This article from October 2017, contributed to INMR by Dr. Jens M. Seifert of IEC TC 36 ‘High-Voltage Insulators’, assessed the state of technology in this field and how suitable standards allow for continued progress.
TC 36 Insulators
Scope: Standardization of insulators for high voltage systems and equipment including bushings, insulators for overhead lines, substations and their couplings.
SC 36A Insulated Bushings
Scope: Standardization of insulated bushings and of couplings of these insulators.
Technical Committee 36 was established in June 1949 to deal with high-voltage testing, wall bushings and insulators. In 1955, the part of the scope regarding high voltage testing was assigned to the newly created Technical Committee 42 and the scope of TC 36 became ‘to prepare IEC recommendations on bushing insulators, line insulators and related equipment’. In 1966, TC 36 was reorganized and three Subcommittees established:
• Subcommittee A: “Insulated bushings”;
• Subcommittee B: “Insulators for overhead lines”;
• Subcommittee C: “Insulators for substations”.
The present scope of TC 36, re-worded in 1998, is “Standardization of insulators for high voltage systems and equipment including bushings, insulators for overhead lines and substations and their couplings.” The field of activity of each Subcommittee regards the insulator types falling under their scope. The technical activity of the parent committee regards only the preparation of international standards on topics common to insulators for different use (bushings, lines, substations), such as general test methods (artificial pollution tests, power arc tests, etc.), general guidelines for the selection of insulators (for instance under polluted conditions). In 2013, Subcommittees B (Insulators for Overhead Lines) and C (Insulators for Substations) were absorbed in TC 36.
Back in 1999, those working in the insulator industry were confronted with IEC, ANSI, VDE and many other national standards. At the time, standards for “classical” insulators made of glass or porcelain (e.g. IEC 60383, IEC 60233, and IEC 60168) were of high maturity, well established and accepted by manufacturers, customers and test institutes. In 1992, the first IEC standard (61109) for composite insulators was published, similar to ANSI C29.11 that became available some years earlier. Keeping in mind that the basic design principle behind composite insulators was invented in 1965, a period of 25 years to the first IEC standard seems very long. Why? In 1965, composite insulators were totally unknown, with only limited experience with polymeric (epoxy cast) insulators available and even more limited for outdoor applications. During the 1970s, only a few manufacturers for composite insulators existed, dealing with a couple of alpha/test customers – mainly European and U.S. utilities that tested these innovative products in pilot installations at 110 to 765 kV. The promise of advantages such as light weight, high strength and extreme dynamic impact resistance (e.g. against gunshot and during load transposition) were high and in many cases these products were promoted as ‘miracle’ insulators supported by a trend in favor of ‘synthetic materials’, as was also seen in other industries at the time. Standards are based on experience and best practice. But both were unavailable at the time. On the other hand, new customers asked for specifications and standards. So, knowledge was developed and collected step-by-step and then compiled into standards. Some examples:
• The water diffusion, tracking & erosion and low voltage power arc resistance (material) tests were established in the German VDE 0441 already in 1982 (the first standard for composite insulators at all). The tests were selected out of different VDE standards, mainly for epoxy casted materials. The acceptance levels were reversely adjusted or calibrated to the service life of 15 years (the longest experience at that time) in terms of “minimum requirements” with the philosophy “if this test is passed – a life time expectancy of min. 15 years can be assumed for the material in outdoor application”. Later on, exactly these tests and the acceptance levels were refined and then defined in IEC 61109:1992.
• Load-time curves: it was believed that composite insulators have a very high short-term strength and then loose strength over time if stressed at different mechanical load-time points (see Fig. 1). The diagram was derived from a theoretical slope assumption that resulted in a 33% SML load capability at 50 years load-time which is regarded as the maximum working load over 50 years of service life.
