According with IEC-60050-471, an insulator is: “a device intended for electrical insulation and mechanical fixing of equipment or conductors which are subject to electric potential differences”. An insulator is therefore a mechanical support. Once that basic requirement has been fulfilled, the next key issue relates to the electrical requirements necessary to offer good service performance. But several different types of insulators are available for application on new overhead transmission lines (OHTL) – from glass and porcelain string insulators, to porcelain long-rods to composite insulators. Within each category, there are also a range of designs, quality levels, materials and prices. There are also remedial measures to improve performance of insulators operating on lines in areas of high pollution, e.g. special shed profiles, RTV silicone-coatings. Given these options, is there always one best material or best design for any particular service environment? Moreover, what criteria must engineers take into account when deciding on the optimal insulator material for a new project and which parameters will help them best evaluate expected service performance? Finally, how can end of life of an insulator be predicted?
There are no simple answers to these questions. Still, it is possible to identify different steps engineers should consider when evaluating and comparing different insulator options. This edited contribution to INMR by Javier García of La Granja Insulators in Spain reviews some of the basic considerations when selecting an insulator for application on HV overhead lines.
An insulator is primarily a mechanical support and only once all mechanical aspects of a design have been finalized should electrical characteristics be addressed. If the insulator does not support the line, electrical issues mean little. In fact, mechanical characteristics are so vital to function that they constitute the only real common factor when selecting an insulator, irrespective of type. Another issue to take into account is consequence of mechanical failure: is it only loss of leakage distance or is it a line drop? IEC 60797, for example, establishes mechanical residual test methods and acceptance criteria for both glass and porcelain string insulators under breakage of the dielectric component. The user must then determine maximum loading that a line will apply to the insulator, including weight of conductor and hardware, ice and wind loading as well as any other load factors.
Suspension insulators are rated in terms of their Specified Mechanical Load (SML) and manufacturers typically recommend that insulators never be loaded to more than 50% of their SML. As such, an insulator’s SML rating is basically 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 using statistical criteria. Routine test load is the proof load applied to each unit and also the maximum load that an insulator should see in service. IEC 60383-1 and IEC 61109 establish mechanical test methods and acceptance criteria for porcelain or glass insulators and composite polymeric insulators respectively.
Electrical characteristics of an insulator are determined largely by the air space around it and defined mainly by arcing distance, i.e. 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 both based on dry arcing distance. Some might argue that wet power frequency withstand/ flashover characteristics are determined by leakage distance. But that only holds true within a narrow band and the main role of leakage distance is as a contributing factor. IEC 60071-1 recommends withstand voltages associated with the highest voltage for equipment
Techniques for Selecting & Dimensioning HV Insulators for Polluted Areas
The latest edition of series IEC TS 60815 proposed techniques for selecting and dimensioning high voltage insulators. After a well-established process, these enable determination of the most efficient insulation. Three alternative approaches are recommended to select suitable insulators based on system requirements and service conditions:
• Approach 1: Use past experience
• Approach 2: Measure & test
• Approach 3: Measure & design
Applicability of each approach depends on available data, time and project economics. Some parameters required for these different approaches are:
Determining Reference Unified Specific Creepage Distance
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 provided for use with Approach 3 (i.e. measured and design). If SPS is available, it is recommended to take the RUSCD that corresponds to the position of the SPS measurements within the class, following the curve in Fig. 1.
For Type A pollution (i.e. inland, desert or industrially polluted areas), SPS is calculated from ESDD/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).
Choice of Profile: Glass & Porcelain Insulators
Different types of insulators and different positions along the same type of insulator accumulate pollution at different rates – even in the same environment. In addition, variations in nature of pollutants can make some insulator shapes more effective than others. Table 1 of IEC TS 60815-2 offers a summary of the relative advantages and disadvantages of the different profiles with respect to pollution performance.
Profile Suitability: Glass & Porcelain Insulators
Tables 2 & 3 of IEC TS 60815-2 give simple merit values for porcelain and glass insulator profiles. Table 2 gives suitability of profile, relative to standard profile, assuming the same creepage distance per unit or string while Table 3 does so assuming same insulating length. There is also a 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 little or no non-soluble components.
Also IEC TS 60815-2 provides profiles parameters to take in account:
• Alternating sheds & shed overhang;
• Spacing versus shed overhang;
• Minimum distance between sheds;
• Creepage distance versus clearances;
• Shed angle;
• Creepage factor.
Composite/Polymeric Insulator Profiles & Parameters
Chapters 8 & 9 of IEC TS 60815-3 offer recommendations for composite/polymeric insulator profiles and the parameters to take into account when specifying them, including:
• Alternating sheds & shed overhang;
• Spacing versus shed overhang;
• Minimum distance between sheds;
• Creepage distance versus clearances;
• Shed angle;
• Creepage factor.
