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by on March 9, 2019
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All transmission lines consist of structures mostly of the same material but of different sizes, insulator, line angle and loading configurations. These can be grouped into a “family’’ of structures specific for the project. Once the basic structure type (usually tangent) has been established, the “family’’ can be set up by adding systems of other configurations, specifically angle and deadend structures. In other words, the structure family basically contains tangent, angle (small, medium, large) and deadend structures. For example, if a single wood pole with horizontal post insulators is chosen as the primary tangent structure for a specific voltage, then the extended “family’’ would consist of the following guyed structures to complement the tangent system:

1. Small Angle Structure for line angles 0◦ to 20◦

2. Medium Angle Structure for line angles 21◦ to 45◦

3. Large Angle/Deadend Structures for line angles 46◦ to 90◦

Similarly, if a wood H-Frame with suspension insulators is chosen as the primary tangent structure, then the extended “family’’ would consist of the following guyed structures which complement the tangent system:

1. 3-Pole Small Angle Structure for line angles 0◦ to 20◦

2. 3-Pole Medium Angle Structure for line angles 21◦ to 45◦

3. 3-Pole Large Angle/Deadend Structures for line angles 46◦ to 90◦

PLS-POLE contains a library of standard RUS structure families and models for ready use. These families cover single pole systems, H-Frames and 3-pole systems for various voltages. The structures refer to RUS/USDA Bulletin 810 (1998) up to 69 kV and Bulletin 811 (1998) up to 230 kV. The user can also adopt a particular RUS model as a base and modify it to suit the requirements of the project, for voltages above 230 kV.

Structure families based on usage and wire configuration are shown in Figures 3.5 and 3.6 while Figure 3.7 depicts a typical family of H-Frames wood structures designed for various voltages up to 345 kV. Note that the footprint of the structure (i.e.) width of ROW needed, increases with increasing voltage. Figure 3.8 shows two- and three-pole guyed structures employed at running angles and deadends.

1-) Structure Models

In the US, most transmission structures are currently modeled on PLS family of programs. Models can be created to the level of detail required by the engineer. Where available, assembly units can be directly incorporated into the model (example: standard cross arms, pole top angle, davit arms etc.). For realistic representation, the nature of the connection of the unit to the pole must be correctly modeled. For instance, cross

arms are affixed to the poles of an H-Frame with through bolts, which offers partial fixity. If the PLS Family of programs are used, models can be created on PLS-POLETM or TOWERTM for analysis as a single independent structure or for future export to the PLS-CADDTM program. For the latter option, the set and phase numbers assigned to each wire/insulator must be clearly defined.

1-a) Insulator attachment to structure

The attachment of insulators to various structures depends on the structure configuration as well as material. Hardware associated with insulator connections is a function of pole material. Also, the end fittings of the insulators depend on what type of attachment is being sought. As mentioned earlier in Chapter 2, the effective length of the

insulator string is critical in determining the sag-clearance in the span as well as the swing. This in turn will affect the required height of the pole/structure.

Attachment to Steel Structure: Suspension insulators are usually attached to the structure via steel tubular davit arms (tapered) or horizontal posts. In steel H-Frames, the insulators are typically attached via vertical vangs welded to the steel cross arm. In sub-station structures, tubular steel cross arms are generally used to support the deadend/terminal wire loads, in which case the vangs are horizontal. Angle and strain insulators are attached to the pole via welded vang plates. Where moderate line angles pose a challenge along with narrow ROW, the insulators are attached to the davit arms via swing brackets. Horizontal posts are attached to the pole via support brackets, flat base or gain base.

Attachment to Wood Structure: Suspension insulators are attached to the structure via davit arms or horizontal posts. In H-Frames, the insulators are attached via proper end fittings to the wood cross arm. Angle and strain insulators are attached to the pole directly via eye bolts or guying tees. Where moderate line angles pose a challenge along with a narrow ROW, the insulators are attached to the davit arms via swing brackets. Horizontal posts are attached to the pole via support brackets, flat base or gain base.

Figures 3.9a, b and c show various insulator attachments (braced line post, cross arm and davit arms).

Attachment to Concrete Structure: Insulator attachment to concrete is a bit more complex than other structures since the poles are prestressed (i.e.) contain stressed tendons inside the core wall. Holes to attach insulator hardware are pre-drilled carefully. For example, insulators are usually attached to the structure via steel brackets supporting horizontal posts (Figure 3.10). Angle and strain insulators are attached to the pole via bolted tees. Where moderate line angles pose a challenge along with narrow ROW, the insulators are attached to the davit arms via swing brackets. Horizontal posts are attached to the pole via support brackets, usually with a gain base inclined at 12 degree angle.

Attachment to Lattice Towers: Suspension insulators are usually attached to the structure via proper end fittings at the ends of the lattice arms. To control insulator swing and to maintain wire-structure surface clearances, 2-part insulators are often used. Angle and strain insulators are attached to the tower via vangs welded at the end of the lattice arms.

 

The engineer is referred to various manufacturer catalogs (Ohio Brass, NGK Locke, MacLean, Hubbell etc.) for more information on insulators, end fittings and hardware. Figures 3.11 to 3.13 show insulator attachment in typical structural systems. Another situation where insulator attachment pattern is important is at locations where the wires transition from a vertical to a horizontal configuration or vice-versa. One example is from a vertical angle or deadend to a H-Frame. Figure 3.14 indicates a preferred way of connecting the phases so that potential for short circuiting (phase wire contact) is minimized.