The diagram in Fig. 1 led to a lot of confusion. For example, some users derived from these load curves that composite insulators lose strength over time, which is not correct. With Edition 2 of IEC 61109:2008, this understanding was improved with the philosophy of the Damage Limit Level (DLL) (Fig. 2). This was confirmed by tests and experience that the mechanical lifetime of composite insulators seems to be infinite as long as the permanent load is below this critical damage limit DLL that is seen in most designs between 60-80% of SML but depends greatly on how tight the design was calculated.
The discussion about the interpretation of load-time curves and the special role of the SML as characteristic value for all relevant type and design tests in relation to the “nominal load” is still ongoing. This clearly shows that standards must be written also in a comprehensive way or need some guidance/application guides. For example, the deficit regarding load co-ordination and mechanical component performance triggered the recent revision of IEC 61109: Edition 3. Following this review, after 1992, there was a lot of activity regarding development of the first standards for all kinds of composite insulator products such as IEC 61466:1998 for OHL composite insulators, IEC 61462:1998 for composite hollow insulators and IEC 61952:2002 for composite line post insulators. The work was mainly handled by WG 10 of SC 36B, a high performance working group with experts such as Clive Lumb, Ray Houlgate, Wallace Vosloo, then late Claude de Toureil, Alberto Pigini, K. Naito, Martin Kuhl, Richard Martin and Igor Gutman (and many others). Also, the idea to define test methods to avoid brittle fracture was already discussed in 1996, but the Technical Report IEC TR 62662 was published 14 years later in 2010 due to lack of consensus. This standardization phase in the 1990s reflected industry need for innovation-supportive standards that have been entering the market phase (Fig. 3). Defining common terms, test methods and acceptance criteria allowed customers to issue specifications and purchase agreements. The market was now formally opened for composite insulators in general. This phase was also characterized by the industrialization of the product from smaller individual pilot productions to larger quality controlled serial operations as it is today.
Acceptance was high in the initial phase (1970s and 1980s) but then dropped due to the fear of material ageing, some negative experience with early designs of the first/second generation and the lack of standards that serve to prevent use of non- suitable designs and materials. Professor Hermann Kärner, TC 36 Chairman of the years 1994 to 2000 expressed these demands in his Kärner Matrix (see Fig. 4) and asked for better standards for insulator designs and tighter material tests.
As reaction, during the 2000 TC 36 General Meeting in Stockholm it was decided that a common clause standard for polymeric insulators should be developed to improve and simplify cost demanding design testing, including accelerated ageing tests and to cover all design tests in one IEC document (common clauses). These design tests shall then be valid for all kinds of insulators (long rod, hollow, post) of same design and technology. This task was taken over by WG 12, a Working Group convened by Jens Seifert, which published IEC 62217 Ed 1.0 in 2005 and other standards such as IEC TR 62662, IEC TR 62730 and IEC TS 62896. This was an influential WG but disbanded in 2016. With the next revision of IEC 62217:2012, ageing tests were discussed again and separated from the document. Professor Josef Kindersberger, TC 36 Chairman from 2004 to 2014 clearly insisted on the IEC rule “only one test for one property”, so the wheel test and the 5000 h test had to disappear from IEC 62217 and were documented in IEC TR 62730. Both tests were considered as “Tracking and Erosion” design tests. With the triggered revision of IEC 62217:Ed. 3.0 in 2016 the 5000 h test may come back as an ageing test with modified test conditions because the community indicated demand for an cyclic ageing test. In the same time, the frame for the standard family of IEC 60815 for AC and DC for classical and composite insulators was developed. The total project took 16 years until the final publication of IEC 60815-4. This of course raised the question whether TC 36 standardization process might be too slow considering the relative fast and emerging development of HVDC technology in the last 2 decades.
During the 2000s, the product became a commodity. Starting with only a few manufacturers in the 1980 and 90s, there are now well over 100 manufacturers worldwide, and growing. In 2006, the IEC standard system for composite insulators was fully developed with available test and product standards – maybe not perfect but mature enough to serve market needs. It can also be observed that development of standards for classical insulators more or less stopped in 1993 with the latest revision of IEC 60383:1993.
