BY KATHLEEN DAVIS, SENIOR EDITOR
There is a tendency to get caught up in the march toward the horizon as the Obama administration wades through grant applications for smart grid technology, the renewables and integration discussion heats up and energy efficiency gains supporters. And in the rush to this future grid, proponents sometimes forget that to get to the horizon, the industry must build on today.
There are two ways a utility can build a better transmission system: Go soft or go hard.
Going soft requires a change in thinking, mathematics, computer systems, software and the numbers that rule the estimates, ratings and regulations that govern transmission. Going hard is usually simpler: a change in hardware.
To look at the hard and soft options available to utilities, Utility Automation & Engineering T&D spoke with two companies about products already available: General Cable’s T-2 conductor (the hard) and The Valley Group’s real-time rating analysis (the soft).
Twisted pair (TP) conductors have been around since the 1970s and were designed to take the bounce of Midwestern gusts. They are specifically geared to shore up lines in those wide, open plains.
“The chief enemy of overhead power lines is wind,” said Dennis Doss, director of technical marketing with General Cable.
Doss described two problem categories that can arise when the wind blows: small amplitude movement like Aeolian vibration and sub-conductor oscillation or a violent, large-amplitude wave known as galloping.
Aeolian vibration is a low-amplitude motion that hits the line at lower wind speeds, say 5 to 15 mph. Think of the vibration in the conductor like that of a guitar string vibrating. The more the line vibrates, at each insulator attachment point along the line the aluminum strands bend, tighten, bend again—and metal fatigue sets in. If left long enough, the strands will snap.
Galloping is a phenomenon when the entire conductor span between the poles bounces up and down. It results from ice buildup on a conductor paired with winds typically blowing more than 15 mph across the conductor. Freezing rain blowing in the wind forms ice along the face of the conductor forming “an air foil shape,” Doss said. And what are air foils designed for? Perfect lift. The more ice, the more wind, the bigger the “air foil” and the greater that lift effect. This can result in violent up and down movement of the entire conductor span. In the end, galloping can cause power outages. During severe events, there might be significant damage to conductors, structures or even complete line failure along one or more spans of a cable, Doss said.
Professional engineer Robert O. Kluge, P.E., transmission line services engineer with American Transmission Co., has seen 345 kV conductors gallop, he said.
“The event occurred on a line with a round conductor and spans that were 800 feet long,” Kluge said. “A review of the physics reveals that at 2.5 pounds per foot times 800 feet, a single span of wire weighs 2,000 pounds, not counting the weight of the ice. This is equivalent to a small Volkswagen flying 24 feet in the air.”
Uncontrolled conductor motions can be destructive, Doss said.
“TP was invented to combat these detrimental and costly consequences,” he said.
At Kluge’s company, more than 10 percent of the total system circuit miles consist of TP conductor, and that figure is increasing. (The company has more than 9,350 circuit miles of transmission.) American Transmission Co. installs predominantly TP conductor on new lines of all voltage levels (69 kV through 345 kV) throughout the system, and galloping is one reason why.
“Today, system reliability is too important. We believe twisted pair conductor is the best available mitigation option to reduce galloping on new lines,” Kluge said.
TP conductors help control the destructive pattern of wind in two ways, Doss said.
“First, the constant varying diameter prevents buildup of resonant vibration,” he said. “Second, low torsional stiffness dissipates the energy from motion-causing wind forces.”(See Figure 1 and Figure 2.)
General Cable’s T-2, a registered trademark describing the company’s TP conductor product, was first patented by Kaiser Metals in 1972. The following year, field testing started with the company running comparisons and lab work for another eight years.
These days, TP conductors like the T-2 have found homes in several Midwestern states, including the Dakotas, Wisconsin, Illinois, Idaho, Nebraska, Oklahoma and Texas. Doss doesn’t expect that to change soon.
“TP has been used by utilities for 30 years,” he said. “The design has a proven record and the emphasis on system reliability continues to make TP an indispensable tool in the transmission and distribution line design toolbox.”
ASTM International, originally known as the American Society for Testing and Materials (ASTM) and one of the largest voluntary standards development organizations, created a specification just for TP conductors in 2000.
As the industry and country upgrade to a smarter grid, it’s more important to keep hardware like the TP in mind, Doss said. The smart grid might require a lot of software, but the transfer of power itself requires a lot of hardware—and the tougher the hardware, the better.
