WO1995008684A1 - Tower constructed of pultruded composites - Google Patents

Abstract

A high voltage electrical transmission line tower (22) having leg members (15) and cross bracing members (19, 21) is constructed using glass reinforced pultruded composites. Using only pultruded composite material in the vicinity of the conductors (18) permits the elevation of the tower to be reduced and closer spacing of conductors when compared to conventional steel towers. Also, EMF at ground level is substantially reduced.

Description

TOWER CONSTRUCTED OF PULTRUDED COMPOSITES

BACKGROUND OF THE INVENTION

The disclosed tower is the subject of United States Patent Application No. 715,912 filed June 14, 1991, issuing September 28, 1993, Patent No. 5,247,774. The invention combines the fields of high voltage transmission towers and pultruded composite construction, and continues a series of developments relating to the fabrication of relatively large structures formed from pultruded composites. Particular attention has been directed toward the development of effective jointing techniques in a field in which steel construction has dominated, but using materials for which the steel joining techniques of bolting and welding are unsuitable. Currently, virtually all transmission towers are made of steel. However, certain undesirable performance limitations are inherent in steel, the foremost being high electrical conductivity. Inasmuch as it is the role of the tower to support, and isolate from ground, conductors carrying more than 200,000 volts, the towers must be quite large, both to separate the individual conductors from each other and from the steel structure, and to accommodate inter-tower line sag. The high conductivity of the steel structural supports mitigates against these goals, increasing flashover potential and posing a chronic safety hazard to the line maintenance crew as well. Steel towers also inevitably suffer from deterioration from rust and corrosion and must be coated regularly and eventually replaced, often at great expense if the site is remote and inaccessible.

A consideration involving transmission towers of current and increasing concern regards the powerful, EMF radiation field in the immediate vicinity of the lines. EMF is suspected of being linked to cancer in animals and humans who live or work under the conductors on a daily basis. Whether or not this alleged link is ever substantiated, the current public perception that it may be true causes problems right now, involving land values, lawsuits, the anxiety for property owners and those who work or dwell in the immediate vicinity of high voltage lines. While steel used in towers may not directly enhance EMF radiation, partly due to its conductivity the out-of-phase conductors must be widely spaced apart, which minimizes the flashover potential but also reduces the natural phase cancellation that occurs between out-of-phase fields.

These factors warrant consideration of alternative techniques and materials for tower construction, and composites have characteristics that make them worth investigating. A pultruded composite is made by drawing a bundle of fibers through a resin bath and then through a die, in which it is heat-cured to a smooth, hard member that is usually a thermal and electrical insulator and as well as being resistant to corrosive chemicals. There are few large structures made of pultruded composites. This is due at least in part to problems in joining composite members with structurally sound joints. When composite members are fastened with conventional fasteners such as bolts and screws, joint strength is generally unacceptable.

Using steel, pre-punched, drilled or torched holes can be used without significant loss of strength other than that due to the absence of the removed material itself. Alternatively or in addition to bolts, welded steel joints are of strength equal to or greater than bolted joints and comprises a routine field alternative, generally chosen based on the ready availability of materials and access rather than structural requirements.

These fastening techniques which give steel construction high joint strength do not transpose well to composites. Although composites may be chemically welded with epoxy, such bonding is supplemental only and does not approach the strength of a steel weldment. Nuts and bolts would be a logical alternative but boltholes destroy fiber continuity in composite members, creating points of debilitating stress concentration.

However, although structural engineers have been accustomed to working with steel and naturally tend to apply steel joining techniques to other materials, fortunately nuts, bolts and welding are not the only joint-forming alternatives. A number of high-strength interlocking joint construction techniques that do not require welding or penetration have been developed and are used in the construction of the disclosed composite tower.

SUMMARY OF THE INVENTION

The invention is a high-voltage transmission tower made substantially completely from composites. It is superior to typical towers currently in use in size and weight, weighing about half as much as a conventional tower of equal power rating and having overall dimensions on the order of two-thirds of comparable towers. The narrower right-of-way required for a transmission line supported by these compact towers represents a significant economic advantage in an era in which power consumption is universally increasing and transmission lines are being replaced by larger, higher capacity lines daily. The reduction of EMF resulting from the elevated level of phase cancellation due to the narrower spacing between the conductors is largely a windfall gain over and above the advantages of greater longevity inherent in the highly chemical- and UV-resistant composite columns and lattice members, reduced tower size, increased safety to maintenance workers and improved aesthetics, that characterize the tower.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a diagrammatic view of a tower made according to the invention;

Figure 2 represents diagra matically a prior art tower;

Figure 3 is a prior art tower of a different style of construction than the tower of Figures 1 and 2, with a similarly configured composite tower superimposed on the prior art tower for size comparison purposes; and,

Figure 4 illustrates the relationship between wet insulator flashover and the size of the gap between the conductor and ground, that is the distance between the conductor and the grounded steel structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A typical prior art tower is indicated at 10. It consists of a lattice structure 12, sometimes called a "cage" with extended support arms 14. Strings of insulators indicated at 16 support the power lines 18. A "goat peak" 20 supports a lightning shield wire, not shown. The structure in Figure 2 is all L-angle steel, and is bolted together with as many as 1500 bolts.

