Increasing HVDC voltages requires bigger and heavier equipment, increasing the mechanical strength required from support insulators. These must support high compression forces, combined with increased flexural stresses coming from seismic requirements, high wind-loads or short circuit forces due to the magnetic coupling between reactors.

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Resin-impregnated paper bushings offer a safer solution in terms of fire risk in case of failure and require reduced maintenance. But they can suffer from thermal degradation and moisture penetration in aged transformer oil with high water content. They also need careful storage to prevent humidity absorption.

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While it has long been known that corona can lead to insulation failure, not all aspects of the problem are still fully understood. These include magnitude and duration of corona to initiate degradation, best detection methods and most suitable tests to predict performance in its presence.

When it comes to composite insulators, corona activity can originate from hardware, voids within the material or from interfacial defects. Most of the light produced by such corona has a wavelength shorter than 400 nm and therefore falls in the UV range. By contrast, most solar radiation is in the 400-700 nm visible range – the shorter wavelengths filtered by the earth’s ozone layer. In fact, some peaks in the UV region of the corona spectrum match or exceed those in the solar spectrum.

Polymeric materials are more susceptible to degradation from UV produced by corona than from solar radiation, particularly if the corona takes place close to the material. Corona ruptures stable oxygen molecules (O2) to create radicals that combine with the molecules to form ozone (O3). The ozone then attacks double and triple bond sites in elastomeric materials such as silicone rubber or EPDM. The result is cracking. Even tiny amounts of ozone in the ppm range are sufficient to initiate cracks, although the time required for this may depend on material formulation.

Although modern elastomers are typically stabilized against this threat, some eventually succumb to ozone attack should the concentration become sufficiently high. Corona also produces oxalic and nitric acids in the presence of surface moisture from humidity, dew or fog. Depending on pH, this can also locally degrade polymers.

RHM International

USA

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Italy

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Corona can even ‘drill’ holes in a material, suggesting that degradation is not solely due to chemical attack by ozone. Past research calculated the temperature at the tip of the discharge and showed it to be high enough to cause ‘evaporation’ of even inorganic materials. There is also suggestion of mechanical attack, much like sandblasting, due to the impact of repeated discharges on a material. It is rare in power engineering that any one physical phenomenon has the potential to trigger so many modes of degradation.

For example, past research on impact of corona involved ground-based inspection of composite insulators on 115 kV, 230 kV and 500 kV lines in the U.S. southwest using a specialized camera as well as simple binoculars. Several 230 kV towers were equipped with composite insulators with and without corona rings on adjacent phases. The 115 kV insulators did not have corona rings whereas the 500 kV insulators had rings on the line as well as on the ground end hardware.

Given the dry conditions of the service territory, the utility operating these lines had decided not to replace the 230 kV insulators not equipped with corona rings. This allowed comparative study of the effects of corona on composite insulators since all other factors were the same (i.e. same design, material, manufacturing details, location and system voltage). The corona observed on these insulators, many of which had been in service for over 25 years, was sporadic and originated from hardware.

The 230 kV insulators being evaluated comprised 3 generations of composite insulator technology. Some were removed for further examination and it was found that the shed closest to the line end without a corona ring exhibited minor to serious changes in the form of hardening, cracking and discoloration. It was clear that these insulators were approaching their end of useful life and indeed would have probably already failed in locations having more precipitation.

By contrast, none of the insulators equipped with corona rings showed such degradation. This demonstrated forcefully that when it comes to composite insulators, design and need for a corona ring depend not only on voltage level but also service environment. In the case of insulators in environments with heavy contamination, high altitude or frequent wetting, it is prudent to have a line end corona ring even at lower transmission voltages.

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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 past contribution to INMR by Javier García at La Granja Insulators in Spain, reviewed important considerations when selecting the most suitable type of insulator for application on an overhead transmission line.

Wenzhou Yikun Electric

China

Norsk Teknisk Porselen Products AS

Norway

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Mechanical Considerations

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 60797 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 60383-1 and IEC 61109 respectively establish mechanical test methods and acceptance criteria for porcelain insulators, glass insulators and composite insulators.

Electrical Considerations

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 60071-1 recommends the withstand voltage associated with the highest equipment voltage.

Selecting & Dimensioning HV Insulators for Polluted Service Areas

The past edition of the IEC TS 60815 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 60815-1. 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 60815-2 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 60815-2 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 60815-2 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 60815-3 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 60507 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 60507 solid layer test);
• Quick flashover (QF, based on IEC 60507 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, 2007 “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.

Corrosion on Insulators

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.

Phenomena Linked to Corrosion of Metal Parts

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-60383-1 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-61365 specifies minimum requirements for a zinc sleeve but this can also be improved to increase corrosion performance.

Also, IEC-61365 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-60383-1 (for AC lines).

Operating Parameters

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.

Estimated End of Life: Glass & Porcelain Insulators

Insulators are expected to perform with high reliability over long periods of time. Various design parameters, as discussed above, choice of material as well as mastering the manufacturing process are all required to maintain 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 time to replace insulators is key to optimizing maintenance costs and there are several potential modes of degradation. 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, 2006 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.

Estimated End of Life: Composite Insulators

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.

Powertech Labs Inc.

Canada

ICMET Craiova – National Institute For Research, Development And Testing In Electrical Engineering

Romania

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Conclusions

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 consdiered 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

[1] IEC 60050-471: International Electrotechnical Vocabulary. Part 471: Insulators
[2] IEC 60071-1: Insulation co-ordination – Part 1: Definitions, principles and rules
[3] IEC 60575: Artificial pollution tests on high-voltage ceramic glass insulators to be used on a.c. systems.
[4] IEC 60797: Residual strength of string insulator units of glass or ceramic material for overhead lines after mechanical damage of the dielectric.
[5] IEC-60383-1: 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 60815-1: Selection and dimensioning of high-voltage insulators intended for use in polluted condition – Part 1: Definitions, information and general principles
[7] IEC/TS 60815-2: 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 60815-3: Selection and dimensioning of high-voltage insulators indented for use in polluted condition – Part 3: Polymer insulators for a.c. systems
[9] IEC 61109: 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-61365: 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 2014.
[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 2014.
[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-2012, 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, 1999
[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, 2006
[16] CIGRE WG B2.03, “Guide for the establishment of naturally polluted insulator testing stations” CIGRE Technical Brochure No. 333, 2007
[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, 2011
[18] CIGRE WG D1.44, “Pollution Test of Naturally and Artificially contaminated insulators” CIGRE Technical Brochure No. 691, 2017
[19] Javier GARCIA /Philippe Platteau. Glass insulators in polluted environment: design, test, experiences and benchmarking with other materials. Cigre regional meeting outdoor insulation. Tunisia, 2010
[20] R. García Fernández, M.A. Perez Louzao, I. Serrano “REE’s insulator global maintenance policy”, CIGRE 2014; B2-206.

