Key Questions to Ask When Ordering Aerodynamic Glass Insulator

Apr. 14, 2025

Selecting Insulators for Polluted Environments: Utility Perspective -

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.

Using IEC/TR

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.

Although it is advised to determine pollution type and severity and then use laboratory tests to help select insulators, many utilities have instead simply chosen to standardize on one or more values of minimum Specific Creepage Distance. Here, SCD (the ratio of the leakage distance measured between phase and earth over the r.m.s. phase to phase value of the highest voltage for the equipment) is recommended for various pollution levels (I to IV), namely 16, 20, 25 and 31 mm/kV(Um) respectively for Light, Medium, Heavy and Very Heavy pollution environments. Pollution levels I to IV are determined mainly from IEC/TR Tables (shown in Fig. 1), service experience or by doing site pollution severity (SPS) measurements using Equivalent Salt Deposit Density (ESDD) or Directional Dust Deposit Gauges (DDDG) methods. Table 1 shows typical ESDD and DDDG values used to classify site pollution level and minimum required SCD.

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.

Case Study Using IEC/TR for Selecting Insulators

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).

Using Latest Specification IEC/TS “Selection & Dimensioning of HV Insulators for Polluted Conditions”

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.

Practical Example Using Three Approaches as per IEC/TS for Insulator Selection

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: Measure & Test

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;

Contact us to discuss your requirements of Aerodynamic Glass Insulator. Our experienced sales team can help you identify the options that best suit your needs.

• 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;

• 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.

Discussion

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.

Conclusions

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.

Insulators Requirements: Design Criteria, Operation Parameters and ...

MECHANICAL CONSIDERATIONS

An insulator is a mechanical support. After the mechanical aspects of a design have been finalized, electrical characteristics are added. If the insulator doesn’t keep your line in the air, all electrical characteristics in the world mean nothing. Mechanical characteristics are so important to the functioning of an insulator that they’re the one commonality in the marking of all insulator. Another question to take in account is the consequences of a mechanical failure: Loss of leakage distance only or cable fall. This depend of the design of the insulator. IEC stablish the mechanical residual test methods and acceptance criteria for glass or porcelain string insulators with the dielectric part breakage.

The use must determine the maximum loading that the line will ever apply to its insulator, including conductor weight, hardware weight, ice and wing loading, and other overload factors.

Suspension insulators are rated in terms of their Specified Mechanical Load (SML). Most manufacturers recommend that you never load the insulator to more than 50% of the Specified Mechanical Load (SML). The SML rating is a guaranteed minimum ultimate strength rating. Each batch of insulators produced is sampled for mechanical strength and all samples must meet or exceed the stated SML value and the statistical criteria. The routine test load is the proof load applied to each unit and also the maximum load that the insulator should ever see in service.

IEC -1 & IEC stablish the mechanical test methods and acceptance criteria for Ceramic or glass insulators units and Composite insulators respectively.

ELECTRICAL CONSIDERATIONS

The electrical characteristics of the insulator are imparted by the air around it. This is principally defined by the arcing distance of the insulator defined as “shortest distance in the air external to the insulator between the metallic parts which normally have the operating voltage between them”.

The impulse withstand/flashover characteristics and dry power frequency characteristics are all based upon the dry arcing distance.
Some would argue that the wet power frequency withstand/ flashover characteristics are determined by leakage distance, but that argument only holds within a narrow band of leakage distances. Leakage distance plays a role, but only as a contributing factor.
IEC -1 recommend the withstand voltages associated with the highest voltage for the equipment

TECHNIQUES FOR SELECTION AND DIMENSIONING OF HIGH VOLTAGE INSULATORS FOR USE IN POLLUTED AREAS

Last edition of series IEC TS has developed new techniques for the selection and dimensioning of high voltage insulators. It enables, after an established process, to determinate the most efficient insulation. The technical specification recommends three approaches to select suitable insulators based on system requirement 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 economics involved in the project.

Some of the parameters required for these approaches are mentioned on the following sub-chapter:

Determination of the reference Unified Specific Creepage Distance (RUSCD)

Figure 1 shows the relation between site pollution severity (SPS) class and RUSCD (reference unified specific creepage distance) for insulators. The bars are preferred values representative of a minimum requirement for each class and are given for use with approach 3 (measured and design) of the IEC/TS -1.

If the site pollution severities (SPS) are available, it is recommended to take a RUSCD (reference unified specific creepage distance) which corresponds to the position of the site pollution severity (SPS) measurements within the class by following the curve in Figure 1.

