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Guyed structures for transmission lines
Guyed structures for transmission lines H. Brian White P.O. Box 939, Hudson, Quebec, Canada
Guyed structures have become the dominant type for use on transmission lines remote from urban areas or where land use restrictions are prohibitive. All other attributes, including cost of materials, erection and performance are positive and the continuing pace of innovations of types and techniques is adding to their attractions. This review of old and new types and practices illustrates adaptability and some of the many new ideas produced in response to frequently changing demands and conditions.
Keywords: guyed structures or towers, transmission lines Three different types of guyed structures are used in modern transmission line design and the line designer should recognize and be aware of the seemingly minor but significant differences between them. They are: • • •
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single poles or masts rigid frames and hinged masted structures plus several guyed structures
This paper will attempt to demonstrate the issues common to the three and point up the differences that must be well understood if the few pitfalls are to be avoided and then full advantage is to be taken of what can be done with some but not all of them.
Guyed transmission towers vs. guyed communication masts A structural engineer who is familiar with the behaviour and analysis of very tall guyed radio or TV masts must recognize the great differences with the guyed structures used to support the wires of very high (VH) and extra high voltage (EHV) transmission lines. The transmission line towers are usually 2 0 - 5 0 m in height and the guys are thus short enough that wind or ice loads on the guys are of little consequence in analysis or performance and the sags in the guys are also very small so that, with only a few exceptions that will be discussed below, slack and strain changes in the guys can be neglected.
tures the most popular supports for HV and EHV lines in countries around the world. Their advantages are many and, when conditions of use are suitable, large economies can result if the best type is selected to suit the many and varying conditions that can face the line designer. However, a guyed structure is not always the best solution as restrictions on, or perceived problems with, the use of guys are sometimes very real and decisive and correctly lead to the selection of other types of support. For example, guyed structures will almost always require a larger site area for the anchoring of the guys and this alone may rule out their use on farm lands where large equipment is used. The guys can present a hazard to the equipment and conversely the large machinery can pose a serious threat to the towers. In urban and suburban areas, the right-of-way for a new line may depend on minimizing the occupied land area and this may force the use of very compact rigid lattice constructions or embedded and cantilevered tubular poles of steel, concrete or other materials. The line designer will find that the correct selection requires both an appreciation of all of the conditions of the line to be built as well as a full knowledge of the characteristics of the many different guyed tower types that have been developed and very often used with success to meet the conditions of different lines. Thus the suitability of a specific guyed tower type for a given project may depend on an assessment of some of the following:
Basic guyed tower concepts The old but still oft quoted three basic rules of structural engineering are that (i) the most efficient structural component is a wire rope in tension; (ii) that the most efficient in compression is a lattice mast; and (iii) do not try to push (i). Consideration of these three rules, which might be adjusted slightly by removing the whimsical third and substituting 'and every effort should be made to minimize bending in the second' are the precepts that have been used by line designers to make guyed struc0141-0296/93/040289-14
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• • •
Terrain; flat, rolling, mountainous. The guyed portal towers (Figure 1) fit well on flat terrain, the guyed Ys and Vs (Figures 2 and 3) were developed for rougher terrain Access and transport of equipment and materials (and workmen), by mule, truck or helicopter Erection of towers, piece by piece, with gin pole or mobile crane, by small or large helicopter Procurement of easements: guyed towers usually require larger site areas for placement of anchors
© 1993 Butterworth-Heinemann Ltd
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Guyed structures for transmission lines." H. B. White.
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110 kV guyed portal in Finland about 1930 designed to replace traditional wood pole H frames with a structure of greater height, strength and more longitudinal stability. It is possible that several countries were moving in the same direction at that time
and possibly temporary whole tower ground assembly space. However, some guyed tower types such as the cross-rope suspension in Figure5(d) may permit greatly reduced rights-of-way requirements in the spans between towers Structural loadings, ice and wind load severity Voltage of the line: as voltage increases, tower (crossarms) widths increase faster than heights. At 138 kV, the crossarm of a rigid or guyed Y or V presents a small and innocuous problem while at 500 and 800 kV, the crossarm can be more than 50% of
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tower weight resulting in a top-heavy construction problem that may yield to the CRS solution. Thus the selection of the appropriate guyed tower type may be very dependent on the voltage or scale of the line Importance of variable electrical parameters: magnetic field strengths below the line, corona effects such as radio interference and line reactance (power transfer capability) are dependent on phase spacings and arrangements that may be limited by the selected tower type
Guyed structures for transmission lines. H. B. White
Figure 2 Guyed Y of aluminium for Ontario Hydro's 500 kV line in 1963. Shown under test at Aluminum Company of Canada (Alcan) test site at Kingston, Ontario
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Figure 3 Guyed V designed in aluminium by Alcan for 765 kV Project UHV of General Electric at Pittsfield MA, USA in 1961
Basic types of guyed structures
Guyed single poles or masts
The guyed tower types discussed in this paper are those known to the writer and no slight is intended to anyone whose concept has been overlooked. The writer's personal library contains many interesting and useful writings on some of the 18 types that are discussed but many of the documents are from trade magazines or from conferences with no published transactions: thus no reference can be quoted.
Single guyed masts or poles find many applications on modern lines and are often the most efficient means of handling the large horizontal loads of large angle positions or at points where dead ending is required. The more common use of guyed single poles or masts relies on an effective pinned connection to the footing. Attempts to apply guys to fixed masts or poles will, of course, introduce indeterminancies and predictable per-
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Guyed structures for transmission lines: 14. B. White formance will depend on the degree and control of pretensioning of the guys and close control on any future movement of guy anchors or rotation of the mast foundation. When the poles or masts are used as dead ends, the line or wire tensions are usually taken by in-line back guys, while at line angles, the angle pulls are resisted by bisector guys or by pairs of approximately in-line guys. Galloping can be a problem. Galloping is caused by strong winds acting on the airfoils of ice coated wires. Single or two loop galloping can produce tension changes of 2:1 in the conductors. At a dead end or 90 ° guyed single mast, the in-line back guy can respond as a spring system to these pulsating tension changes and, with suitable resonant response of the mass of the mast or pole, the result can be amplified motions that can lead to the destruction of line elements 1. Furthermore, it is important that the lines of action of the major conductor loads and the guy loads are as close to centric as possible. This is not always easy with lattice masts as a mast designed and detailed for centric lines of action when installed at a 60 ° line angle may just as likely be installed at a lesser line angle with resulting noncentric lines of action. Either the dynamic pulsating loads of an adjacent span in gallop, or simply the case of an in-cloud ice load on only one of the adjacent spans can apply a torque load that can rotate the otherwise freely pivoting mast. This may cause problems with clearances to electrical jumpers. Awareness of the potential for twisting led to an important change of the wire attachment detail for guyed single masts used for the large line angles of a 735 kV line in Canada. The guys and phase wires were brought to a single point within the mast in order to ensure that neither static nor dynamic unbalances would cause rotation.
Guyed rigid frames Guyed rigid frames, as typified in Figure 4 are very popular and are frequently used at voltages from 69 kV to 230 kV and infrequently as high as 500 kV. A single mast and a set of guys and anchors (usually four) replaces the lower tower body and foundations while the upper part of the structure remains practically the same as it would for a totally rigid framing. The significant difference between these guyed rigid frames and the guyed single masts mentioned earlier is that each pair of side guys is attached at widely separated points so that torsional stability is inherent, an imperative with at least three phases attached to the structure. Furthermore, the two separated points of attachment automatically result in equal guy tensions in the absence of externally applied horizontal loads, an important point that is discussed in greater detail later in the paper. Figure 4(a-h) shows some of the framing and guying arrangements that have been used by designers in attempts to attach the guys and, in some cases, to thread the guys between phases, maintaining the required electrical clearances while minimizing the eccentricities of the loads with the guys and the mast. The overall objective is to keep the lines of action of the guy system centric with the centre of effort of the major load combinations and the centre line of the mast. With a significant eccentricity, a very large horizontal reaction
will be applied to the mast footing and large shears and bending moments can be put into the mast. On some projects, the shear and bending loads applied to the single mast of the structures in Figure4(e,g) were large enough to produce a single leg that weighed more than the two masts of a guyed V tower. The oft prescribed demand for torsional strength to resist the possible loads of broken wires or to resist the complex of loads that might be imposed by failure of an adjacent structure (the duty of failure containment) can create a problem as some of the guy systems with crossed guys, as in Figure 4 (e, g) and Figure 6(d), will permit a rotation that reduces the spacings of the resisting guys, the guy tensions and strains then increase with more rotation etc. until failure or snap through may occur. For this reason some designers have used auxiliary or longitudinal secondary crossarms that will ensure that under torque forces, the spacings of the acting guys will move apart and stabilize the problem as shown in Figure 4(h) and Figure 6(e). These extra arms do not add significantly to tower weights as they keep some of the heavier guy loads out of the upper part of the structure but they can and do make the tower lay out and assembly on the ground more difficult. The tower shown in Figure 4(a) was used in Finland as early as 1927, fabricated as a lattice work in steel. In the 1960s, a similar tower of aluminium was being used in North America for helicopter transport and erection on lines of remote or difficult access. Thousands were installed in Western Canada and Alaska in areas of permafrost or unstable ground conducive to frost heave. A structural fuse was placed in one of the guys to prevent crushing of the tower from excessive guy tensions if the mast footing was lifted. Figure 4(b) shows an aluminium version without ground or shield wires that made use of an extruded aluminium tube as a crossarm. The towers of Figure 4(c), also described in References 14 and 15, are guyed rigid towers fabricated of Corten Hollow Structural Shapes, both round and square. More than 4000 have been used at 138 kV in Manitoba and Saskatchewan, Canada, being installed by helicopter in some areas otherwise accessible only when frozen in the winter. The guyed Y of Figure 4(d) has been used for voltages of 230 kV and up to 500 kV but has not proved to be very useful at the higher voltages. The costs of accommodating the large moments that are developed within the frame from the eccentricities of the major forces being one of the costly features. The towers of Figure 4(e,g) are well suited to the aluminium/helicopter marriage, the present author having witnessed the transport and erection of more than 75 towers of 345 kV in one day by one medium sized helicopter.
