Apr. 28, 2025
Past technical recommendations such as IEC/TR as well as newer specifications such as IEC/TS are both used by power utilities for insulator selection in polluted outdoor environments.
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This edited contribution to INMR by Dr. Wallace Vosloo, retired from Eskom, Richardo Davey of Eskom Research Testing and Johannes Bekker at the University of Stellenbosch in South Africa reviewed different possible approaches in this regard.
Selecting porcelain and glass insulators for three-phase a.c. systems up to 525 kV phase-to-phase using IEC/TR (Guide for the selection of insulators in respect of polluted conditions) has been based on service experience as well as laboratory testing under naturally and artificially polluted conditions. As stated in this document: Simple general rules should assist choosing the insulator, which should give satisfactory performance under polluted conditions.
Fig. 2 and Table 2 summarize basic profiling rules recommended in IEC/TR , which are normally followed.
Influence of insulator diameter on pollution performance is also considered for insulators with average diameters of between 300 and 500 mm. In the case of diameters greater than 500 mm, it is recommended that specific creepage distances be increased by 10% and 20%. Laboratory tests, in accordance with IEC 507, are also recommended to evaluate an insulator’s pollution performance but are rarely used by most power utilities. Greasing or washing is recommended for areas with severe pollution and/or low natural washing.
For a.c. porcelain and glass insulators, a utility would typically specify maximum connection length taking into account live line work, minimum dry arcing distance, and minimum creepage distance. Then, it would be stated that insulator profile must comply with IEC/TR , the focus being on simplicity and ease of use.
It is proposed to erect 132 kV lines in areas with pollution levels ranging from Light to Very Heavy using standard glass cap & pin disc insulators (U120B, F12/146). Assuming the parameters given below, how many discs (n) would be required per string?
System highest voltage (Um) = 145 kV
Minimum required Dry Arcing Distance (DAD) of the insulator string is mm (i.e. for a high lightning area)
Disc spacing (s) = 146 mm
Arcing distance per disc (a) = 210 mm
Creepage distance (CD) per disc = 320 mm
The arcing distance of a disc insulator string = a + (n – 1) s
Thus, = 210 + (n – 1) x 146 and n = ( – 210) / 146 + 1 = 9.84 (in other words, 10 discs are required)
In terms of pollution level, as shown in Table 1, the specific creepage distances needed for Light, Medium, Heavy and Very Heavy pollution areas are 16, 20, 25 and 31 mm/kV (Um) respectively.
Number of discs required per string = (CD)/320 where CD = (Um x SCD)
Table 3 gives values calculated using (145 x SCD)/320.
Utilities have typically been using 7 to 14 standard glass discs per string for 132 kV lines. More recently, with the advent of new ‘active’ polymeric insulation materials that interact with the environment, utilities have continued to use IEC/TR since nothing else was available in IEC.
Typically, for a.c. polymer insulators, utilities such as Eskom would specify maximum connecting length (taking into account live line work), minimum dry arcing distance and minimum creepage distance. They would also state that insulator profile must:
1. comply with IEC/TR based on in-service and test station experience;
2. have open aerodynamic alternating shed profile with S/P ratio ≥ 1; and
3. that the material must be hydrophobic, with good hydrophobicity transfer capabilities.
Then, for e.g. ease of stock, minimum SCDs of 20 mm/kV for Light to Medium and 31 mm/kV for Heavy to Very Heavy pollution areas would be specified.
Insulation requirements for both UHV a.c. and d.c. insulators are more complex and normally determined along with technical experts from manufacturers (mostly members of Cigré WGs).
The following major changes have been made with respect to IEC/TR :
• Encouraging use of site pollution severity measurements, preferably over at least a year, in order to classify a site instead of the previous qualitative assessment (see below).
• Recognition that ‘solid’ pollution on insulators has two components: one soluble and quantified by ESDD; the other insoluble and quantified by NSDD.
• Recognition that, in some cases, measuring layer conductivity should be used for SPS determination.
• Using results of natural and artificial pollution tests to help with dimensioning and to gain more experience in order to promote future studies to establish a correlation between site and laboratory severities.
• Recognition that creepage length is not always the sole determining parameter.
• Recognizing the influence of other geometry parameters and of the varying importance of parameters according to the size, type and material of insulators.
• Recognition of the varying importance of parameters according to type of pollution.
• Adoption of correction factors to attempt to take into account influence of the above pollution and insulator parameters.
Fig. 3 shows three approaches proposed for selection and dimensioning of insulators.
