Sag increases resulting from conductor creep on medium-voltage transmission lines, and the problem of measuring sag on live overhead lines

From experiences and laboratory tests it can be seen that the sag increases considerably during the operational period of overhead transmission lines. Because of lack of knowledge of this fact, the changes of sag is not always taken into calculation in the phase of planning. The authors show how the results of long-time creep strain measurement in the laboratory can be used in planning of overhead transmission lines. An example illustrates how the alteration in sag caused by conductor creep is dependent on the span length. The increases in sag are extrapolated for an operational period of 30 years. The example shows that for a medium voltage overhead transmission line in a 200 m span the sag after 30 years can increase up to 70 cm. The sag control of existing overhead lines is possible only when an adequate knowledge of the conductor temperatures is available. On disconnected lines, the air temperature at conductor height requires to be taken into account. The authors show possible ways of measuring the temperatures of conductors on live transmission lines