Tower Configurations & Composite Cross-Arms
A typical twin circuit tower of the type used in the United Kingdom and elsewhere is illustrated in Fig. 1. Here, tower height is effectively determined by factors such as statutory ground clearances, conductor sag, insulator length, conductor-to-conductor separation, conductor-to-tower clearance and lightning shielding requirements. Depending on system voltage, span length, conductor and service environment, some of these factors become more important than others. In addition, possible blowout conditions (i.e. when an insulator assembly breaches the required air gap from the tower) contribute to determining tower width and also when making for conductor-to-tower calculations.
Among the key benefits of composite cross-arms is that insulator swing under windy conditions is reduced to a minimum and instead determined by metal clamping assemblies. There is also no requirement for additional tower height to accommodate the length of the insulator string itself. Therefore using composite insulating cross-arms can effectively raise heights of conductors by this same distance, i.e. about 4 m in the case of a 400 kV line. Basically, such a solution can:
1. resolve ground clearance problems on existing lines;
2. allow greater sag on existing or new conductors, critical to improving power transfer capacity since it enables conductors to run at highest rated temperatures while still not infringing ground clearances;
3. facilitate voltage upgrading due to improved clearances from towers, especially since risk of blow out is mitigated;
4. permit more compact towers with smaller foundations and therefore reduced costs (see Fig. 3).
Mechanical Requirements
In normal operation, the higher elements of a cross-arm are in tension and the lower elements in compression (as in Fig. 4). It has also been noted by experts that the fundamental limit to application of such a cross-arm is the compressive strength of its lower member. If this limit is exceeded, the cross-arm will buckle. Typically the most extreme and limiting situation for design is under broken wire conditions, in which case high asymmetric stresses are experienced in the cross-arm. This is less of a problem for cross-arms designed to be able swing to the side, as seen on compact lines supported by steel poles. Composite insulator cross-arms have therefore become popular for such applications. Still, even in this case, insulators may need to be ‘doubled-up’ to provide sufficient compressive strength (as in Fig. 5). This is because traditional composite insulators are not able to provide sufficient compressive strength since their diameters would have to increase to the point that they become too heavy or too costly to produce. In cases of steep terrain, galloping or ice shedding, the circumstances whereby a cross-arm is exposed to uplift also has to be considered in power line design.