Basic USCD (mm/kV*)

* r.m.s. value of the highest operating
voltage across the insulator.

Site pollution severity (SPS) classes:
a: Very light
b: Light
c: Medium
d: Heavy
e: Very heavy

For Type A pollution (inland, desert or industrially polluted areas), SPS is calculated from ESDD/NSDD values.

For Type B pollution (coastal areas where salt water or conductive fog is deposited onto the insulator surface), SPS is calculated from SES (site equivalent salinity)

Choice of profile – glass and ceramic insulators

Different types of insulator and even different positions of the same insulator type may accumulate pollution at different rates in the same environment. In addition, variations in the nature of the pollutant may make some shapes of insulator more effective than others.

IEC TS -2 – Table 1 below shows a brief summary of the principal advantages and disadvantages of the main profile types with respect to pollution performance.

Antipollution profiles

Profile suitability – glass and ceramic insulators

IEC TS -2 – Tables 2 & 3 give simple merit values for ceramic and glass insulator profiles. Table 2 give the profile suitability, relative to standard profile assuming the same creepage distance per unit or string, Table 3 assuming the same insulating length.

Below tables show a brief summary of the principal advantages and disadvantages of the main profile types with respect to pollution performance.

Type A pollution: Solid pollution with non-soluble components.

Type B pollution: Liquid electrolytes with very little or no non-soluble components.

Also IEC TS -2 give profiles parameters to take in account:

Alternating sheds and shed overhang;
Spacing versus shed overhang;
Minimum distance between sheds;
Creepage distance versus clearances;
Shed angle;
Creepage factor.

Polymer profile and parameter

IEC TS -3 – Chapters 8 & 9 give recommendations for polymer insulators profiles and parameter to take in account:

Alternating sheds and shed overhang;
Spacing versus shed overhang;
Minimum distance between sheds;
Creepage distance versus clearances;
Shed angle;
Creepage factor.

Pollution tests standards

Pollution tests on glass and porcelain insulators on a laboratory can be carried out for two main objectives:

To obtain information about the pollution performance of insulators (comparison of different insulator types/profiles)
To check the performance in a configuration as close as possible to the in-service one.

IEC prescribe procedures for artificial pollution tests applicable to ceramic and glass insulators for overhead lines.

Two categories of pollution test methods are recommended for standards tests:

Salt fog method in which the insulators is subjected to a defined ambient pollution:
Solid layer method in which a fairly uniform layer of a defined solid pollution is deposited on the insulators surface.

This standardized laboratory pollution test methods are not applicable for composite (polymeric) or RTV-coated insulators. A proposal for a test method for artificially polluted composite insulators is covered by the recent CIGRE TB 555: “Artificial Pollution Test for Polymer Insulators”.

In the case of naturally polluted insulators removed from the service a recent CIGRE TB 691 (WG D1.44), “Pollution Test of Naturally and Artificially contaminated insulators” summarizes the recent experience with 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 both ceramic (glass and porcelain) and composite insulators for both AC and DC.

The objective of these tests is a need for reliable diagnostic of naturally polluted insulators to evaluate their residual dielectric strength and also a general trend to make testing more cost-effective and time-efficient, even for the case of artificially polluted insulators.

A reduction of performance can be due to pollution in case of ceramic insulators or due to a combination of pollution and ageing in case of polymeric insulators. In both case cases the residual pollution strength should be quantified in terms of a flashover voltage, not a withstand voltage; because the withstand voltage does not provide the user by the information about the probability for flashover or standard deviation of flashover voltage.

Insulator testing stations

Sometime, the combination of the many variable environmental parameters which influence an insulator’s behaviour over its lifetime are difficult to artificially simulate and, moreover, to accelerate. The validity of laboratory tests is thus often questions as the procedures adopted may not take into account significant factors which would be encountered in service or they may over-emphasise others.
The evaluation of insulator performance in naturally polluted outdoor test stations is becoming more important and popular. Although involving a longer test duration, and still requiring care in the correct interpretation of the test data, the results tend to be accepted with more confidence.

Outdoor test stations is also a good tool for new insulation technologies in which there is as yet no technical or normative specification for its testing or characterization.