Guyed and hinged (or pinned) masted structures The guyed portal, guyed V and the cross-rope suspensions (CRS) of Figure 5 are the three most used versions of this family, each of them comprising two tripods of a mast and two guys, the apexes of which are held apart by the crossarm of the portal tower, held together by the crossarm of the guyed V and also held together by the tensioned wire rope suspension assembly of the CRS.
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Guyed structures for transmission lines: H. B. White The guyed portal arrangement in Figures 1 and 5(a) has been used to advantage since the 1920s for lines from 138 kV to 500 kV in Scandinavia 2 and in what was the USSR and associated countries with extensive use in the last few decades in the latter at 800 kV. The guyed portal fits well on flat ground, and if it is not too high, the four guys can be brought to two guy anchors for a saving in cost and land use. However, at a given voltage and as the height increases, the transverse spacing between mast footing and the guy anchors does not increase proportionally with the result of high loads in the mast, guy and anchors for the higher towers of a set. The height limit can be overcome if the guys are crossed and four separate anchors are used but then one of the greater attractions of the guyed portal is removed. Use on rolling or hilly terrain poses the biggest problems for the portal arrangement, for with unequal length masts, ground assembly and rotation up into place becomes very difficult if not impossible. The tower is also sensitive to the torque load problem as the lines of action of the resisting guys come closer together as twisting increases. The guyed V tower 34~2 in Figures 3 and 5(b) was created in Canada in 1 9 5 8 - 5 9 after line engineers became aware of the many values of the guyed portal tower then in use in Scandinavia but conscious also of the difficulties of using the portal type in some of the rougher terrain in parts of Canada. An initial attempt was made in 1958 to create a V masted lorm with a wire rope suspension (what eventually became the cross-rope suspension) but available wire rope fittings (among other things) did not appear to be adequate at that time for the loads and the guyed V was the end result of the studies. A similar concept was also under study in France in 1958, where a 225 kV line of nine towers of a V shape with a cross-suspension of steel bars tested the limits of available fittings and was not pursued further. The cross-rope suspension concept later proved useful at higher voltages where the crossann is more of a problem but the V of the masts had to be eliminated and the masts moved further apart onto separate footings. The guyed V, like the portal, is also made of two tripods but with one footing for the two equal length masts, a footing that can be located on the smallest bit of rock and offering the advantage of cutting and fitting the guys to suit the roughest terrain. The adaptability to taller structures and thus longer spans led to major efficiencies in building long cross-country lines. A 230 kV steel guyed V line erected in Newfoundland Labrador, Canada in the 1960s was built to an average span length of ahnost exactly 500 m (1650 ft) and the recent Alicura 500 kV line te in Argentina had an almost exactly similar average span. Guyed V lines proliferated, further aided by development of towers fabricated of structural grade aluminium. A few line designers quickly recognized the potential of marrying the low weight of the guyed mast designs, the further reduction of more than 50% resulting from the change to lighter weight aluminium and the possibility of helicopter transport and erection. The guyed V offered the chance of yard assembly, transport of and then lowering the completely assembled structure onto one footing. However, the weight savings of the guyed V combined with ever increasing helicopter capacities
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soon found steel guyed Vs offering almost the same advantages and market conditions subsequently brought to an end what was, for more than a decade, a major thrust by aluminum into line construction. The guyed V tower has shown an amazing versatility as line designers have responded to the flexibility and inherent tolerance of the guyed principle in general and the guyed V in particular to produce ever improving efficiencies. Steel 500 kV guyed towers have been landed on very small rock projections in British Columbia, Canada, at sites accessible only by foot or by small helicopter. Guy anchors have had to be located far down the slope and at one site, as much as 100 m lower than the mast footings. Mixes of short and long guys are readily tolerated as are small movements of anchor position to accomodate uncovered ground anomalies. On a line traversing a steep cross-slope, the downhill anchors were moved uphill and inwards to shorten what would have been many excessively long guys: the more highly stressed guys and masts of the changed geometry were accommodated by a reduction in the wind span capacities of the towers involved. However, such special calculations or adjustments are usually needed for only the most extreme cases. At present the guyed V tower is possibly the most used tower type for EHV lines in the world, the height and span advantages and the ease with which it can accommodate rough terrain moving it ahead of the guyed portal which tk~r many years was the standard for guyed structures. The guyed V with haunches 5 in Figure 5(c) is a modification of the V that was developed to solve a special problem at very high voltages. The desire to keep the phases as close together as possible conflicts in the guyed V with the space taken up between phases by the mast which can have a different slope with each height of tower. To reduce the phase spacing to an absolute minimum, designs have been introduced in both the United States and South Africa where haunches have been added to the underside of the crossarm to provide a narrow and fixed hinge point for the mast and for attachment of the guys. This modification with haunches had not been widely adopted and it may be that the advantage of reduced phase spacing has not overcome the complexities and costs added to the crossarm and the large bending moments that are introduced to the masts by longitudinal loads at the conductor or ground wire positions. The cross-rope suspension tower or CRS in Figure 5(d) was conceived in 1974 as the proposed solution for the difficult conditions of a new, major line project and also to overcome some of the problems that were becoming evident in the use of guyed V towers at voltages up to 800 kV. It was noted previously that as voltages increase, tower widths or crossarm lengths can increase faster than the heights. It was becoming apparent to some who were aware of the experiences with the use of 800 kV guyed V towers in Canada and elsewhere that the towers were becoming top heavy; they required a very large and level open assembly area and were difficult to erect except under the best of conditions. They had become heavy beyond the lifting capacity of all but the largest helicopters, they required good road access for a large mobile crane and, in the event of a tower failure, a structure could not be replaced until the road access was renewed. The very advantages of the guyed
Guyed structures for transmission lines: H. B. White V for use in rough terrain that led to the original development of the tower were now producing real limits or deterents to its use at EHV. This writer had a good experience with the design and construction of a large cross-rope suspension system6 in the mountains of British Columbia, Canada in 1955 where about 2030 m (6700 ft) of a 'very large and heavily loaded double circuit 300 kV line was suspended along and over an avalanche swept valley by a crossrope assembly of two wires that spanned 1110 m (3660 ft) transverse to the valley. Thus, in focusing on the difficult problems of access, transport and erection for the forthcoming line and with the experience of the above mentioned suspension system, of almost two decades of work with guyed towers of many kinds and still retaining the three objectives of the ultimate structure, the thought process led almost inevitably to the CRS Tower 7"8 in Figure 5(d). The CRS was first designed for Hydro Quebec, where it became known as the Chainette and it is simply another pair of tripods (each of a pair of guys and a mast) held together at their tops, in this case by a wire rope assembly from which are suspended the insulator strings and from them the conductors. The CRS concept is simple and almost primitive and, to this date, analysis of the governing loading conditions and other assessments of static behaviour can be completely carried out on a peg board model or by simple graphical analysis. More exact methods have been developed which are useful in probing dynamic problems 9. However, before acceptance for first use on a major line, the CRS concept was subjected to intensive testing as part of a full-line system with particular attention to dynamic response from galloping and from ice shedding from the wires. The possibility that a single cross-wire support could transmit the galloping of one phase of conductors into the other phases was the reason for proposing the six-part triangulated arrangement. The test program demonstrated convincingly that, with this configuration, there would be no transfer of motion. The CRS was used for the 4th and 5th lines of the 735 kV James Bay system in Quebec and for an 85 mile section of 500 kV line in Oregon, USA. The latest news is that it is being employed on 500 kV lines across the 'Mongol Prairie' in China and it is a serious contender for use on new lines in South Africa at 400 kV and 800 kV and in South America at 500 kV. The proposal for South Africa and other lines under study for nonice areas (no galloping) is for a single cross-rope to replace the six-part suspension of the original design as shown in Figure 5(e). Although simple in concept and design, the CRS posed a serious problem for construction because there was no way to erect the masts, align them and tension the guys until the conductors were strung and had contributed the weight necessary to tension the system. The key to the CRS was, and still is, a construction spacer cable, cut and fitted to an exact length, which is inserted between the mast tops and used for aligning and tensioning the guy system and, of great importance, for ensuring that the spacing between mast tops is correct so that the cross-rope assembly, also precut and fitted to an exact length, will hang with the correct sag when the conductors are installed. The CRS requires a large area at each site for the spread of the guys but almost all its
other characteristics are advantages, including the fact that, with no structural material between the phases, the phases can be put as close together as wind induced motions will permit, and close phase spacing will reduce the needed width of the rights-of-way in the spans between towers and also improve the power transfer capability of the line. The ground or shielding wires attached to the tops of the masts give the best possible shielding against lightning strokes, the two masts fitted with guys can be assembled in a small area and can be erected with a small crane or gin pole or flown to site and installed with a small (less costly) helicopter. Small angle suspension towers can be created from the same masts and guys by merely changing the lengths of the pieces of the cross-rope suspension and the heights of the two masts. At 800 kV, the structural material of the two masts weighs about 45 to 50% of a comparable guyed V which is, in turn, about 50% of the weight of a rigid tower. Improved procedures for the construction of the CRS with adoption of the single rope suspension for nonice areas and of the method of precutting guys and thus automatic alignment of the masts (see below) should lead to much greater use of this crossarmless type of tower for future lines.
