Electrical Power Plant Design: Transmission System

WEEK 06: TRANSMISSION: OVERHEAD AC

Suspension Clamps These designs are fairly well standardized for the usual conductors. Simple, light, well-designed clamps in both malleable iron and forged steel are available for almost any conductor. The suspension clamp is intended primarily to support the weight of the conductor and to prevent any longitudinal movement from accidental unequal tensions in adjacent spans. Trunnion-Type clamps are designed to give an almost completely flexible connection by supporting the clamp on a pivot, approximately on the axis of the conductor. Slip or Releasing clamps is a clamp that would hold under all ordinary conditions but would slip at something like one-half the maximum conductor tension in case of a break. Dead-End clamps are of the bolted type and are available for all copper and aluminum conductors. This is very similar to the compression splice used on ACSR.

Armor Rods are generally used on ACSR lines as protection against fatigue of the aluminum strands from vibration. They consist of a bundle of aluminum rods, somewhat larger in diameter than the strands of the conductor, laid parallel to the length of the conductor and arranged to form a complete covering. They are spirally twisted by a tool to lie approximately parallel with the lay of the strands in the cable and are clamped in place at each end. The bending stresses caused by vibration are reduced by the increased diameter and area of metal and distributed over a longer section of conductor.

Vibration Dampers are devices for damping vibration out of the entire span. The commonly used is the Stockbridge damper.

Hardware Many items have become fairly well standardized. The dimensions of the eye of the eye bolts, the length of thread on various-length bolts, end links, and hardware for suspension insulators are quite uniform. Many other items such as shackles, guy clamps, and crossarm braces are furnished in such a wide variety but suitable stock items.

Insulating Braces Steel crossarm braces have been replaced with wood on a number of lines. Reinforced fiberglass is receiving increased favor in place of wood and as crossarm braces.

Guys are braces or cables fastened to the pole. There are various grades of guy strands. Siemens-Martin strand is most commonly used for the lighter lines and high-strength strands for heavy construction. The 3/8-in size is most generally used both for guys and for overhead ground wires. The most commonly used form of guying is the anchor guy. The major components of an anchor guy are the wire, clamps, the anchor and sometimes a strain insulator. The wire is usually Copperweld or galvanized or bethanized steel. The guy is usually firmly attached to the pole by a thimble-eye or by a guy eye bolt and a stubbing washer. Guy guards are usually employed to make the wire more visible to prevent pedestrians from tripping and accidentally being cut by the wire, and protect the guy wire from damage by cars or vandals. Anchors are of several types: no-wrench screw, swamp screw, and wrench-type screw. Installation can be through expanded, single-plate, cross plate, cone, or expanding rock anchoring. Aside from anchor guy, other forms of guying includes: the span, head, stub, crossarm, dead-end, side, and storm guys. Span Guy transfers some strain from one pole to another. the Head Guy, a variation of the span guy, differs in that the wire runs to a point somewhere below the top of the sustaining pole. When a pole is located on the slope of a hill, a head guy is used to counteract the downhill pull of the conductors. Straight Stub Guy which employs a stub, a piece of wood, parallel to the pole, to which the guy wire is attached. The stub guy wire must allow enough clearance for traffic. The Raked Stub Guy, similar to the straight type but uses cribbing logs and the stub is usually installed diagonally away from the pole. Dead-End Guys are wires applied to where the conductor terminates (dead-ends) on a pole to counteract the pull of conductors. Crossarm Guys are applied on crossarms at dead-ends from the side of the crossarm undergoing the worst strain to the adjacent pole. Side Guys are a pair of anchor guys attached to a pole located on a corner. The two guys counteract the pull in two directions. When a branch line shoots from a pole, the weight of the branch conductors causes a side pull. This side pull is counterbalanced by side guys. Storm Guys are usually four guys: two line guys and two side guys or four anchor guys -- used on lines that maybe subjected to storms or other strong atmospheric disturbances. All four guys are fastened to the same pole.

Connectors are sometimes spliced by overlapping the ends and twisting the ends together. Types include: parallel clamp; twisted sleeve, compression sleeve, automatic line splice and the split bolt connector. The parallel clamp inserts the two ends into a double sleeve. Twisted sleeve are used when the two conductors are parallel and adjacent to each other. Compression sleeve are used with the large ACSR. Conductors are inserted from both ends until they butt and the sleeve is crimped in several places. The automatic splice has the conductor ends inserted in each end where they are gripped by wedges held together by a spring. The split-bolt or bug-nut connector is a copper or tin-plated-copper bolt with a channel cut into a shank, but conductors fit into the channel and are compressed together by a nut.

Overhead Ground Wires should receive no less care in erecting than the conductors because the usual zinc or copper protective coating is very easily destroyed. Ground wires should be sagged in the same way as the conductors except that the important factor in ground-wire sags is to maintain ample clearance tot eh conductor. Generally, ground wires are sagged to about 80% of the conductor sags. They are also subject to lightning damage and burning at connection points due to power frequency fault currents. Ground wires are also equipped with jumper connectors at tower attachment points to bypass fault currents.

Foundation