Jun. 05, 2025
Past service experience and laboratory testing revealed risk of great variance in the quality of glass insulators from different manufacturers, depending on raw materials, process and know-how. This finding applied to all components – from the glass shell to metal fittings to cement. Moreover, experts report that it has not always been possible to distinguish insulators of inferior quality based solely on past standard tests. This is because insulators sent for type and sample testing can be specially selected for this purpose. Moreover, requirements for routine testing during production are not always stringent enough to screen out insulators of inferior quality.
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Combined with this development, demand for uninterrupted transmission has increased worldwide and especially in Europe. Improved modelling and specialized software have also made it possible to optimize insulator selection and dimensioning, resulting in more highly stressed insulators. All these factors have meant greater need for uniform high quality since the failure of any disk in a string becomes that much more serious.
A decade ago, large power supply companies in Europe co-operated to establish test procedures and criteria that would better reveal the true quality of toughened glass insulators. This edited past contribution to INMR by Kjell Halsan (Statnett), Igor Gutman and Johan Lundengård (then at STRI) provided the impetus for changes to standards, now implemented, and also discussed findings from laboratory testing of glass insulators.
Norwegian transmission system operator, Statnett, has had a policy of closely monitoring the quality of its glass insulators and representatives have attended sample testing for all major deliveries. In addition, operating data on shatter rate per 100,000 glass insulators per year has been collected and analyzed for long transmission lines. These statistics showed that shatter rate of discs can vary significantly among suppliers and sometimes even for different batches delivered by the same supplier. Given this, Statnett conducted a project to ensure purchase of only high quality glass insulators from both existing and new suppliers – with emphasis more on life cycle cost than on acquisition cost.
In Sweden, past refurbishment of a 220 kV line by TSO, Svenska Kraftnät, employed glass cap & pin insulators selected on the basis of usual tendering procedures including compliance with IEC requirements. Insulators were installed in a mostly clean but humid area, typical of Scandinavia. Abnormal discharge activity and audible noise were reported on these insulators after only a short time in service, most often when humidity was highest. Field inspection of the line with a daylight UV-camera confirmed the activity and suggested that the insulators could be suffering from defects in their cement.
These observations became a concern since such unexpected initial behaviour for new insulators could influence long-term performance. Detailed investigations were performed, including voltage tests at operating voltage, and revealed discharge activity on the glass surface around the pin, just outside the cement. This was especially evident if the electric field distribution in this part of the insulator was affected by water sprayed onto it, to simulate humid service conditions. Resistance measurements using a hand-probe showed that the weakness observed was due to unusually high conductivity as well as unfavourable geometry of the cement. RIV levels, for example, had a wide spread among insulators from different manufacturers and this was interpreted as an indicator of poor quality control.
Given this, it was proposed to expand test requirements for tenders to include RIV measurements at 20 kV and 24 kV to reduce risk of similar problems in the future. Such a requirement is easy to implement and would help ensure that all insulators purchased are more robust with respect to discharge activity under humid conditions.
The aim of the first stage in the investigation was to identify effective yet practical test methods that could be used to qualify manufacturers in terms of the quality of toughened glass insulators they supply. This included a workshop with experts, discussions with leading test laboratories and manufacturers and a review of the literature. The production process for glass insulators was also examined closely to identify critical issues that can affect final quality. This was then used to identify tests that could be applied to differentiate between good and inferior quality insulators. The following points were highlighted:
• Overview of the components in a glass insulator disc;
• Walk-through a glass disc fabrication process;
• Quality control aspects during manufacture and assembly;
• Relationship between key properties and test methods to verify these.
Subsequently, a combination of the following tests was identified as suitable for more detailed investigation:
1. Steep-front puncture test in air to test the dielectric strength of the glass shell. This test method might need refinement with respect to steepness of the voltage wave applied.
2. RIV testing to verify pin cavity design and quality of cement used. This test was seen as maybe needing modification to verify the longevity of the RIV control measures that manufacturers use, e.g. coating with bitumen.
3. Ultimate tensile test to verify mechanical design. This is a standard test for insulators but could be modified for larger required sample size to better understand failure modes as well as how a manufacturer can control mechanical failure.