Fig. 4 provides Eskom’s specifications for determining site pollution severity and pollution performance curves.
It has been proposed to erect a 132 kV substation and interconnecting lines at Koeberg Nuclear Power Station (KNPS) along the South African west coast. The area is approximately 600 m from the coastal high-water mark and is exposed to strong coastal winds, low rainfall and regular salt fog events. The insulators (substation: 189 – posts (4 kN), 294 – hollow cores (average diameter <400 mm), 387 – long rods/strings (120 kN, ball and socket) and lines: 768 – long rods/strings (120 kN, ball and socket)) should have a maximum connecting length of 1.48 ± 0.02 m, minimum insulation length of 1.2 m, minimum dry arcing distance of 1.5 m and minimum specific creepage distance of 31 mm/kV (CD ≥ mm). In addition, insulators with open aerodynamic alternating shed profiles, S/P ratio ≥1, hydrophobic material and good hydrophobicity transfer capabilities are preferred.
The question is what insulation is required to ensure risk of insulator flashover is minimal, with mean time between flashover (MTBF) of at least 50 years?
Approach 1: Use Past Experience
Duinefontein 132 kV Substation (≈600 m from the coastal high-water mark) is situated only ≈1 km from the new proposed 132 kV substation and interconnecting lines area at KNPS.
In , the 60 porcelain station post insulators installed at the Duinefontein Substation that have a specific creepage distance of 32.4 mm/kV were upgraded (along with all other insulation) using room temperature vulcanized silicone rubber coating (RTV SR A). Prior to this upgrade (since washing did not work and greasing had only a 6 to 12 month effective lifespan) the substation experienced flashovers on an annual basis, including the Type B instantaneous event of February . Since the upgrade, no flashovers occurred to date (18 years later), which includes the catastrophic Type B instantaneous pollution event experienced in February . Indeed, the RTV SR A coating still has excellent hydrophobic properties.
Personnel from the KNPS Weather Station (≈250 m from Duinefontein Substation) classify the area as follows:
• Average ambient temperatures between 14° and 20°C (minimum 4°C and maximum 36°C);
• Exposed to strong coastal winds (gusts up to 35 m/s);
• Low rainfall area (≈320 mm per year) with only 3 to 5 rainy days in summer (≈80 mm)
• High humidity levels at night/early morning and regular salt fog events (≈40 per year).
ESDD, NSDD and DDDG pollution measurements at the Duinefontein Substation from March to date, were used to calculate ESDD2% = 0.165 mg/cm2 (STDEV = 0.57), average ratio of NSDD/ESDD = 1.1 and monthly average DDDGave = 382 µS/cm.
The flashover of a bare porcelain 132 kV breaker support insulator (having specific creepage distance 32.4 mm/kV) during the catastrophic Type B instantaneous pollution event experienced in the Cape in Feb was used to estimate minimum uniform pollution present on insulation during this event, namely ESDD = 0.4 mg/cm2 and NSDD = 0.182 mg/cm2. Field experience has shown that, while instantaneous pollution events will not occur annually, it can be conservatively assumed that one event occurs each year.
Pollution levels at Duinefontein Substation are as follows:
Type A: 40 natural pre-deposited pollution events per year with critical wetting (ESDD2% = 0.165 mg/cm2 with STD deviation of 0.57 and ESDD/NSDD ratio = 1.1).
Type B: One instantaneous conductive fog pollution event per year (ESDD2% = 0.4 mg/cm2 and NSDD = 0.182 mg/cm2).
The SPS levels obtained from ESDD and NSDD measurements at Duinefontein Substation (see Fig 5) classify the area for Type A pollution (E6/7) d – Heavy and Type B (E7) e – Very Heavy. DDDG measurements, after climatic factor correction, classify the area as Very Heavy.
The 60 RTV SR A coated porcelain station post insulators with SCD of 32.4 mm/kV at Duinefontein Substation (≈1 km from the new proposed 132 kV substation and lines area) have provided excellent performance for 18 years. Note: these same 60 bare porcelain station post insulators had flashed over more than once annually. No washing or greasing was recommended.
Koeberg Insulator Pollution Test Station (KIPTS) (≈50 m from the coastal high-water mark) lies about 2 km from the new proposed 132 kV substation and lines area. Monthly average DDDGave = µS/cm measured at KIPTS is Extreme (≈6.6 times higher compared to Duinefontein Substation). Findings of some insulator research and tests done at KIPTS over 15 years are presented in “Power Utility Perspective on Natural Ageing and Pollution Performance Insulator Test Stations”.
