Transmission Line Design
The towers and conductors of a transmission line are familiar elements in our landscape. However, on closer inspection, each transmission line has common components with unique characteristics, beginnings and ends.
Transmission towers are the most visible component of the bulk power transmission system. Their function is to keep the high-voltage conductors separated from their surroundings and from each other.
Higher voltage lines require greater separation. The unintended transfer of power between a conductor and its surroundings, known as a fault to ground, will occur if an energized line comes into direct contact with the surroundings or comes close enough that an arc can jump the remaining separation.
The first design consideration for transmission towers is to separate the conductors from each other, from the tower, and from other structures in the environment in order to prevent faults. This requirement and the electrical potential (voltage) define the basic physical dimensions of a tower, including its height, conductor spacing, and length of insulator required to mount the conductor.
Given these basic dimensions, the next design requirement is to provide the structural strength necessary to maintain these distances under loading from the weight of the conductors, wind loads, ice loading, seismic loads, and possible impacts.
Of course, the structure must meet these requirements in the most economical possible manner. This has lead to the extensive use of variants on a space frame or truss design, which can provide high strength with minimal material requirements. The result is the ubiquitous lattice work towers seen in all regions of the country.
The last design requirement is to provide a foundation adequate to support the needed tower under the design loads.
Some of the environmental implications of a transmission line result directly from these transmission tower design requirements.
In addition, excavation, concrete pouring, and pile driving are required to establish foundations.
All of these tasks require access roads and service facilities with dimensions and strength sufficient to handle large, heavy tower components, earthmoving equipment, and maintenance equipment.
Figure 1 shows a lattice-type tower with a single-circuit 765-kV line. A close look at the figure reveals twelve conductors strung from insulators suspended on the crossbar, but this is a single-circuit line. A single-circuit AC line transfers power in three phases.
The voltage in each phase varies sinusoidally with a period of 1/60 second, and each of the phases is separated from the others by 120 degrees.
Thus, there are three isolated conductors for a single AC transmission circuit. In addition, some high-capacity circuits at up to 345 kV use multiple (bundled) conductors for each phase rather than a single larger conductor.
The lattice tower in Figure 1 uses groups of four conductors to carry each of the three phases. Above 345 kV, bundled conductors are normally used to reduce corona discharge.
Lattice Tower Construction on Big Eddy – Knight 500kV River Crossing
There are several other features to note in Figure 1 above. The conductors are supported in a horizontal configuration. This configuration requires broad towers to achieve adequate line separation, which is about 45 feet between conductors for 765 kV.
The horizontal configuration requires a correspondingly greater cleared width for the ROW than a vertical configuration, which stacks the conductors in a vertical plane. The vertical configuration results in higher, narrower towers.
An alternative to the lattice tower, the monopole tower, is also used in this power corridor.
The monopole structures shown here actually support two circuits of three conductors each, for a total of six isolated conductors. Just barely visible at the top outer edges of these towers are grounding lines that are connected directly to the towers and that serve as lightning protection.
Finally, it is important to recognize that Figure 1 represents an important type of shared energy corridor, a power corridor with multiple circuits supported on separate towers. Because of spacing requirements to avoid faults, substantial width is required to separate the tower lines.
Figure 2 shows another example of a shared corridor. Here, a high-voltage distribution line is flanked by much higher-voltage transmission lines. Note that the lattice towers each carry two (three-phase) circuits in a vertical configuration and that single rather than bundled conductors are used.
The point of view of the photograph obscures the fact that the lattice towers are twice the height of the wood pole structures.
However, where a change of direction is required, the conductor tension is unbalanced and a stouter tower, called a deviation tower, is required. This tower is likely to have a broader footprint than the other towers.
Figure 3 shows a 765-kV deviation tower located less than 50 yards from a new two-story home.
The illustration provides a good indication of the size of these towers. The footprint for towers along straight segments is smaller because the balanced conductor load reduces the bending moment that must be supported at the foundations.