 

 2-) Structure Types

Transmission structures are divided into 4 functional categories for defining strength requirements and based on the manner in which the wire loads are resisted (See Figure 3.3).

Suspension or Tangent Structure: where all wires are attached to the structure using suspension insulators and clamps not capable of resisting tension on the wires.

Strain Structure: primarily used at running angles where all wires are attached to the structure using suspension or strain insulators and clamps where the transverse forces resulting from wire tensions are resisted by guy wires and anchors (or an unguyed system if steel poles are used).

 

Deadend Structure: primarily used at large angles and deadends where all wires are attached to the structure using strain insulators and botted deadend clamps (or compression deadend connectors) where the structure must have the ability to safely resist a situation where all wires are broken on one side, in addition to loading from intact wires.

Terminal Structure: where all wires are attached to the structure using strain insulators on one side only. This situation usually occurs at substation frames where wires are installed at a reduced tension on the spans coming into the substation.

Configuration-wise, the most basic structure type is the single pole system which is extensively employed for tangent, angle and deadend applications, in wood, steel, concrete and composite. Apart from lattice-type systems, the only other unique configuration popularly used is the 2-Pole H-Frame.

2-a) H - FRAMES

H-Frame structures are commonly used in 69 kV to 230 kV (and above) single or double-circuit high voltage transmission lines. They are often used in situations where spans are relatively moderate and ROW adequate. Design with H-Frames is generally performed in terms of “Allowable Spans’’ where the maximum allowable horizontal (and vertical) spans are determined as a function of several variables. Spans are often limited by X-brace and cross arm strengths, insulator swings or uplift. Design is often governed by the setting depth needed to resist lateral overturning forces in case of unbraced structures. Wood is the predominant material in most H-Frames although steel and composites are also being increasingly employed. Since asymmetrical bending is often involved, factors like backfill material often control the overturning resistance of the structure at ground line. Also, if the ratio of Vertical Span/Horizontal Span is less than 1.0 (excessive elevation difference), then the effects of the vee/knee braces also become predominant.

Cross Arms connect the two (or three) poles of the H-Frame and provide locations for attaching insulators. A double cross arm is often used to resist large vertical loads due to large spans or when the frame is a tangent deadend. Cross arm lengths range from 12 ft. to 40 ft. (3.7 m to 12.2 m) depending on voltage, phase separation etc.

X-bracing in H-Frames helps increase the allowable horizontal spans by increasing the structure strength. They also help in enhancing the lateral stiffness of the structure to resist transverse deflections. Design strength of typical RUS braces ranges from 20,000 lbs to 40,000 lbs (89 kN to 178 kN) in either tension/compression. All wood cross arms and braces used in RUS standard H-Frames are typical RUS units, defined by the following pole separations:

69 kV − 10½ ft. (3.2 m)

115 kV − 12½ ft. (3.8 m)

161 kV − 15½ ft. (4.7 m)

230 kV − 19½ ft. (6.0 m)

The reader is referred to Example 3.6 showing situations where various H-Frame types are chosen.

For 3-pole systems, the arm lengths vary from 25 ft. to 35 ft. (7.6 m to 10.7 m). For other pole spacing, the axial capacity of the X-braces or minimum brace size can be found using the catalogs from various manufacturers such as Hughes Brothers (2012).

2-b) Guyed Structures

Wood structures at running angles and deadends are characterized by strain insulators and guy wires linked to an anchor. In case of single poles (vertical angles), the guys are usually “bi-sector’’ guys (i.e.) they are oriented along a line bisecting the line angle. For larger 3-pole angle systems, the guys and anchors are located on either side of the structure. Anchors can be individual (one anchor per guy wire) or combined (one anchor for two guy wires). At line locations where there is a change in wire tension, in-line guying is adopted.

For poles stabilized by guy wires, the wires are considered an integral part of the structural system. Design specifications include guy type, size, modulus of elasticity, rated tensile strength (RTS), allowable load (often as a% of RTS), installation tension (usually as a% of RTS) and location of attachment on pole and anchor on ground (guy slope or angle). The recommended guy angle to pole is 45◦. Utilities specify several sizes of storm guys for wood poles, namely, 3/8 in., 7/16 in., ½ in. etc. up to ¾ in. (9, 11, 13 mm up to 19 mm) with ultimate tensile strengths from 10.8 kips to 58 kips (48 kN to 258 kN), respectively.

Anchors come in a variety of sizes and configurations (single log, double log, plate), helical screw and rock anchors. Virtually all guy-anchor systems provide means for grounding the overhead ground wire by connecting it to the anchor and therefore embedded in the ground.

At locations where guying at a pole is prevented for various reasons (lack of space, for instance), the system is guyed by means of a stub pole usually installed across the street or road. The guying here includes overhead wires from pole to stub pole and then the anchor guys from the stub pole to the ground.

From analysis perspectives, any structural system with a cable element (i.e.) a guy wire is predominantly a non-linear system. Therefore, such systems when analyzed on any computer program (such as PLS-POLETM) must use the non-linear option.

 

 

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