From natural insulator pollution performance experience gained at KIPTS the following:
Posts: Porcelain post insulators with SCD = 38 mm/kV will flashover more than 3 times per year at KIPTS;
RTV SR A coated porcelain post insulators with SCD ≥ 24 mm/kV will have no flashovers per year at KIPTS. RTV SR A coated porcelain transformer bushings at KIPTS with SCD = 30 mm/kV will perform well for 15 years;
Open aerodynamic alternating shed profile with S/P ratio ≥ 1 and a hydrophobic material with good hydrophobicity transfer capability will work best.
Hollows: SR hollow core insulators with SCD ≥ 28 mm/kV will have no flashovers per year at KIPTS;
Open aerodynamic alternating shed profile with S/P ratio ≥ 1 and hydrophobic material with a good hydrophobicity transfer capability will work best.
Longrods: SR longrod insulators with SCD ≥ 22 mm/kV will have no flashovers per year at KIPTS;
Open aerodynamic alternating shed profile with S/P ratio ≥ 1 and hydrophobic material with a good hydrophobicity transfer capability will work best.
Note: Corona rings should be installed on 132 kV longrod insulators.
Strings: Standard glass cap & pin disc insulator (F12/146) strings with SCD of 27 mm/kV will flashover more than 3 times per year at KIPTS. The same insulator string with RTV SR A coating applied will have no flashovers per year;
Fog-type glass cap & pin disc insulator (F120P/146) strings with SCD of 37 mm/kV will have no flashovers per year at KIPTS. The same insulator string with RTV SR C coating applied will experience similar leakage currents to SR longrod insulators.
Note: Expect poor natural washing/cleaning and pin erosion problems.
Approach 2 (as per Fig. 3) is used along with Section 12 of IEC/TS -2 for porcelain and glass, and Section 12 of IEC/TS -3 for polymeric insulators.
The pollution and climate at Duinefontein Substation (as in Approach 1) give the expected pollution severity levels and climate in the area of the proposed 132 kV substation and interconnecting 132 kV lines as follows:
Type A: (E6/7) d – Heavy as 40 natural pre-deposited pollution events per year with critical wetting (ESDD2% = 0.165 mg/cm2 with STD deviation of 0.57 and ESDD/NSDD ratio = 1.1).
Type B: (E7) e – Very Heavy as one instantaneous conductive fog pollution event per year (ESDD2% = 0.4 mg/cm2 and NSDD = 0.182 mg/cm2).
Candidate insulators were selected as shown in Table 4, where possible with a maximum connecting length of 1.48 ± 0.02 m, minimum insulation length of 1.2 m, minimum dry arcing distance of 1.5 m, minimum specific creepage distance of 31 mm/kV (CD ≥ mm). Insulators with open aerodynamic alternating shed profile with S/P ratio ≥ 1, and hydrophobic material with good hydrophobicity transfer capabilities. Porcelain post, standard glass and fog type glass insulators were included as reference.
Laboratory pollution U50% flashover voltage (using the rapid flashover test method) curves at three pollution levels (as in Fig. 4) – SDD of 0.06; 0.12 and 0.48 mg/cm2 with NSDD ≥ 0.1 mg/cm2 was obtained using:
• for porcelain and glass insulators the Solid Layer Test Method (see Table 5) according to IEC using Procedure B and spray gun for applying the Kaolin composition and Annex B.3.2. The degree of pollution on the test insulator was determined using the SDD method. Recommendations as given in Annex D and E were followed.
• and, for polymeric insulators according to modified Solid Layer Test Method (see Table 5) with pre-conditioning procedure, and with/without recovery according to Cigre TB 555 and Cigre TB 691.
The rapid flashover laboratory solid layer pollution test done on SR Longrod A insulator to determine U50% = 225 kV at SDD = 0.12 mg/cm2 and NSDD = 0.1 mg/cm2 with 48-hour hydrophobicity recovery is shown in Figure 6 as example. The rapid flashover laboratory solid layer pollution test results for all the candidate insulators are shown in Table 4.
The candidate insulators’ pollution U50% flashover voltage results in Table 4 were then converted into flashover stress along the test insulation length HT = 1.2 m as in kV/m and is presented as a three-point approximated inverse power law curves against pollution level SDD in mg/cm2 in Fig. 7.