4. A test verifying residual mechanical strength can be an important tool to verify that, even if an insulator breaks, the conductor will not be dropped.
5. Thermal shock test to check the quality of the glass shell.
The first two, i.e. impulse puncture (called further steep-front) test and RIV test were subsequently chosen for the preliminary verification.
From discussions during this phase of the project, it was also concluded that testing alone might not always guarantee quality. Buyers would also need to implement a process for them to evaluate the quality control procedures used by the manufacturer. This would include regular factory inspections to ensure that these procedures are always implemented during production. In addition, a library of photos was created to assist the inspector to visually identify potentially problematic units for subsequent sample testing.
In this project phase, an overview of the entire manufacturing process was created to highlight important process parameters to be scrutinized during any factory visit. This guide (or questionnaire) would make it easier for a representative from the power supply company to go through the production and quality control processes of any manufacturer.
Limited testing was performed on 3 types of glass insulators selected from inventory at Statnett and Svenska Kraftnät. The aim was to conduct comparative testing on insulators assumed to have different levels of quality, i.e. ‘high’, ‘acceptable’ and ‘low’. These insulators were subjected to only two types of tests, selected as the most promising based on the literature, opinion of experts and previous tests performed for these power companies. The two tests were: RIV testing in dry/wet conditions followed by resistivity measurements of the cement; and steep-front testing (i.e. impulse puncture withstand test in air).
The RIV test was performed according to Swedish standard SS 447 10 11 (). A pre-stress voltage of 30 kV was applied for 5 minutes after which the RIV level was read at test voltage reduced to 24 kV. Then, test voltage was decreased to 20 kV and the RIV level was read once more. According to this standard, the RIV level must not exceed 60 dB (µV) at 20 kV.
Steep-front testing was performed according to IEC cl. 5.6 and Canadian standard CSA C411.1-10 cl. 6.6 and witnessed by representatives from STRI and Statnett. The results (Table 1) demonstrated that both tests were able to reveal insulators with ‘low’ quality in two of their elements, i.e. cement and glass. As such, it was recommended that these tests be part of an extended test matrix.
As one useful finding, results of initial laboratory testing showed that test insulators should be randomly selected since it seemed that manufacturers could produce a smaller batch with better quality insulators. Another practical output was that 8 out of 10 insulators randomly selected from one batch in storage failed the steep-front test by shattering. This confirmed not only the need for further work to prevent such insulators from ever reaching the stocks of power utilities but also the importance of carrying out steep-front as a sample test on randomly selected unit.
The aim of this phase of the project was to evaluate whether or not the test methods selected would be able to reveal insulators of variable quality. This would be accomplished by collecting a selection of insulators of differing quality such that some would be expected fail (thereby confirming that the test matrix can catch these) while some would be expected to pass (verifying that the test matrix was not unreasonably stringent). It was decided not to request the test insulators from manufacturers. Instead, these were selected at random from batches already purchased by participating power companies for line construction and refurbishment. In total, batches from 5 different suppliers with factories all over the world were chosen. Moreover, based on the specific request of participating utilities, the mechanical strength classes were limited to: 120 kN, 210 kN and 300 kN.
The results of 3 types of tests, i.e. RIV test (performed at STRI), steep-front test and thermal-mechanical test (both performed at EGU) were presented in a paper at the CIGRE General Session in . In order to verify how different tests can affect and reveal insulators of poor quality, a test program according to the scheme proposed by the CSA (outlined in Fig. 2) was performed using 3 different types of glass insulators, with 30 units of each type.
When comparing test results from the mechanical failing load test on the same type of insulators, no significant variation was found between the 3 groups of insulators going through different numbers of tests (batch 1, 2 or 3). Problems with quality of glass discs were revealed during the thermal-mechanical test, steep-front test and mechanical failing load test. Thus, it was proposed not to use the thermal-mechanical test to pre-stress the glass before the steep-front test since it was felt this would be overly complicated.