Some special guyed towers Some guyed structure types do not fit easily into the categories of guyed masts, guyed rigid frames nor of the guyed and pinned masts of Figures 4 and 5. These special types are shown in Figure 6. Several interesting attempts have been made to reduce the lengths of the masts of the guyed V tower by converting into forms of pinned or hinged Ys for the reason that three half-length masts should be and are much less costly than two full-length masts while retention of the pinned or hinged mast principle prevents the problems of the guyed rigid structures. The guyed V on a guyed post is shown in Figure 6(a) where the short centre mast requires four small guys to hold it in position. Three half-length masts replace two full-length masts at the expense of an extra four small guys. The saving in mast material is evidently greater than the cost of the extra guy system although the tower is now critically vulnerable to the failure of one of these small guys. This contravenes a principle that holds for almost all other guyed towers in that after stringing, they will remain upright and still be able to carry at least 50% of design loads with one guy removed. This tower would drop at once if one of the small guys were removed or broken. The tower was designed for helicopter erection in two pieces, the upper V and crossarm being lowered onto the post and pulled into position by a line that is fed through a hole in the top of the post. The hinged guyed Y is shown in Fioure 6(b). At a first glance it looks very much like the V on a post. It was installed on hundreds of miles of line under severe terrain and load conditions in Eastern Canada in the 1960s. This is probably the most efficient of all the hinged masted structures, with the exception of the CRS, but the mysteries of its behaviour and the evident difficulty of following very strict procedures during maintenance or repairs have been points against its further use. An unusual feature is that no guys are attached to the hinge point and they are not needed. The usual
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Guyed structures for transmission lines: hi. B. White four outer guys of the guyed V are supplemented by four interior guys that cross over to the opposite sides of the crossarm. The secret of its stable behaviour is understood when it is realized that the hinge point cannot move transversely and pop out unless the crossarm can lift and it is prevented from doing so by the interior guys which hold the crossarm down. This tower will remain standing with the removal of one anchor or two of eight guys (but not two similar guys). It is an interesting structural feature that the hinged connection to the lower vertical leg ensures that the loads in the two upper legs will always be equal and this reduces the uncertainties regarding the load distribution into the crossarm and guy system and ensures that each o f t h e mast sections works fully under all loads. Erection was made simple by attaching two of the inner guys (one from each side of the tower) to the base of the centre mast to form a temporary rigid Y, erecting and plumbing the tower with the four outer guys and then transferring the two inner guys from the base of the mast to their anchor positions. The guyed V and inverted delta or 'T' ~0 is shown in Figure 6(c). The need to build compact lines and to control the magnetic fields beneath a line that some believe may have influences on health have focused attention on the inverted delta or T arrangement of the phases. One method of supporting a compact T is within a guyed V, a V of two tripods with a simple spreader strut to hold the mast tops apart. This tower holds great promise for magnetic field control, for building on narrow rights-ofway and for the benefits that compaction of the phases bring in reducing the reactance of long distance lines and increasing power transfer capability. So far, no one seems to have found a totally acceptable way of suspending the three phases, as there is an evident reluctance to suspend the lower phase from the other two with phase-to-phase insulators. However, it is but a matter of time before someone finds an acceptable suspension arrangement and the new guyed V with T becomes one of the standards for EHV lines. Two guyed single mast direct current (two pole) towers are shown in Figure 6 (d, e). Though but single masts, these towers are more appropriately classified as guyed rigid frames as the four guys do not converge on a single point. At first glance this is a simple and straightforward solution but there are difficult spatial problems of trying to maintain electrical clearances between guys and conductors and at the same time of trying to have the guys centric with the line of action of the resultant of the major loads and the centre line of the mast. If the guys extend beyond the mast and attach to the opposite crossarms, the tower under torque load will rotate and the lines of action of the resisting guys will come closer and guy tensions will increase, a problem mentioned above with regard to the guyed portal. It is common practice with this tower to use extra secondary arms to keep the guys apart although the ground assembly problems with these arms are causing some designers to look again at the need for the torque loading. The guyed X tower is shown in Figure 6(f). The problems of frost heave that are common to many northern lands have led to the development in Alaska of a special type of X tower that behaves as a rigid tower under vertical and transverse loads and is guyed longitudinally. Many hundreds of miles of these X towers are in place,
some of lattice steel, of lattice aluminum and of tubular steel in some of the areas considered more environmentally sensitive. The usual footings are two single H piles on each of which is attached, by U bolts, the bracket on which the tower can pivot back and forth. Pairs of guys are set ahead and behind to maintain longitudinal stability and the anchor assembly of each pair includes tension fuses to ensure that excessive frost heave does not increase guy tensions enough to crush the towers. If frost heave raises one of the piles, the maintenance crews can loosen the U bolts and regain the correct elevalion the next spring.
Special techniques for guyed towers Some guyed tower types allow use of special techniques or practices that increase their effectiveness and lower the overall costs of a line. Five are worthy of note.
Construction tolerances and techniques: necessary standards Major savings can be made in erection time and costs by taking advantage of the relaxed tolerances permitted by the flexibility of the usual type of guyed structure. Furthermore, when a full understanding of the issues is reached, additional savings are possible by reducing the need for the complexity of the fittings and adjustments normally put into the guy systems. These frequently unnecessary turnbuckles and threaded fittings of many types are costly, a possible attraction for random mischief, and in simply adding to the number of components of a structure, reduce the line reliability. Many lines of guyed structures are erected with the same attention to precision that is necessary for the erection of rigid latticed structures and it has proved difficult to break the ingrained habits of the engineers, erection crews and teams of inspectors who take pride in the degree of precision specified, installed and verified. However, it is necessary to relax these standards and tolerate what may appear to some to be poor workmanship if full advantage is to be taken of the guyed principle.
Rigid latticed towers: The costly dimensional controls for the construction of rigid latticed towers are focused on the installation of the foundations. Stress-free erection of the bolted tower assembly is only possible with the four (usually four) footing stubs set to horizontal dimensional tolerances of about 1/1000 or 1/8th of an inch in 10 ft and vertical tolerances that would limit warping of the plane of the footings to about the same 1/1000. If setting errors exceed these by very much, the assembly of the tower will become very difficult, specially in the upper stages, and the built-in stresses could have marked effect on the subsequent load carrying capacity. Although specifics are frequently given for the plumbing of the completed rigid structure, any deviation from the vertical is not produced by the erection process but is evidently the result of a tilting of the plane of the footings.
Guyed tower tolerances: There are two aspects of guyed tower construction that may influence the behaviour of the final structure: the placement of the mast footings
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Guyed structures for transmission lines: H. B. White and guy anchors and the subsequent adjusting of the guy lengths and the tensioning of the guys. Before attempting to set limits or tolerances on any of these, the necessary and sufficient performance criteria should be established. With no known exceptions, the plumbing for verticality of a guyed structure has negligible effect on the structural performance or load carrying capacity unless the inclination far exceeds what is noticeable to and offends the eye of the beholder. A viewer can detect about a 2 ° inclination from the vertical in a single mast or flag pole and variances of maybe ! ° can be noticed in a closely spaced line of poles. It is proposed (and has in tact been used on major guyed tower projects) that the l ° limit (approx. 1/50) be accepted as the general tolerance criteria; thus, for a tower such as the guyed V or the guyed portal, the centre point of the crossarm should not be more than 1/50 of the height from directly above the tower centre point on the ground and the crossarm should be transverse to the line with an error of not more than 1/50 of the crossarm width, etc. Attempts have been made to plumb guyed towers to greater accuracies by adjusting the guy lengths and tensions with turnbuckles or other threaded devices only to find that as the sun came out and heated the guys on the one side, the tower moved sufficiently that the precise criteria had to be discarded. The criteria should be based only on what is necessary and this seems to reduce to the limiting requirement that the structure not appear to be crooked to the naked eye. The criteria of 1 ° tolerance can also apply to anchor locations with the anchor acceptable if lying within a 1 o cone projected from the guy to tower attachment point. This acceptance of less than 'obtainable perfection' will be found to have a significant influence on both labour and material costs when the subject of guy pretensions is also rationalized (see below).
