Electrical Power Plant Design: Transmission System

WEEK 06: TRANSMISSION: OVERHEAD AC

Requirements The electrical operating performance of a transmission line depends primarily on the insulation. An insulator not only must have sufficient mechanical strength to support the greatest loads of ice and wind that maybe reasonably expected, with an ample margin, but must be so designated as to withstand severe mechanical abuse, lightning, and power arcs without mechanically failing. It must prevent a flashover for practically all power-frequency operating condition and many transient voltage conditions, under any conditions of humidity, temperature, rain or snow, and with such accumulations of dirt, salt, and other contaminants which are not periodically washed off by rains.

Materials The majority of insulators are made of glazed porcelain. Porcelain is a ceramic product obtained by the high-temperature vitrification of clay, finely ground feldspar, and silica. The insulator glaze seals the porcelain surface and is usually dark brown, but other colors such as gray and blue are used. Porcelain insulators for transmissions maybe disks, posts or long-rod types. Glass insulators are also used, made from toughened glass, usually clear and colorless or light green. For transmission voltages they are available only as disk types. Most glass disk insulators will shatter when damaged, but without mechanically releasing the conductor. Synthetic insulators, pioneered by GE in 1963 are used for high-voltage transmission lines. Most consist of a fiberglass rod covered by weather sheds of skirts of polymer (silicon rubber, polytetrafluoroethylene, cycloalipathic resin, etc. Other types include cast polymer concrete called Polysil R (i.e., polymer bonded silica) and a coreless type with alternating metal and insulating sections (i.e., metapol insulator).

Types Transmission insulators maybe strings of disks (either cap and pin, or ball and socket), long rods, or line posts. Posts are only infrequently applied above 230 kV. The pin-type insulator is designed to be mounted on a pin which in turn is installed on the crossarm of the pole. The insulator is screwed on the pin and the conductor is mounted on the insulator. The pin insulator can weigh anywhere from 1/2 to 90 lb. This type of insulator is applicable for rural and urban distribution circuits. Larger, stronger pin-type insulators are used for high-voltage transmission lines. These differ in construction in that they consists of two or three pieces of porcelain cemented together. Post-type insulators are somewhat similar to pin type. They are generally used for higher voltage applications. They maybe mounted horizontally as well as vertically, although their strength is diminished when mounted horizontally. The insulator is made of one piece of porcelain and its mounting bolt or bracket is an integral part of the insulator. The suspension insulator, also called ball and socket type hangs from the crossarm. The line conductor is attached to its lower end. The entire unit of suspension insulators is called a string. How many insulators this string consists depends on the voltage, the weather conditions, the type of transmission construction and the size of insulator used. It is important to note that in a string of suspension insulators one or more insulators can be replaced without replacing the whole string. Strain insulators often consists of an assembly of suspension insulators. They are constructed so that the cable will compress and not pull apart the porcelain. They are particularly used at a corner, at a sharp curve or at a dead end, as well as in guy cables, where it is necessary to insulate the lower part of the guy cable from the pole for the safety of the people on the ground. Spool insulators is usually used for secondary mains. The spool insulator may be mounted on a secondary rack or in a service clamp. Both the secondary low voltage conductors and the house service wires are attached to the spool insulator. The use of this type has decreased greatly since the introduction of cabled secondary and service wires.