The test program concentrated on RIV and steep-front tests and details on both test methodologies are as follows:
RIV Test
A pre-stress voltage of 30 kV was applied first for 5 minutes (before the RIV level was read). Test voltage was then decreased to 24 kV and RIV level was read again at this voltage. Then, test voltage was decreased to 20 kV and RIV level read once more. The same procedure was applied for dry and for wet insulators. Wetting the insulators was done by placing them upside-down and pouring de-ionized water onto the cement area close to the pin. The insulators were moistened this way for 15-20 hours before the test. The acceptance criterion to check if the lot passes the RIV test will be according to the formula: Average RIV level ≤60 dB-Risk factor x Standard deviation. Risk factor for 20 kV is considered as 1.6 and 1.2 for 24 kV. Application of the risk factor formula will make it more difficult for insulators having a large spread to pass. As such, there is already additional quality control.
Steep-Front Test
Tests were performed according to IEC cl. 5.6 with a series of 5 positive, 5 negative, 5 positive and 5 negative impulses. Based on preliminary results, it was decided that no impulses with reduced amplitude should be applied between polarity changes. Test voltage for the steep-front was according to IEC . At relevant steep-front test voltages, each impulse resulted in flashover. The acceptance criterion was according to C411.10.1, i.e. the insulator passes if every steep-front impulse voltage application results in external flashover and the test record or peak voltage indicator does not show a large reduction in voltage between impulse applications. To evaluate results from the complete batch tested, it was decided to apply acceptance criteria in accordance with IEC , cl. 5.8, i.e. requiring that no punctures are accepted. If only a single unit is punctured, a re-test procedure shall be applied. Also, punctured units shall be included in the mechanical failing load test.
After the steep-front test, all insulators were subjected to the mechanical failing load test performed according to IEC -1 cl. 19.2. The acceptance criterion according to CSA C411.1-10, cl. 6.13.2 was used. Similar to the risk factor formula applied for results of RIV testing, the CSA has a formula to evaluate results of the mechanical failing load test: Quality factor Q=4≤ (Average failing load-Rated failing load)/(Standard deviation). A summary of these tests is presented in Table 2 that outlines results for 16 insulator types. It is clear the tests revealed different qualities of insulators.
Conductivity measurements provide some indication of possible issues with cement under humid service conditions, i.e. high RIV levels. However, there is no clear correlation to RIV testing. For example, some Type ‘A’ insulators that failed the RIV test showed little or no resistivity of their cement and partial discharge activity came from the bottom of the cap. During the latest series of tests, performed in , it was found that two cameras should ideally be used to allow simultaneous observation of discharges from the top or bottom as this could occur with different insulator designs.
There is information in the literature that glass insulators experiencing partial discharges between the cap and disc (i.e. at the top) will also have a higher shatter rate due to deterioration of the surface. Indeed, in this project those insulators with high RIV activity between the cap and disc had an abnormally high shatter rate in service.
It therefore seems logical that when evaluating an insulator for purchase, a power supply company should carry out an RIV test under dry conditions. This should be done at the maximum possible level of voltage for the shortest typical string length for that voltage class. For example, in the case of the TSO in Sweden, this can be 24 kV for 400 kV overhead lines, whereas it might be 20 kV for Statnett or CEPS, the TSO in the Czech Republic. It would also seem reasonable to apply the same criterion for the RIV test under humid conditions, although results in this case suggest that this might prove too strict a test criterion. Therefore, further analysis of all results will be made, taking into account which insulators were considered as acceptable and then changing the risk factors of 1.2 and 1.6.
From all test results presented, it can be seen that the batches where glass shells shatter during the mechanical failing load test also have a higher probability of failing the steep-front test. However, at present both tests are still felt necessary. This is because of the distinct difference between a steep-front test and a mechanical failing load test. In the case of the former the glass shell is put under very high electrical stress, whereas in the latter mechanical load is increased to the maximum tension stress that results in mechanical separation of the insulator.
The proposed test matrix should consist of 3 tests: RIV, steep-front and residual strength. The guidelines for the requirements and criteria have been developed in the framework of this project and are shown in Fig. 3.
The two key issues under focus, i.e. high RIV levels and varying shatter rate, can be revealed by additional tests. Based on a comprehensive research program performed between and , the following tests have been verified and are recommended to verify the quality of glass cap & pin insulators supplied by different manufacturers: modified RIV measurements under dry and wet conditions; modified steep-front test; and modified residual strength test. Actual test methods have been discussed and an important point is to ensure that insulators for these tests are selected in a random manner.
