The advent of composite insulators began during the 1950s, first in the U.S. and soon after in Germany and France. User acceptance was slow at the start and these products went through the ‘teething’ problems common to most technological innovations. Moreover, their initial pricing made them far too costly for widespread application. But all this changed during the past two decades. Today, these insulators account for about half of the total world market. Production volumes have soared and, with this, acquisition costs are now often below those of porcelain and glass counterparts.
One of earliest applications for composite insulators was as insulating cross-arms, which are indispensable for design of compact lines and so-called aesthetic towers. The former, in particular, are rapidly gaining ground as an alternative to building traditional lines due to higher public acceptance. Moreover, composite insulators play a growing role in cases of line uprating to increase power transfer capacity of existing lines. Konstantin O. Papailiou, former Chairman of CIGRE’s Study Committee on Overhead Lines and longtime executive in the insulator industry, explains the necessary properties of composite insulators as well as recent examples of such applications.
Compact lines were first developed in the 1970s but only started to become popular during the late 1990s due to rapid growth in the availability of composite insulators. Insulated cross-arms, which are indispensable for installation of a compact line, are loaded primarily by compression, which means that they are subjected to relatively large deformations. These deformations can better be sustained by composite materials than by conventional porcelain and glass insulators. Specifically, the following key properties of composite insulators are advantageous for application in insulated cross-arms: high bending strength; elastic limit in the region of ultimate strength; high ultimate strain; and non-brittle behavior
Options for Line Compaction
The basic idea behind line compaction is to suppress horizontal movement of the classical suspension string. This way, line supports can become more slender and, at the same time, the right-of-way dimensions needed are reduced. Over time, four different insulator arrangements have come to be used for line compaction: V-strings; horizontal posts; suspended posts; and insulated cross-arms. These four arrangements are shown in Figs. 1 to 4.
Fig. 5 shows the loads that act on an insulated cross-arm. These are:
• vertical loads, V, from the conductor and from ice, if present;
• horizontal loads, H, from wind and, in the case of light-angle supports, from angular pull; and
• longitudinal loads, T, possibly from non-uniform conductor tension in adjacent spans or from a conductor failure – a rare exceptional load.
The vertical loads are taken up largely by the brace, depending on the angle, α, between the brace and post. By contrast, horizontal loads acting in compression, load the post in buckling. The insulator forces, i.e. the compression force, P, on the post and tensile force, B, on the brace are calculated, assuming T = 0, using the following formulas:
For voltages up to 245 kV, the post is often rigidly connected to the support (as in Fig. 6). CIGRE WG 22-03 used commercial finite element software to calculate the loading diagram, also called the application curve, for a 63 mm post of 2000 mm length with inclination angle to horizontal of 15° (see Fig. 8). Coupling angle of the 16 mm brace to tower was 45°, this brace being assumed to pivot at either end.
The load on the brace should not be negative (compression) so as to prevent buckling of the brace that would lead to contact between the metal fittings of the two insulators. With a horizontal angle of the post insulator of 15°, as used here, this condition leads to the inequality: V > H tan 15°. In this diagram, the lower straight line corresponds to the equality V = H tan 15°, i.e. the brace is not loaded along this line or, in other words, the insulated cross-arm should not ‘work’ below this line. The upper straight line in the diagram extends parallel to the lower straight line and corresponds to maximum allowable tensile load of the brace. It is good practice to use a so-called fail-safe base for the post, which will plastically deform in case of overload thus protecting the more sensitive – and more expensive – post.