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Collecting experience with DC stress began some 25 years ago. The common result of all these investigations was: if identical test parameters for both AC and DC are chosen, DC stress leads to more intense tracking and erosion than at AC, especially so if the positive polarity is used during testing.

This higher severity can result in too harsh a testing of investigated materials and their disqualification, in a non-differentiable manner, which often contrasts to actual service performance. Therefore, various modifications of test parameters such as reduction in test voltage or flow rate have been conducted to decrease test severity and evaluate and differentiate between materials.

This edited past contribution to INMR by C. Baer, J. Lambrecht and K. Hindelang of Wacker Chemie in Germany reported on past research comparing erosion resistance of silicone rubber materials under DC stress.

Guangzhou MPC Power International Co. Ltd.

China

EMCO Industries

Pakistan

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Past work within CIGRE WG D1.72 has concentrated on basic investigations with the IPT to develop a standardizable test method in IEC with respect to representativeness, repeatability, reproducibility and cost efficiency.

The general aim of the IPT is an accelerated testing of tracking and erosion resistance by continuous discharge activity. Under AC, test parameters, i.e. voltage, flow rate and series resistance, are set such that continuous discharge activity is achieved. Such parameter studies have so far been missing for DC application. Afterwards, parameters for DC set-up were defined during pre-investigation such that continuous discharge activity was achieved and overly severe test conditions were prevented.

In case of severe erosion, formation of degradation products can influence test conditions by interrupting continuous discharge activity. This is not desirable.

Two round robin tests with constant voltage, one at 4.5 kV DC and another at 3.5 kV DC, both at positive polarity, using one silicone elastomer formulation were conducted at 7 test laboratories.
It was observed that test results between different laboratories but also within one test series show a wide spreading of the results from passing the test with slight erosion only or failing by digging erosion and hole formation. Thus, a statistical evaluation showed a mixed distribution as shown for the maximum erosion depth in Fig. 1. Further analysis showed that the degradation process is non-linear with time and is accelerated once digging erosion was initiated. Therefore, further tests were conducted with reduced test durations of 2 h and 4 h (Fig. 2) and it could be shown that most specimens fail between 2 h and 4 h of the test time. Thus, testing with application of constant voltage was found to be unsuitable for an international material testing standard.

To reduce the scatter and to avoid the occurrence of a mixed distribution of test results, a stepwise increase of the test severity was thought to be advantageous in evaluating and ranking different materials under DC voltage stress. Therefore, the aim of this contribution is an evaluation of the DC tracking and erosion resistance of different silicone elastomer formulations by using a stepwise increase of the test voltage. Results are compared with investigations conducted under AC stress according to IEC 60587. For a stepwise increase of the test voltage, a utility step-test specification with up to seven voltage levels is taken. Furthermore, a simplified method with three voltage levels only is applied.

Test Methods & Procedures

Additional tests under DC voltage of positive and negative polarity involving different silicone elastomers were conducted according to the following parameters:

• Test setup and test parameters according to IEC 60587 (see Fig. 3);

• Flow rate of 0.3 ml/min for any voltage level
and procedures:

• Stepwise increase in test voltage in seven steps, starting from 2.15 kV DC+ and increasing every 60 min for 0.25 kV until 420 min of test duration (10 samples per material formulation);

• Stepwise increase in test voltage in seven steps, starting from 2.90 kV DC- and increasing every 60 min for 0.25 kV until 420 min of test duration (10 samples per material formulation);

• Stepwise increase in test voltage in three steps, starting from 2.45 kV DC+ and increasing every 120 min for 0.5 kV until 360 min of test duration (5 samples per material formulation).

As reference, an AC test was carried out with constant voltage application at 4.5 kV for a maximum of 360 min with parameter selection according to IEC 60587. This was done for the material formulations chosen as well (i.e. 5 specimens per material formulation).

As reference, an AC test was carried out with constant voltage application at 4.5 kV for a maximum of 360 min with parameter selection according to IEC 60587. This was done for the material formulations chosen as well (i.e. 5 specimens per material formulation).

Table 1 shows the different silicone elastomer formulations tested (VMQ, vinyl-methyl silicone elastomer).

Materials have been selected such, that the erosion resistance is weakening within the groups A and B, from material VMQ 1 towards VMQ 3 and VMQ 4 towards VMQ 6 each.

So, while VMQ 1 and VMQ 4 are established material formulations with a good resistance to tracking and erosion and positive long-term service experience, VMQ 2 and VMQ 3 as well as VMQ 5 and VMQ 6 are modified variants with high and low hardness of peroxidic respectively addition curing systems and have been created for these investigations only.

A voltage level counts as passed if the leakage current does not exceed 60 mA for 2 s, no deep erosion with formation of a hole occurs, and samples don’t start burning.

For a better comparison of the tested materials, the time to failure, the mass loss and the maximum erosion depth have been measured. Since silicone elastomers don’t show typical tracking characteristics but erosion, it was decided not to evaluate their track length.

Test Results for Silicone Elastomer Formulations Investigated
Reference results (4.5 kV AC stress acc. to IEC 60587) and results of all DC step tests are presented and discussed in the following:

• time to failure (see Fig. 4);

• mass loss (see Fig. 5);

• maximum erosion depth (see Fig. 6).

Evaluation of AC Test Results
As expected, at 4.5 kV AC, VMQ 1 and VMQ 4 pass the tracking and erosion test by showing low average values of the mass loss and moderate values for the maximum erosion depth. These materials are classified as tracking and erosion resistant with class 1A4,5.

In comparison to these results, the other material formulations do not pass the 4.5 kV AC tracking and erosion test and fail at least for one sample by exceeding the leakage current criterion with a switch-off before the full test duration of 360 min is reached. Additionally, VMQ 5 and VMQ 6 show severe erosion with a high mass loss and formation of a hole.

Evaluation of DC Test Results
Under DC voltage, most of the materials show more severe erosion for the chosen test procedures compared to tests at AC voltage.
By comparing the three different DC test procedures, the procedure with seven different voltage levels with positive voltage results in the most severe erosion, followed by testing with three voltage levels at positive DC voltage stress. The conducted tests under negative DC show less severe erosive degradation than the tests at positive polarity, which is consistent with previous investigations ‎[8].

The test procedure with three voltage steps at positive polarity was conducted in the attempt to reduce the required test effort, the test severity, and the spread of the test results. This was achieved for the majority of the tested material versions and allows a better differentiability between the tested materials.