The invention is shown in Figure 1. Although similar in appearance to the prior art in the illustrated embodiment, it is vastly different, being made of pultruded composites, and having no bolts.

The spacing of the phases, or individual cables, is fairly narrowly defined by the line voltage rating. The rule is expressed as, The wet-insulation flashover should be four times the line-to- ground voltage. For example a three-conductor tower carrying 345 kilovolts is first divided by V3 , which equals approximately 200 kilovolts. Since four times two hundred equals 800 kilovolts, an insulator string is selected to space the conductor from the metal tower sufficiently to have a minimum of 800 kilovolt flashover with wet insulation. The chart in Figure 4 illustrates how to convert from a flashover voltage to air gap spacing. Therefore, at 345 kv, the conductors must be approximately 110 inches from ground potential, which includes all of the tower in addition to the actual ground. Referring to the drawing of Figure 3, a modified form of the new tower 24 can be seen superimposed on the equivalent metal tower 10. By eliminating the conductive material in the tower, it can be seen that the wires can be brought in to approximately half of their former spacing in the new composite tower, from spacing "D" in the steel tower to "1/2 D" in the composite tower. The same approximate ratio of reduction in spacing applies to the conductor-to-frame spacing "B" and the vertical conductor-to-conductor spacing "A".

This same efficiency in spacing is apparent in Figure 1 as the tower 22 is approximately 80% as high as the tower of Figure 2. The closest conductor to ground level, 18, remains at the same height in both configurations, to ensure with conductor sag, the minimum safe height above ground level is achieved. However, in Figure 1, a compaction of conductors, or phases, is possible because the tower is a fully insulative composite and the design criteria of Figure 4 is no longer a limiting criteria. Thus the lengths of the insulators 17 in Figure 1 are half the length of the insulators 16 in Figure 2.

The insulator length of Figure 1 could be reduced to half the typical length required of a steel tower as shown, but it could alternatively be eliminated as a separate unit. This could be achieved by adding the silicone rubber sheds, a common "tracking" resistant skirt material used in high voltage polymer insulators, to extended rods which are an integral structure of the tower as shown at 26 in Figure 1. In lieu of separate insulators, the sheds that are generally installed on insulator rods will be installed directly on a portion of the tower adjacent to the attachment point of the conductor. This is shown on just one arm of the tower in Figure 1 at 26 but would of course replace all of the hanging insulators.

By compacting the conductors, the tower height is reduced, the right-of-way owned by the entity transmitting electricity is more compact, energy is transmitted more efficiently due to lower inductive reactance, the electromagnetic field (EMF) at ground level is reduced, and further reduction in weight is achieved.

IT IS HEREBY CLAIMED:

Claims

1. A high voltage electrical transmission tower for use in areas where maximum limits on EMF at the ground level are desirable in the vicinity of transmission lines comprising:

(a) a vertically oriented three dimensional support lattice structure having a top end and a bottom end, said support lattice structure having at least three upright oriented leg members that are structurally connected to each other by cross bracing members;

(b) means for supporting at least three high-voltage wire conductors, for conducting at least partially mutually out-of-phase currents, adjacent the top end of said support lattice structure in a predetermined configuration in which said high-voltage wire conductors are each spaced from the next closest of said wire conductors at least a minimum predetermined distance B from said vertically oriented three dimensional support lattice structures, at least a portion of said support lattice including said vertical leg members and substantially all of said cross bracing members in the vicinity of said three high voltage wire conductors being constructed of pultruded material permitting the compacting of said conductors whereby the resulting ground level EMF is reduced and said three dimensional lattice is formed with reduced vertical height.

2. A high-voltage electrical transmission tower recited in Claim 1 wherein said high voltage wire conductors are designed to carry at least 115 kilovolts and said lattice structure is configured and dimensioned to support said wire conductors spaced with minimum distances A and B as predetermined by industry standards from 115- KV voltage class towers having the electromagnetic and electrical characteristics of pultruded composites of the type used in said tower.

3. A high voltage electrical transmission tower as recited in Claim 1 wherein substantially all of the leg members and bracing members of said three dimensional support lattice structure are made of pultruded composite material.

4. A high voltage electrical transmission tower as recited in Claim 1 wherein said means for supporting said high voltage wire conductors is a single laterally extending composite support arm and a plurality of strings of insulators depending therefrom which support said three high voltage wire conductors.

5. A high voltage electrical transmission tower as recited in Claim 1 wherein said means for supporting said high voltage wire conductors are three vertically spaced laterally extending composite support arms each of which has a string of insulators depending therefrom and which support a respective one of said conductors.