Introduction

Overhead conductors are manufactured in a variety of sizes and strandings, as well as from several different materials. This range of choices permits selection of combinations of conductor characteristics suitable for specific lines. Conductor characteristics include conductance, diameter, strength, weight and coefficient of thermal expansion, as well as stress-strain, creep and thermal loss-of-strength characteristics. Proper conductor selection takes into account the interaction of these characteristics with requirements of the line: its voltage, capacity and load factor, the terrain it will traverse and the environmental loads that it must withstand. Optimum conductor selection is beyond the scope of this book, since it does not cover several areas involved in the required tradeoffs, such as electrical and thermal behavior. Discussion of procedures for making this selection can be found in numerous papers, of which LaForest & Lindh [1], Bontempo et al [2], Chang & Zinn [3] and Douglass [4] are examples. Selection procedures have been embodied in computer programs such as EPRI's TLOP [5].

This book deals with several areas involved in conductor fabrication, beginning with a brief description of the machinery used in stranding. Discussion of the geometry of conventional conductors follows. The strains and stresses imposed on the strands during stranding are analyzed, including the impact of the type of strander employed. The book ends with a treatment of the springback behavior of conductors following cutting.

All overhead conductors are made by winding layers of strands about a central core, usually a single strand. The strands of these layers are formed into helices during stranding, so the layers are roughly cylindrical in shape and the axes of all the strands in each layer trace out helices on a common circular cylinder. All standard overhead conductors are concentric-lay, that is, the strand axis cylinders for all layers are concentric with the core. All standard overhead conductors are reverse-lay: the direction of rotation of each layer is made opposite to that of the layer below, as illustrated in Fig. 1. By convention, the outer-most layer in the energized, or phase conductors has the direction of lay of a right-hand screw, while that of the "earth" or "ground" wires that shield the phase conductors from lightning have left-hand lay.

The advantages of the type of construction used in overhead conductors become intuitively obvious with only a little experience in handling them. However, it is perhaps worthwhile to note briefly the reasons why they are made the way they are.

Using stranded conductors permits them to be flexible enough to be reeled for shipping, and makes them more tolerant of minor damage than solid rods would be. It also makes it possible to manufacture them in great lengths, even though the maximum size of ingots or spool loads, involved in the manufacturing process, may limit the weight of individual component strands. Helical lay, and concentric lay in particular, is used to realize fully the flexibility offered by multiple strand construction. Without it, the shape of the conductor would distort badly when it was bent. In addition, there would be no "skin" to hold it together.

Reverse lay improves the conductor's structural integrity by defining the space for each layer. Laying successive layers in opposite directions creates a dense pattern, or "matrix," of inter-layer contact points where their strands cross. These contact points give definition to the boundary separating the layers. In parallel-lay, or bunched conductors, the strands from one layer can invade an adjacent layer. In fact, when parallel-lay conductors have been tried in overhead lines, inner layers have sometimes thrust through the conductor surface during stringing. Strands were damaged, and it was sometimes not possible to restore the conductor to its intended shape.

Use of reverse lay reduces the torque that is created in conductors when they are tensioned. This torque can cause problems during stringing. Reverse lay in addition improves electrical characteristics, in particular self-inductance and ac resistance.

Figure 1. Reversed lay construction

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