Although the silicone elastomer formulations do not exactly show the same performance and ranking compared to AC voltage, both VMQ 1 and VMQ 4 show the highest time to failure, lowest mass loss and lowest maximum erosion depth within their material groups, especially with respect to the DC step test with 3 steps.

Material formulations VMQ 5 and VMQ 6 show a quite different performance at AC and DC stress. While both materials perform quite poor under AC stress with respect to the mass loss and the maximum erosion depth, they show a better performance under DC stress, also compared to VMQ 2 and VMQ 3. The root cause for this is not clear yet and needs to be further investigated to avoid improper material selection

Summary & Conclusions

As a conclusion from the conducted investigations, and in line with previous investigations, a separate DC tracking and erosion test is seen to be required to properly select and rank materials for high-voltage DC outdoor application since test results conducted under AC stress cannot be transferred to DC application.

As it could be observed in recent CIGRE WG D1.72 activity with a high scatter of test results within one test series in one but also between different laboratories, the current approach with constant voltage application is not seen to be sufficient and satisfying to fulfil the requirements to become a standardised IEC test method.

A potential solution is seen in the application of a stepwise voltage increase which increases the test severity during testing and shall reduce the high test scatter. In this contribution, three different test procedures for a stepwise increase of a DC test voltage of both polarities were presented. While testing with seven voltage levels at positive DC voltage resulted in the most severe erosion during these investigations, much less severe erosion was observed for the negative polarity. From this perspective, it is concluded that testing with negative polarity may not be required and testing at positive polarity only may be sufficient, which is consistent with previous investigations.

It could be shown furthermore that testing with three DC voltage levels and positive polarity reduces the test effort, test severity, the scatter of test results and may therefore allow a differentiation and ranking between investigated material formulations.

Further work basing on a stepwise increase of the DC test voltage as presented in this paper is planned in a future CIGRE D1 WG including international round robin testing and finally preparing a recommendation for an internationally standardized test method in IEC.

References
[1] IEC TR 62039:2021 – Selection guide for polymeric materials for outdoor use under HV stress, March 2007.
[2] IEC 60587:2007 – Electrical insulating materials used under severe ambient conditions – Test methods for evaluating resistance to tracking and erosion, Mai 2007.
[3] G. P. Bruce, S. M. Rowland, A. Krivda: “Performance of Silicone Rubber in DC Inclined Plane Tracking Tests”, IEEE TDEI, Vol. 17, No. 2, pp. 521-532, 2010.
[4] C. Bär, R. Cervinka et al.: “On a Comparative Evaluation of the Retention of the Hydrophobicity and the Tracking Resistance of Silicone Elastomers under AC and DC Stresses”, 17th International Symposium on High Voltage Engineering, Hannover, 22nd -26th of Au-gust 2011.
[5] R. A. Ghunem, S.H. Jayaram, E.A. Cherney, “Inclined Plane Initial Tracking Voltage for AC, +DC and –DC”, Conf. Record of the 2012 IEEE Int’l. Sympos. Electr. Insul. (ISEI), pp. 459-463, 2012.
[6] J. V. Vas, B. Venkatesulu, M. J. Thomas: “Tracking and Erosion of Silicone Rubber Nanocomposites under DC Voltages of both Polarities”, IEEE TDEI, Vol. 19, No. 1, pp. 91-98, 2012.
[7] R. A. Ghunem, S. H. Jayaram and E. A. Cherney, “Erosion of Silicone Rubber Composites in the AC and DC Inclined Plane Tests”, IEEE Trans. Dielectr. Electr. Insul., Vol. 20, No. 1, pp. 229- 236, 2013.
[8] CIGRE WG D 1.27, TB 611: “Feasibility Study for a DC Tracking and Erosion Test“, 2015.
[9] DLT 810-2002 – Technical specification for ± 500 kV D.C long rod composite insulators, 2002-09-01.
[10] “IEEE Guide for DC Inclined Plane Tracking and Erosion Test for Outdoor Insulation Applications,” in IEEE Std 2652-2021, vol., no., pp.1-22, 7th of October 2021
[11] S. Kuehnel, S. Kornhuber, J. Lambrecht, K. Hindelang: Evaluation of the Tracking and Erosion Resistance at DC-Stress using the Example of Silicone Elastomers, 8th Conference on Silicone Insulation, Burghausen, Germany, 2022.

The post Comparative Evaluation of Erosion Resistance of Silicone Elastomers Under DC Stress appeared first on .

This edited contribution to INMR by Igor Gutman presented a comprehensive review of this topic by diverse international experts representing manufacturers, users, consultants and researchers. The aim was to summarize the state-of-the-art of pollution testing of HTM insulators across the globe.

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EB Rebosio SRL, A Gruppo Bonomi Company

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The first CIGRE document on this topic (Technical Brochure (TB) 142 published in 1999) presented a historical review of pollution test methods for polymeric insulators. A subsequent CIGRE TB 158 appeared in 2000 and reviewed accumulated knowledge on polluted insulators. Test procedures for polymeric insulators were summarized in a short paragraph. CIGRE TB 361, published 8 years later, was dedicated to pollution dimensioning of insulators for AC and a special Appendix for laboratory test methods for polymeric insulators was included.

A modified version of the Solid Layer Test was outlined and considered testing various levels of hydrophobicity from ‘worst’ through ‘intermediate’ to ‘best’. In 2011, two other CIGRE TBs (455 and 481) were issued and both referred to pollution test methods for polymeric insulators used in practice, mostly for testing under natural pollution. The salt fog test using the rapid procedure (Quick Salt Fog) and the modified Solid Layer Test were recommended for polymeric insulators. CIGRE TB 518 was published in 2012 and dedicated to pollution dimensioning of insulators for DC. It mentioned that several organizations have utilized modified test methods based on existing standards for ceramic insulators.

The first detailed description of an entirely new test method arrived in the long-anticipated CIGRE TB 555 (published in 2013) with its pre-conditioning procedure before pollution application, which included pollution by dry kaolin powder. Two days of elapsed time between the pollution and voltage test was proposed to simulate recovery of hydrophobicity. According to TB 555, while this procedure met CIGRE/IEC requirements for testing, several aspects warranted further consideration. Among these was wetting rate and applicability for DC.

A published document on this issue, i.e. CIGRE TB 691, investigated aspects of rapid methods for pollution testing of ceramic as well as composite insulators for both natural and artificial pollution. The conclusion was that the Rapid Flashover Test (simulating solid layer pollution) and the Quick Flashover Test (simulating wet salt fog pollution) can both be applied for ceramic and for composite insulators and for both AC and DC.

