Fretting Fatigue Of ACSR Conductors And Running State Estimation Of The Aged ACSR Conductors

In China, distribution of the electric power resources and the load centers is extremely imbalance, and the power supply and consume in different areas and/or seasons varies widely. In order to solve this energy supply and demand structural contradiction, state strategy i.e., Interconnection in the whole country, transmission electric power from West to East and exchange electric power between South and North, emerges as the times require. The extra high voltage (EHV) and Ultra high voltage (UHV) overhead transmission lines have been constructed in some major transmission projects in the central and western China. They are mostly located in high altitude areas having a very severe natural condition. The overhead lines are nearly always keeping in a state of vibration caused by a dynamic load induced by wind, rain, snow, and etc. And thus, the conductors usually suffer serious fretting fatigue, and it may be the main reason resulting in the damage of the service conductors. Therefore, fretting fatigue and running state assessment of the ACSR conductors in the overhead lines were continuously studied based on the previous research results by our group and others. The research results are valuable to the operation maintenance of the overhead transmission line conductors, theoretically and technologically.In this paper, series of fretting fatigue experiments and aluminium armour tape enwrapped conductors’fretting damage experiments were carried out aimed at the LGJ150/20ACSR conductors. Mechanisms of fretting fatigue fracture and fretting damage of the conductors were determined through the observation and analysis of the fretting fatigue fractographies and the damage surface morphologies of the Al strands of the conductors. And thus, a new fretting fatigue life prediction model was established. A LGJ240/40conductor serviced for26years in the110kV Zhu-Xie463transmission line, Tongling, was investigated for the running state assessment of the aged conductor. Damage conditions and mechanisms of the conductor under the strain clamp and damper were focused on. And, the running state of the conductor was evaluated, mainly based on the mechanical property test and metallurgical observation.Study on the fretting fatigue of the conductor indicates that fractural Al strands only take place on the condition of the fretting amplitude equal to1.0mm, all the fracture positions locates at the last contact point of the conductor and the suspension clamp. The number of the fractured strands increases with the fretting cycle. At a low fretting cycle number, only plane fracture occurred,45°fracture and V-type fracture happen when the fretting cycle increases. Plane fracture is caused by bending fatigue, and is divided into three types of uniaxial bending fatigue, dual bending fatigue and rotating bending fatigue. After several Al strands fractured, the stress distribution in the conductor changes, torsional fatigue then happens to form the45°fracture and V-type fracture. All the fatigue fracture surfaces of the Al strands consist of three regions, i.e., the fatigue source region, the fatigue crack extension region and the instant fracture region, respectively. A fretting fatigue life prediction model was established based on the fracture mechanics in the current study, the relative error between the prediction results and trial value was smaller than any other models.Study on the aluminium armour tape enwrapped conductors’fretting damage shows that the aluminium armour tape can effectively protect the ACSR conductor, the external Al strands are injured by the scratch of the aluminium armour tape and the abrasive wear. As for the inner Al layer, the damage is the same as that of the conductors without enwrapped aluminium armour tape. The finite element method (FEM) analysis results are quite consistent with the experiments results, indicating that the stress concentration and great shearing strain at the contact edges of the Al strands are two major causes of the fatigue crack initiation.The external Al strands of the aged ACSR conductor under the damper and strain clamp mainly subject to the incision from the aluminum armour tape and the adhesion and abrasive wear. As for the inner Al layer strands, the primary damage mechanism was the adhesion wear and the abrasive wear. All the elliptical contact marks deflect and overlap on the surface of the inner Al strands. The conductor under the strain clamp is fastened tightly, so the vibration amplitude of the conductor under the damper is much bigger than that of the conductor under the strain clamp, and that’s why the damage degrees of both outer/inner Al strands of the conductor under damper is much higher than that of the conductor under the strain clamp. The crack expanding depth of the conductor under the strain clamp was much bigger, so, the mechanical properties of the conductor under the damper are superior to those of the conductor under the strain clamp, and all of them are much higher than the required value according to the relative state standard of China. To the free conductor, its damage conditions are similar to the slipping region in the suspension clamp and conductor system. And to the steel core strands, only partial zinc coating injuries, no distinct mechanical property reduction takes place. So, the aged conductor can still be used, but more intensive inspection and maintenance is suggested.