Toughened glass has been increasing its share of the total insulator business through growing application of glass strings on overhead lines. At the same time, there has been increased ‘turbulence’ in this market sector. Competition has intensified with the entry of new suppliers, some of who have built a strategy around low selling price.
Recent service experience as well as laboratory research have revealed that there can be great variance in the quality of glass insulators coming from different manufacturers, depending on raw materials, process and know-how. This applies to all components of the insulator, from the glass shell to the metal fittings to the cement. Moreover, experts report that it is not always possible to distinguish insulators of inferior quality based solely on present IEC standard tests. This is because insulators sent for type and sample testing can be specially selected for this purpose by their manufacturer. Moreover, requirements for routine testing during production are thought not to be stringent enough to always identify insulators with inferior quality. Combined with this development on the supply side is the fact that the demand for uninterrupted transmission, without faults, has increased worldwide and especially in Europe. At the same time, improved modelling and specialized software has made it is possible to optimize insulator selection and dimensioning. This, in turn, has resulted in shorter, more highly stressed insulators. Together, all these factors are placing greater need for uniform high quality since the failure of any disk in a string becomes that much more serious.
Faced with these challenges, power supply companies in Norway, Sweden, the Czech Republic and Finland began co-operating to develop an additional test matrix to supplement existing IEC requirements. Their goal was to find practical test procedures and criteria that would better reveal the true quality of the toughened glass insulators being installed on their networks.
This article based on a contribution by Kjell Halsan of Statnett and Igor Gutman and Johan Lundengård of STRI in Sweden, discusses possible changes in existing standards. It also presents findings from extended laboratory testing on different batches of glass insulators.
Norwegian transmission system operator, Statnett, has a policy of closely monitoring the quality of glass insulators it purchases. Its representatives attend sample testing for all major deliveries and have also evaluated such test results spanning many years. In addition, operating data on shatter rate per 100,000 glass insulators per year is collected and analyzed for long transmission lines in Norway. These statistics have shown that shatter rate of discs can vary significantly among suppliers and sometimes even for different batches delivered by the same supplier. This is true even for the most reputable manufacturers with exceptionally good service records. Given this, Statnett initiated a project to ensure that it will purchase only high quality glass insulators from both new and existing suppliers – with emphasis more on life cycle cost than on acquisition cost
To the east, in Sweden, refurbishment of a 220 kV line by the TSO, Svenska Kraftnät, employed glass cap & pin insulators selected on the basis of the usual tendering procedures, including compliance with IEC requirements. The insulators were installed in a mostly clean but humid area, typical of Scandinavia. Abnormal discharge activity and audible noise were reported on these insulators after only a short time in service, most often when humidity was highest. Field inspection of the line with a daylight UV-camera confirmed the activity and suggested that the insulators could be suffering from defects in their cement.
These observations became a concern since such unexpected initial behaviour for new insulators could influence long-term performance. Detailed investigations were performed, including voltage tests at operating voltage. These revealed discharge activity on the glass surface around the pin, just outside the cement. This was especially evident if the electric field distribution in this part of the insulator was affected by water being sprayed onto it, to simulate humid service conditions. Resistance measurements using a hand-probe showed that the weakness observed was due to unusually high conductivity as well as unfavourable geometry of the cement. RIV levels, for example, had a wide spread among insulators from different manufacturers and this was interpreted as an indicator of poor quality control.
Given this, it was proposed to expand test requirements for tenders to include RIV measurements at 20 kV and 24 kV to reduce risk of similar problems in the future. Such a requirement is easy to implement and would help ensure that all insulators purchased are more robust with respect to discharge activity under humid conditions.
The aim of the first stage in the investigation was to identify effective yet practical test methods that could be used to qualify manufacturers in terms of the quality of toughened glass insulators they supply. This included a workshop with experts, discussions with leading test laboratories and manufacturers and a review of the literature. The production process for glass insulators was also examined closely to identify critical issues that can affect final quality. This was then used to identify tests that could be applied to differentiate between good and inferior quality insulators. The following points were highlighted:
• Overview of the components in a glass insulator disc;
• Walk-through a glass disc fabrication process;
• Quality control aspects during manufacture and assembly;
• Relationship between key properties and test methods to verify these.