In short, CIGRE recommendations can be summarized as follows:

1. Both Modified Solid Layer and Salt Fog Procedures are applicable;
2. Different hydrophobicity surface conditions should be considered.

Pollution Testing of HTM Insulators

Experience from Sweden

Test Method

The HV laboratory at STRI in Sweden (now Hitachi Energy) has been at the forefront of developing pollution test methods. Initial focus since 2000 was on test methods that do not require conditioning of polymeric insulators, e.g. the Dry Salt Layer Method and the Dust Cycle Method. Since 2005, STRI began using modifications of the Solid Layer Test from IEC 60507, with addition of pre-conditioning for pollution testing of HTM insulators. This method, based on pre-conditioning by dry kaolin, was finally developed and has since been used. Elapsed time between pollution and voltage testing to simulate recovery of hydrophobicity was typically 2 days for composite insulators and 3 days for RTV-coatings. After this, the voltage for flashover tests was applied either by the Up-and-Down or the Rapid Flashover Procedure.

Practical Applications

Many test results were obtained and used in the statistical dimensioning procedure included in IEC 60815-1 (see illustration of the stress/strength concept in Fig. 1 left). A cumulative distribution function describing the strength of the insulation is normally obtained via laboratory tests. Fig. 1 (right) presents typical laboratory results in the form of a graph depicting the 50%-flashover voltage gradient (50%-flashover voltage divided by insulation length) versus SDD. The strength curve (Fig. 1 left) can be created based on this pollution flashover curve and standard deviation of flashover voltage.

Several collaborative international projects have used this stress/strength concept supported by the flashover voltage curves shown in Fig. 1 using the above test procedure (illustrations are shown in Fig. 2), e.g.:

• South Africa: Refurbishment of AC and DC OHLs in Western Cape. The re-installation was completed and the network has been performing well;

• Norway: A feasibility study was done to upgrade a 300 kV AC OHL to DC, also involving selection of insulation for a new ±525 kV DC OHL, which is now in full operation;

• Sweden: Refurbishment of a 400 kV AC substation;

• United Kingdom: Dimensioning for a new 400 kV AC OHL with T-Pylon towers, where a diamond-like insulation structure of silicone rubber insulators is used.

Experience from France

Test Methods

Sediver operates two high voltage laboratories in France, both having pollution testing capabilities and practical experience in IEC salt fog and solid layer test methods as well as their modifications up to 350 kV AC or DC. The background of this company with polymeric insulators and the growing interest in silicone-coated (RTV) glass insulators has generated growing experience in pollution testing of HTM insulators as well as development of techniques and procedures for pollution testing. However, the protocols for conducting these tests must be described precisely. Differences between findings by laboratories highlight the urgent need for standardization across this industry.

Lessons from Salt Fog Tests

Strict adherence to the Salt Fog Test method from IEC 60507 for HTM materials raises the question of what pre-conditioning is required by the standard. In most cases, the initial phase requiring flashovers will alter hydrophobicity to a certain degree. This can result in discrepancy between some test results and also the real condition of insulators in service, which likely preserve some HTM property along the string (see Fig. 3 for RTV-coated insulators returned from the field and Fig. 4 for silicone rubber apparatus insulators measured directly in the field). This question is under discussion within IEC TC 36 63414.

Lessons from Solid Layer Tests

This test method requires special care in preparation and deposition of the layer as well as consistency in approach, i.e. with or without a rest time to allow for some transfer of hydrophobicity through the pollution layer. The entire concept behind the superior performance of HTM materials in this case relies on the ability of the insulator surface to transfer Lower Molecular Weight (LMW) components but this process is not instantaneous. Without this consideration, pollution testing of HTM insulators is essentially useless. Several details are important:

• Preparation of the surface to enable the pollutant to remain on the surface with consistent surface levels of ESDD and NSDD, as desired;

• Rest time to enable at least some transfer of the hydrophobic property into the pollution layer;

• Test procedure for voltage application to determine the withstand or flashover performance of the insulator (50%-flashover voltages, i.e., U_50% are preferred).

Generally, Sediver has been performing pollution tests in line with CIGRE TB 555 and has also customized a technique that produces consistency with no need for “repairs”. This is not a dip but rather a spray technique based on automatic rotation of the test specimen with the slurry established at given viscosities by controlling temperature. Another technique (i.e. the Spray Deposit Airborne Method, “SDAM” developed by Sediver) simulates service representative wind-driven dust deposit using nozzles that generate a cloud of contaminants in a chamber. The contaminant progressively accumulates on the insulator in a consistent layer and the structure of such a solid layer is more representative of actual service conditions. This method also directly allows for CUR (top/bottom) variations, thereby avoiding approximation linked to correction factors, as currently described in IEC 60815.

Typically, the average accepted rest time between contamination and testing is 2 or 3 days, although this is the commonly accepted strategy for pollution layers that are not too thick. There is a real challenge when heavy or very heavy pollution levels are required, starting with NSDD > 0.2 mg/cm2. The HC on the top of an artificially polluted silicone surface with a NSDD 1 mg/cm2 after 26 days is HC 6 (see Fig. 5). If recovery of hydrophobicity is estimated on the surface of the pollution layer for such thick layers, this will take 100 days (see Fig. 7). Today, there is still no consensus on what is most suitable for testing such heavy pollution layers. Transfer time can also differ with application method.

Several tests by different laboratories have shown the benefit of using a Quick or Rapid Flashover Method, which allows for relatively easy and fast knowledge of the critical voltage levels for any given pollution condition. Using such methods to obtain a relatively precise window on where pollution performance becomes critical is preferred. However, sometimes the classical Withstand/Flashover Voltage is found to be higher than the results estimated through any Rapid Method. There are also indications that the average U_50% found in the Rapid Method is similar to the maximum withstand value from the Up-and-Down Test. This should be further investigated.

Experience from Germany

Experience with Housings for Apparatus Insulators

Composite (HTM) housings are now a widespread solution implemented at electrical grids worldwide and are often preferred in severe environments because of superior pollution performance versus porcelain insulators. The design of composite housings depends mainly on pollution level, diameter, shed profile, material and creepage distance. To achieve optimal design, one must consider:

• possible shed bridging due to rain;

• preventing local short circuiting between sheds,

• aiding self-cleaning;

• avoiding pollution “traps”; and

• limiting local electric field stress.

The pollution performance is always the dimensioning case for DC applications.

