Compact lines with pivoted insulated cross-arms. General stability design criteria

Abstract

Line compaction is a subject of increasing interest due to the difficulties of obtaining new Rights-of-Way (ROW) and therefore the need of reducing ROW widths and increasing the power transmitted within a certain ROW. Among all the line compaction options, the highest degree of compaction can be obtained through the use of insulated cross-arms. For certain voltage levels and, to reduce loading in case of longitudinal events, is it very common the use of pivoted insulated cross-arms (horizontal vees). The pivoted configuration prevents breaking failure of the cross-arms by means of cross-arm pivoting when sustained to longitudinal events. Under certain winds speeds and directions, a complete overhead line section can become unstable thus producing a power failure. The stability condition not only will depend on wind speed and direction but also on section and span lengths, the presence of inclined spans, the cross-arm geometry and the conductor used. To study general stability and, to provide general design criteria guidelines, a 3-D corotational finite element model has been developed. The behavior of the line under wind load could be highly non-linear, especially for winds over 50 kmh. In order to avoid convergence problems a robust formulation of the element is required. The formulation developed includes the drift and drag effects of the wind load and the contribution to the stiffness matrix of the element to the wind load in a co-rotational framework. In this first approach the load of the wind is considered as static load acting uniformly in the entire overhead line. This model has enabled several simulations that will lead to general design guidelines, study and understanding of the stability problem and the sensitivity analysis of stability versus all the variables mentioned above. The paper presents a preliminary results of the static and dynamic behavior of different overhead sections and compare the results between the configurations and the type of load (static or dynamic). Therefore, conclusions are obtained to manage the design process of this particular configuration of overhead compact lines that include mitigation methods or guidelines for the prevention of instability events.