Different types of insulators are available for application on overhead transmission lines, including glass or porcelain string insulators, porcelain long-rods and composite/polymeric insulators. Within each category there are also a range of designs, materials, qualities and prices. At the same time, there are also several alternatives available to improve performance of insulators intended for polluted service areas, from advantageous shed profiles to coating with RTV silicone material.
Given all these options, there are a host of questions when deciding on which insulators to select for any new project, such as: What is the best design for that environment? What is the best material and what criteria must be taken in account when selecting it? Which parameters are most suitable for in service evaluation of condition? What will be the estimated service life? and so on. Unfortunately, there is no simple answer to all these questions. But it is possible to note the different elements to be considered when evaluating and comparing all possible solutions.
This edited contribution to INMR by Javier García, an expert at La Granja Insulators in Spain, offers his views on what type of considerations must be taken into account when selecting an insulator for application on an overhead transmission line.
An insulator acts mainly as a mechanical support. As such, only after all mechanical aspects of any design have been finalized are the required electrical characteristics added. In fact, mechanical characteristics are so important to the function of an insulator that they are the only commonality found in all markings on insulators. Another issue to consider is consequence of mechanical failure, e.g. is it only loss of leakage distance or is it a dropped conductor. This of course depends on design of the insulator. IEC establishes the mechanical residual test methods and acceptance criteria for glass and porcelain string insulators under dielectric breakage.
The user must also determine maximum loading that the line will ever apply to the insulators, including weight of conductor and hardware, ice and wind loading and any other load factors. Suspension insulators are rated in terms of their Specified Mechanical Load (SML). Manufacturers usually recommend that the insulator never be loaded to more than 50% of its SML, which is a guaranteed minimum ultimate strength rating. Each batch of insulators produced is sampled for mechanical strength and all samples must meet or exceed stated SML value based on statistical criteria. The routine test load is the proof load applied to each unit and also the maximum load that the insulator should ever experience in service. IEC -1 and IEC respectively establish mechanical test methods and acceptance criteria for porcelain insulators, glass insulators and composite insulators.
The electrical characteristics of an insulator are imparted to it by the surrounding air. This is defined principally by its arcing distance, namely “the shortest distance in the air external to the insulator between the metallic parts which normally have the operating voltage between them”. Impulse withstand/flashover and dry power frequency characteristics are all based on dry arcing distance. Some might argue that the wet power frequency withstand/flashover characteristics are determined by leakage distance but that argument only holds within a narrow band. Leakage distance plays a role but as a contributing factor. IEC -1 recommends the withstand voltage associated with the highest equipment voltage.
The past edition of the IEC TS series developed new techniques for selecting and dimensioning high voltage insulators and established a process to determine the most efficient insulation. This technical specification recommends three approaches to select suitable insulators based on system requirements and environmental conditions:
• Approach 1: Use past experience
• Approach 2: Measure and test
• Approach 3: Measure and design
The applicability of each approach depends on available data, time and the economics of a project. Some of the parameters required for these approaches include:
1. Determining Reference Unified Specific Creepage Distance (RUSCD)
Fig. 1 shows the relationship between site pollution severity (SPS) class and reference unified specific creepage distance (RUSCD) for insulators. The bars are preferred values representative of a minimum requirement for each class and are given for use with Approach 3 (i.e. measure and design) of IEC/TS -1. If site pollution severities are available, an RUSCD is recommended that corresponds to the position of the SPS measurements within the class, following the curve.
For Type A pollution (i.e. inland, desert or industrial areas), SPS is calculated from ESDD and NSDD values. For Type B pollution (i.e. coastal areas where salt water or conductive fog is deposited onto insulator surfaces), SPS is calculated from SES (site equivalent salinity).
2. Choice of Profile: Glass & Porcelain Insulators
Different types of insulators and even different positions on the same insulator type accumulate pollution at different rates in the same environment. In addition, variations in the nature of pollutants may make some shapes of insulator more effective than others. Table 1 from IEC TS -2 briefly summarizes the principal advantages and disadvantages of the main profiles with respect to pollution performance.