The following deals with DC pollution tests and completes previous studies. Several artificial pollution tests were performed at FGH Engineering & Test (part of CESI Group) in cooperation with Siemens Energy and the Bushings Div. of GE Power Grid Solutions in Italy. The test objects were composite insulators and testing included both the Solid Layer Method and the Salt Fog Method. The intention was to verify the expected behavior of insulator designs with different housing materials under varying types and levels of pollution. The average diameters of the housings that were tested were in the range 417 to 875 mm.

Test Methods

In the Solid Layer Method, pre-conditioning of the insulators and the test procedure were adjusted based on the method from CIGRE TB 555. To pollute the hydrophobic housing material, a thin layer of dry kaolin powder was applied on the insulator surface before application of the main pollution layer. The main contamination of the insulator surface was from a mixture of kaolin, water and sodium chloride. After a defined drying period of 20 to 48 h, the polluted insulator was placed into the test chamber and steam fog was used to wet the pollution layer. During the tests, voltage was applied according to the rapid procedure. For the Salt Fog Tests, there was no additional preparation or pre-conditioning of the insulator surface. The test procedure was also based on the rapid procedure.

Lessons from Solid Layer Tests

For purposes of comparison, the many different results from these DC tests according to the above procedure were normalized for both flashover voltage and pollution severity. Flashover voltages were presented in SCD0, FO format, which means flashover specific creepage distance corrected for diameter. Pollution severity was presented in RSSD format, which is SDD corrected by NSDD. Fig. 6 illustrates the results and compares these with the recommendation in IEC 60815-4.

Most of the results obtained for HTM insulators using the results of pollution testing were in line with IEC recommendations or even allowed for shorter insulators – especially in more heavily polluted areas (only for insulator 3 was consistently lower flashover performance obtained, as typical for non-HTM insulators). This is an important practical conclusion since many users and manufacturers believe that dimensioning based on direct pollution testing of HTM insulators can only lead to requirements that are higher than recommended by IEC.

Lessons from Salt Fog Tests 

Fig. 7 shows the results from the Salt Fog Tests using the same normalization procedure as for the Solid Layer Method. However, here dimensioning for lower pollution based on direct testing sometimes requires a higher creepage distance than recommended by IEC. By contrast, insulators can be much shorter for higher pollution than recommended by the IEC. These results under DC voltage showed that the recommendations of IEC 60815-4 for the Salt Fog Method require additional research in regard to insulators with large diameters.

Experience with Overhead Line Insulators

AC and DC pollution design of HTM insulators for overhead lines, based on pollution testing, was performed in Germany over the past decade. In most cases, the Solid Layer Method was applied because the sites of greatest interest were inland. Some utilities with lines close to the North Sea also used the Salt Fog Test method, mostly as a withstand test. In special cases (e.g. 420 kV lines and 400 kV AC/DC Ultranet), the statistical method for insulation dimensioning was applied. As part of this method, two or three representative points with different pollution severities were selected and flashover tests were performed at these points (normally by Up-and-Down voltage application with 10 valid points). Tests were performed at AC or DC test voltages depending on the respective application. Pollution flashover performance curves (flashover voltage over SDD) were derived for these applications and implemented into design software used for pollution performance design of insulation.

In general, experience with pollution on German networks has been diverse. For example, in the past pollution level was estimated as medium to high, in accordance with IEC 60815. But since the mid 1980s pollution level has fallen to between light and medium. For most inland applications, a solid layer deposit is typical for HTM insulators (see Fig. 8). Hydrophobicity levels on the often thick industrial pollution layers on samples removed from service was normally in the range of HC 3 to HC 4.

This service experience confirmed that the dynamics of hydrophobicity need to be simulated in any representative pollution test. Given this, pollution tests in some cases were performed with the polluted surface adapted to HC 3 to HC 4 to simulate pollution performance under typical local service conditions. Such adaptation was based on visual HC evaluations however discussions are ongoing as to which criteria to apply to best simulate electrical performance of HTM insulators in service. Aside from visual estimation of HC using the spray method in IEC 62073, electrical conductivity of the pollution layer penetrated by LMW components as well as static contact angle and receding angle, etc. are all under discussion.

Experience from Italy

Background

Pollution tests on composite insulators began in Italy at CESI Laboratories during the 1970s, along with the first installation of composite insulators on AC lines. These investigations continued in the 1980s with focus on pollution performance of composite insulators for DC applications. Pollution tests were carried out on composite insulators when new, after severe laboratory ageing and on insulators removed from service. Generally, these tests were performed in Salt Fog with a Quick Flashover Procedure. A few Solid Layer Tests were conducted by RSE, after inheriting part of the CESI Laboratory, using the procedure proposed in CIGRE TB 555.

More recently, systematic tests with Salt Fog (Quick Flashover Procedure) and Solid Layer (Rapid Procedure) were made on composite insulator housings of large diameter. This was done in collaboration with different manufacturers and performed by FGH Laboratory, part of the CESI Group. Because most of the comprehensive Italian experience is with the Salt Fog Quick Flashover Method, only this is presented in the following.

Application of Quick Flashover Salt Fog Test

Fig. 9 presents results of pollution tests under AC voltage obtained after 3000 h ageing tests (artificial ageing multi-stress ENEL/CESI test) using salinity of 80 kg/m3 expressed as a function of Unified Specific Creepage Distance (USCD). These findings confirm the expected superior pollution performance versus ceramic insulators.

Quick Flashover Methods were also applied to evaluate composite insulators of different types and materials at DC voltage. Fig. 10 presents results of these tests at a salinity of 80 kg/m3 as a function of USCD along with present IEC recommendations. In the case of all silicone rubber insulators, recommended USCD based on testing is remarkably lower than for cap & pin insulators. However, while the non-aged insulators required lower USCD values than those suggested for composite insulators by IEC, insulators after 3000 h ageing required USCD values that corresponded well with IEC recommendations.

Similar results were also obtained for HTM housings for bushings. It was shown that, based on results of pollution tests, HTM insulators can be dimensioned in a more optimal way than by applying those recommended by the IEC. For example, tests on large diameter composite insulators under DC voltage confirmed that dependence of USCD on insulator diameter for HTM insulators is much lower than for ceramic insulators (see Fig. 11).

Experience from Czech Republic

Background

Pollution tests on ceramic and composite insulators started at EGU-HV Laboratory in the Czech Republic almost 15 years ago. Practical results obtained using the Quick Flashover Salt Fog Test Procedure have already been summarized in CIGRE TB 691. The focus below is therefore on the procedure based on Solid Layer Test Method.

Test Method for Solid Layer Test

The test procedure followed CIGRE TB 555 and consisted of application of dry kaolin and blowing off any excess, followed by application of contamination by spraying (see Fig. 12).

