Insulation Characteristics of GFRP Cross-arms Subjected to Lightning Impulse Testing

HV/HP Testing

The trend toward growing use of composite materials in electrical applications is also evident in the increasing use of glass fiber-reinforced plastic (GFRP) composites for manufacturing complete towers as well as key elements such as cross-arms. GFRP is recognized for being lightweight, strong and robust as well as offering good electrical insulation properties. Combining all these characteristics offers significant advantages when designing a compact and reliable transmission system where right-of-way presents a challenge.

At the same time, in spite of these advantages, this material can be vulnerable to ageing and degradation related to environmental and electrical stresses. Indeed, past studies have revealed that ultraviolet radiation and heat could progressively degrade a fibreglass structure, causing fibre blooming, delamination and water ingress, eventually shortening service life. According to some research, stiffness of a GFRP composite was predicted to reduce by up to 35% after a year of loading. This can grow to 100% after 50 years.

Application of composite materials in electrical insulation began when these were first used to manufacture insulators during the 1960s. Gradually, this technology, applied first in North America and Europe, gained acceptance worldwide for an extensive range of voltage levels. Use of composites was later extended in Japan to manufacture composite cross-arms, involving a simple structure of bare GFRP bars. These were subsequently discovered to be vulnerable to surface discharges, especially when contaminated. More recently, GFRP composites have also been used as the core structure for insulating cross-arms used on compact transmission lines. Meanwhile, the basic GFRP structure is still being used in several countries recognized as a GFRP tube structure, having various surface treatments and coatings for improved performance. Wide application of GFRP composites for insulators and cross-arms has invigorated more and more studies of insulation performance in terms of the electrical stresses that can lead to electrical breakdown.

Structure with bare GFRP bars.
Example of composite insulating cross-arm design

In power networks, an overvoltage might occur under different circumstances, i.e. transient and temporary overvoltages that can be recognized by their amplitude, shape and duration. Generally, transient overvoltage can be caused by impulse voltage due to lightning strikes on power lines and structures. By contrast, temporary overvoltages are usually initiated by faults to ground, resonance conditions, load rejection, energization of unloaded transformers or some combination of these. Withstand tests against both types of overvoltages as per IEC 60060-1 and IEC 60383-2 have been adopted in acceptance testing of GFRP cross-arms.

Previous studies have successfully demonstrated a good reference for insulation performance of a solid, cylindrical GFRP cross-arm against both types of overvoltage. CFO and AC flashover voltage have been obtained considering both wet and dry conditions. However, the performance shown may be limited only to the specific cross-arm being tested since stress distribution is unique depending on a structure’s type, material and shape. Therefore, dedicated testing must be conducted for every individual design of cross-arm.

Plan to attend the 2022 INMR WORLD CONGRESS in Berlin to listen to a lecture by Professor Mohd Zainal Abidin Ab Kadir, an expert in electromagnetic and lightning protection from Malaysia. He will present results of lightning impulse testing conducted on samples of GFRP cross-arms as well as the impact on external insulation strength.