Need of standardized adhesion test for composite insulators: lessons learned from service experience and testing

In the beginning of 2016 two failures of line composite insulators were experienced on the 130 kV transmission line. The two failed insulators were inspected in the laboratory and the root cause of the damage was considered as weak adhesion between the fibreglass rod and housing which finally resulted in the so-called “flashunder” type of failure. The results of IR inspection of the rest of the insulators in service were in line with this assumption, revealing a lot of hot spots on different insulators. This indicated some kind of internal conducting path inside the insulators (in the interface silicone rubber/fibreglass rod). Two batches of new composite insulators of two different types were then evaluated in the laboratory by sample testing. The program for sample testing included: ? Visual inspection and hydrophobicity classification. ? Test on interfaces according to the appropriate selection of tests from the IEC standard (reference dry power frequency test, water immersion pre-stressing, visual examination, steep-front impulse voltage test, dry power frequency voltage test). ? Adhesion test not defined by the IEC. A special procedure was developed and applied, which will be described in detail. Based on general insulator knowledge good adhesion should result in separation in rubber material before the separation between the fibreglass and the silicone rubber housing. This method is generally accepted and performed by many manufacturers as an internal check of the quality during the production of composite insulators. All insulators have passed the selection of electrical tests according to the IEC standard aiming for test of interfaces. However, obvious weakness in adhesion has been revealed for one type of the insulators (the same as being detected in service), which would influence the long-term performance. This indicates that the requirements of the present standard cannot reveal the insulators with internal weakness of such type. Revealing of poor adhesion should be an important topic for CIGRE/IEC in the future, i.e. to develop new representative test methods and to revise relevant IEC standards.