Studies for the diagnostics of composite insulators

Studies for the diagnostics of composite insulators are needed, because insulator strings of distribution and transmission overhead lines are made in most of silicone rubber instead of porcelain material.

composite insulators

Studies for the diagnostics of composite insulators Abstract:
During the last decades the use of composite insulators has grown all over the world. These insulators have advantages from both structural and operational points of view. Diagnostic techniques for these insulators have been studied and developed during the last years. However, in LLM (Live Line Maintenance) international standard, the procedures and distances to adopt for composite insulators have not yet been defined. This paper, after reporting some data on the characteristics and diffusion of composite insulators, describes the diagnostic techniques available for the assessment of their conditions, with particular reference to laboratory activities carried out in Italy. With reference to these experimental activities, indications are given about in field applications. Particular attention is given to electric field measurement, operation that need to be carried out by LLM techniques. Description is given of the future activities that TERNA and CESI RICERCA intend to carry in order to deepen the argument; a common effort at international level is also encouraged in order to overcome the present lack of standard for LLM in presence of composite insulators.

Introduction studies for the diagnostics of composite insulators:
Insulator strings of distribution and transmission overhead lines in Italy are made in most of the cases by ceramic (glass, and in some cases porcelain) units. The knowledge gained through long years of experience with these insulators and steady improvements in materials and production methods have made products offering good reliability and long running life guarantees available.

The development and improvements obtained in the last decades in the polymeric material field have made new classes of these materials available featuring such electric and mechanical characteristics as to make them suitable for use in manufacturing HV and MV line insulators. These insulators are made up of an inner glass-fibres-reinforced plastic rod (lengthwise glass fibres buried in a polymeric matrix), an external housing supplied with sheds enclosing the rod (generally consisting of silicon rubbers or in some cases of ethylene-propylene elastomers, both loaded with mineral fillers), and two metal fittings connected to the ends of the rod. The glass-fibres-reinforced plastic rod, in addition to the dielectric withstand, serves to support the mechanical load to the suspension or anchor of the line conductor. The external housing is fit to ensure the electrical surface withstand and to provide the rod protection. By the metal fittings the mechanical load from the conductor is transmitted to the insulator rod, and from the latter to the tower or pole. These insulators currently called "composite insulators", offer some advantages when compared with the corresponding strings of conventional ceramic insulators, both from a structural point of view (smaller size and weight, possibility to obtain higher values of the ratio creepage distance-insulation length, possibility to have single units with considerable length, etc.) and an operational point of view (relatively good behavior under pollution conditions, installation or replacement made easier and faster). However some cautions still exist concerning an extensive use of composite insulators intended to replace the ceramic ones.

Service experiences have pointed out that polymeric materials, in both the external housing and the rod, may undergo ageing phenomena in time under effects of environmental and operative stresses. The external housing may be subject to puncturing, splitting, tracking and erosion, with the ensuing risk of exposing the rod to the environmental agents. The glass-fibers-reinforced plastic rod may be subjected to mechanical fractures (brittle fractures), the cause of which have been till today not entirely explained, even if important findings have recently been carried out [1,2].

Moreover, the possibility to adopt live line maintenance techniques on this type of insulators is still under consideration and needs further developments on both technical and normative sides.

These aspects will be analyzed in this paper in order to identify the problems that need to be faced to carry out live line works on lines equipped with composite insulators.

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