Pretensioning of guys: With the same exceptions as noted previously, most guyed structures are very insensitive to pretensioning of the guys. The obvious upper limit is that the pretension should not be more than 50% of the maximum loaded tension to ensure that the leeward or back side guys go slack under maximum horizontal loading and do not contribute to the maximum stresses in the structure. There are no rigorous or standard lower limits on what is needed although a well used criteria is a pretension sufficient so that the leeward guys should not go slack and flap around unnecessarily in frequently occuring winds: possibly a yearly wind speed. Thus, the pretension would be about 50% of the tension needed to resist that yearly wind load. As was seen earlier, the tensions in all four guys of a tangent 'in line' structure (except for guyed single masts) will be equal in the absence of a transverse wind load and efforts to equalize the pretensions are usually fruitless exercises that merely establish the variances of the tension measuring devices. Precut and fitted guys: Guyed towers are typically erected with guys cut approximately to length with a suitable fitting applied to the top end which is then attached to the tower before erection. Preliminary tension is then applied through temporary grips to each of the four guys to steady and approximately plumb in the
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tower, the guys cut, bottom fittings installed and then all guys brought to tension and the final tower position checked by adjusting the threaded devices, turnbuckles or U-bolts, one in each of the four guys. Guyed tower erection costs are very dependent on the time taken by this operation with much tightening and loosening, verification by optical devices and acceptance or rejection by the inspection crew who follow and who usually have to duplicate the instrument set ups. However, several projects with large guyed towers have demonstrated that most of these costly steps can be eliminated by using precut and prefitted guys and tension adjustment in only one of the four guys. The technique is possible with the acceptance of the tolerances proposed above and the awareness that tensioning of one guy puts equal load in all four. After installation of the mast footings and the anchors (rods), the elevation of each is measured, the required guy length calculated according to the height of the mast or tower and the guys cut and both end fittings applied at a field shop. The completely fitted guys are attached before the mast or tower is erected. A tolerance on guy length of about 0.3% should be readily obtained and a simple graphical exercise will demonstrate that the worst possible combinations of pluses and minuses will still position the tower within the positional limits outlined above. A quick connection of shackles or equivalents and a tensioning at the fourth guy which is the only one with a tensioning device will find the tower erected and in satisfactory position at a saving of many man hours and the saving of at least three unnecessary tensioning units.
Modular towers A guyed tower with pinned masts, such as the guyed V, is specially suited to modular design concepts as the structure can be thought of as an assembly of three distinct structural components that may be combined in different arrangements to provide maximum efficiency of use of materials ~. The only requisite is that all the masts can be fitted to all crossarms. The potential of the system becomes apparent when it is realized that the three components respond to different design specifics or parameters. The three major and interchangeable components are: firstly, the crossarms which are governed by length and the vertical applied loads which depend on the weight span carried by the tower. The length of the crossarm is controlled by the swing of the insulator assemblies and the swing angle and thus the length is at a maximum with a small weight span to wind span ratio and at a minimum with a large weight span. Thus the conditions at each site give the opportunity to use as required: either a normal crossarm, a very strong (vertically) but short crossarm or a long but not very strong crossarm. Secondly, the important compression load in the mast is created by the guys resisting the wind pressure on the wind span of conductors and the bending in the mast is dependent on the wind on the mast and thus critically affected by the height of the mast. The P-6 effect is a product of the two. The weight of the spans of wires has little effect on the mast design. Thus two or more mast designs can be used and selected on the basis of wind span and height.
Guyed structures for transmission lines: H. B. White Thirdly, the loads in the guy/anchor system are directly responsive only to the wind span at each tower site and two or more guy/anchor systems can be used as warranted by the number of towers of the line and the range of spans encountered. A modern tower spotting program can have as input the cost and span limits of combinations of 3 crossarms x 3 masts x 2 guy/anchor systems for a total of 18 different structures and the efficiency of use can be very high if the 'limits are set judiciously.
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Eccentric masts A tower, such as the guyed portal or guyed V, is relatively efficient compared to some rigid equivalents but, in fact, while under the maximum and usually critical load of high transverse wind, only half (one of the tripods) is working and the other tripod is idling and supporting only the vertical weight of half of the wires. For the working tripod, the maximum compression in the mast (load from the guys) combined with the maximum bending moment caused by wind on the mast can only result from wind from one direction. This provides an opportunity to make noncentric masts with an induced bending moment counteracting the bending of the wind on the mast. The concept has been used in Romania and with the guyed V towers of the Alicura project j2.
Footing connection The pinned connection(s) common to all but the fixed mast type of guyed structures range in their detailing from the very simple to the very complex. The movement or action as a pinned connection is very limited once the tower is in place as the masts seldom deflect more than a fraction of a degree during their lifetimes. However, they must be able to rotate freely about the footing (pin) to eliminate torque from the masted part of the structure. The angle of inclination of the masts of the guyed portal or the CRS or any of the guyed rigid towers is a fixed angle although the angle of the masts of the guyed V will change with the height. The angle of the masts of a guyed V set of towers will vary about -4-30-4 ° . Quite large ball and socket castings of aluminum or steel that permit large movements have been used as well as some very simple fittings, as shown in Figure 7. This detail consists of a steel pin, embedded in concrete or directly into rock and set at an appropriate angle, a grout pad as needed, a steel plate to distribute the bearing pressure and a spherical washer to distribute the bearing load from the base of the mast to the washer. The bearing pressure is on a ring of contact and is infinite until the base plate of the mast deforms slightly as the tower is put in place and the guys tensioned. This deformation is then accepted as a permanent and nonthreatening condition and, as long as it is restricted to a few millimetres at most, is negligible compared to the elasticity of the guy system.
Guy sizing and stresses It is presumed that all the analyses of the guyed structures are based on ultimate or limit loads, i.e. the loads that will threaten some component of the system. Thus,
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the computed guy loads represent the maximum demands to be placed on the guy system. The steel strandings in common use for guys are usually limited to about 65 to 70% of the rated tensile strengths (RTS) under the loads of ice and/or wind that may be assumed to be applied to all towers during their lifetimes. This produces a margin on failure of about 50% (an apparent factor of safety of 1.5) and such a margin can be justified. First of all, the guy fittings will seldom develop the RTS of the guy and many will be limited to 9 0 - 9 5 % of the guy RTS. Furthermore a loading much beyond 70% will approach the yield point of the guy with the result that working to such a high stress level might require retensioning of the guy systems of many towers after a major wind storm. The overriding justification comes from the sequence of failure concept that a critical component of little cost should not be the item that instigates a costly failure. The one departure from the 6 5 - 7 0 % limit is that some designers take the guy anchor system to about 85% under failure containment conditions (usually of a single phase longitudinal load). The justification is based on reliability concepts and the attempts to rationalize and economize in the placing of the guys. The major load of wind on the wires will probe all the towers of the line over the lifetime and a single deficient guy or guy fitting will be found and trigger an outage. The demands of failure containment will be imposed on, at most, the two towers adjacent to an already failed structure or a failure of some kind. If one of the two guy assemblies being severely tested by containment duty proves to be deficient, the containment duty merely passes to the next tower where the odds are in favour of there not being another deficient guy or component. This is not a minor point as the line designer is always trying to find ways of improving his guying angles; he wants to move them transverse to the line to reduce the tensions under wind loads while not being able to forget the need to resist the infrequent anticascade loads. A justified increase of 20% (70 to 85) in allowable stresses under longitudinal loading may permit a more efficient guy deployment.
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References 1 White, H. B. "Some destructive mechanisms activated by galloping conductors', IEEE PES Winter Meeting, New York, NY, February 1979, Paper A79 106-6 2 Voipo, E. 'Power generation and transmission system of Finland', IEEEPES Winter Meeting, New York, NY, 1966, Paper 31 TP 66-48 3 White, H. B. 'Chute des Passes 345 kV Transmission Line', AIEE Winter Meeeting, New York, February 1960, Paper 60-72 4 McMutrie, N. J., Murphy, B. R. and Markowsky, M. 'Engineering design features of the P i n a r d - H a n m e r 500 kV transmission line', IEEE Winter Meeeting, New York, February 1964, Paper 64 93 5 Samuelson, A. J. 'American electric power 765 kV transmission line project', IEEE EHV Transmission Conf., Montreal. Canada, 1968, Paper 68 C 57-PWR 6 White, H. B. 'Cross suspension system Kemano-Kitimat transmission line', AIEE Winter Meeting, New York, NY. February 1958, Paper: CP-58-432 7 White, H. B. 'Structural system for the James Bay transmission lines', Hydro-Quebec Symposium on Transmission of Electrical Energy at EHV and UHV, AC, Varennes, Quebec, October 1973 (First disclosure of cross-rope suspension tower) 8 Ghannoum, E. and Lamarre, M. 'Hydro Quebec's experience with the design and construction of 1500 km of 735 kV Chainette
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9
10
11
12
13
14
15
transmission lines', IEEE PES Winter Meeting, New York, NY. 1985, Paper 85 WM 200-1 Peyrot, A. H., Lee, J. W., Jensen, H. G. and Osteraas, J. 'Application of cable elements concept to a transmission line with cross rope suspension structures', IEEE Trans. 1981, PAS-100, (7) Gidlund, J. I. et al., "Swedish State Power Board adopts the T-tower design for 420 kV lines', International Conference ml blrge Electrical Networks, CIGRE SC22 (WG8)-I3, Paris, France, June 1988 White, H. B. "A modular design system for guyed V towers', IEEE PES Winter Meeting, New York, NY, January 1978. Paper F78-151-3 Behncke, R. and White, H. B. 'The Alicura 500 kV transmission system', hlternational Cotl~'rence on l~rge Electrical Network,~, CIGRE, Paris, France, September 1984, Paper 22-02 Bateman, L. A., Haywood, R. W. and Brooks, R. F. "Nelson Rivcr DC transmission project', lEEk? EHV Transmis,~ion Cot~ference. Montreal, Canada, 1968. Paper 68 C 57-PWR Staudzs, A. 'Design considerations of the Radisson-Churchill 138 kV transmission line', Canadian Electrical Association. Toronto, Ontario, March 1986 Mathur, R. K., Shah, A. H., Trainor, P. G. S. and Popplewell, L. "Dynamics of a guyed transmission tower system', IEEE Sumnte~ Meeting. Mexico City, Mexico 1986, Paper 86 SM 414 7
Guyed structures have become the dominant type for use on transmission lines remote from urban areas or where land use restrictions are prohibitive. All other attributes, including cost of materials, erection and performance are positive and the continuing pace of innovations of types and techniques is adding to their attractions. This review of old and new types and practices illustrates adaptability and some of the many new ideas produced in response to frequently changing demands and conditions.
