TECH PRIOR

Polymeric Insulators - Introduction

Electrical insulator is a very important component in the electric power systems such as sub-stations and distribution & transmission lines. In-depth study and practical knowledge on this subject is necessary for the electrical professionals in the electrical power field. In the early days, insulators were made of ceramic and glass materials. But in 1963, polymeric insulators were developed and its improvements in design and manufacturing in the recent years have made them attractive to utilities. It is consist of a fiberglass core rod covered by weathersheds of skirts of polymer such as silicone rubber, polytetrafluoroethylene, EPDM (ethylene propylene diene monomer) and equipped with metal end fittings. It is also called composite insulators, which means made of at least two insulating parts – a core and housing equipped with end fittings. Polymeric insulators have many advantages over the ceramic and glass insulators such as good performance in contaminated environment, light weight, easy handling, maintenance free, and considerably low cost etc. Because of these properties it is gaining popularity worldwide and replacing the conventional ceramic and glass insulators. The scope of this technical paper is to discuss about construction, types, designing, testing, and selection of polymeric high voltage insulators.

Historical Development

The first polymers used for electrical insulators were bisphenol and cycloaliphatic epoxy resins. Introduced commercially in the mid 1940s, bisphenol epoxy resins were the first polymers used for electrical insulators, and are still used to make electrical insulators for indoor and outdoor applications. Cyloaliphatic epoxys (CE) were introduced in 1957. They are superior to bisphenol because of their greater resistance to carbon formation. However, the first commercial CE insulators failed shortly after installation in outdoor environments. Since then, new CE formations have resulted in improved electrical performance. In the early 1960s, distribution class (CE) insulators were first sold commercially in the U.S. under the name GEPOL. These units failed due to surface damage and puncture. CE was used later in experimental 500 kV station breaker bushings, and in 115 kV bushings in the 1970s, and for suspension insulators by Transmission Development Limited (TDL) of England. The TDL suspension insulators used slant sheds to provide natural washing of contamination. From the mid 1960s on, CE insulators were tested at up to 400 kV service voltage as suspension / strain insulators and cross-arm in the United Kingdom. For various reasons, including poor cold temperature performance and insufficient weight reduction, CE did not gain acceptance in the US for outdoor high voltage suspension insulators. But today, CE is widely used in indoor and even semi-enclosed power systems. In the 1960s an insulator having porcelain sheds supported by an epoxy resin fiberglass rod was developed. It was not widely used because of further developments in lighter-weight polymeric insulating materials.