The candidate insulator pollution flashover performance curve constants A in kV/m and α was determined for equation U50%/Ht = A · SDD-α and the values are shown in Table 4.
Um-ph/H was calculated as 70 kV/m using the specified insulation length H = 1.2 m, and Um-ph = 83.7 kV (the highest system r.m.s. phase to ground voltage that the insulator to be supplied will be subjected to).
The candidate insulator was accepted for further calculation if U50%/Ht> 70 kV/m in the SDD range of 0.12 to 0.48 mg/cm2.
Insulator pollution flashover performance curve constants A in kV/m and α, of the candidate insulators was used along with pollution severity levels and climate in the area of the proposed 132 kV substation and interconnecting 132 kV lines in the statistical approach as per Annex G of IEC/TS -1 in order to optimize insulation selection. Fig. 8 shows the Insulator Selection Tool, a commercially available statistical software.
As example, using the IST for MTBF of 50 years 768 – 132 kV SR Longrod A insulators require:
• SCD of 20.2 mm/kV when exposed to 40 natural Type A pre-deposited pollution events per year with critical wetting (ESDD2%= 0.165 mg/cm2 with STD deviation of 0.57 and ESDD/NSDD ratio = 1.1) (see Fig. 9);
• and SCD of 22 mm/kV when exposed to one Type B instantaneous conductive fog pollution event per year (ESDD2%= 0.4 mg/cm2 and NSDD = 0.182 mg/cm2) (see Fig. 10).
The MTBF obtainable within the required connecting length of 1.48 ± 0.02 m and SCD needed for MTBF of 50 years was calculated for the reference and candidate insulators using the IST. Results are shown in Table 6.
Approach 3: Measure & Design
As per Fig. 5, the area proposed for the 132 kV substation and interconnecting lines can be classified for Type A pollution as Class d – Heavy and for Type B pollution as Class e – Very Heavy. Thus, in the worst case, a minimum SCD of 31 mm/kV is needed for the reference glass disc insulator.
As per IEC/TS -2, the following is recommended for porcelain and glass insulators:
• Aerodynamic, alternating sheds on long rod insulators, post insulators, hollow core insulators;
• Anti-fog profile for disc insulators;
• p1 – p2 ≥ 15 mm, s/p ≥ 0.65, c ≥ 25 mm, l/d ≤ 5, 5˚ ≤ α ≤ 25˚ and CF ≤ 4 (lowest risk options used);
• No altitude correction required;
• 10% increase in SCD for hollow core insulators with average diameter > 300 mm and < 400 mm.
As per IEC/TS -3, the following is recommended for hydrophobicity transfer polymeric insulators:
• SCD could be reduced or increased depending on the environment or pollution level (no clear advice given);
• Aerodynamic alternating sheds;
• p1 – p2 ≥ 18 mm, s/p ≥ 0.75, c ≥ 40 mm, l/d ≤ 4.5, 5˚ ≤ α ≤ 25˚ and CF ≤ 4 (lowest risk options used);
• No altitude correction required;
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• 10% increase in SCD for hollow core insulators with average diameter > 300 mm and < 400 mm.
Insulators must have a minimum SCD of 31 mm/kV (CD = mm) and hollow core insulators with minimum SCD of 31 x 1.1 = 34.1 mm/kV (CD = mm) are recommended. Table 7 provides a summary and comparison of results obtained for these three Approaches.
As per Table 7, the findings of Approach 1 and 2 correlate with one another, showing that both approaches would work if correct data required as per Fig. 3 is available. The practical examples demonstrate that both approaches lead to a selection having good accuracy. Moreover, as per Table 7, Approach 3 in general would result in over-dimensioning of required insulation.
Table 8 offers a comparison of the recommendations as per IEC/TR and specifications as per Approach 3 in IEC/TS -1 for the practical examples. Findings are similar in regard to specific creepage distance. However, Approach 3 takes into account use of hydrophobicity transfer materials, which could result in a one class lower SCD, different profile parameters and anti-fog instead of open aerodynamic disc insulators.
In summary it has been demonstrated with practical examples, how the old technical recommendation, i.e., IEC/TR and the new technical specification i.e., IEC/TS are being successfully applied by a utility, in this case, Eskom, to select and dimension outdoor insulators for polluted environments (Note: Ageing and failure modes are not taken into consideration in this discussion).
The importance is also shown of site pollution severity measurements, climatic conditions, identifying pollution type and practical use of data collected from natural pollution test sites, in-service insulators, laboratory pollution flashover tests and statistical evaluation.
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
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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