3. Profile Suitability: Glass & Porcelain Insulators
Tables 2 & 3 in IEC TS -2 give simple merit values for porcelain and glass insulator profiles. Table 2 gives profile suitability, relative to standard profile assuming the same creepage distance per unit or string. Table 3 assumes the same insulation length. Both review the principal advantages and disadvantages of the main profile types with respect to pollution performance.
Moreover, IEC TS -2 also gives profile parameters to take into account, e.g.:
• Alternating sheds and shed overhang;
• Spacing versus shed overhang;
• Minimum distance between sheds;
• Creepage distance versus clearances;
• Shed angle;
• Creepage factor.
4. Polymeric Insulator Profiles & Parameters
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Chapters 8 & 9 of IEC TS -3 give recommendations for polymeric/composite insulators profiles and parameters to take into account, including:
• Alternating sheds and shed overhang;
• Spacing versus shed overhang;
• Minimum distance between sheds;
• Creepage distance versus clearances;
• Shed angle;
• Creepage factor.
5. Pollution Test Standards
Pollution tests on glass and porcelain insulators in a laboratory can be carried out with two main objectives:
• To obtain information about the pollution performance of insulators (i.e. comparing different insulator types/profiles);
• To verify performance in a configuration as close as possible to that in-service.
IEC prescribes the procedures for artificial pollution tests applicable to porcelain and glass insulators for overhead lines. Two categories of pollution test methods are recommended for these standard tests:
• Salt fog method in which the insulators are subjected to a defined ambient pollution;
• Solid layer method in which a fairly uniform layer of a defined solid pollution is deposited onto the insulator surface.
These standardized laboratory pollution test methods are not applicable for composite (polymeric) or RTV coated insulators, although a proposed test method for artificially polluted composite insulators is covered in CIGRE TB 555: “Artificial Pollution Test for Polymer Insulators”. In the case of naturally polluted insulators removed from service, a recent CIGRE TB 691 (WG D1.44), “Pollution Test of Naturally and Artificially Contaminated Insulators” summarized recent experience with the so-called rapid flashover test methods:
• Rapid flashover Test (RFO, based on IEC solid layer test);
• Quick flashover (QF, based on IEC salt fog test).
Both tests can be applied for glass and porcelain as well as for composite insulators for AC and DC applications. The objective of these tests is based on the need for a reliable diagnostic of naturally polluted insulators so as to evaluate residual dielectric strength. Also considered is the trend to make testing more cost-effective and time-efficient, even for artificially polluted insulators.
Any reduction in performance can be due to pollution in the case of ceramic insulators or due to a combination of pollution and ageing in the case of polymeric insulators. In both cases, however, residual pollution strength should be quantified in terms of flashover voltage and not withstand voltage. This is because withstand voltage does not provide the user with information about the probability of flashover or the standard deviation in flashover voltage.
6. Insulator Test Stations
Sometimes, the combination of all the varying environmental parameters that influence insulator behaviour over its lifetime are difficult to simulate and accelerate. The validity of laboratory testing is thus often questioned since the procedures adopted for these tests may not take into account significant factors that would be encountered in service; or they may overemphasize others.
Given this, evaluation of insulator performance at naturally polluted outdoor test stations is becoming more important. Although involving longer test durations and still requiring care in correct interpretation of test data, results tend to be accepted with more confidence. An outdoor test station is also a valuable tool for new insulation technologies for which there is still no technical or normative specification for testing or characterization.
CIGRE Technical Brochure No. 333, “Guide for the establishment of naturally polluted insulator testing stations” serves as a general guide for establishing natural test stations that will facilitate comparison of various insulator designs, exploration of particular aspects of insulator performance and/or selection of the most appropriate insulation for a particular application. While such testing relates specifically to insulators intended for use under AC conditions, certain aspects are applicable to DC as well. Typical goals for such testing could be one or more of the following:
• To compare performance of insulators of different design;
• To compare performance of insulators from different manufacturers;
• To dimension insulators for a particular environment or application;
• To examine behaviour of insulators of different dielectric materials;
• To compare performance of insulators in different orientations;
• To explore effects of specific parameters such as profile geometries or insulators diameters;
• To identify possible weaknesses or failure mechanisms of an insulator design;
• To estimate life expectancy of various insulators;
• To serve as a qualification test for potential suppliers;
• To establish effectiveness and service life of special insulator treatments such as washing, greasing, silicone rubber coating, shed extenders, etc.;
• To assess performance of other outdoor equipment insulation such as transformer bushings, surge arresters, cable terminations, etc.
