Written by faizul hadi on 12:50 AM
The towers and conductors of a transmission line are familiar elements in landscape. However, on closer inspection, each transmission line has unique characteristics that have correspondingly unique implications for the environment. In this section, we list design specifications (line characteristics) that are commonly required to define a transmission line. Many of these specifications have implications for the net environmental effects.3 For the purpose of this report, a range of values is considered for these specifications, with the exception that a fixed nominal voltage of 500 kV is assumed.
1. Overall Descriptive Specification
The most basic descriptive specifications include a line name or other identifier, nominal voltage, length of line, altitude range, and the design load district. The line identifier is commonly taken from endpoint names, e.g., Inland−Macedonia on the Cleveland Electric Illuminating Co. system. The endpoint names are generally geographic points, but may be substation names or major industrial facilities. The nominal voltage is an approximation to actual line voltage that is convenient for discussion. Actual voltage will vary according to line resistance, distance, interaction with connected equipment, and electrical performance of the line. For AC lines, the nominal voltage is close to the RMS (root mean square) voltage.4 The altitude range is a rough surrogate for weather and terrain. This is important, since nearly all aspects of line design, construction, and environmental impacts are linked to weather.
The design load district is another surrogate for weather. These districts are defined by the National Electrical Safety Code (NESC) and by some local jurisdictions. These districts include NESC Heavy Loading, NESC Medium Loading, NESC Light Loading, California Heavy Loading, and California Light Loading. The design wind and ice loading on lines and towers is based on the design load district. This affects insulator specifications as well as tower dimensions, span lengths, tower design, and conductor mechanical strength and wind dampening.
2. Tower Specifications
The towers support the conductors and provide physical and electrical isolation for energized lines. The minimum set of specifications for towers are the material of construction, type or geometry, span between towers, weight, number of circuits, and circuit configuration. At 500 kV, the material of construction is generally steel, though aluminum and hybrid construction, which uses both steel and aluminum, have also been used. The type of tower refers to basic tower geometry. The options are lattice, pole (or monopole), H-frame, guyed-V, or guyed-Y. The span is commonly expressed in the average number of towers per mile. This value ranges from four to six towers per mile. The weight of the tower varies substantially with height, duty (straight run or corner, river crossing, etc.), material, number of circuits, and geometry. The average weight of 670 towers for 500-kV lines included in the EPRI survey (EPRI 1982) is 28,000 lb. The range of reported tower weights is 8,500 to 235,000 lb. The type of tower (specific tower geometry) is very site-dependent, and, for any given conditions, multiple options are likely to exist. The number of circuits is generally either one or two. The circuit configuration refers to the relative positioning of conductors for each of the phases. Generally the options are horizontal, vertical, or triangular. The vertical orientation allows for a more compact ROW, but it requires a taller tower.
3. Minimum Clearances
The basic function of the tower is to isolate conductors from their surroundings, including other conductors and the tower structure. Clearances are specified for phase-to-tower, phase-toground, and phase-to-phase. Phase-to-tower clearance for 500 kV ranges from about 10 to 17 feet, with 13 feet being the most common specification. These distances are maintained by insulator strings and must take into account possible swaying of the conductors. The typical phase-to-ground clearance is 30 to 40 feet. This clearance is maintained by setting the tower height, controlling the line temperature to limit sag, and controlling vegetation and structures in the ROW. Typical phase-to-phase separation is also 30 to 40 feet and is controlled by tower geometry and line motion suppression.
4. Insulators
Insulator design varies according to tower function. For suspension towers (line of conductors is straight), the insulator assembly is called a suspension string. For deviation towers (the conductors change direction), the insulator assembly is called a strain string. For 500-kV lines, the insulator strings are built up from individual porcelain disks typically 5.75 inches thick and 10 inches in diameter. The full string is composed of 18 to 28 disks, providing a long path for stray currents to negotiate to reach ground. At this voltage, two to four insulator strings are commonly used at each conductor connection point, often in a V pattern to limit lateral sway.
5. Lightning Protection
Since the towers are tall, well-grounded metallic structures, they are an easy target for lightning. This puts the conductors, other energized equipment, and even customer equipment at high risk. To control the effects of lightning, an extra set of wires is generally strung along the extreme top points of the towers. These wires are attached directly to the towers (no insulation), providing a path for the lightning directly to and through the towers to the ground straps at the base of the towers. The extra wires are called shield wires and are either steel or aluminum-clad steel with a diameter of approximately ½ inch.
6. Conductor Motion Suppression
Wind-induced conductor motion, aeolian vibration, can damage the conductors. A variety of devices have been employed to dampen these oscillatory motions. By far, the most common damper style on 500 kV lines is called the Stockbridge damper. These devices look like elongated dumbbells hung close to and below the conductors, a few feet away from the point of attachment of the conductors to the tower. The weighted ends are connected by a short section of stiff cable, which is supported by a clamp to the conductor immediately above. Dampers can prevent the formation of standing waves by absorbing vibrational energy. Typically, a single damper is located in each span for each conductor.
A line seperation of at least one span (perhaps 700 spans) has proven effective in avoiding multicircuit outages. A seperation of 2000 feet from adjacent 500KV lines was planned for the Los Banos-Gates 500 KV line in California. This seperation requirement implies a substantially wider ROW than would be required for electrical protection and line maintenance.
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