By Steven M. Brown, editor in chief
According to the most recent “Long-term Reliability Assessment” from the North American Electric Reliability Council (NERC), electricity demand is expected to grow by about 69,500 MW between 2005 and 2009. Projected resource additions over this same period total about 49,000 MW. The report goes on to state that more than 12,400 miles of new transmission are proposed to be added over the 2005 to 2014 time frame, which represents a 5.9 percent increase in the total amount of installed transmission over the assessment time frame.
According to NERC, while the transmission system is expected to operate reliably throughout North America, “some portions of the grid will not be able to support all desired electricity market transactions. Some well-known transmission constraints are recurring, while new constraints appear as electricity flow patterns change. As customer demand increases and transmission systems carry increased power flows, portions of these systems will be operated at or near their reliability limits more of the time.”
While certainly no cause for panic, the NERC reliability assessment does point to the continued need for increases in transmission system capacity-a goal many utilities are trying to achieve within their existing transmission rights of way. New composite conductors and equipment designed to manage reactive power are bringing the goal within reach.
High-capacity Conductor
Several large utility companies are turning to new high-capacity conductors to solve problems with transmission constraints. Installations at such utilities as Xcel Energy, Alabama Power, Arizona Public Service and other locations show that the new-style lines from such companies as 3M and Composite Technology Corp. (CTC) are ready for real-world use.
3M says its Aluminum Conductor Composite Reinforced (ACCR) overhead conductor can provide transmission capacity 1.5 to 3 times greater than conventional conductors, while using lighter weight material. The ACCR conductor contains a multi-strand core of heat-resistant aluminum matrix composite wires, retains its strength at high temperatures and is not adversely affected by environmental conditions, such as moisture or ultra-violet exposure, according to 3M. Due to its light weight and reduced thermal expansion properties, the conductor can be installed on existing towers in existing rights of way-a major plus, given the difficulty generally associated with obtaining new rights of way.
3M’s ACCR is currently in use at Xcel Energy and Alabama Power. At Xcel, the conductor has been installed on the 10-mile Black Dog-Blue Lake line in Minnesota. It was the ACCR’s first commercial application. The installation at Xcel is intended to support expansion of the utility’s Blue Lake power plant, which will provide power during peak demand periods in Xcel’s upper Midwest service territory.
Doug Jaeger, Xcel Energy’s transmission vice president, said the ACCR provided a quick, cost-effective way to link Xcel’s 1.5 million upper Midwest customers to the power produced at the Blue Lake plant. “Without it (the ACCR), we would have had to replace existing towers to accommodate larger sized conventional conductors,” Jaeger said. “Use of the new conductor allowed us to boost capacity on the line while avoiding major construction in an area with sensitive wetlands.”
At Alabama Power, the ACCR is expected to replace a 10-mile line in northeastern Alabama. The change is being made because the existing conductor would be at capacity for certain contingencies resulting from the addition of new generation during summer peak loads, beginning in 2008. Alabama Power is expected to being installation of the ACCR in January 2006.
Andy Wallace, transmission line manager at Alabama Power, said the use of the 3M conductor allowed the utility to avoid replacement of 22 transmission structures and installation of eight new structures.
Composite Technology Corp. also provides a new high-capacity conductor. The company says its Aluminum Conductor Composite Core (ACCC) cable can double the current carrying capacity of conventional cable, virtually eliminates high-temperature sag and requires fewer support structures due to its increased strength. Both CTC’s and 3M’s products replace the steel core common in traditional transmission line cores with lighter composite material.
In March 2005, Arizona Public Service Company (APS) completed an installation of CTC’s ACCC cable at the Gavilan Peak Substation in Phoenix. Arizona Public Service engineers and the Electric Power Research Institute will be monitoring the 69-kV line to document the conductor’s performance throughout changing seasonal conditions.
In May, American Electric Power ordered 49 miles of General Cable Corporation’s TransPowr ACCC from General Cable, which utilizes CTC’s technology, for installation in Texas. The 138-kV line located about 60 miles southwest of San Antonio will connect the cities of Leon and Pleasanton, Texas, by way of a connection to City Public Service. Due to the low-sag characteristics of the TransPowr ACCC conductor, AEP was able to eliminate the need to replace approximately 60 percent of the wooden support structures which would have required replacement had the project used conventional aluminum conductor steel reinforced (ACSR) product.
