Composite insulators are considered mature products and a broad range of insulator types is nowadays available, including line insulators, hollow-core insulators and apparatus insulators, for applications ranging from distribution voltages to the highest transmission voltages. This development has created a need to elaborate diagnostic methods and tools for determining the quality of composite insulators. A review on recognized insulator faults and methods for their detection is provided.

Diagnostic Methods for Outdoor Polymeric Insulators

Aging of polymeric and composite materials is reviewed mainly from the standpoint of their interfacial performance. Insulating materials can be divided into simple polymers and composites. Polymers for power cables, transformers, insulators and rotating machines consist of thermoplastics such as PE, PET and PPS, elastomers such as silicone, EPR and EPDM, and thermosets such as epoxy. Composites for GIS, rotating machines and insulators comprise epoxy/glass, epoxy/silica/alumina, and epoxy/mica systems. Aging processes are complicated in general, and take place under simultaneous multiple stresses such as electrical, thermal, mechanical and environmental stresses. Some of the phenomena covered in this paper are associated with the degradation by tracking and erosion and the loss of hydrophobicity in case of surface properties, and with PD, electrical treeing, water treeing and combined phenomena. The quality of cable insulation such as XLPE has been improved extensively from the standpoint of design electric strength. Interfacial problems will emerge for modification of cable joints. It is expected that polymers for outdoor use and filled epoxy resin systems should be improved from their environmental stability and from their design stress enhancement, respectively.

Aging of polymeric and composite insulating materials. Aspects of interfacial performance in aging

Although the transfer of hydrophobicity to the outer surface of the pollution on silicone rubber insulators is found to be a slow process, the findings of this work indicate that its completion is not necessary to achieve near-full benefit. It is proposed that the techniques used for these studies can be readily adopted to quantify the effect of different types of pollutants and of the size plus shape of water drops on partially hydrophobic surfaces. Further, these experimental findings can help to establish an internationally accepted artificial pollution test for silicone rubber insulators and coatings. For such insulation in service, it is concluded that the average value of the contact angle of such water drops that had been evenly positioned along its length would be the most useful parameter when assessing the partially hydrophobic state of these adhering pollutants

Hydrophobicity transfer from silicone rubber to adhering pollutants and its effect on insulator performance

An approach to high field control, particularly in the areas near the high voltage (HV) and ground terminals of an outdoor insulator, is proposed using a nonlinear grading material; Zinc Oxide (ZnO) microvaristors compounded with other polymeric materials to obtain the required properties and allow easy application. The electrical properties of the microvaristor compounds are characterised by a nonlinear field-dependent conductivity. This paper describes the principles of the proposed field-control solution and demonstrates the effectiveness of the proposed approach in controlling the electric field along insulator profiles. A case study is carried out for a typical 11 kV polymeric insulator design to highlight the merits of the grading approach. Analysis of electric potential and field distributions on the insulator surface is described under dry clean and uniformly contaminated surface conditions for both standard and microvaristor-graded insulators. The grading and optimisation principles to allow better performance are investigated to improve the performance of the insulator both under steady state operation and under surge conditions. Furthermore, the dissipated power and associated heat are derived to examine surface heating and losses in the grading regions and for the complete insulator. Preliminary tests on inhouse prototype insulators have confirmed better flashover performance of the proposed graded insulator with a 21 % increase in flashover voltage.

Stress control on polymeric outdoor insulators using Zinc oxide microvaristor composites

We report the development of a measurement system capable of monitoring and characterizing the main features of the leakage current flowing on a polluted insulator string of a high-voltage transmission line. The system comprises a fiber-optic sensor, directly connected to one insulator of the string, that emits a sample of the leakage current waveform to a processing module via an optical link. A capacitive sensor connected to the processing module allows recording the relative humidity of the outer environment. At the present stage, six measurement units have been deployed on CHESF's electrical transmission system in the Northeast of Brazil, and studies are underway to further correlate the degree of environmental pollution with humidity and leakage current activities on high-voltage insulator strings

Novel sensor system for leakage current detection on insulator strings of overhead transmission lines

The formation and development of dry bands can best be studied by modifying the standard test procedures. When such controlled behavior is allied with synchronized optical and electrical recordings, then characterization of the pre-formative leakage current, the transient phenomena associated with partial arcs across dry bands, the location of partial arcs and the voltage drop across dry bands can be determined. Interpretation of test data is greatly aided by finite element computation of insulating structures with a conducting surface layer. When this layer is continuous, this allows straightforward prediction of dry band formation under wetting conditions. Following formation, dry bands can be represented by discontinuities in this layer. Simulation of dry bands with various lengths, when combined with the test data, enables partial arc voltage gradients to be quantified. These results will be discussed in the context of previous work on the pollution flashover mechanism of ceramic insulators.

Formation and characterization of dry bands in clean fog on polluted insulators

This literature review identifies ageing factors and their effects on polymeric insulators. According to their nature, these factors are classified as electrical, mechanical, ultra-violet radiation, chemical, thermal and water ingress. The effects are manifested as external or internal damage with loss of electrical and mechanical viability. These processes are relevant to the definition of artificial and accelerated ageing tests.

Survey of polymeric insulator ageing factors

A simple mathematical model has been developed to investigate the effect of the profile of an insulator on its pollution performance. The creepage length is the standard criterion used to predict pollution performance but the profile shape and subsequent current density distribution were also considered as these have a direct influence on the development of dry bands.

H. M. Young

Papers

Formation and characterization of dry bands in clean fog on polluted insulators

Survey of polymeric insulator ageing factors

Effect of shape factors on the performance of polluted polymeric insulators

A new technique for pollution testing of polymeric insulators under clean-fog conditions

Quantification of polymeric insulator hydrophobicity using image processing technique