Experience from Japan

Background & Test Method

Various test procedures have been tried in Japan for HTM insulators exactly as had earlier been conducted for ceramic insulators. These procedures included:

• Fog withstand test (Solid Layer Method as per IEC 60507, Procedure B, and JEC);

• Equivalent fog method (as per JEC);

• Repeated flashover method (a traditional way to test and described in JEC Appendix.

In the past, the Equivalent Fog Method was adopted for non-HTM (i.e. ceramic) long rod insulators since this yielded similar results to the Fog Withstand Test Method but with much shorter testing time. However, the Equivalent Fog Test results for HTM insulators tended to give the same flashover results as for ceramic non-HTM insulators. This is because such tests do not simulate the influence of hydrophobicity on flashover performance. Therefore, the need for correction of test results for HTM insulators is now under discussion among Japanese experts.

The Equivalent Fog Method for HTM insulators is performed with a pre-conditioning procedure. The main artificial pollution consists of a suspension of Tonoko and salt, which is normally applied by a flow-on process. Testing starts before drying of the pollution layer and voltage is increased gradually until flashover occurs, usually within 0.5 to 3 min after contamination. Fig. 13 illustrates the Equivalent Fog Test procedure being applied to an HTM hollow core insulator.

Application of Test Results

Results from these pollution tests have been directly adopted for practical selection of HTM insulators in both clean and polluted service areas. However, loss of hydrophobicity has been reported under rapid pollution conditions. Moreover, the Japanese environment is characterized by warm and humid conditions and biological growths such as algae are often observed on HTM composite insulators, independent of pollution severity. A surface covered by algae will lose hydrophobicity and lead to lower surface resistance compared to standard contamination by Tonoko. To take this into account, it has been proposed to define design NSDD as 3 times higher than for non-HTM ceramic insulators. However, to allow for some recovery of surface hydrophobicity in service, the specific creepage distance for HTM insulators can be reduced to 90% in comparison to ceramic insulators.

Experience from South Africa

Background

For many years, Eskom has followed international guidelines when selecting transmission and distribution insulators for use in polluted environments. The proliferation of non-ceramic (i.e. polymeric, resin and coated) insulators in recent years has led to their increased application in South Africa in spite of concerns about their long-term electrical performance and material longevity.

Eskom’s experience has led engineers to conclude that only field testing will expose any insulator to a realistic combination of pollution and ageing. Given this, the 11-132 kV Koeberg Insulator Pollution Test Station (KIPTS) was established to determine ageing performance of insulators under natural pollution. Results from KIPTS have also been complemented by pollution tests on artificially and naturally polluted insulators, mainly to compare different insulation options. Unfortunately, severe sand dune movement required KIPTS to be decommissioned and a new insulator pollution test station (11-400 kV) is planned. In the meantime, the following pollution performance test approach is being used to help qualify MV and HV insulation.

Test Approach

Eskom’s approach is based in general on Approach 2 (“Measure and Test”) from IEC TS 60815-1. Pollution performance curves are required, only from recognized, independent laboratories, on test specimens having identical insulation to that of an insulator to be supplied for the network. These are U_50% flashover voltage curves (using the Rapid Flashover Test Method) at three pollution levels of SDD, with NSDD of ≥ 0.1 mg/cm² and using:

• the Solid Layer Test method for glass and porcelain insulators, according to IEC TS 60507 and using a spray gun to apply the kaolin composition;

• the Modified Solid Layer Test method (with a pre-conditioning procedure with and without recovery) for polymeric insulators according to CIGRE TBs 555 and 691.

The U_50% flashover voltage obtained is then converted into flashover stress along the test insulation length H_T as _50%/ (in kV/m) and presented as three-point approximated power law curves versus pollution level (SDD in mg/cm² as per Fig. 14). The U_m is the highest system r.m.s. phase-to-ground voltage that the insulator to be supplied will be subjected to. The insulator will be accepted if _50%/ > _m/ in the SDD range:

• 06 to 0.12 mg/cm² for application in ‘Light’ to ‘Medium’ environments, with minimum specific creepage distance of 20 mm/kV in terms of phase-to-phase voltage;

• 12 to 0.48 mg/cm² for use in ‘Heavy’ to ‘Very Heavy’ environments, with minimum specific creepage distance of 31 mm/kV in terms of phase-to-phase voltage.

Insulator pollution flashover performance curve constants “A” and “α” should be derived for the equation _50%/ =•DD⁻. This will be used by Eskom along with Site Pollution Severity values in the statistical approach to optimize insulation selection (as per Annex G of IEC TS 60815-1).

Application of Test Results

Practical applications of the statistical approach for insulation dimensioning (i.e. pollution flashover performance curves mentioned above together with site severity measurements and target outage rate) are summarized as follows:

• Insulation level requirements for new 132 kV and 400 kV outdoor substation;

• Refurbishment of existing 400 kV AC line equipped with glass cap & pin insulators with silicone rubber units. The re-insulation of 21,000 glass cap & pin insulator strings with composite insulators was performed in a short time by live line work and the network in West Cape has since been operating successfully;

• Refurbishment of a 533 kV DC line with composite insulators. Statistical dimensioning using the IST (Insulator Selection Tool) Program and with a given targeted outage rate made it possible to insulate all poles with composite insulators at 40 mm/kV and with a 4712 mm connection length for operation at 533 kV. Refurbishment was completed and the line is now in operation.

Experience from China

Background

Experience with HTM insulators in China has exceeded 35 years and it is estimated that over 9 million composite line insulators are now in service at voltage levels between 110 to 1000 kV AC and ±400 kV to ±1100 kV DC. One of the important reasons for the rapid increase in application of composite insulators in that country has been their excellent pollution performance. As such, research on pollution testing of composite insulators has been ongoing since their introduction. In the late 1990s, an original test method was developed and verified and presently is included in China Electric Power Industrial Standard DL/T 859-2015 as well as in National Standard GB/T 34937-2017.

Test Method

Traditional artificial pollution test methods for ceramic insulators, such as found in IEC 60507, are not deemed suitable for composite type insulators because it is difficult to apply a uniform layer of artificial pollution on the surface of highly hydrophobic silicone rubber material. After continuous attempts, however, this problem was eventually resolved using a pre-conditioning procedure. Before applying the pollution, a dry sponge or soft brush is used to paste a uniform thin layer of dry kieselguhr on the surface of the test insulator. A blower-like device is then used to remove excess kieselguhr, while also enabling a very thin layer of hydrophilic substance to remain attached to the surface. Since the layer of kieselguhr is extremely thin, it does not influence desired NSDD in the main pollution phase. The test insulator can now be easily contaminated by any standard pollution application method, which shall be completed within 1 h after pre-conditioning.