Pollution Test Standards
Pollution tests on glass and porcelain insulators in a laboratory are carried out with two main objectives:
1. To obtain information about the relative pollution performance of different insulators (i.e. comparing alternative types and profiles);
2. To verify performance in a configuration that is as close as possible to that to be used in-service.
IEC 60507 prescribes procedures for artificial pollution tests applicable to porcelain and glass insulators for overhead lines. Two categories of pollution test methods are recommended for standard tests:
• Salt fog method, whereby insulators are subjected to defined ambient pollution;
• Solid layer method, whereby a fairly uniform layer of a defined solid pollution is deposited onto the insulator surface.
But these standardized laboratory pollution test methods are not applicable to composite (polymeric) insulators nor to RTV-coated porcelain or glass 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 service, a recent CIGRE TB 691 (WG D1.44), “Pollution Test of Naturally and Artificially contaminated insulators” summarizes recent experience with the so-called ‘rapid flashover test’, e.g.:
• Rapid flashover test (RFO, based on the IEC 60507 solid layer test);
• Quick flashover (QF, based on the IEC 60507 salt fog test).
Both tests can be applied for glass and porcelain and also for composite insulators for AC and DC. The objective behind these tests is meeting the need for a reliable diagnostic of naturally polluted insulators that will evaluate their residual dielectric strength. This is part of the general trend to make testing as cost-effective and time-efficient as possible – even in the case of artificially polluted insulators. Reduction in performance can be due to pollution for ceramic insulators or due to a combination of pollution and ageing for polymeric insulators. In both cases, residual pollution strength should be quantified in terms of flashover voltage, not withstand voltage. Withstand voltage does not provide the user with information about the probability of flashover or about standard deviation of flashover voltage.
Insulator Test Stations
Sometimes the combination of many environmental parameters that influence behaviour over the service life of an insulator are difficult to artificially simulate and even more difficult to accelerate. The validity of laboratory tests in such cases is therefore questioned. For example, procedures adopted may not have taken into sufficient account significant factors encountered in service; or they may have over-emphasized others. For these reasons, evaluation of insulator performance in naturally polluted outdoor test stations has become more popular. Although involving longer test duration and requiring care in correctly interpreting test data, results tend to be viewed with greater confidence. An outdoor test station is also a valuable tool for new insulation technologies for which there is not yet any technical or normative specification for testing or characterization.
CIGRE Technical Brochure No. 333, 2007 “Guide for the establishment of naturally polluted insulator testing stations” serves as a general guide for establishment of natural test stations that facilitate comparing insulators designs, exploring aspects of insulator performance and selecting the most appropriate insulation for a specific application. While it relates specifically to insulators intended for use under AC, aspects are applicable to DC as well. Typical goals can be one or more of the following:
• Comparing performance of insulators of different design;
• Comparing performance of insulators from different manufacturers;
• Dimensioning insulators for a particular environment or application;
• Examining the behaviour of insulators of different dielectric materials;
• Comparing performance of insulators in different orientations;
• Exploring the effects of parameters such as profile, geometry or diameter;
• Identifying possible weaknesses or failure mechanisms of an insulator design;
• Estimating life expectancy of various insulators;
• Serving as a qualification test for potential suppliers;
• Establishing effectiveness and expected service life of treatments such as washing, greasing, silicone rubber coating, shed extenders, etc.;
• Assessing performance of other outdoor equipment insulation such as transformer bushings, surge arresters, cable terminations, etc.
Severity of the pollution and the prevailing climate must be carefully studied and should be representative of conditions found on the system. As is the case for laboratory tests, over-acceleration of ambient stresses can produce misleading results. Contamination severity assessment by means of ESSD and NSDD measurements and/or directional dust deposit gauges should all be undertaken to ensure that an appropriate site is selected.
Insulator test stations have a wide range of sizes and levels of sophistication and can be broadly 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 usually monitored. In addition, performance of test samples should be judged by regular inspection, including: close visual examination of surfaces; assessing hydrophobicity of the dielectric and viewing any electrical activity.
Potential Insulator Problems Due to Pollution: Corrosion
Insulator Fitting Corrosion Mechanism
Corrosion can occur if the surface of insulators is polluted and subjected humidity. Whenever the surface is covered by a deposit of wet pollution, leakage currents start – 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 in string insulators. Impact of leakage current is most harmful when frequency and duration of wet periods are high (e.g. tropical climates) and also when pollution finds a hygroscopic surface. Hence the importance of inert contaminants that absorb or retain humidity.
Risk of corrosion is more important on DC than AC voltage at the same site due to unidirectional current and electrostatic phenomena that cause the greatest pollution deposition. For insulators, dominant electrolytic corrosion effects add to those of atmospheric corrosion and those due to the formation of oxidizing agents from the presence of arcs near the fittings. Corrosion phenomena result in:
a) attack on galvanization.
b) attack on internal steel structure, with formation of a conductive rust deposit that can flow onto the dielectric.