Keywords: guyed structures or towers, transmission lines Three different types of guyed structures are used in modern transmission line design and the line designer should recognize and be aware of the seemingly minor but significant differences between them. They are: • • •
Guyed Guyed Guyed special
single poles or masts rigid frames and hinged masted structures plus several guyed structures
This paper will attempt to demonstrate the issues common to the three and point up the differences that must be well understood if the few pitfalls are to be avoided and then full advantage is to be taken of what can be done with some but not all of them.
Guyed transmission towers vs. guyed communication masts A structural engineer who is familiar with the behaviour and analysis of very tall guyed radio or TV masts must recognize the great differences with the guyed structures used to support the wires of very high (VH) and extra high voltage (EHV) transmission lines. The transmission line towers are usually 2 0 - 5 0 m in height and the guys are thus short enough that wind or ice loads on the guys are of little consequence in analysis or performance and the sags in the guys are also very small so that, with only a few exceptions that will be discussed below, slack and strain changes in the guys can be neglected.
tures the most popular supports for HV and EHV lines in countries around the world. Their advantages are many and, when conditions of use are suitable, large economies can result if the best type is selected to suit the many and varying conditions that can face the line designer. However, a guyed structure is not always the best solution as restrictions on, or perceived problems with, the use of guys are sometimes very real and decisive and correctly lead to the selection of other types of support. For example, guyed structures will almost always require a larger site area for the anchoring of the guys and this alone may rule out their use on farm lands where large equipment is used. The guys can present a hazard to the equipment and conversely the large machinery can pose a serious threat to the towers. In urban and suburban areas, the right-of-way for a new line may depend on minimizing the occupied land area and this may force the use of very compact rigid lattice constructions or embedded and cantilevered tubular poles of steel, concrete or other materials. The line designer will find that the correct selection requires both an appreciation of all of the conditions of the line to be built as well as a full knowledge of the characteristics of the many different guyed tower types that have been developed and very often used with success to meet the conditions of different lines. Thus the suitability of a specific guyed tower type for a given project may depend on an assessment of some of the following:
Basic guyed tower concepts The old but still oft quoted three basic rules of structural engineering are that (i) the most efficient structural component is a wire rope in tension; (ii) that the most efficient in compression is a lattice mast; and (iii) do not try to push (i). Consideration of these three rules, which might be adjusted slightly by removing the whimsical third and substituting 'and every effort should be made to minimize bending in the second' are the precepts that have been used by line designers to make guyed struc0141-0296/93/040289-14
•
• • •
Terrain; flat, rolling, mountainous. The guyed portal towers (Figure 1) fit well on flat terrain, the guyed Ys and Vs (Figures 2 and 3) were developed for rougher terrain Access and transport of equipment and materials (and workmen), by mule, truck or helicopter Erection of towers, piece by piece, with gin pole or mobile crane, by small or large helicopter Procurement of easements: guyed towers usually require larger site areas for placement of anchors
© 1993 Butterworth-Heinemann Ltd
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Guyed structures for transmission lines." H. B. White.
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110 kV guyed portal in Finland about 1930 designed to replace traditional wood pole H frames with a structure of greater height, strength and more longitudinal stability. It is possible that several countries were moving in the same direction at that time
and possibly temporary whole tower ground assembly space. However, some guyed tower types such as the cross-rope suspension in Figure5(d) may permit greatly reduced rights-of-way requirements in the spans between towers Structural loadings, ice and wind load severity Voltage of the line: as voltage increases, tower (crossarms) widths increase faster than heights. At 138 kV, the crossarm of a rigid or guyed Y or V presents a small and innocuous problem while at 500 and 800 kV, the crossarm can be more than 50% of
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tower weight resulting in a top-heavy construction problem that may yield to the CRS solution. Thus the selection of the appropriate guyed tower type may be very dependent on the voltage or scale of the line Importance of variable electrical parameters: magnetic field strengths below the line, corona effects such as radio interference and line reactance (power transfer capability) are dependent on phase spacings and arrangements that may be limited by the selected tower type
Guyed structures for transmission lines. H. B. White
Figure 2 Guyed Y of aluminium for Ontario Hydro's 500 kV line in 1963. Shown under test at Aluminum Company of Canada (Alcan) test site at Kingston, Ontario
Engng. Struct. 1 9 9 3 , V o l u m e 15, N u m b e r 4
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Figure 3 Guyed V designed in aluminium by Alcan for 765 kV Project UHV of General Electric at Pittsfield MA, USA in 1961
Basic types of guyed structures
Guyed single poles or masts
The guyed tower types discussed in this paper are those known to the writer and no slight is intended to anyone whose concept has been overlooked. The writer's personal library contains many interesting and useful writings on some of the 18 types that are discussed but many of the documents are from trade magazines or from conferences with no published transactions: thus no reference can be quoted.
Single guyed masts or poles find many applications on modern lines and are often the most efficient means of handling the large horizontal loads of large angle positions or at points where dead ending is required. The more common use of guyed single poles or masts relies on an effective pinned connection to the footing. Attempts to apply guys to fixed masts or poles will, of course, introduce indeterminancies and predictable per-
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Guyed structures for transmission lines: 14. B. White formance will depend on the degree and control of pretensioning of the guys and close control on any future movement of guy anchors or rotation of the mast foundation. When the poles or masts are used as dead ends, the line or wire tensions are usually taken by in-line back guys, while at line angles, the angle pulls are resisted by bisector guys or by pairs of approximately in-line guys. Galloping can be a problem. Galloping is caused by strong winds acting on the airfoils of ice coated wires. Single or two loop galloping can produce tension changes of 2:1 in the conductors. At a dead end or 90 ° guyed single mast, the in-line back guy can respond as a spring system to these pulsating tension changes and, with suitable resonant response of the mass of the mast or pole, the result can be amplified motions that can lead to the destruction of line elements 1. Furthermore, it is important that the lines of action of the major conductor loads and the guy loads are as close to centric as possible. This is not always easy with lattice masts as a mast designed and detailed for centric lines of action when installed at a 60 ° line angle may just as likely be installed at a lesser line angle with resulting noncentric lines of action. Either the dynamic pulsating loads of an adjacent span in gallop, or simply the case of an in-cloud ice load on only one of the adjacent spans can apply a torque load that can rotate the otherwise freely pivoting mast. This may cause problems with clearances to electrical jumpers. Awareness of the potential for twisting led to an important change of the wire attachment detail for guyed single masts used for the large line angles of a 735 kV line in Canada. The guys and phase wires were brought to a single point within the mast in order to ensure that neither static nor dynamic unbalances would cause rotation.
Guyed rigid frames Guyed rigid frames, as typified in Figure 4 are very popular and are frequently used at voltages from 69 kV to 230 kV and infrequently as high as 500 kV. A single mast and a set of guys and anchors (usually four) replaces the lower tower body and foundations while the upper part of the structure remains practically the same as it would for a totally rigid framing. The significant difference between these guyed rigid frames and the guyed single masts mentioned earlier is that each pair of side guys is attached at widely separated points so that torsional stability is inherent, an imperative with at least three phases attached to the structure. Furthermore, the two separated points of attachment automatically result in equal guy tensions in the absence of externally applied horizontal loads, an important point that is discussed in greater detail later in the paper. Figure 4(a-h) shows some of the framing and guying arrangements that have been used by designers in attempts to attach the guys and, in some cases, to thread the guys between phases, maintaining the required electrical clearances while minimizing the eccentricities of the loads with the guys and the mast. The overall objective is to keep the lines of action of the guy system centric with the centre of effort of the major load combinations and the centre line of the mast. With a significant eccentricity, a very large horizontal reaction
will be applied to the mast footing and large shears and bending moments can be put into the mast. On some projects, the shear and bending loads applied to the single mast of the structures in Figure4(e,g) were large enough to produce a single leg that weighed more than the two masts of a guyed V tower. The oft prescribed demand for torsional strength to resist the possible loads of broken wires or to resist the complex of loads that might be imposed by failure of an adjacent structure (the duty of failure containment) can create a problem as some of the guy systems with crossed guys, as in Figure 4 (e, g) and Figure 6(d), will permit a rotation that reduces the spacings of the resisting guys, the guy tensions and strains then increase with more rotation etc. until failure or snap through may occur. For this reason some designers have used auxiliary or longitudinal secondary crossarms that will ensure that under torque forces, the spacings of the acting guys will move apart and stabilize the problem as shown in Figure 4(h) and Figure 6(e). These extra arms do not add significantly to tower weights as they keep some of the heavier guy loads out of the upper part of the structure but they can and do make the tower lay out and assembly on the ground more difficult. The tower shown in Figure 4(a) was used in Finland as early as 1927, fabricated as a lattice work in steel. In the 1960s, a similar tower of aluminium was being used in North America for helicopter transport and erection on lines of remote or difficult access. Thousands were installed in Western Canada and Alaska in areas of permafrost or unstable ground conducive to frost heave. A structural fuse was placed in one of the guys to prevent crushing of the tower from excessive guy tensions if the mast footing was lifted. Figure 4(b) shows an aluminium version without ground or shield wires that made use of an extruded aluminium tube as a crossarm. The towers of Figure 4(c), also described in References 14 and 15, are guyed rigid towers fabricated of Corten Hollow Structural Shapes, both round and square. More than 4000 have been used at 138 kV in Manitoba and Saskatchewan, Canada, being installed by helicopter in some areas otherwise accessible only when frozen in the winter. The guyed Y of Figure 4(d) has been used for voltages of 230 kV and up to 500 kV but has not proved to be very useful at the higher voltages. The costs of accommodating the large moments that are developed within the frame from the eccentricities of the major forces being one of the costly features. The towers of Figure 4(e,g) are well suited to the aluminium/helicopter marriage, the present author having witnessed the transport and erection of more than 75 towers of 345 kV in one day by one medium sized helicopter.
