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Monitoring of the Electrical and Mechanical Performance of the Transmission Lines


In Brazil, the vast majority of conductive cables, used in transmission lines, are composed of aluminum wires coiled in the shape of a propeller around a core made of steel wires; These cables are commonly referred to as ACSR conductive cables (Aluminum Conductor Steel Reinforced). Recently, the ACSR cables have been replaced by the aluminum cables with the core composed of carbon fiber reinforced composite material. These new conductor cables are lighter and have higher mechanical strength and temperature.

Conductive cables are subjected to mechanical tensile stresses of the order of 20 kN or approximately 20% of their mechanical bursting load. In addition to the mechanical tensile load (static load), the conductive cables are also subject to dynamic loads from the wind action on them. For example, wind induced mechanical vibration in conductive cables (called wind vibration) is an extremely complex phenomenon and is responsible for increasing the mechanical stress levels to which the conductive wires, towers and chains of insulators are subjected.

Cyclic loading from the continuous wind action can lead to complete cable breakage and interruption in the supply of electricity due to the degradation of the material by a mechanism known as fatigue.

The measurement and monitoring of magnitudes such as wind speed and direction, mechanical stress applied to conductors and their amplitude of vibration are challenging and important activities for the evaluation of the mechanical performance of the line, and are part of the scope of theoretical and experimental research activities. In terms of research on wind vibrations in conductor cables, Cepel developed a computer program to predict vibration levels in conductive cables and to optimize its damping system, aiming at minimizing the risk of fatigue rupture of the conductor.
The measurement of vibration levels in a transmission line is carried out using instruments called vibrators. These instruments are fixed to the conductor cables and equipped with electronic circuits (properly protected and shielded against the action of the electromagnetic field of the line) that allow to measure and store the values of displacement of the conductor cable as a function of the frequency of excitation. This measurement allows a rapid evaluation of the wind vibration intensity in the line and a more accurate diagnosis of the susceptibility to cable conductor fatigue and its remaining life. This information is important for the decision making to intervene immediately on the line to mitigate the effects of this vibration or to postpone its shutdown until the scheduled maintenance program.

Mechanical stress on cables and tower

The growing demand for electric power overloads the transmission system by requiring the construction of new transmission lines. This process is facing great difficulties due to the obtaining of environmental licenses for new areas of servitude. The need arises to redesign the line to meet the challenges of development.

The monitoring of the parameters that determine the cable-ground distance of transmission lines, be it the mechanical traction, the arrow or the temperature of the conductive cables, increase the current levels of ampacity without decreasing the operational reliability.

The research on wind speed monitoring and the stresses on transmission towers allows the triggering of alarm in critical situations of strong winds in order to prevent against a possible accident in the transmission line.


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