CIGRE Technical Brochure No. 333, “Guide for the establishment of naturally polluted insulator testing stations” serve as a general guide for the establishment of natural test stations which will facilitate the overall comparison of various insulators designs, the explorations of particular aspects of insulator performance and/or the selections of the most appropriate insulation for a particular application. It is relates specifically to insulators intended for use under AC conditions but aspects are applicable to DC stations as well.
Typical test aims may be one or more of the following:

To compare the performance of insulators of different design.
To compare the performance of insulators from different manufacturers.
To dimension insulators for a particular environment or application.
To examine the behaviour of insulators of different dielectric materials.
To compare the performance of insulators in different orientations.
To explore the affects of specific parameters such as profile geometries or insulators diameters.
To identify possible weaknesses or failure mechanisms of an insulator design.
To estimate the life expectancy of various insulators.
To serve as a qualification test for potential suppliers.
To establish the effectiveness and life of insulator treatments such as washing, greasing, silicone rubber coating, shed extenders, etc.
To assess the performance of other outdoor equipment insulation such as transformer bushings, surge arresters, cable terminations, etc..

The severity of the pollution and the prevailing climate must be examined and should be representative of conditions found on the system. As is the case for laboratory tests, an over-acceleration of the ambient stresses can produce misleading results. Contamination severity assessment by means of ESSD and NSDD measurements and/or directional dust deposit gauges should be undertaken to ensure that an appropriate site is selected.

Insulator test stations have a wide range of size and sophistication and may be categorized as:

Research station;
Simplified, on-line station;
In service Test Structure;
Mobile insulator Test Station.

Photo 1: Permanent insulator research station at Martigues (France) and examples of Verescence La Granja Insulators tests

Photo 3: In-service insulator test structure

Leakage current activity (include number of flashovers experienced), climatic effects and pollution severity are usually monitored. In addition, the performance of the test samples should be judged by a regular inspection of the insulators: Close visual examination of the surface; an assessment of the hydrophobicity of the dielectric material and the viewing of electrical activity.

OPERATION PARAMETERS

The main objective of the overhead line maintenance policy in an electric company is to maintain the number of fault outages at reasonable values. A Data Base with the principal information of the insulation is an efficiency tool for the evaluation of performance. The information that this database should contain is:

Type / Sub-type of string;
Type of insulation: Glass, ceramic, composite, coated glass, etc..
Sub-type of insulator: Standard profile, pollution profile…
Number of insulator per string;
Manufacturer of the insulation;
Insulator trazability data (Production order, production date…)
Standard;
Installation year;
Silicone manufacture / applicator;
Estimated end of life
Degradation environment: Normal, hard and very hard.

Several maintenance indicators are habitual used by the utilities:

Number of faults
Insulator breakage rate
Washing frequency

Also numerous methods of maintenance procedures are well-known by the utilities:

Aerial inspection
Patrol inspection
HD recording
Infrared inspection

The evolution of maintenance indicators together with the results of inspections linked with the data base of the line help both the decision making regarding the maintenance or replacement of the insulation and the valuation / comparison of the different types of materials, profiles and manufacturers quality.

Estimated end of life – glass and ceramic insulators

Insulators are high-technology products expected to work with high reliability over a long period of time. A great number of successfully balanced design parameter, analyzed in the previous chapters, choice of material as well as mastering of manufacturing processes are required to ensure this high long term reliability.

An insulator comes to the end of its working life either when it fails mechanically, flashover at unacceptably high frequency or gives evidence of deterioration to a condition likely to lower its factor of safety in service. All insulators are affected to some extend by impact, cycling but thermal and mechanical, deterioration from weathering and electrothermal causes, flexure and torsion, ionic motion, corrosion and dement growth.

Determinate the exactly time to replace if it necessary is important to optimise the cost of maintenance. There are a fairly large number of degradation modes. Some modes are easily detectable by visual inspection other, especially for porcelain insulators, need sophisticated methods. Degradation modes causes by easily detectable mechanism like slip of metal fitting, pin corrosion or surface erosion are considered to be reason for replacement of insulators.

CIGRE has established a test procedure to determinate the state of cap and pin and long-rod insulators and to decide on the safe time for their 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, .

This document stablishes a testing sequence with a number of non-destructive tests as visual tests (e.g. degree of corrosion) as well as dimension and thermal and the combined thermal-mechanical tests. This first series of tests is followed by destructive mechanical tests. A probability diagram based on normal distribution is used to analyse the failing load test results. With the probability (risk) for failure on the ordinate and the failing load on the abscissa, the failing load characteristics are represented as straight lines. In that way changes in strength become easily visible.