“An outage from galloping can last for days while utilities scramble to survey the damage, procure needed repair materials, repair or replace downed poles or steel structures and install conductors and hardware,” Doss said. “The best defense is a good offense, and TP offers that.”
“To the extent that future technological innovations rely on a highly reliable system,” he said, “TP conductors help support a higher level of reliability.”
Real-time ratings can feed into the positive advantages of TP conductors, among other hardware upgrades for transmission, said Sandy Aivaliotis, senior vice president of operations at The Valley Group.
“Real-time monitoring maximizes the value of any conductor by operating at true capacity, not a static rating,” Aivaliotis said. “The conductor thus delivers its full power transfer capability in addition to its other design benefits.”
Real-time monitoring gives more bang for the buck out of hardware already in place by drawing out real perimeters rather than assumed ones (see Figure 3). Transmission lines have a required minimum clearance, a height they must maintain above the ground for safety reasons, Aivaliotis said. While the ground rarely moves, the line does, especially in the burning sun. Sun warms the line, the line sags, and suddenly it might push the limits of that clearance.
If a utility can’t monitor that line by the hour, minute and second, it must factor in all conditions that might make the line bounce below that required clearance. Even in moderate weather, the assumed physical perimeters of the line are drawn as if that line is sweltering in summer heat or freezing in winter wind—leaving a lot of room most of the time and lacking needed room in extreme situations where the worst case was miscalculated.
“Those [weather] conditions rarely occur,” Aivaliotis said, “so the utilities are consistently sending less current down the line than the line can handle. Real-time monitoring determines exactly how the line is responding to actual weather conditions permitting the utility to safely operate the line at higher power while deterministically maintaining statutory clearance above ground.”
There should be a 10 to 30 percent increase in capacity being available 90 to 98 percent of the time with real-time rating (vs. traditional static rating), Aivaliotis said.
As soft processes like real-time rating and hardware like TP conductors work together, it should not be long before implementation of an intelligent grid system. In tandem, existing technologies such as these make for a healthy body in which to place the brain of a smart grid. The conductors make that body stronger, and the real-time rating brings awareness of the body’s possibilities and limitations. (For more on smart grids and monitoring, see the sidebar.)
“Before the transmission grid can be managed by intelligent entities, the grid’s true capacity must be known,” Aivaliotis said. “Capacity is the foundation upon which all grid management is built.”
Kathleen Davis is senior editor for Utility Automation & Engineering T&D magazine.
The Smart Grid and Monitoring
By Sandy Aivaliotis, The Valley Group
The smart grid seeks to maximize the asset utilization and efficiency of the grid. Real-time monitoring (RTM) maximizes three efficiencies: market, operational and electrical.
1. Market efficiency: RTM removes constraints on transmission equipment that prevent utilities from maximizing the results and thus limit consumer access to low-cost power, such as wind power. Using static assumptions, utilities typically underutilize the capacity of wind turbines for fear of overheating lines. The faster the wind blows, the more power can be generated and the cooler the lines will be. But static assumptions rely only on fixed, worst-case weather conditions, including assumed low wind speeds, and cannot accommodate more favorable wind patterns; they force utilities to underutilize the lines. RTM on the other hand, allows them to take full advantage of changing wind patterns.
2. Operational efficiency: The electric grid is an interlaced network of transmission lines. Generally, utilities know day-to-day which generators will be online, approximate power demand, etc., and they configure the grid for optimum reliability and delivery of least-cost power. If something changes, such as if a generator unexpectedly goes offline or demand substantially and unexpectedly increases, their plans no longer work if the changes result in transmission lines seeing increases in power beyond static limits. An operator must reconfigure the grid to accommodate these changes, and the resulting configuration likely will be less reliable and limit access to the most efficient, lowest-cost power. With real-time ratings, an operator might not need to change the plan because he or she can easily see that the transmission line can safely handle the added power. If something dramatic happens, the operator will have more room to maneuver and make better-informed decisions to avoid repercussions such as a blackout.
3. Electrical efficiency: The higher the voltage rating of a transmission line, the greater the line’s efficiency (i.e., the less power that is lost over distance during transmission). Because of this, utilities try to use high-voltage lines when possible. Using RTM to verify the true line capacity, a utility can put even more power through the line and the losses are even lower. This benefits utilities and consumers because less power is wasted.
Sandy Aivaliotis is senior vice president, operations with The Valley Group, a Nexans company.