A variety of conductor compositions and constructions are currently in use to meet a variety of specific requirements. In the early years of the industry, copper was used almost exclusively because of its high electrical conductivity, but cable diameters with copper were determined more by the need for mechanical strength than by the need for improved conductivity.
The low strength-to-weight ratio of copper limited the acceptable span length (distance between towers).
The first aluminum transmission lines were installed in the last 5 years of the 19th century. An additional incentive favoring aluminum conductors in more recent times is that aluminum is more economical to use than copper, even though aluminum has only 60% of the conductivity of copper. Typical aluminum conductors are composed of multiple 1/8-inch-thick strands twisted together.
There are about 50 varieties of multistrand conductor cables, which are named after flowers, perhaps because the cross sections suggest flower-like patterns and symmetry. The Narcissus is a 61-strand conductor that can carry over 1,100 amperes.
In 1907, aluminum-steel composite cables were introduced to achieve an even higher strength-to-weight ratio while maintaining the electrical performance of aluminum. These cables have a central core of steel strands surrounded by aluminum strands.
While steel is relatively poor conductor, its high strength makes it possible to increase span lengths, which reduces tower investments. These composite conductors are designated by stranding combinations. For instance, 84/7 has 84 aluminum strands surrounding a central core of 7 steel strands.
These aluminum conductor steel reinforced (ACSR) composite conductors have been given bird names, rather than flower names. For example, the 26/7 ACSR conductor is known as the Starling.
Very recently, a new type of composite using ceramic fibers in a matrix of aluminum has been introduced that has lighter weight and higher strength. These ACCR cables (aluminum conductor composite reinforced) were the first technology tested at the Electric Power Research Institute’s Powerline Conductor Accelerated Testing Facility, which opened in 2003.
This new conductor format has the advantage of high strength even at elevated temperatures, and the addition of zirconium to the aluminum alloy makes it more resistant to degradation at high temperatures.
As indicated, the voltage required for economical transmission of electric power exceeds the voltage appropriate for distribution to customers.
First, customer equipment generally operates at only a few hundred volts, rather than at the hundreds of thousands of volts used for transmission. Second, if high voltages were maintained up to the point of customer connection, fault protection would be extremely expensive.
If the new transmission line were high-voltage direct current (HVDC), the origin substation would be expanded to accommodate AC-to-DC converters. Intermediate substations may also be required if there is a voltage change along the route, say, from 500 kV to 230 kV.
Figure 4 shows a Midwestern substation that supplies a 765 kV long-distance transmission line from 345 kV feeders connected to area power plants. The site occupies approximately 10 acres.
Figure 5 shows a substation of comparable size under construction. This substation, which is now complete, is the terminus of a 500 kV, 600 MW line in the Bonneville Power Authority System.
A ROW is a largely passive but critical component of a transmission line. It provides a safety margin between the high-voltage lines and surrounding structures and vegetation. The ROW also provides a path for ground-based inspections and access to transmission towers and other line components, if repairs are needed.
A ROW generally consists of native vegetation or plants selected for favorable growth patterns (slow growth and low mature heights). However, in some cases, access roads constitute a portion of the ROW and provide more convenient access for repair and inspection vehicles.
Drone Transmission Line Right-of-Way
Demonstration of the ability to quickly, cost effectively and safely inspect right-of-way easements for utility companies.
Access routes to transmission line structures for both line construction and maintenance use existing roads wherever possible. At least a portion of existing roads along the route is likely to be paved. New roads constructed for access would be gravel.
This refers to existing roads that need improvements in order to meet the loads expected for line construction and maintenance. Roads are also classified as temporary or permanent. A temporary road will be decommissioned after construction is complete, and the ROW will be restored.
The roadway includes the traffic-bearing traveled way, the shoulders, and areas adjacent to the road that have been excavated or filled to provide drainage and support. Beyond the roadway are the clearing width and the outer boundary of the ROW.
Electricity Transmission Project – Germany 2013
Providing temporary access roads and pads for the construction of a brand new power line in Germany.
Reference // The design, construction and operation of long-distance HV electricity transmission technology by J.C. Molburg, J.A. Kavicky and K.C. Picel (Argonne National Laboratory)