Other installations of composite-core conductors, like those from 3M and CTC, have been made or planned at Pacific Gas & Electric, the Western Area Power Administration, PacifiCorp, Austin Energy and Kingman Municipal Utility in Kansas.
Static VAR Compensators
Stringing up new higher-capacity conductor is one way utilities are increasing transmission capacity, often in existing rights of way. But another type of technology can serve to boost the capacity of the power grid’s existing lines.
Using power electronic devices to manage reactive power, or “VARs,” utilities can boost transmission capacity, without the need for new lines. One such device being used to strengthen the transmission system in a number of locations worldwide is ABB’s static VAR compensator (SVC).
According to information from ABB, “Electrical loads both generate and absorb reactive power. Since the transmitted load varies considerably from one hour to another, the reactive power balance in a grid varies as well. The result can be unacceptable voltage amplitude variations, a voltage depression, or even a voltage collapse.
“A rapidly operating SVC can continuously provide the reactive power required to control dynamic voltage swings under various system conditions and thereby improve the power system transmission and distribution performance. Installing an SVC at one or more suitable points in the network will increase transfer capability through enhanced voltage stability, while maintaining a smooth voltage profile under different network conditions.”
ABB, which has more than 400 SVC installations either in service or under construction worldwide, says its SVCs can boost transmission capacity by tens of percents in most cases. In one particularly pronounced case, an ABB SVC implementation in Thailand improved transmission capacity by more than 50 percent over existing lines.
In that case, an ABB SVC was installed in 1994 in the Bang Saphan 230-kV substation in Thailand. At the time, the power system in Thailand was undergoing rapid expansion. One weak link in the Thai bulk power system was found in a tie-line linking the load center around Bangkok with generation to the south. The length of the interconnection was about 700 km. To strengthen this weak link, an SVC was implemented at Bang Saphan, about halfway down the line.
Before the SVC installation, transmission capacity was limited to less than 200 MW due to transient stability limitations on the tie line. Once the SVC was put into operation at Bang Saphan, transmission capacity was increased to more than 300 MW.
Stateside, in late September 2005, American Electric Power (AEP) announced it had awarded a contract to ABB for three SVCs to be used in AEP’s Texas territory. That implementation is intended to help control and stabilize voltage in areas of high load growth and wind generation.
“AEP was one of the first utilities to install SVC technology in 1980,” said Anders Sjoelin, senior vice president and general manager of ABB Power Technologies. “This technology is in strong demand by utilities as they seek to improve reliability.
The SVC project at AEP is scheduled for completion in July 2006.
SuperVAR Machines
Another device used to manage reactive power, with a goal of increasing transmission capacity, is American Superconductor’s SuperVAR machine. The SuperVAR machine, which was developed in conjunction with the Tennessee Valley Authority (TVA), uses superconductor technology and serves as a reactive power “shock absorber” for the grid, dynamically generating or absorbing reactive power depending on the transmission system’s voltage level.
SuperVAR machines are “dynamic synchronous condensers”-rotating machines, much like motors or generators. They differ, however, from conventional synchronous condensers in their use of high-temperature superconductor (HTS) technology-an area in which American Superconductor has a great deal of expertise. The company says the use of HTS technology makes its SuperVAR machines more compact and efficient than conventional synchronous condensers. They are also not as subject to heat damage.
According to American Superconductor, its SuperVAR machines respond instantly to protect grids against voltage sags and surges, which can be caused by lightning storms, short circuits caused by tree branches momentarily touching lines, animals contacting transmission elements, and other sources. The SuperVAR devices stabilize voltage and provide utilities with a means to actively increase the reliability and maximize the capacity of transmission grids.
The first SuperVAR prototype was installed in January 2004 in Gallatin, Tenn. Based on results from that prototype installation, which was conducted in collaboration with TVA, engineers produced an advanced prototype which has also been successfully tested under live conditions. TVA has committed to buying five of the SuperVAR devices upon further successful evaluation and testing of the prototypes. ࢮࢮ