Guyed and hinged (or pinned) masted structures The guyed portal, guyed V and the cross-rope suspensions (CRS) of Figure 5 are the three most used versions of this family, each of them comprising two tripods of a mast and two guys, the apexes of which are held apart by the crossarm of the portal tower, held together by the crossarm of the guyed V and also held together by the tensioned wire rope suspension assembly of the CRS.
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Guyed structures for transmission lines: H. B. White The guyed portal arrangement in Figures 1 and 5(a) has been used to advantage since the 1920s for lines from 138 kV to 500 kV in Scandinavia 2 and in what was the USSR and associated countries with extensive use in the last few decades in the latter at 800 kV. The guyed portal fits well on flat ground, and if it is not too high, the four guys can be brought to two guy anchors for a saving in cost and land use. However, at a given voltage and as the height increases, the transverse spacing between mast footing and the guy anchors does not increase proportionally with the result of high loads in the mast, guy and anchors for the higher towers of a set. The height limit can be overcome if the guys are crossed and four separate anchors are used but then one of the greater attractions of the guyed portal is removed. Use on rolling or hilly terrain poses the biggest problems for the portal arrangement, for with unequal length masts, ground assembly and rotation up into place becomes very difficult if not impossible. The tower is also sensitive to the torque load problem as the lines of action of the resisting guys come closer together as twisting increases. The guyed V tower 34~2 in Figures 3 and 5(b) was created in Canada in 1 9 5 8 - 5 9 after line engineers became aware of the many values of the guyed portal tower then in use in Scandinavia but conscious also of the difficulties of using the portal type in some of the rougher terrain in parts of Canada. An initial attempt was made in 1958 to create a V masted lorm with a wire rope suspension (what eventually became the cross-rope suspension) but available wire rope fittings (among other things) did not appear to be adequate at that time for the loads and the guyed V was the end result of the studies. A similar concept was also under study in France in 1958, where a 225 kV line of nine towers of a V shape with a cross-suspension of steel bars tested the limits of available fittings and was not pursued further. The cross-rope suspension concept later proved useful at higher voltages where the crossann is more of a problem but the V of the masts had to be eliminated and the masts moved further apart onto separate footings. The guyed V, like the portal, is also made of two tripods but with one footing for the two equal length masts, a footing that can be located on the smallest bit of rock and offering the advantage of cutting and fitting the guys to suit the roughest terrain. The adaptability to taller structures and thus longer spans led to major efficiencies in building long cross-country lines. A 230 kV steel guyed V line erected in Newfoundland Labrador, Canada in the 1960s was built to an average span length of ahnost exactly 500 m (1650 ft) and the recent Alicura 500 kV line te in Argentina had an almost exactly similar average span. Guyed V lines proliferated, further aided by development of towers fabricated of structural grade aluminium. A few line designers quickly recognized the potential of marrying the low weight of the guyed mast designs, the further reduction of more than 50% resulting from the change to lighter weight aluminium and the possibility of helicopter transport and erection. The guyed V offered the chance of yard assembly, transport of and then lowering the completely assembled structure onto one footing. However, the weight savings of the guyed V combined with ever increasing helicopter capacities
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soon found steel guyed Vs offering almost the same advantages and market conditions subsequently brought to an end what was, for more than a decade, a major thrust by aluminum into line construction. The guyed V tower has shown an amazing versatility as line designers have responded to the flexibility and inherent tolerance of the guyed principle in general and the guyed V in particular to produce ever improving efficiencies. Steel 500 kV guyed towers have been landed on very small rock projections in British Columbia, Canada, at sites accessible only by foot or by small helicopter. Guy anchors have had to be located far down the slope and at one site, as much as 100 m lower than the mast footings. Mixes of short and long guys are readily tolerated as are small movements of anchor position to accomodate uncovered ground anomalies. On a line traversing a steep cross-slope, the downhill anchors were moved uphill and inwards to shorten what would have been many excessively long guys: the more highly stressed guys and masts of the changed geometry were accommodated by a reduction in the wind span capacities of the towers involved. However, such special calculations or adjustments are usually needed for only the most extreme cases. At present the guyed V tower is possibly the most used tower type for EHV lines in the world, the height and span advantages and the ease with which it can accommodate rough terrain moving it ahead of the guyed portal which tk~r many years was the standard for guyed structures. The guyed V with haunches 5 in Figure 5(c) is a modification of the V that was developed to solve a special problem at very high voltages. The desire to keep the phases as close together as possible conflicts in the guyed V with the space taken up between phases by the mast which can have a different slope with each height of tower. To reduce the phase spacing to an absolute minimum, designs have been introduced in both the United States and South Africa where haunches have been added to the underside of the crossarm to provide a narrow and fixed hinge point for the mast and for attachment of the guys. This modification with haunches had not been widely adopted and it may be that the advantage of reduced phase spacing has not overcome the complexities and costs added to the crossarm and the large bending moments that are introduced to the masts by longitudinal loads at the conductor or ground wire positions. The cross-rope suspension tower or CRS in Figure 5(d) was conceived in 1974 as the proposed solution for the difficult conditions of a new, major line project and also to overcome some of the problems that were becoming evident in the use of guyed V towers at voltages up to 800 kV. It was noted previously that as voltages increase, tower widths or crossarm lengths can increase faster than the heights. It was becoming apparent to some who were aware of the experiences with the use of 800 kV guyed V towers in Canada and elsewhere that the towers were becoming top heavy; they required a very large and level open assembly area and were difficult to erect except under the best of conditions. They had become heavy beyond the lifting capacity of all but the largest helicopters, they required good road access for a large mobile crane and, in the event of a tower failure, a structure could not be replaced until the road access was renewed. The very advantages of the guyed
Guyed structures for transmission lines: H. B. White V for use in rough terrain that led to the original development of the tower were now producing real limits or deterents to its use at EHV. This writer had a good experience with the design and construction of a large cross-rope suspension system6 in the mountains of British Columbia, Canada in 1955 where about 2030 m (6700 ft) of a 'very large and heavily loaded double circuit 300 kV line was suspended along and over an avalanche swept valley by a crossrope assembly of two wires that spanned 1110 m (3660 ft) transverse to the valley. Thus, in focusing on the difficult problems of access, transport and erection for the forthcoming line and with the experience of the above mentioned suspension system, of almost two decades of work with guyed towers of many kinds and still retaining the three objectives of the ultimate structure, the thought process led almost inevitably to the CRS Tower 7"8 in Figure 5(d). The CRS was first designed for Hydro Quebec, where it became known as the Chainette and it is simply another pair of tripods (each of a pair of guys and a mast) held together at their tops, in this case by a wire rope assembly from which are suspended the insulator strings and from them the conductors. The CRS concept is simple and almost primitive and, to this date, analysis of the governing loading conditions and other assessments of static behaviour can be completely carried out on a peg board model or by simple graphical analysis. More exact methods have been developed which are useful in probing dynamic problems 9. However, before acceptance for first use on a major line, the CRS concept was subjected to intensive testing as part of a full-line system with particular attention to dynamic response from galloping and from ice shedding from the wires. The possibility that a single cross-wire support could transmit the galloping of one phase of conductors into the other phases was the reason for proposing the six-part triangulated arrangement. The test program demonstrated convincingly that, with this configuration, there would be no transfer of motion. The CRS was used for the 4th and 5th lines of the 735 kV James Bay system in Quebec and for an 85 mile section of 500 kV line in Oregon, USA. The latest news is that it is being employed on 500 kV lines across the 'Mongol Prairie' in China and it is a serious contender for use on new lines in South Africa at 400 kV and 800 kV and in South America at 500 kV. The proposal for South Africa and other lines under study for nonice areas (no galloping) is for a single cross-rope to replace the six-part suspension of the original design as shown in Figure 5(e). Although simple in concept and design, the CRS posed a serious problem for construction because there was no way to erect the masts, align them and tension the guys until the conductors were strung and had contributed the weight necessary to tension the system. The key to the CRS was, and still is, a construction spacer cable, cut and fitted to an exact length, which is inserted between the mast tops and used for aligning and tensioning the guy system and, of great importance, for ensuring that the spacing between mast tops is correct so that the cross-rope assembly, also precut and fitted to an exact length, will hang with the correct sag when the conductors are installed. The CRS requires a large area at each site for the spread of the guys but almost all its
other characteristics are advantages, including the fact that, with no structural material between the phases, the phases can be put as close together as wind induced motions will permit, and close phase spacing will reduce the needed width of the rights-of-way in the spans between towers and also improve the power transfer capability of the line. The ground or shielding wires attached to the tops of the masts give the best possible shielding against lightning strokes, the two masts fitted with guys can be assembled in a small area and can be erected with a small crane or gin pole or flown to site and installed with a small (less costly) helicopter. Small angle suspension towers can be created from the same masts and guys by merely changing the lengths of the pieces of the cross-rope suspension and the heights of the two masts. At 800 kV, the structural material of the two masts weighs about 45 to 50% of a comparable guyed V which is, in turn, about 50% of the weight of a rigid tower. Improved procedures for the construction of the CRS with adoption of the single rope suspension for nonice areas and of the method of precutting guys and thus automatic alignment of the masts (see below) should lead to much greater use of this crossarmless type of tower for future lines.