The severity of pollution and prevailing climate of an outdoor test station should ideally be representative of conditions found on the system. As is the case for laboratory tests, over-acceleration of ambient stresses can yield misleading results. Contamination severity assessment by means of ESDD and NSDD measurements and/or directional dust deposit gauges should be undertaken to ensure that the appropriate site has been selected.
Insulator test stations have a range of sizes and levels of sophistication and can be categorized as:
• Research stations;
• Simplified, on-line stations;
• In service test structures;
• Mobile insulator test stations.
Leakage current activity (including number of flashovers experienced), climatic effects and pollution severity are all usually monitored at these sites. In addition, performance of test samples should be judged based on regular inspection of insulators, including close-up visual examination of surfaces, assessment of the hydrophobicity of the dielectric material and evidence of electrical activity.
Insulator Fitting Corrosion Mechanism
Insulator corrosion generally occurs whenever an insulator is polluted and there is presence of humidity. Leakage currents start when the surface is covered by a deposit of wet pollution, with amplitude a function of degree of pollution (i.e. amount of soluble salts). Polluted and wet insulators energized with AC voltage display a biased leakage current having a DC component that causes electrolytic corrosion of pins. Impact of leakage current is most harmful when frequency and duration of wetting periods are high, such as in tropical climates, and also when pollution finds a hygroscopic surface. Hence the special importance of monitoring for inert contaminants that absorb or retain humidity.
Such corrosion is more important for DC than for AC voltages given the same site due to unidirectional current and electrostatic phenomena that contribute to pollution deposition. For insulators, dominant electrolytic effects only add to atmospheric initiated corrosion, particularly those due to formation of oxidizing agents caused by presence of arcs near fittings. These can be initiated and maintained during periods of humidification and drying that precede and follow critical conditions or whenever the insulator is more humid. Protective field dispatch accessories can be beneficial to limit such humidification and drying periods, which accelerate insulator fitting corrosion in those units that are most electrically stressed. Corrosion can result in:
1. attack on galvanization;
2. attack on internal steel structure with formation of a conductive rust deposit that can flow onto the dielectric
The most severe cases of corrosion can be found in tropical areas with heavy marine pollution and in areas where pollution by dust accumulation occurs over long periods without rain in combination with high environmental humidity.
Corrosion of insulator fittings can have the following effects:
1. Impact on mechanical resistance
This applies particularly to the pin of the insulator when the section of the corroded part becomes reduced, such as reduction in pin diameter;
2. Impact on electrical resistance due to formation of rust deposit on surface
This deposit can also cause damage to the insulation due to concentrated electric field around this new electrode.
3. Breakage of dielectric due to expansion of corroded pin
Remedies to improve resistance to corrosion on insulators typically involve special metal protection developed to avoid or delay this phenomenon.
These remedies consist of reinforced galvanized fittings and use of sacrificial zinc sleeve protection.
• Reinforced galvanized fittings
Ch. 26 of IEC--1 standardizes minimum average coating mass for the metal fitting of insulators: 600 g/m2 (85 µm) but this value can increase to 140 µm for insulators installed in high corrosion areas in order to prolong service life.
• Zinc sleeve is galvanically positive and has a large potential difference from iron
This works as a sacrificial electrode at the cement boundary where current flows. The zinc sleeve is free from accumulation of corrosive products.
IEC- specifies minimum requirements for a zinc sleeve but this can also be improved to increase corrosion performance.
Also, IEC- specifies a test method for control of the zinc sleeve. Future work in standards and norms would have to include zinc sleeve requirements and tests methods within IEC--1 (for AC lines).
Among the principal objectives of any overhead line maintenance policy is to maintain the number of fault outages at acceptable levels. In this regard, a database containing key information on line insulation is an efficient tool to track and evaluate performance. The information this database should contain includes:
• Type/sub-type of strings;
• Type of insulation: glass, ceramic, composite, coated glass, etc.