Another challenge has been simulating the hydrophobicity state of the surface. Various non-soluble pollutants were investigated to choose a material that best simulates different levels of hydrophobicity. These included kieselguhr, kaolin, Tonoko, SiO₂, metal oxides (e.g. ZnO, Fe₂O₃, Al₂O₃) and inorganic salts with different solubilities, such as BaSO₄, CaSO₄, ZnSO₄, (NH₄)₂SO₄, K₂SO₄, NH₄Cl, CaCl₂, NaCl and KCl. The most commonly used pollutant in China has become a combination of kieselguhr/kaolin and NaCl.

A pollution layer based on kieselguhr is much faster in recovering hydrophobicity than one based on kaolin. For example, after only 2-4 h of ‘rest’ time, the contact angle (CA) on the surface becomes higher than 90°. In the case of kaolin, CA remains lower than 90° even after 5 days of ‘rest’ time. As such, different surface hydrophobicity classes for polluted silicone rubber insulators could be simulated – from HC 7 (totally hydrophilic) to HC 1 (completely hydrophobic). Fig. 15 shows the pollution flashover gradient along creepage distance E_L over SDD for different levels of HC. Change in hydrophobicity from HC 1 to HC 4 had almost no influence on pollution performance. As hydrophobicity deteriorated, E_L gradually decreased. When hydrophobicity became totally lost (i.e. HC 7), the E_L curve decreased significantly in comparison to HC 6.

Choosing relevant laboratory pollution severity is another key issue. Based on experience in China, valid for both ceramic and composite type insulators, measured in-service ESDD value cannot be directly translated into SDD in artificial pollution tests. In fact, for many cases of natural pollution, making ESDD equal to SDD will provide different flashover voltages for the same insulators tested under natural and artificial pollution. Correction from measured ESDD to laboratory SDD is therefore very important.

Finally, the test method includes four steps (illustrated in Fig. 16):

• Step 1 (Pre-conditioning);

• Step 2 (Pollution application): Brushing method is recommended for line insulators and dipping method is recommended for insulators with large diameters;

• Step 3 (Waiting for hydrophobicity transfer): Different hydrophobicity levels at the surface of the contaminated insulator can be achieved by applying different pollutants and recovery times;

• Step 4 (Pollution withstand/flashover test): Standard artificial pollution test procedures shall be applied.

Application of Test Results

In China, artificial pollution testing of HTM composite insulators is used for evaluating pollution performance, thus guiding future insulator selection and dimensioning. Practically-speaking, dimensioning is based conservatively on a low hydrophobicity state (i.e. HC 5 to HC 6) to reflect impact of pollution, hydrophobicity reduction and possible ageing.

Two decades have now passed since this artificial pollution test method was first standardized in 2002. At present, this mature method has been fully accepted and is widely being used in practice for evaluating pollution performance and dimensioning of AC and DC silicone rubber composite insulators. Average pollution outage rates for all AC and DC OHLs in China (110 kV to 1000 kV) has decreased by two orders of magnitude compared to the peak of 0.12 outages/100 km·year back in 2001 – even as the total length of OHLs equipped with composite insulators has increased dramatically. This confirms that application of this artificial pollution test method for HTM composite insulators has been a success.

Summary

Results of comprehensive practical experience using artificial pollution tests on HTM insulators across different countries and at different commercial high voltage laboratories makes possible to reject a number of myths and misconceptions:

1. The first myth is that such a test is not necessary. In fact, many countries/laboratories are using it for insulation dimensioning which has then been verified by service experience;

2. The second myth is that it is complicated to create a relevant test procedure. Actually, most countries/laboratories use principles already summarized in two published CIGRE brochures (TB 555 and TB 691) which propose slightly modifications to known standard procedures. Basically, both the Modified Salt Fog and the Solid Layer Test are proposed;

3. The third myth is that practical application of such a test will lead to over-dimensioning. Results from Germany and Italy, in fact, have shown that dimensioning based on test results is in line with IEC recommendations. Even shorter insulators can be proposed based on comprehensive testing. Results from China, widely using pollution dimensioning based on testing, are also well supported by service experience. Application of statistical dimensioning based on laboratory flashover performance also makes it possible to optimize insulator length/creepage. This has been confirmed across several practical projects worldwide.

Table 1 summarizes experience with different test methods across different countries. The colors illustrate which tests are most used in each: GREEN illustrates major experience, YELLOW illustrates other experience.

Level of practical application of laboratory pollution tests differs by country:

• China and Russia have already included these tests in their national standards and use them regularly;

• South Africa includes these tests as part of the acceptance process for any new insulator entering the market;

• Many other countries/laboratories use these tests for dimensioning and comparison purposes, exactly as has been their main purpose for ceramic insulators. There is no intention to require these tests as type tests.

The intrinsic feature of HTM insulators, i.e. surface hydrophobicity, should be included in any future pollution test method. The test procedures described above are similar to those for ceramic insulators and two basic procedures are preliminarily proposed:

1. Modified Salt Fog Procedure, as described in CIGRE TB 691, i.e., Quick Flashover Procedure. A standard salt fog withstand procedure, which does not significantly influence hydrophobicity level of insulators in the absence of repeated flashovers, would in many cases allow insulators to pass under any salinity;

2. Modified Solid Layer Procedure, as described in CIGRE TB 555 and TB 691, i.e. including preconditioning and elapsed time between contamination and voltage test;

3. Proposals (1) and (2) are basic while the exact test procedure can be slightly different, e.g. using different contaminants (as for ceramic insulators in IEC), different criteria for recovery of hydrophobicity (e.g. on the top or inside the pollution layer), etc. Special attention should be paid to preconditioning (i.e. recovery simulated by transfer time) for very heavy pollution conditions. All these can be finetuned based on discussions within IEC TC 36 63414 that aims to develop a new standard “Artificial pollution tests on high-voltage insulators made of hydrophobicity transfer materials to be used on a.c. and d.c. systems”.

4. It is important to stress that, in contrast to ceramic insulators, HTM insulators cannot be dimensioned based solely on pollution performance since possible ageing must also be considered. But this is a separate issue for another technical review.

