Sheath-to-Rod Bonding in Polymeric Insulators

Insulators

Polymeric insulators have been used on overhead lines and in substations for decades, predominantly for AC but also in some DC applications. While choice and development of suitable housing material formulations and production methods have improved greatly since the first generation, their principle design concept has remained the same. This has required synergistic technical performance among different elements and materials. For example, apart from preventing possible ageing of the housing, it has also remained necessary to protect the glassfibre reinforced core rod against entry of moisture. This avoids risk of insulator puncture or flashunder, which can cause full mechanical disintegration and dropped conductor.

Reliable long-term performance of polymeric insulators can only be achieved if the insulator is designed with respect to:

• Consistently reliable and high-quality manufacturing process;
• Adequate design reflecting actual service conditions;
• Appropriate choice of housing material that matches these service conditions; and
• Functional and reproducible quality of insulator interfaces in terms of long-term adhesion.

Polymeric insulators have now achieved a high technical maturity due to service experience across all voltage classes and applications as well as continuous development of applicable IEC standards. But there has also been a relatively fast transition from the time when these were exclusive and costly to their present status as a basic commodity. This transition can be attributed to enhanced application knowledge, automation of production and the impact of increasing demand on economies of scale.

The positive overall development has nonetheless been accompanied by observations and negative field experience that together call for improvements or enhanced precision in certain applicable standards. Based on this, questions have been raised whether present standards and common procedures for design interface testing are sensitive enough and whether any improved design test result can be transferred to routine test behavior.

Service experience has confirmed that poor adhesion in the rod-housing interface is a key trigger for development of internal tracking and failed such interfaces have been found to be one of the most critical aspects determining service life of polymeric insulators. Internal tracking can result in insulator puncture if the tracking path achieves sufficient length that the remaining unaffected length can no longer withstand an overvoltage or even line-to-ground voltage.

Results in terms of failures during design testing of polymeric insulators have been analyzed. According to long-term laboratory experience testing polymeric insulators, only about 10% fail during design and type testing. Fig. 1 summarizes the distribution of failed insulators per design test.

mech: mechanical test / steep: steep front impulse test / AC: 30 min power-frequency withstand test / WDT: water diffusion test / DPT: dye penetration test / erosion: 1000h tracking and erosion test

Fig. 1: Distribution of failure modes in design testing polymeric insulators at EGU HV Laboratory.

Attend the 2022 INMR WORLD CONGRESS in Berlin where testing expert, Dr. Jan Lachman, of EGU HV Laboratory will review results of 30 years testing polymeric insulators. He will also offer recommendations on improved testing of the quality of the sheath-to-rod interface of these insulators to assure long-term performance.