for INMR WEEKLY TECHNICAL REVIEW
Utilities have three proven insulator technologies they can deploy on their networks. Porcelain and toughened glass insulators have been around since the first days of overhead power delivery and have a demonstrated record of high reliability. Composite insulators, by contrast, were introduced on a large scale only about 35 years ago but have since made rapid strides to be considered alongside with porcelain and glass. Still, there are important differences in these insulators. For example, both porcelain and glass are easily wettable, brittle and heavy materials, whereas composite insulators are light, non-brittle and discourage water filming for extended periods of time. However since the outdoor conditions and the conductors they are supporting are the same, line design in terms of insulator dimensions as well as electrical and mechanical strengths is virtually the same for the different insulator technologies. They also have several common national and international standards within ANSI and IEC. While many users feel that operation of lines with aged insulators and maintenance practices on these lines should also be the same for all insulator technologies, nothing could be further from the truth. All similarities between the different technologies should end once insulators are no longer in their new condition. This is because the various service stresses (electrical, mechanical, environmental, etc.) can produce significant differences in insulator performance over time.
Many users expect these insulators to last over 50 years and this has indeed been demonstrated for toughened glass and some porcelain. Still they are known to fail occasionally due to problems originating from manufacture, misapplication, contamination and vandalism. At the same time, failure modes of porcelain, toughened glass and composite suspension insulator are different and relevant failure rates vary accordingly. In most cases, differences in failure modes are related to the materials and the manufacturing methods used. More important than failure rate is impact of a single failure on system reliability.
This 2011 overview by Prof. Ravi Gorur, explains why there can be important differences in the types and incidence of failures for porcelain, glass and composite insulators.
Electrical grade, high strength porcelain is made using a wet process. The raw materials are clay, quartz, feldspar and corundum, which are all mined and can vary in quality, impurities and consistency. These raw materials are mixed in water to facilitate intimate blending of the constituents and help shape the product into the final form. Water is slowly removed in several progressive steps up to firing the porcelain in a kiln, where temperature is tightly controlled. A glaze is applied prior to firing and its role is to form a smooth surface and improve mechanical strength. Hardware is attached to each disc or bell using cement, typically Portland cement. At the microscopic level, porcelain is heterogeneous, with crystals of different sizes and chemistries, pores and grain boundaries.
Raw materials used in manufacturing glass insulators include soda ash, feldspar and cullet (small pieces of glass) as well as minor amounts of other agents. These are crushed to form powder and melted in a furnace. The molten glass drops into a mold and is immediately pressed into shape and subjected to toughening, which is a rapid and controlled cooling of the still hot glass shell. The purpose of toughening is to increase the mechanical strength of ordinary annealed glass by a factor 4 to 5, in order to make it suitable for manufacturing high strength cap & pin suspension insulators.
Composite insulators consist of a central fiberglass core, polymeric housing and metal end fittings. The core provides the mechanical and electrical strength. For suspension insulators, the core is made by pultrusion of unidirectional glass fibers that are bonded by an epoxy resin. Since materials used degrade in the presence of moisture and electric stress, the core must be protected at all times and this is done by the external housing and the seals used to bond the housing to the core and end-fittings. Housing materials are expected to resist weathering as well as damage from electrical activity such as corona and surface discharges that can result under wet and contaminated conditions. The method of fixing the housing over the core is different among manufacturers. Some mold the housing over the rod in one or more sections, whereas others use a combination of extrusion and individually molded sheds that are positioned over the extruded sheath and chemically bonded. Seals (single or multiple) at hardware junctions to prevent moisture ingress are commonly employed, although some manufacturers mold the housing over the hardware at the triple junction of hardware, rod and air dielectrics to perform the function of a seal.
Each failure mode has specific consequences on the serviceability of the insulator and the string that contains the failed units. In some cases, the failure mode will result in loss of service e.g. separation of the cap and the pin or the fiberglass rod breaking with resulting line drop. In other cases, less severe failures might be noted, such as partial damage to the bells or housing, shattering of the toughened glass insulator shell (for which there is no immediate consequence on the safety and integrity of the string). Therefore, when comparing failure rates of different insulator types it is important to also qualify them based on their associated failure modes, risks and consequences.
Some modes of failure, such as pin corrosion, can occur in places with severe contamination. This is associated with hardware and independent of dielectric material. In the case of composite insulators, the long rod between points of different potential limits the current to very small values; hence hardware corrosion is not as serious a problem as for porcelain and glass insulators. In any case, use of sacrificial zinc sleeves for corrosion protection is a proven technology.
This mode of failure occurs external to the insulator and results in a temporary loss of insulation strength. It depends mainly on pollution conditions and lightning activity as related to the electrical design parameters of the string (leakage distance and dry arc distance) as well as the overall insulation coordination of the system. For any given leakage distance, flashover is more common in porcelain and glass insulators due to their easy wettability compared to composite insulators that utilize low surface energy materials able to resist water filming over long periods. No significant difference in external flashover failure rates have been reported between porcelain and glass insulators of given leakage distance and shell profile as well as comparable strings design parameters. The power arc current typically causes the glaze in the porcelain to melt leaving behind a rough surface on those units in contact with the power arc. Composite insulators are usually more resistant to power arcs; only superficial burning and/or ablation of the housing material are observed. Some melting of the hardware is usually common but there is no internal damage.
This failure mode is seen in porcelain insulators and is primarily related to the quality control of raw materials and manufacturing process employed. Some manufacturers have better process and quality control: consequently there are differences in macroscopic properties such as color and microstructure of the porcelain body. Fig. 1 shows the microstructure from insulators that pass all tests in their new condition. The presence of pores (voids) and numerous grain boundaries between the various crystals are common to porcelain. Sometimes tiny cracks or micro-cracks can exist and these grow over time under the multiple stresses generated by service conditions. Eventually, they can lead to electrical breakdown through the dielectric (as shown in Fig. 2). These internal punctures of porcelain insulators are only detectable with close, in-line inspection using measuring instruments. Reliable statistics for this type of failure are not available on a global scale but field experience shows that this event is generally rare on new lines but not so rare with older lines, especially if the porcelain insulators are not of high quality.
In one study, for example, porcelain insulators removed from service were subjected to insulation resistance measurement and power frequency puncture test, as described in ANSI 29.2. Some bells showed extremely low insulation resistance (less than 10 MΩ) and were not subjected to the puncture test. Of those insulators that showed high resistance (>100 MΩ), some could not withstand more than 10 kV in the puncture test and were cut open with a water jet. Fig. 2 shows the internal degradation and puncture paths observed.