The most severe cases of corrosion occur in tropical areas near to marine pollution and also in areas where the pollution mechanism is by dust accumulation over long dry periods yet with periodic high humidity.
Phenomena Linked to Corrosion
Insulator fitting corrosion can have the following impact on insulators:
• Affecting mechanical resistance: this applies particularly to the pin when the corroded section becomes reduced (e.g. reduction of pin diameter);
• Affecting electrical resistance due to formation of a deposit of rust on the surface. This deposit could also cause damage the insulation component due to concentration of electric field around this new electrode;
• Causing breakage of the dielectric due to expansion of the corroded pin. This phenomenon is specific for porcelain cap & pin insulators.
Remedies to Improve Corrosion Resistance
Metal part protections have been developed to avoid or delay corrosion phenomena.
These consist of reinforced galvanized fittings and of a sacrificial zinc sleeve protection.
• Reinforced galvanized fittings. IEC-60383-1 Chapter 26 standardizes minimum average coating mass for metal fitting of insulators: 600g/m2 (85µm). This value can increase to 140µm in the case of insulators installed in areas of high corrosion risk. This will extend estimated end of life.
• The 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 by-products.
IEC-61365 specifies minimum requirements for the zinc sleeve but this can be increased to improve performance.
Also, IEC-61365 specifies a test method for zinc sleeve control. Future standardization work would have to include zinc sleeve requirements and tests methods for IEC-60383-1 for AC lines.
The main objective of an overhead line maintenance policy by an electricity supply company is to maintain number of fault outages at reasonable values. A database with key information on line insulation is an effective tool to evaluate performance. Information this database should contain includes:
• Type/Sub-type of string;
• Type of insulation: Glass, ceramic, composite, coated glass, etc.;
• Sub-type of insulator: Standard profile, pollution profile;
• Number of insulators per string;
• Manufacturer of insulation;
• Insulator traceability data (production order, date, etc.);
• Applicable standards;
• Year of installation;
• Silicone manufacturer/applicator;
• Estimated end of life;
• Degradation environment: Normal, difficult or extreme.
Several maintenance indicators are also used, such as:
• Number of faults;
• Insulator breakage rate;
• Washing frequency.
Also different methods of maintenance are available:
• Aerial inspection;
• Patrol inspection;
• HD recording;
• Infrared inspection.
These maintenance indicators, together with results of inspections linked with the database of a line, help decision-making in regard to maintenance or replacement of insulation. They also allow comparison of different types of materials, profiles and manufacturer quality levels.
Estimated End of Life: Glass & Porcelain Insulators
Insulators are high-technology products expected to perform with high reliability over long periods of time. Many carefully balanced design parameters, analyzed above, as well as proper choice of material anf mastering of the relevant production process are required to ensure such long-term reliability. Still, an insulator reaches its end of service life when it fails mechanically, flashes over at unacceptably high frequency or shows ample evidence of deterioration. All insulators are affected to some extent by impact, thermal and mechanical cycling, deterioration from weathering, flexing and torsion, ionic motion, corrosion and cement growth.
Determining the exact time to replace an insulator is important in order to optimize cost of maintenance. There are a number of degradation modes, some easily detectable by visual inspection while other (especially for porcelain insulators), require more sophisticated methods. Degradation caused by easily detectable mechanisms such as slipping of metal fitting, pin or surface erosion are considered valid reasons for replacing insulators. CIGRE established a test procedure to determine the state of cap & pin and long-rod insulators and to decide the best 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, 2006. This document establishes a testing sequence with a number of non-destructive visual tests (e.g. degree of corrosion) as well as dimensioning, thermal and combined thermo-mechanical tests. This series of tests is followed by destructive mechanical tests. A probability diagram based on normal distribution is used to analyse 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.
In the above example, Reference scenario F1, the reductions in strength shown are representative of poor quality products. Ageing and TMP-test should have negligible influence on high quality products.
Estimated End of Life: Composite Insulators
A Technical Brochure published by CIGRE assists evaluating the 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 that enable some conclusion regarding residual life of composite insulators of the same age and design. This document also gives indications in case of investigation of a failure or of a unit recognized as high risk to fail. Recommendations are provided on a sequence of tests to be performed on samples removed from different stress zones along the line.
The selection of the most suitable type of insulator is not a simple task, especially if it will be installed in a highly polluted area. Fortunately, several documents (IEC Standards, CIGRE Technical Brochures) are available to help selection of the most appropriate insulator, monitoring behaviour in service and determining end of life. Different solutions are available to improve insulator performance in high corrosion areas. Several factors must be taken into account in selecting the optimal insulator type:
• Most effective design/material;
• Maintenance costs, i.e. inspection, cleaning, replacement;
• Expected service breakage rate to be guarantee/certified by the supplier;
• Severity of consequence in case of failure (mechanical breakage, electrical failure);
• Expected end of life.