• Sub-type of insulator: standard profile, pollution profile, etc.
• Number of insulators per string;
• Manufacturer of the insulation;
• Insulator traceability data (production order, date, etc.)
• Standards;
• Year of installation;
• Manufacturer/applicator of silicone material;
• Estimated end of life;
• Degradation environment: Normal, hard or very hard.
Several maintenance indicators are normally used by utilities:
• Number of faults;
• Insulator breakage rate;
• Washing frequency.
Also a range of maintenance methods and procedures are known:
• Aerial inspection;
• Ground patrol inspection;
• HD recording;
• Infrared inspection.
Trends in maintenance indicators together with findings from inspections can then link with the database to help decision-making with respect to maintenance or replacement of insulation. There is also the opportunity for evaluation and comparison of different types of materials, insulator profiles and manufacturer qualities.
Insulators are expected to perform with high reliability over long periods of time. A large number of design parameters (discussed above), choice of material as well as mastering manufacturing processes are all required in order to maintain such reliability over the long-term.
An insulator comes to the end of its working life when it fails mechanically, flashes over with unacceptably high frequency or gives evidence of deterioration to a condition likely to lower its safety factor in service. All insulators are affected to some extent by impact, cycling (both thermal and mechanical), weathering, conductor motion, corrosion and cement growth. Determining when is the right time to replace insulators is key to optimizing maintenance costs and there are a large number of possible degradation modes. Some are easily detectable by visual inspection while others, such as porcelain and composite insulators, may require more sophisticated methods. Degradation modes can also be due to easily detectable mechanisms such as slip of metal fittings, pin corrosion or surface erosion – all considered to be valid reasons for insulator replacement.
CIGRE has established a test procedure to determine the state of cap & pin as well as long-rod insulators and to decide on time for replacement: “Guide for the assessment of old cap and pin and long-rod transmission line insulators made of porcelain or glass: What to check and when to replace”. CIGRE Technical Brochure No. 306, established a testing sequence with a number of non-destructive tests including visual tests (e.g. degree of corrosion) as well as dimensional, thermal and combined thermo-mechanical tests. This first series of tests is followed by destructive mechanical testing. A probability diagram based on a normal distribution is used to analyze failing load test results. With probability (risk) of failure on the ordinate and failing load on the abscissa, failing load characteristics are represented as straight lines. That way, changes in strength are easily seen. To help users, the document includes a number of typical cases of analysis of test result called “Reference Scenarios” that are useful to assess the condition of the insulator.
Failing load characteristics are represented by:
• dashed line for an insulator sample tested when new;
• solid line for insulators as received from a line;
• dashed/dotted line for insulators that have been submitted to thermo-mechanical testing (TMP test).
The SFL (specified failing load) is marked with a solid vertical line. For the example of ”Reference scenario F1”, the reductions in strength in this diagram are not representative of high quality products. Ageing and TMP tests should have only negligible impact on products of high quality.
Parallel to this document, another Technical Brochure published by CIGRE assists evaluation of the technical condition of aged, old or failed composite insulators: “Guide for the assessment of composite Insulators in the laboratory after their removal from service” (CIGRE Technical Brochure No. 481). Different methods, philosophies and tools are described which enable some conclusion regarding the residual lifetime of composite insulators of the same age and design family. The document also gives indications for research and evaluation in the case of investigating a failure or a unit considered at high risk of failing. This is based on a recommended sequence of testing on samples removed from different stress zones on the line.
Selection of insulator type is not a simple task, especially if the insulator will be installed in a highly polluted area. Numerous documents (e.g. IEC standards, CIGRE Technical Brochures, etc.) are available to help select the most appropriate insulator, to monitor its behaviour in service and to determine when it is nearing end-of-life.
Different solutions are available to improve insulator performance in high corrosion areas.
Several factors must be taken into account when it comes to optimizing selection of insulator type:
• More effective designs/materials;
• Maintenance costs: Inspection cost, cleaning cost, replacement cost, etc.;
• Breakage rate in service, to be guaranteed by the supplier;
• Severity of consequences in case of failure (mechanical breakage or electrical failure;
• Expected end-of-life.
References
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