Transmission Line V-String Insulator Calculations

A V-string insulator consist of two insulator strings or rods that are attached, at their tops, to two separate points of a structure and, at their bottoms, provide a shared wire attachment. The elevation view looks like a letter “V”, hence the name. The main advantage of this insulator type is that the configuration of the V-string, unlike an I-string, restricts transverse (into the structure) swings/deflections of the wire, allowing horizontal framing within the structure to potentially be more compact and eliminating the insulator swing component from wire blowout calculations, potentially reducing the required right-of-way width. In addition, the V-strings are still allowed to swing longitudinally (into the adjacent spans), allowing the same tension imbalance and broken wire adjustments that an I-string insulator affords.

While insulator swing calculations are generally not necessary for V-String insulators unless the longitudinal swing under a loading scenario is of interest, an allowable load angle calculations is generally performed to ensure that no leg of the insulator will go into compression, since the insulator bells and rods are generally not designed to support significant compressive loads, if any. This post will develop a variety of equations necessary for calculating the geometry of a V-string insulator, as well as evaluating the allowable load angles and internal loads for strength calculations. The geometry of interest for the subsequent development is shown in Figure 1, which displays two insulators of length $L_1$ and $L_2$ attached to two structure attachments at separations $\Delta x$ and $\Delta z$. In addition, load $P$ is some wire resultant load applied at angle $\phi$ from vertical.

Statistical Evaluation of PLS-CADD Batch Thermal Calculator Results

This is a presentation I gave on using statistics to identify outliers and potential input errors in PLS-CADD batch thermal calculations.

Transmission Line I-String Insulator Swing Calculations

I-string insulators are insulators that attach to a structure at one end, via a pinned connection, and support a conductor at the other end, via a suspension clamp or other hardware. They typically consist of a series of porcelain or glass insulator bells (Figure 2) or a polymer rod (Figure 1). Their attachment to the structure via a pinned connection allows the insulator to freely swing with imbalances in wire tension loads. This flexibility helps reduce the lateral loads on transmission line structures due to temporary tension imbalances. In the event of a broken wire on a tangent or angle structure, the insulator is also capable of swinging until a new equilibrium is obtained, dissipating energy and reducing the tension that the structure must support under a broken wire scenario.

While the ability of an I-string insulator to swing is beneficial due to its flexibility, excessive flexibility can result in the insulator swinging too far toward the structure or other nearby objects, developing electrical clearance issues. To verify that the swing provides adequate clearance to the structure, designers typically calculate the allowable minimum and maximum swing angles that an insulator of a specified length must reside between in order to maintain clearances. The expected insulator swings due to loads (wire tensions, wind loads, etc.) are then calculated and compared versus these limiting angles to ensure that the design is in conformance.

This post will present derivations of the equations necessary to perform I-string insulator swing checks, as well as to determine the deflected wire attachment location in space for use in sag-tension or other calculations.

Transmission Line Jumper Clearance and Minimum Line Angle Calculations

In order to maintain electrical clearances within transmission line dead-end structures, jumper supports, such as horizontal posts or I-string insulators supported on davit arms, are occasionally required in order to restrain the jumper away from the structure. Most often, they are required for in-line dead-ends (structures with no line angle) or dead-ends with small line angles; whereas, structures with large line angles, such as that shown in Figure 1, do not require jumper supports since the length of the insulators alone provide an adequate offset distance from the structure.

This post will present equations to assist in evaluating the clearance between a jumper and pole with and without jumper supports. In addition, an equation to calculate the minimum line angle required in order to maintain electrical clearances to the pole for a structure without a jumper support will be presented, as this value can be beneficial in making general design decisions or specifying limits for structures within design standards.