Design Three basic design considerations of line insulation are in practice. The Power-Frequency Design considers that flashover shall not occur for normal operating conditions, including reduced clearances to the structures from high wind. Design for contamination, salt deposits and water-soluble conducting liquids, is usually expressed as inches of creepage per kilovolt, where the creepage distance is the length of the shortest path for a current over the insulator surface and ranges up to 2 in/kV or more for heavy contamination. Standard insulator disks (10 X 5-3/4 in) have a typical creepage length of 11.5 in per disk. The glaze has a surface resistivity of about 10 MW per square. The contaminants can be removed by hot-line washing, using high-pressure water and insulated nozzles and hoses. Another method is dry-cleaning by the use of an abrasive powder such as a limestone mixture or biodegradable plastic pellets discharged at high pressure. In Switching Surge Design considers that the operation of a circuit breaker on a transmission line can cause transient overvoltages, although flashovers due to such switching surges are rare in lines below 500 kV. If the breaker is opening, this may be due to restrikes across the breaker contacts as they separate, although restriking has been nearly eliminated with present technology. If the breaker is closing, the cause maybe unequal voltages on each side of the breaker, including the effect of residual charge on the line from a recent deenergization. The crest magnitudes of switching surges are normally defined in per unit of nominal power-frequency-crest phase-to-ground voltage. Thus insulators are designed using (a) the maximum expected surge, (b) 2% of the statistical level, (c) a statistical approach for a low number of flashovers per switching event and (d) statistical distribution of switching surge crests. In Impulse Surge Design considers the fact that impulse surges on a line are caused by lightning strokes to or near the line. At transmission insulation levels, only strokes that directly intercept the line are capable of causing flashovers. Methods to curb this effect tend to deal more in changing the parameters such as shield wires or grounding than to add insulation. Also, the use of surge arresters and lightning arresters are employed.

Line and Structure Location

The cost of preparing for transmission-line construction is a considerable part of the total cost, even as much as 25%. Right-of-way and clearing are more or less fixed by local conditions, but the cost of surveys, accompanying maps, profiles, and engineering layout is governed by judgment.

Location Following a general reconnaissance by ground or air, for which 10 to 20 days per 100 mi should be allowed, and the assembling of all available maps and information, control points can be established for a general route or areas selected for more detailed study which may prove to be determining factors in the location of the line. The policy as to such matters as right-of-way condemnation, electrical environmental assessments, telephone coordination, navigable-stream crossing, air routes, airports, and crossings with other utilities must be decided as definitely as possible. Preliminary specifications should be issued before the final survey is started. These should include (1) outline drawings of the various structures with the important dimensions, (2) conductor sag curves and a sag template, (3) the maximum spans and angles for each type of structure, and (4) the requirements for right-of-way and clearing. Location Survey follows with at least four divisions of work (1) an alignment party, choosing the exact location and cutting out the line; (2) a staking party, driving stakes at 100-ft stations and locating all obstructions; (3) a level party, taking elevations and side slopes; and (4) a property and topography party, locating property lines. Generally, right-of-way is not purchased in fee, but a perpetual easement is secured in which the owner grants the necessary right to construct and operate the line but retains ownership and use of land. The easement must provide for (1) a means of access to each structure; (2) permission to erect all structures and guys; (3) all trees and brush to be cleared over a specific width for erection; (4) the removal of trees which would not safely clear the conductor if they were to fall; (5) the removal of buildings, lumber piles, haystacks, etc., which constitute a fire hazard.

Tower Spotting The efficient location of structures on the profile is an important component of line design. Structures of appropriate height and strength must be located to provide adequate conductor ground clearance and minimum cost. Manual Tower Spotting involves the use of a celluloid template, shaped to the form of the suspended conductor, is used to scale the distance from the conductor to the ground and to adjust structure locations and heights to (1) provide proper clearance to the ground, (2) equalize spans, and (3) grade the line. The template is cut as a parabola on the maximum sag (usually at 49°C) of the ruling span and should be extended by computing the sag as proportional to the square of the span for spans both shorter and longer than the ruling span. The template must be used subject to a "creep" correction for aluminum conductors. Creep is a nonelastic conductor stretch which continues for the life of the line, with the rate of elongation decreasing with time. Creep also causes continuous slow increase in the sag of the line. Uplift is seen especially on steep inclined spans in which the low point may fall beyond the lower support, this indicates that the conductors in the uphill span exerts a negative or upward pull on the lower tower. Swing defines the minimum weight that should be allowed on any structures by determining the traverse angle to which the insulator string may swing without reducing the clearance from the conductor to the structure too greatly. The insulator will swing in the direction of the resultant of the vertical and horizontal forces acting on the insulator string. Computer Tower Spotting considers the fact that there are a very large number of possible permutations of tower spotting. These programs are of different type, including look ahead, dynamic, reiterative dynamic and computer-aided design. Computers has advantages in terms of speed, reduced labor and convenience of redesign but it is data extensive, huge computer costs and software limited.

Mechanical Design Considerations