POWER SYSTEM LOAD MODELING APPLICATIONS BASIC AND.DISTRIBUTION CIRCUIT FEEDERS VOLTAGE DROP AND LOSS.However, the interaction of conductors in the bundle due to slightly different tensions and increased damping from spacers results in lower vibration levels for bundles than for single conductors in the same wind exposure. Higher tensions are routinely used in areas where the line is parallel to existing lines and the higher tension on the existing line has not resulted in problems. The use of vibration dampers or special anti vibration conductors can also allow the use of higher tension levels.Īs with single conductors, bundled conductors are subjected to eolian vibration. As a practical approximation, stringing conductors to a final unloaded tension of 15% or less at the minimum seasonal average temperature (usually 0 to 30 F) will prevent vibration fatigue problems. The addition of dampers to the conductor has been established as an effective means of control. Special conductors such as SDC and SSAC have also been shown effective in reducing the strand stress levels.Īnother effective means of limiting vibration fatigue problems is to increase the self-damping of standard conductors by reducing tension. The amplitude of eolian vibration is fixed by the balance of energy input from the wind-induced vortex shedding forces and the energy loss due to conductor, accessory, and structure damping. Stress and amplitude of vibration can be related by analytical means such as the Poffenberger Swart formula. The stress is related to the amplitude of conductor vibration and is the amplitude normally measured by field recording devices. The maximum alternating stress resulting in strand fatigue normally occurs at the conductor clamp. Practical wind speeds cause vortex shedding forces of greater than 10 Hz, eliminating frequencies below this level, and frequencies above 100 Hz are not present because of the rapid increase in conductor selfdamping for these higher frequencies. This is verified by actual conductor performance where significant amplitudes are usually observed for frequencies in the range of 10 to 100 Hz. Therefore, the eolian vibration force will be unlikely to excite a fundamental span mode. The fundamental frequency of vibration of the suspended conductor is in the range of 0.1 to 1.0 Hz. To develop significant amplitudes, there must be a resonance between this oscillating wind force and the vibrating catenary (conductor). The frequency at which this alternating pressure occurs is given by the expressionįor a 1.0-in-diameter conductor exposed to a 10-mi/h wind, the vortex shedding force oscillates at 32.6 Hz. The vortex shedding is accompanied by a varying pressure on the top and bottom of the conductor that encourages cyclic vibration of the conductor perpendicular to the direction of wind flow. As wind blows across a conductor, vortices are shed from the top and bottom of the conductor. The amplitude of vibration can be reduced by reducing the conductor tension, adding damping by using dampers (or clamps with damping characteristics), 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.Įolian Vibration. If the amplitude of such vibration is sufficient, it can result in strand fatigue and/or fatigue of conductor accessories. Hence, the conductor in the span under consideration exert an upward force, and the conductor tends to swing clear of the lower support.Eolian vibration can occur when conductors are exposed to a steady low-velocity wind. Equation (8) shows that the lowest point lies outside the span AB. The value of x obtained above may be substituted in equation (5) to calculate sag at OA. Vertical reaction at the higher support – w.(l-x)įor calculation, it is assumed that the AOB is the parabola. The maximum tension will occur at B since (l-x) is greater than x as seen in the figure above. The sag at OA and OB is expressed by the equations L – x – a horizontal distance of B from the lowest point O. X – the horizontal distance of A from the lowest point O. L – horizontal span length between A and B H – difference span length between A and B The portion of OA and OB may be treated as catenaries of half span x and l-x respectively shown in the figure below. For the calculation of sag and tension at unequal supports level consider a conductor AOB. In hilly areas or sloping grounds, the supports are not usual at the same level. Calculation of Sag and Tension at an unequal level supports
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