The task for the future is to improve standards in terms of a continuous improvement process (CIP), as the same is applied for the product development. Additionally, standards must react on technology trends such as hybrid insulators (already done with IEC TS 62896), HVDC and field controlled products in cooperation with CIGRE D1, B2, A3 working bodies. Some challenges also have to be resolved, e.g. development of artificial pollution tests for housings made of hydrophobicity transfer materials (HTM) and artificial ageing tests more representative of actual service conditions.
During the last General Meeting in 2016 in Frankfurt, several revisions of existing standards were made by the Committee. Revisions are done by Maintenance Teams (MT) while new work is channeled through Project Teams (PT) or Working Groups (WG). TC 36 has 1 Sub Committee (SC), 1 WG, 3PTs, 9MTs and one Joint WG (JWG) with TC 42 “Insulation Coordination”. The work program has 16 active projects (Fig. 6) and TC 36 made more than 100 publications since 1949 (47 are presently actively used in the market).
With these activities and number of publications, TC 36 is an active Committee that reflects interest in good, stable standards for insulators. What can also be seen on the Project List is that most of composite insulator standards are again in revision (some highlights):
IEC 61109: Ed. 3.0 Motivations:
– Materials – need to make requirements clearer
– Interface testing – how to improve long-term quality
– Tracking/erosion – to make it comprehensive for customers
– A/C and D/C – does it need a D/C section?
– load-time curves and load coordination of the components
IEC 61952-1: Ed. 1.0 Motivations:
– create a standard with dimensions and characteristics
– define typical base and end fittings applied for the products
IEC 62217: Ed. 3.0 Motivations:
– better definition / improvement of the dye penetration test
– tests for HTM materials
– inclusion of an ageing test (e.g. modified 5000 h test)
– considering modified (tighter) interface tests
IEC 60815 -1…-4 Motivations:
– adjust the documents to latest knowledge and experience
– better definition / characterization of HTM property
– review and adjustment of profile factors, especially for DC
IEC 60383-1 Motivations:
This is a good example of how improvements can be initiated and revisions triggered. Another relevant example: Glass disks are ‘in principal’ a 50 year proven technology. During the period from 1960 to 2000 there were only four major suppliers (mostly European based) serving the market. Materials and designs were more or less standardized in terms of ‘Best Practice’, including shape of assembly interfaces, cementing processes and material/glass composition. By 2014, the number of suppliers had grown to more than 20, mostly cost-driven from ‘low cost’ countries. More manufacturers had come to the market with different design approaches (glass formulation, cement, assembly technology) and production moved more and more into Asia. In reality, present production samples show large scatter in quality due to non-standardized materials and production methods and lack of quality control and process stability. The circumstances and the deficits of the present IEC 60383:1993 standard to detect these weaknesses were expressed in two CIGRE papers during the Paris sessions of D1 in 2014 and 2016. At the moment, no IEC standard is available to detect the variance of quality. Since 1992, all new standards (design tests) were developed for composite insulators in order to implement test methods to reject insufficient designs and materials. IEC 60383:1993, however, has not been ‘touched’ since 1993 when quality and standards were all in basic agreement. So, the market situation for glass insulators changed during these past 25 years, but the relevant standard did not. As this insight came to light at several utilities worldwide, the Swedish National Committee proposed a revision of IEC 60383 during the last plenary meeting in 2016. The project was allocated to MT 20. In this case it has to be said “standards” follow market changes.
Some other information about the work in TC 36: Fortunately new experts and professionals are presently supporting CIGRE/IEC and are in balance with experienced experts. This is important to secure resources and conserve industry knowledge. Interest is high, as reflected by participation in active projects. In future, IEC standards will be organized with the following general document structure:
Main Part: e.g. IEC 61952: Terms, definitions, test methods and acceptance criteria
Part 1 – IEC 61952-1: Dimensions and characteristics
Part 2 – IEC 61952-2: Application guide
The application guide will give further information for products and for applications such as braced line posts, or to help better understand complex topics such as load-time curves and load coordination.