![]() ![]() ![]() In extreme cases, the cross-arm could fail mechanically. Particularly in the case of long cross-arms for higher voltages, it may be that cross-arms become de-stabilized, leading to considerable deflections with associated reduction in safety distance between conductor and tower. as in mountainous terrain) and short circuit forces. These movements could be caused by gusts of wind, irregular icing, spans of considerably different length (e.g. Such movements could take place should temporary differential line tension occur at the tip of a cross-arm on an overhead line section consisting of a number of spans. One particular advantage of pivoting insulated cross-arms is their ability to stabilize in the event of sudden conductor movements. (left) test arrangement (right) deflections vs. 10: Full-scale test of 420 kV insulated cross-arm (courtesy Pfisterer Sefag). 9: Test results of buckling tests on insulated cross-arms having 63 mm posts of different length compared to theoretical Euler load. Coupling angle of the 16 mm brace to tower was 45°, this brace being assumed to pivot at either end.įig. 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. 5: Geometry and forces.įor voltages up to 245 kV, the post is often rigidly connected to the support (as in Fig. the compression force, P, on the post and tensile force, B, on the brace are calculated, assuming T = 0, using the following formulas: Fig. By contrast, horizontal loads acting in compression, load the post in buckling. The vertical loads are taken up largely by the brace, depending on the angle, , between the brace and post. longitudinal loads, T, possibly from non-uniform conductor tension in adjacent spans or from a conductor failure – a rare exceptional load.horizontal loads, H, from wind and, in the case of light-angle supports, from angular pull and.vertical loads, V, from the conductor and from ice, if present.5 shows the loads that act on an insulated cross-arm. These four arrangements are shown in Figs. Over time, four different insulator arrangements have come to be used for line compaction: V-strings horizontal posts suspended posts and insulated cross-arms. This way, line supports can become more slender and, at the same time, the right-of-way dimensions needed are reduced. The basic idea behind line compaction is to suppress horizontal movement of the classical suspension string. 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.Īdvertisement Options for Line Compaction These deformations can better be sustained by composite materials than by conventional porcelain and glass 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. Papailiou, former Chairman of CIGRE’s Study Committee on Overhead Lines and retired longtime executive in the insulator industry, explains the necessary properties of composite insulators as well as examples of such applications.Ĭompact 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. ![]() Moreover, composite insulators play a growing role in cases of line uprating to increase power transfer capacity of existing lines. The former, in particular, are rapidly gaining ground as an alternative to building traditional lines due to higher public acceptance. 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. Production volumes have soared and, with this, acquisition costs are now often below those of porcelain and glass counterparts. Today, these insulators account for about half of the total world market. But all this changed during the past two decades. Moreover, their initial pricing made them far too costly for widespread application. User acceptance was slow at the start and these products went through the ‘teething’ problems common to most technological innovations. The advent of composite insulators began during the 1950s, first in the U.S. ![]()
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