Some special guyed towers Some guyed structure types do not fit easily into the categories of guyed masts, guyed rigid frames nor of the guyed and pinned masts of Figures 4 and 5. These special types are shown in Figure 6. Several interesting attempts have been made to reduce the lengths of the masts of the guyed V tower by converting into forms of pinned or hinged Ys for the reason that three half-length masts should be and are much less costly than two full-length masts while retention of the pinned or hinged mast principle prevents the problems of the guyed rigid structures. The guyed V on a guyed post is shown in Figure 6(a) where the short centre mast requires four small guys to hold it in position. Three half-length masts replace two full-length masts at the expense of an extra four small guys. The saving in mast material is evidently greater than the cost of the extra guy system although the tower is now critically vulnerable to the failure of one of these small guys. This contravenes a principle that holds for almost all other guyed towers in that after stringing, they will remain upright and still be able to carry at least 50% of design loads with one guy removed. This tower would drop at once if one of the small guys were removed or broken. The tower was designed for helicopter erection in two pieces, the upper V and crossarm being lowered onto the post and pulled into position by a line that is fed through a hole in the top of the post. The hinged guyed Y is shown in Fioure 6(b). At a first glance it looks very much like the V on a post. It was installed on hundreds of miles of line under severe terrain and load conditions in Eastern Canada in the 1960s. This is probably the most efficient of all the hinged masted structures, with the exception of the CRS, but the mysteries of its behaviour and the evident difficulty of following very strict procedures during maintenance or repairs have been points against its further use. An unusual feature is that no guys are attached to the hinge point and they are not needed. The usual
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Guyed structures for transmission lines: hi. B. White four outer guys of the guyed V are supplemented by four interior guys that cross over to the opposite sides of the crossarm. The secret of its stable behaviour is understood when it is realized that the hinge point cannot move transversely and pop out unless the crossarm can lift and it is prevented from doing so by the interior guys which hold the crossarm down. This tower will remain standing with the removal of one anchor or two of eight guys (but not two similar guys). It is an interesting structural feature that the hinged connection to the lower vertical leg ensures that the loads in the two upper legs will always be equal and this reduces the uncertainties regarding the load distribution into the crossarm and guy system and ensures that each o f t h e mast sections works fully under all loads. Erection was made simple by attaching two of the inner guys (one from each side of the tower) to the base of the centre mast to form a temporary rigid Y, erecting and plumbing the tower with the four outer guys and then transferring the two inner guys from the base of the mast to their anchor positions. The guyed V and inverted delta or 'T' ~0 is shown in Figure 6(c). The need to build compact lines and to control the magnetic fields beneath a line that some believe may have influences on health have focused attention on the inverted delta or T arrangement of the phases. One method of supporting a compact T is within a guyed V, a V of two tripods with a simple spreader strut to hold the mast tops apart. This tower holds great promise for magnetic field control, for building on narrow rights-ofway and for the benefits that compaction of the phases bring in reducing the reactance of long distance lines and increasing power transfer capability. So far, no one seems to have found a totally acceptable way of suspending the three phases, as there is an evident reluctance to suspend the lower phase from the other two with phase-to-phase insulators. However, it is but a matter of time before someone finds an acceptable suspension arrangement and the new guyed V with T becomes one of the standards for EHV lines. Two guyed single mast direct current (two pole) towers are shown in Figure 6 (d, e). Though but single masts, these towers are more appropriately classified as guyed rigid frames as the four guys do not converge on a single point. At first glance this is a simple and straightforward solution but there are difficult spatial problems of trying to maintain electrical clearances between guys and conductors and at the same time of trying to have the guys centric with the line of action of the resultant of the major loads and the centre line of the mast. If the guys extend beyond the mast and attach to the opposite crossarms, the tower under torque load will rotate and the lines of action of the resisting guys will come closer and guy tensions will increase, a problem mentioned above with regard to the guyed portal. It is common practice with this tower to use extra secondary arms to keep the guys apart although the ground assembly problems with these arms are causing some designers to look again at the need for the torque loading. The guyed X tower is shown in Figure 6(f). The problems of frost heave that are common to many northern lands have led to the development in Alaska of a special type of X tower that behaves as a rigid tower under vertical and transverse loads and is guyed longitudinally. Many hundreds of miles of these X towers are in place,
some of lattice steel, of lattice aluminum and of tubular steel in some of the areas considered more environmentally sensitive. The usual footings are two single H piles on each of which is attached, by U bolts, the bracket on which the tower can pivot back and forth. Pairs of guys are set ahead and behind to maintain longitudinal stability and the anchor assembly of each pair includes tension fuses to ensure that excessive frost heave does not increase guy tensions enough to crush the towers. If frost heave raises one of the piles, the maintenance crews can loosen the U bolts and regain the correct elevalion the next spring.
Special techniques for guyed towers Some guyed tower types allow use of special techniques or practices that increase their effectiveness and lower the overall costs of a line. Five are worthy of note.
Construction tolerances and techniques: necessary standards Major savings can be made in erection time and costs by taking advantage of the relaxed tolerances permitted by the flexibility of the usual type of guyed structure. Furthermore, when a full understanding of the issues is reached, additional savings are possible by reducing the need for the complexity of the fittings and adjustments normally put into the guy systems. These frequently unnecessary turnbuckles and threaded fittings of many types are costly, a possible attraction for random mischief, and in simply adding to the number of components of a structure, reduce the line reliability. Many lines of guyed structures are erected with the same attention to precision that is necessary for the erection of rigid latticed structures and it has proved difficult to break the ingrained habits of the engineers, erection crews and teams of inspectors who take pride in the degree of precision specified, installed and verified. However, it is necessary to relax these standards and tolerate what may appear to some to be poor workmanship if full advantage is to be taken of the guyed principle.
Rigid latticed towers: The costly dimensional controls for the construction of rigid latticed towers are focused on the installation of the foundations. Stress-free erection of the bolted tower assembly is only possible with the four (usually four) footing stubs set to horizontal dimensional tolerances of about 1/1000 or 1/8th of an inch in 10 ft and vertical tolerances that would limit warping of the plane of the footings to about the same 1/1000. If setting errors exceed these by very much, the assembly of the tower will become very difficult, specially in the upper stages, and the built-in stresses could have marked effect on the subsequent load carrying capacity. Although specifics are frequently given for the plumbing of the completed rigid structure, any deviation from the vertical is not produced by the erection process but is evidently the result of a tilting of the plane of the footings.
Guyed tower tolerances: There are two aspects of guyed tower construction that may influence the behaviour of the final structure: the placement of the mast footings
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Guyed structures for transmission lines: H. B. White and guy anchors and the subsequent adjusting of the guy lengths and the tensioning of the guys. Before attempting to set limits or tolerances on any of these, the necessary and sufficient performance criteria should be established. With no known exceptions, the plumbing for verticality of a guyed structure has negligible effect on the structural performance or load carrying capacity unless the inclination far exceeds what is noticeable to and offends the eye of the beholder. A viewer can detect about a 2 ° inclination from the vertical in a single mast or flag pole and variances of maybe ! ° can be noticed in a closely spaced line of poles. It is proposed (and has in tact been used on major guyed tower projects) that the l ° limit (approx. 1/50) be accepted as the general tolerance criteria; thus, for a tower such as the guyed V or the guyed portal, the centre point of the crossarm should not be more than 1/50 of the height from directly above the tower centre point on the ground and the crossarm should be transverse to the line with an error of not more than 1/50 of the crossarm width, etc. Attempts have been made to plumb guyed towers to greater accuracies by adjusting the guy lengths and tensions with turnbuckles or other threaded devices only to find that as the sun came out and heated the guys on the one side, the tower moved sufficiently that the precise criteria had to be discarded. The criteria should be based only on what is necessary and this seems to reduce to the limiting requirement that the structure not appear to be crooked to the naked eye. The criteria of 1 ° tolerance can also apply to anchor locations with the anchor acceptable if lying within a 1 o cone projected from the guy to tower attachment point. This acceptance of less than 'obtainable perfection' will be found to have a significant influence on both labour and material costs when the subject of guy pretensions is also rationalized (see below).
