TVA designs a phase-over-phase steel-lattice tower for a narrow right-of-way switch application.
In general, it is getting harder and harder for utilities to site new facilities and structures. Tennessee Valley Authority (TVA) has been facing this problem for several years. In 2011, the utility ran into this issue in Niota, Tennessee, U.S. The location was a customer delivery site with an existing 90-ft (27-m) concrete pole at the tap point, near a substation. The right-of-way was only 75 ft (239 m) wide, which is narrow, compared to TVA's normal 100-ft (30-m) right-of-way.
TVA needed a way to efficiently tap the existing phase-over-phase 161-kV line. While the site in Niota only required two-way switching, TVA wanted to develop a solution that could be used in Niota and at other locations where three-way switching was needed, as well.
If space had been available, TVA would have installed a three-switch solution roughly a span or two away from the tap point. This type of design can take up to a half acre (0.2 hectare) of space, but TVA did not want to buy additional structure rights. So whatever the solution, it needed to fit within the existing right-of-way. A single structure supporting all the needed switches was the obvious answer, but it was not easy to accomplish.
Size, Load and Deflection
The initial problem was designing a structure that could meet the load requirements. At the Niota site, the power line conductor is 954,000 cmil, 45/7 aluminum conductor steel reinforced (ACSR) with 9,000 lb (4,082 kg) of tension. The tap is the same conductor with 1,500 lb (680 kg) of tension. These are standard loads the existing pole accommodated.
The new design also would have to minimize deflection. TVA has had many problems with phase-over-phase switches. Over time, the movement caused by normal switching operations and weather knocked the switches out of alignment.
Most of the phase-over-phase switches on TVA's system are mounted on single-pole structures that vary in height from 80 ft to 140 ft (24 m to 43 m). Of these, a few are found on 69-kV lines and mounted on shorter structures (less than 100 ft). They operate as designed and with few problems. The switches on the 115-kV and 161-kV lines are mounted on structures more than 100 ft tall. Mechanical failures and adjustment problems occur too often. Since this structure at the Niota site would have to be more than 100 ft tall to accommodate all the switches, deflection had to be minimized in some way.
Deflection is caused by several things; the first is flexure, stemming from tall, flexible structures like poles. A rigid tower seemed to be a good answer. The narrow right-of-way eliminated the possibility of a rigid guyed tower, so a lattice tower had to be designed. For the same moment capacity, a lattice structure deflects less — by an order of magnitude — than a pole structure. The steel-lattice design also would minimize deflection because of weather conditions. TVA uses a 1-inch (25-mm) ice-loading criteria and an extreme wind-loading criteria of 90 mph (145 kmph) with 3-second gusts.
Temperature swings also are a concern. Located in east Tennessee, TVA is far enough north to experience severe winters and far enough south to have very hot summers. For design calculations on ACSR conductors, TVA engineers use a maximum operating temperature of 100°C (212°F) and a minimum of -9°C (15.8°F), which is the unloaded, weight-bare temperatures the conductors will operate within. The conductors get tight at -9°C and loose at 100°C. On a pole structure, the difference between those two temperatures can be up to 1 ft (305 mm) of deflection; on a lattice tower, it is limited to a 0.375 inch (9.5 mm).
The Switch Solution
TVA also carefully chose switches whose operation would minimize the deflection and adjustment problems. Engineers intentionally avoided two- and three-way switches that use a common center insulator with side-break switches, since deflection-related problems occur more often with those types of switches. The side-break switches experience a lot of movement during operations. TVA evaluated several types of switches, finally choosing the USCO AVR switch by Hubbell Power Systems Inc. Each AVR switch is mounted independently, and the three-insulator design limits movement during switch operations.
The next problem was configuring the switches. How could the utility maintain proper clearances and put all the switches on one tower? The answer: build a tall tower. The completed structure at the site is 132 ft (40 m) tall, which includes a 55-ft (17-m) distance from the ground to the lowest conductor and a 22-ft (7-m) phase spacing. There also is an overhead ground wire. Therefore, the top switch is at an elevation of 110 ft (33.5 m), and the overhead ground wire is at 132 ft.
While designing this tower, TVA also designed an alternate, shorter tower with a 33-ft (10-m) ground to the lowest conductor distance, which might be useful in other situations. The 132-ft design met the needs of this project and could be used on 161-kV lines and 230-kV lines, as well.
In the new single-structure design, TVA mounted the six vertical break switches horizontally on the main line side, with the switch stacks and the switch blades parallel to the ground. If three-way switching is necessary, TVA can vertically mount three more vertical break switches. This provides enough room for all nine switches and for them to operate normally. Normal operation of multiple switches (mounted on the same structure) has not always been possible on the TVA system.
The operating rods on some of TVA's existing higher-voltage switches are up to 120 ft (37 m) long. Because of torsional deflection in the shaft, on some structures, the switches are designed so the top switch closes before the middle switch and the middle switch closes before the bottom switch. This causes the switches to be intentionally out of time sync during closing, but is necessary to ensure all three phases close completely into the jaws and to achieve proper toggle on the reach rods.
The new design eliminates this problem. The new design includes a 1-to-1 gearbox at the base of the bottom switch that converts the rotational movement of the operating rod into a linear (push-pull) movement. This linear movement allows all three phases to close simultaneously, alleviating the timing issues.
One other difficulty with switch operations was addressed. In the old configuration, most of the switches are operated manually with a swing handle, which can be operated at various speeds and normally requires greater effort on the part of the operator. The new design includes a crank handle with a 30-to-1 gear ratio. As a result, the switches close smoothly and are much easier to operate.
A Solid Switch Structure
From an operational perspective, the new design meets all the requirements, but there was a trade-off. The tower is 132 ft tall, which is remarkable, especially when looking at it from the substation. The entire site — substation and tower — is located on a steep hill. When the substation was built, the lower side required 12 ft (4 m) of fill to level the yard. The tower is on a hill above the substation, and the overhead ground wire attachment point is 100 ft above the ground-wire attachment point on the pull-off structure. The tower has one other special feature. Since TVA has a 100% fall-protection requirement, step bolts were installed on all four tower legs.
Now complete, the steel-lattice tower meets all of TVA's needs and gives TVA one more valuable tool in its transmission arsenal. In the future, this structure can be used to tap any phase-over-phase lines and not have to buy any additional rights-of-way.
The initial tower at Niota was installed in March 2012, and a second tower was installed in Calhoun, Tennessee, in June 2012. Currently, TVA has plans for four additional switch structures, similar to those installed in Niota and Calhoun.
Jeffery L. Phillips (email@example.com) has been the switch specialist at Tennessee Valley Authority for more than 10 years. His responsibilities include approving all new switch designs, contract technical steward for transmission line switches, assisting construction crews with the installation of new switches as well as troubleshooting maintenance problems with existing transmission line switches. His switch experience includes pole- and tower-mounted switches from multiple manufacturers. He is a professional engineer.
Steve Cantrell (firstname.lastname@example.org) is a principal engineer in the transmission line structure and foundation section of Tennessee Valley Authority. Cantrell graduated from Tennessee Technological University in 1990 with a BSCE degree. He is a registered professional engineer in the state of Tennessee and has been with TVA for 21 years, where he is currently responsible for designing and detailing transmission line structures and foundations.
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Tennessee Valley Authority | www.tva.gov