Electrical Power Plant Design: Transmission System

WEEK 06: TRANSMISSION: OVERHEAD AC

Conductor and Structure Loads The span design consists of determining the sag at which the conductors shall be erected so that heavy winds, accumulation of ice and snow, and low temperatures, even if sustained for several days, will not stress the conductors beyond the elastic limit, cause a permanent stretch, or result in fatigue failures from continued vibrations. Wind load is found by the following formula; Wind load (lb/ft) = p × D/12; where p is the wind pressure in pounds per square foot, D is the diameter of the conductor in inches. The wind load is assumed to act horizontally and at right angles to the span. The wind pressure at any height is dependent on the basic wind speed, the force coefficient for cylindrical surfaces, the terrain factor, and the response factor.

Stresses in a Span The high tension levels typical of a transmission line conductor is the result of attempting to support the conductor with tension or nearly at right angles to the direction of the load. The horizontal tension in the wire is equal to the weight supported V (the length of wire l/2 times the weight per foot w) divided by the tangent of the single of slope q. The curves defined by the sag can be of two types: catenary and parabola. The student should be able to solve mathematically the sag and stresses under each conditions.

Span Calculations The problems of span design is divided into three classes: (1) The sag in a span resulting from a conductor of given weight at a given tension. These problems are direct solutions or may be read from the Thomas chart. The Thomas method makes sag calculations that apply to any type of conductor. No approximations are introduced and the accuracy is limited only by the scale to which the charts are drawn. On the other hand, the Martin's Tables is a very complete tabulation of unit-span values used in exactly the same manner as trigonometric functions. The stress factor in the tables is the reciprocal of the stress factor in the Thomas chart. The table also include an additional dimension: the average stress factor between the stresses at the support and at the low point. (2) The sags or tensions resulting from unequal spans or differences in elevation of supports. The sag, measured vertically to a tangent to the conductors which is parallel to a line through the supports, will be very nearly equal to the sag in a level span of a length equal to the slope distance. (3) The sags resulting from changes in loading or temperature. These problems are complicated, but the two simultaneous equations involved are so readily solves as two interesting curves.

Wind-Induced Motion In addition to ordinary "blowout" of overhead conductors, i.e., the swinging motion of the conductor due to the normal component of wind, there are three types of cyclic wind-induced motions that can be a source of damage to structures or conductors or that can result in sufficient reduction in electrical clearance to cause flashover. The categories are aeolian vibration, galloping, and wake-induced oscillation. Aeolian Vibration can occur when conductors are exposed to steady low-velocity wind, If the amplitude of such vibration is sufficient, it can result in strand fatigue and/or fatigue of conductor accessories. The amplitude of vibration can be reduced by reducing the conductor tension, adding damping by using dampers, or by the use of special conductors which either provide more damping than standard conductors or are shaped so as to prevent resonance between the tensioned conductor span and the wind-induced vibration force. Galloping is normally confined to conductors with a coating of glaze ice over at least part of their circumference and thus is not a problem in those areas where ice storms do not occur. It maybe controlled by the use of various accessories attached to the conductor in the span to change mechanical and/or damping characteristics. Wake-Induced Oscillation is limited to lines having bundled conductors and results from aerodynamic forces on the downstream conductor of the bundle as it moves in and out of the wake of the upstream conductor. It is controlled by maintaining sufficiently large conductor spacing in the bundle, unequal subspan lengths, and tilting the bundles.

Supporting Structures