Pretensioning of guys: With the same exceptions as noted previously, most guyed structures are very insensitive to pretensioning of the guys. The obvious upper limit is that the pretension should not be more than 50% of the maximum loaded tension to ensure that the leeward or back side guys go slack under maximum horizontal loading and do not contribute to the maximum stresses in the structure. There are no rigorous or standard lower limits on what is needed although a well used criteria is a pretension sufficient so that the leeward guys should not go slack and flap around unnecessarily in frequently occuring winds: possibly a yearly wind speed. Thus, the pretension would be about 50% of the tension needed to resist that yearly wind load. As was seen earlier, the tensions in all four guys of a tangent 'in line' structure (except for guyed single masts) will be equal in the absence of a transverse wind load and efforts to equalize the pretensions are usually fruitless exercises that merely establish the variances of the tension measuring devices. Precut and fitted guys: Guyed towers are typically erected with guys cut approximately to length with a suitable fitting applied to the top end which is then attached to the tower before erection. Preliminary tension is then applied through temporary grips to each of the four guys to steady and approximately plumb in the
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tower, the guys cut, bottom fittings installed and then all guys brought to tension and the final tower position checked by adjusting the threaded devices, turnbuckles or U-bolts, one in each of the four guys. Guyed tower erection costs are very dependent on the time taken by this operation with much tightening and loosening, verification by optical devices and acceptance or rejection by the inspection crew who follow and who usually have to duplicate the instrument set ups. However, several projects with large guyed towers have demonstrated that most of these costly steps can be eliminated by using precut and prefitted guys and tension adjustment in only one of the four guys. The technique is possible with the acceptance of the tolerances proposed above and the awareness that tensioning of one guy puts equal load in all four. After installation of the mast footings and the anchors (rods), the elevation of each is measured, the required guy length calculated according to the height of the mast or tower and the guys cut and both end fittings applied at a field shop. The completely fitted guys are attached before the mast or tower is erected. A tolerance on guy length of about 0.3% should be readily obtained and a simple graphical exercise will demonstrate that the worst possible combinations of pluses and minuses will still position the tower within the positional limits outlined above. A quick connection of shackles or equivalents and a tensioning at the fourth guy which is the only one with a tensioning device will find the tower erected and in satisfactory position at a saving of many man hours and the saving of at least three unnecessary tensioning units.
Modular towers A guyed tower with pinned masts, such as the guyed V, is specially suited to modular design concepts as the structure can be thought of as an assembly of three distinct structural components that may be combined in different arrangements to provide maximum efficiency of use of materials ~. The only requisite is that all the masts can be fitted to all crossarms. The potential of the system becomes apparent when it is realized that the three components respond to different design specifics or parameters. The three major and interchangeable components are: firstly, the crossarms which are governed by length and the vertical applied loads which depend on the weight span carried by the tower. The length of the crossarm is controlled by the swing of the insulator assemblies and the swing angle and thus the length is at a maximum with a small weight span to wind span ratio and at a minimum with a large weight span. Thus the conditions at each site give the opportunity to use as required: either a normal crossarm, a very strong (vertically) but short crossarm or a long but not very strong crossarm. Secondly, the important compression load in the mast is created by the guys resisting the wind pressure on the wind span of conductors and the bending in the mast is dependent on the wind on the mast and thus critically affected by the height of the mast. The P-6 effect is a product of the two. The weight of the spans of wires has little effect on the mast design. Thus two or more mast designs can be used and selected on the basis of wind span and height.
Guyed structures for transmission lines: H. B. White Thirdly, the loads in the guy/anchor system are directly responsive only to the wind span at each tower site and two or more guy/anchor systems can be used as warranted by the number of towers of the line and the range of spans encountered. A modern tower spotting program can have as input the cost and span limits of combinations of 3 crossarms x 3 masts x 2 guy/anchor systems for a total of 18 different structures and the efficiency of use can be very high if the 'limits are set judiciously.
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Eccentric masts A tower, such as the guyed portal or guyed V, is relatively efficient compared to some rigid equivalents but, in fact, while under the maximum and usually critical load of high transverse wind, only half (one of the tripods) is working and the other tripod is idling and supporting only the vertical weight of half of the wires. For the working tripod, the maximum compression in the mast (load from the guys) combined with the maximum bending moment caused by wind on the mast can only result from wind from one direction. This provides an opportunity to make noncentric masts with an induced bending moment counteracting the bending of the wind on the mast. The concept has been used in Romania and with the guyed V towers of the Alicura project j2.
Footing connection The pinned connection(s) common to all but the fixed mast type of guyed structures range in their detailing from the very simple to the very complex. The movement or action as a pinned connection is very limited once the tower is in place as the masts seldom deflect more than a fraction of a degree during their lifetimes. However, they must be able to rotate freely about the footing (pin) to eliminate torque from the masted part of the structure. The angle of inclination of the masts of the guyed portal or the CRS or any of the guyed rigid towers is a fixed angle although the angle of the masts of the guyed V will change with the height. The angle of the masts of a guyed V set of towers will vary about -4-30-4 ° . Quite large ball and socket castings of aluminum or steel that permit large movements have been used as well as some very simple fittings, as shown in Figure 7. This detail consists of a steel pin, embedded in concrete or directly into rock and set at an appropriate angle, a grout pad as needed, a steel plate to distribute the bearing pressure and a spherical washer to distribute the bearing load from the base of the mast to the washer. The bearing pressure is on a ring of contact and is infinite until the base plate of the mast deforms slightly as the tower is put in place and the guys tensioned. This deformation is then accepted as a permanent and nonthreatening condition and, as long as it is restricted to a few millimetres at most, is negligible compared to the elasticity of the guy system.
Guy sizing and stresses It is presumed that all the analyses of the guyed structures are based on ultimate or limit loads, i.e. the loads that will threaten some component of the system. Thus,
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the computed guy loads represent the maximum demands to be placed on the guy system. The steel strandings in common use for guys are usually limited to about 65 to 70% of the rated tensile strengths (RTS) under the loads of ice and/or wind that may be assumed to be applied to all towers during their lifetimes. This produces a margin on failure of about 50% (an apparent factor of safety of 1.5) and such a margin can be justified. First of all, the guy fittings will seldom develop the RTS of the guy and many will be limited to 9 0 - 9 5 % of the guy RTS. Furthermore a loading much beyond 70% will approach the yield point of the guy with the result that working to such a high stress level might require retensioning of the guy systems of many towers after a major wind storm. The overriding justification comes from the sequence of failure concept that a critical component of little cost should not be the item that instigates a costly failure. The one departure from the 6 5 - 7 0 % limit is that some designers take the guy anchor system to about 85% under failure containment conditions (usually of a single phase longitudinal load). The justification is based on reliability concepts and the attempts to rationalize and economize in the placing of the guys. The major load of wind on the wires will probe all the towers of the line over the lifetime and a single deficient guy or guy fitting will be found and trigger an outage. The demands of failure containment will be imposed on, at most, the two towers adjacent to an already failed structure or a failure of some kind. If one of the two guy assemblies being severely tested by containment duty proves to be deficient, the containment duty merely passes to the next tower where the odds are in favour of there not being another deficient guy or component. This is not a minor point as the line designer is always trying to find ways of improving his guying angles; he wants to move them transverse to the line to reduce the tensions under wind loads while not being able to forget the need to resist the infrequent anticascade loads. A justified increase of 20% (70 to 85) in allowable stresses under longitudinal loading may permit a more efficient guy deployment.
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References 1 White, H. B. "Some destructive mechanisms activated by galloping conductors', IEEE PES Winter Meeting, New York, NY, February 1979, Paper A79 106-6 2 Voipo, E. 'Power generation and transmission system of Finland', IEEEPES Winter Meeting, New York, NY, 1966, Paper 31 TP 66-48 3 White, H. B. 'Chute des Passes 345 kV Transmission Line', AIEE Winter Meeeting, New York, February 1960, Paper 60-72 4 McMutrie, N. J., Murphy, B. R. and Markowsky, M. 'Engineering design features of the P i n a r d - H a n m e r 500 kV transmission line', IEEE Winter Meeeting, New York, February 1964, Paper 64 93 5 Samuelson, A. J. 'American electric power 765 kV transmission line project', IEEE EHV Transmission Conf., Montreal. Canada, 1968, Paper 68 C 57-PWR 6 White, H. B. 'Cross suspension system Kemano-Kitimat transmission line', AIEE Winter Meeting, New York, NY. February 1958, Paper: CP-58-432 7 White, H. B. 'Structural system for the James Bay transmission lines', Hydro-Quebec Symposium on Transmission of Electrical Energy at EHV and UHV, AC, Varennes, Quebec, October 1973 (First disclosure of cross-rope suspension tower) 8 Ghannoum, E. and Lamarre, M. 'Hydro Quebec's experience with the design and construction of 1500 km of 735 kV Chainette
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10
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transmission lines', IEEE PES Winter Meeting, New York, NY. 1985, Paper 85 WM 200-1 Peyrot, A. H., Lee, J. W., Jensen, H. G. and Osteraas, J. 'Application of cable elements concept to a transmission line with cross rope suspension structures', IEEE Trans. 1981, PAS-100, (7) Gidlund, J. I. et al., "Swedish State Power Board adopts the T-tower design for 420 kV lines', International Conference ml blrge Electrical Networks, CIGRE SC22 (WG8)-I3, Paris, France, June 1988 White, H. B. "A modular design system for guyed V towers', IEEE PES Winter Meeting, New York, NY, January 1978. Paper F78-151-3 Behncke, R. and White, H. B. 'The Alicura 500 kV transmission system', hlternational Cotl~'rence on l~rge Electrical Network,~, CIGRE, Paris, France, September 1984, Paper 22-02 Bateman, L. A., Haywood, R. W. and Brooks, R. F. "Nelson Rivcr DC transmission project', lEEk? EHV Transmis,~ion Cot~ference. Montreal, Canada, 1968. Paper 68 C 57-PWR Staudzs, A. 'Design considerations of the Radisson-Churchill 138 kV transmission line', Canadian Electrical Association. Toronto, Ontario, March 1986 Mathur, R. K., Shah, A. H., Trainor, P. G. S. and Popplewell, L. "Dynamics of a guyed transmission tower system', IEEE Sumnte~ Meeting. Mexico City, Mexico 1986, Paper 86 SM 414 7