The valves are connected to the AC system by means of converter transformers. The valves are normally placed in a building, and the converter transformers are located just outside.
The 12-pulse HVDC converter produces current harmonics (11th, 13th, 23rd, 25th, 35th, 37th etc.) on the AC side. These harmonics are prevented from entering into the connected AC network by AC filters, i.e. resonant circuits comprising capacitors, inductances (reactors) and resistors. The filters also produce a part of the reactive power consumed by the converter.
The HVDC converter also gives rise to voltage harmonics on the DC side (12th, 24th, 36th etc.). A large inductance (smoothing reactor) is always installed on the DC side to reduce the ripple in the direct current. In addition, a DC filter is also normally needed to reduce the level of harmonic currents in the DC overhead line. The harmonics may otherwise cause interference to telephone circuits in the vicinity of the DC line.
The power transmitted over the HVDC transmission is controlled by means of a control system. It adjusts the triggering instants of the thyristor valves to obtain the desired combination of voltage and current in the DC system.
Prior to the establishment of the new UHVDC test facility in Sweden, it had been more than 20 years since HVDC voltage had increased. A project in the mid-1990s was aimed at raising HVDC voltage to the “ultra” (800 kV) range, but the basic research needed to achieve this was not available at the time. Making the jump to 800 kV required more research in several areas:
- Development of new insulator materials. Using a higher voltage means there must be a wider gap between voltage and ground to isolate live parts with respect to ground. Porcelain insulators are insufficient, so new polymeric insulators had to be developed;
- Advanced computer-aided design tools for 3D field calculations, primarily for transformer design;
- Refined measurement methods using advanced laser technology to determine the characteristics of insulating materials and insulating systems; and,
- Advanced control systems to control the entire installation. This calls for an extremely high calculation capacity. ABB uses its in-house developed Mach2 system.
Benefits of UHVDC
“Because power is often generated far from consumers, there is-all over the world-an increasing need to be able to transmit large amounts of electricity over long distances,” said Per Haugland, global manager of ABB’s Grid Systems unit, during the inauguration of the new test facility in Sweden. “By increasing the voltage level of transmission, considerable advantages for the environment are gained, such as decreased losses and smaller transmission line highways.”
Comparing 800 kV UHVDC to 765 kV AC and 500 kV HVDC, the combined cost of the converter station, line cost and power line losses is substantially less with 800 kV UHVDC than the other two alternatives (see Figure, this page). In particular, power line losses with UHVDC are significantly lower than with either 765 kV AC or 500 kV DC. For a 2,000-km line, losses for 800 kV DC are about 5 percent; corresponding losses for 765 kV AC are double, at about 10 percent.
There is also an advantage in terms of the amount of real estate required. For UHVDC, smaller is better. ABB reports that the width of the power line track of 800 kV UHVDC is minimal when compared with the other two alternatives. For the transmission of comparable amounts of power, a 2,000-km HVDC line rated at 6,000 MW needs one power line with two suspended conductors, while an equivalent AC link operating at 765-kV requires three power lines each with three suspended conductors.
According to ABB’s Heyman, the first UHVDC station is now up for bidding and scheduled to be in operation in 2009.
ABB Milestones in HVDC development
1954: ABB links the island of Gotland with mainland Sweden, marking the world’s first commercial HVDC installation.
1985: ABB builds 600-kV rated line for Brazil’s Itaipu hydroelectric power plant, then a world record.
2006: ABB Opens 800-kV UHVDC test facility in Ludvika, Sweden.