To help the user, the document includes a number of typical cases of analysis of test result called “Reference Scenarios”. They are useful for the assessment of the present condition of the insulator

The failing load characteristics are represented by:

A dashed line for the insulators sample tested when new.
A solid line for the insulators as-received from the line.
A dashed/dotted line for the insulators which have been submitted to the Thermal-mechanical test (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 shown in this example diagram are not representative of good quality products. Age and TMP-test have only negligible influence on good products.

Estimated end of life – Composite insulators

Parallel to this document, another Technical Brochure published by CIGRE assist to evaluate the technical conditions 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 descripted which enable a conclusion regarding the residual life-time of composite insulators of the same age and design family. The document also give indications of work for the case of an investigation a failure or a unit recognized as high risk and evaluation of units for research.

It is based on a recommended sequence of testing on samples taken out different stress zones of the line.

CONCLUSION

The selection of the insulator type is not a simple job, especially if the insulator will be install in a high polluted area.
Numerous documentations (IEC Standards, CIGRE Technical Brochure…) are available to help on the selection of the most appropriate insulator, for monitoring the behavior in operation and for the determination of the finite insulator life.

Different solutions are available to improve the insulator performance on high corrosion areas.

Several factors must be taken in account on the insulator type selection:

More effective design/material;
Maintenance costs: Inspection cost, clean cost, replacement costs, …
Expected service breakage rate to be guarantee/certify by the supplier;
Severity of consequences in case of failure (mechanical breakage, electrical failure,…);
Expected end of life.

BIBLIOGRAPHY

[1] IEC -471: International Electrotechnical Vocabulary. Part 471: Insulators

[2] IEC -1: Insulation co-ordination – Part 1: Definitions, principles and rules

[3] IEC : Artificial pollution tests on high-voltage ceramic glass insulators to be used on a.c. systems.

[4] IEC : Residual strength of string insulator units of glass or ceramic material for overhead lines after mechanical damage of the dielectric.

[5] IEC--1: Insulators for overhead lines with a nominal voltage above 1 000V: Ceramic or glass insulators units for a.c. systems – Definitions, test methods and acceptance criteria

[6] IEC/TS -1: Selection and dimensioning of high-voltage insulators intended for use in polluted condition – Part 1: Definitions, information and general principles

[7] IEC/TS -2: Selection and dimensioning of high-voltage insulators indented for use in polluted condition – Part 2: Ceramic and glass insulators for a.c. systems

[8] IEC/TS -3: Selection and dimensioning of high-voltage insulators indented for use in polluted condition – Part 3: Polymer insulators for a.c. systems

[9] IEC : Insulators for overhead lines – Composite suspensions and tension insulators for a.c. systems with a nominal voltage greater than 1 000 V – Definitions, test methods and acceptance criteria

[10] IEC-: Insulators for overhead lines with a nominal voltage above 1 000V: Ceramic or glass insulators units for d.c. systems – Definitions, test methods and acceptance criteria

[11] I. Gutman, Wallace Vosloo, “Development of Time-and Cost-Effective Pollution Test Methods Applicable for Difference Station Insulation Option”. IEEE Vol. 21, No. 6. TDEI submitted .

[12] M. Marzinotto, J-M. George, S. Prat, C. Lumb, F. Virlogeux, I. Gutman, and J. Lundengard, “Field Experience and Laboratory Investigation of glass Insulators Having a Factory-Applied Silicone Coating”, TDEI submitted .

[13] I. Gutman, J. Shamsujjoha, C. Lumb, J-M. George, and S. Roude: “Investigation of Rapid flashover solid layer pollution testing as an alternative to current standard method”, IEEE ISEI-, paper 17 pp.73-77

[14] CIGRE Task Force 33.04.07, “Natural and artificial ageing and pollution testing of polymeric insulators” CIGRE Technical Brochure No. 142,

[15] CIGRE WG B2.03, “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,

[16] CIGRE WG B2.03, “Guide for the establishment of naturally polluted insulator testing stations” CIGRE Technical Brochure No. 333,

[17] CIGRE WG B2.21, “Guide for the assessment of composite Insulators in the laboratory after their removal from service”. CIGRE Technical Brochure No. 481,

[18] CIGRE WG D1.44, “Pollution Test of Naturally and Artificially contaminated insulators” CIGRE Technical Brochure No. 691,

[19] Javier GARCIA /Philippe Platteau. Glass insulators in polluted environment: design, test, experiences and benchmarking with other materials. Cigre regional meeting outdoor insulation. Tunisia,

[20] R. García Fernández, M.A. Perez Louzao, I. Serrano “REE’s insulator global maintenance policy”, CIGRE ; B2-206.

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