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).

Figure 3.11 Insulator Attachment to Lattice Tower.

Structural analysis and design 87

Figure 3.12 Insulator Attachment to H-Frame with Double Cross Arms.

Figure 3.13 Insulator Attachment to Steel Pole.

88 Design of electrical transmission lines

Figure 3.14 Phasing Arrangement for Vertical to Horizontal Construction.

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 insu-lators 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.

Structural analysis and design 89 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.

3.2.2.1 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).

3.2.2.2 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

90 Design of electrical transmission lines

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-POLE^TM) must use the non-linear option.