KERI High Power & High Voltage Laboratories

South Korea

Laboratory Center of Koncar Electrical Engineering Institute

Croatia

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References
[1] I. Gutman, P. Cardano, A. Dernfalk, J.-M. George, T. Hayashi, K. Kondo, J. Lachman, S. Li, X. Liang, E. Moal, A. Pigini, J. Seifert, E. Solomonik, W. Vosloo, D. Windmar: “State-of-the-art of Pollution Test Procedures for Insulators with Hydrophobicity Transfer Materials”, CIGRE Science & Engineering, N. 21, June 2021, p.p. 141-169
[2] CIGRE Technical Brochure No. 142, “Natural and artificial ageing and pollution testing of polymeric insulators”, June 1999
[3] CIGRE Technical Brochure No. 158, “Polluted insulators: a review of current knowledge”, June 2000
[4] CIGRE Technical Brochure No. 361, “Outdoor insulation in polluted conditions: guidelines for selection and dimensioning. Part 1: General principles and the AC case”, June 2008
[5] CIGRE Technical Brochure No. 455, “Aspects for the Application of Composite Insulators to High Voltage (≥72kV) Apparatus”, April 2011
[6] CIGRE Technical Brochure No. 481, “Guide for the Assessment of Composite Insulators in the laboratory after their Removal from Service”, December 2011
[7] CIGRE Technical Brochure No. 518, “Outdoor Insulation in Polluted Conditions: Guidelines for Selection and Dimensioning, Part 2: The DC Case”, December 2012
[8] CIGRE Technical Brochure No. 555, “Artificial pollution test for polymer insulators”, October 2013
[9] CIGRE Technical brochure 691, “Pollution test of naturally and artificially contaminated insulators”, July 2017
[10] I. Gutman, J. Lundengård, M. Fairhurst, “Three-phase pollution test of diamond-shaped “suspension” insulator arrangement for T-pylon tower”, 13th INSUKON-2017, Birmingham, UK, 16-18 May 2017, p.p. 233-237
[11] J.-F. Goffinet, I. Gutman, P. Sidenvall, “Innovative insulated cross-arm: requirements, testing and construction”, ICOLIM-2017, Strasbourg, France, 26-28 April 2017, paper 0078
[12] B. Thorsteinsson, K. Halsan, M. Gullo, I. Gutman, “Innovative pollution and ice testing of DC composite insulators for ±525 kV DC line”, ISH-2017, Buenos Aires, Argentina, August 28 – September 01, 2017, paper 156
[13] J.-M. George, S. Prat, C. Lumb, F. Virlogeux, I. Gutman, J. Lundengård, M. Marzinotto, “Field Experience and Laboratory Investigation of Glass Insulators Having a Factory-Applied Silicone Coating”, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 21, No. 6, December 2014, p.p. 2594-2601
[14] I. Gutman, C. Ahlholm, U. Åkesson, A. Holmberg, D. Wu, L. Jonsson, “Long-term service experience and inspection results of HV equipment made of silicone rubber insulators”, CIGRE Symposium, Auckland, New Zealand, September 14-20, 2013, A3-412
[15] E. Moal, “Experience on Composite Hollow Core Insulators Regarding Pollution Performance”, 6th Conference on Silicone Insulation, Burghausen, June 27 – 28, 2017
[16] E. Moal, V. Bergmann, A. Soergel, “Technical Design Requirements for Composite Hollow Core Insulators Regarding Pollution Performance under AC & DC”, 2017 INMR World Congress, Barcelona, November 5-8, 2017
[17] G. Testin, M. Boutlendj, P. Cardano, A. Pastore, M. Saravolac, M. Sehovac, A. Pigini, “Methodologies for pollution tests on composite housings”, CIGRE-2016, A3-115
[18] C. Neumann, “Betriebsverhalten von Silikonisolatoren (Service experience of silicone rubber insulators)”, ETG Workshop, Berlin, 2003 (in German)
[19] M. Cojan, J. Perret, C. Malaguti, P. Nicolini, J.S.T. Looms, A.W. Stannet, “Polymeric transmission insulators: their application in France, Italy and UK”, CIGRE-1980, paper 22.10
[20] F. Bianchi, G. Marrone, C. Masetti, A. Pigini, “Recent laboratory and field experiences on composite insulators in Italy”, L’Energia Elettrica, N.1, 1983 (in Italian)
[21] G.P. Fini, G. Marrone, G. Gallucci, A. Pigini, “Field experience and laboratory ageing test on composite insulators for overhead lines”, L’Energia Elettrica, N. 7-8, 1984 (in Italian)
[22] S. Bossi, A. Pigini, G.P. Fini, A. Porrino, E. Channakeshava, N. Vasudev, M. Ramamoorty, “Study of the performance of composite insulators in polluted conditions”, CIGRE-1994, paper 33.10
[23] G. Pirovano, P. Omodeo Gianolo, A. Pigini, “Assessment of the pollution performance of composite insulators”, ISH-2013, Seoul, Korea, 25-30 August 2013, OC-1
[24] A. Pigini, R. Cortina, M. Marzinotto, G. Lagrotteria, “Pollution tests on composite insulators: the Italian experience”, ISH-2015, Pilsen, Czech Republic, 23-28th August 2015, paper 367
[25] L.L. Vladimirsky, E.A. Solomonik, N.N. Tikhodeev, I. Gutman, “Methods of statistical dimensioning of the outdoor insulation with respect to polluted conditions”, IEEE PowerTech 2005, St. Petersburg, Russia, 27-30 June 2005, paper 670
[26] W.L. Vosloo, I. Gutman, J.P. Holtzhausen, “Statistical determination of insulator performance at Koeberg insulator pollution test station”, The 5th CIGRE Southern Africa Regional Conference, Cape Town, South Africa, 24-27 October 2005, p.p. 180-187
[27] Russian Standard GOST 10390-2015, “Electrical equipment for 3 kV and above. Test methods of outdoor insulation in polluted conditions”, Standardinform, Moscow, 2016 (in Russian)
[28] Russian Standard GOST 28856-90, “Line suspension long rod composite insulators. General technical requirements”, Publishing house of standards, Moscow, 1990
[29] T. Kawamura, T. Seta, K. Nagai, K. Naito, “DC Pollution Performance of Insulators”, CIGRE- 1984, paper 33-10, 1984
[30] JEC (Japanese Electrotechnical Committee) – 0201, “AC Voltage Insulation Tests”, 1988 (in Japanese)
[31] I. Gutman, J. Lundengård, W. Vosloo: “Development of Time- and Cost-Effective Pollution Test Methods Applicable for Different Station Insulation Options”, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 21, No. 6, December 2014, p.p. 2525-2530
[32] I. Gutman, W.L Vosloo, L. Appollis, “Example of refurbishment of overhead lines 400 kV at Western Cape after major pollution event in February 2006”, CIGRE-2012, B2-302
[33] S. Narain, V. Naidoo, R. Vajeth, “Upgrading the performance of the Apollo – Cahora Bassa 533 kV links”, CIGRE Session 2012, B2-301
[34] X. Liang, S. Li, “Looking to the Future of Composite Insulators”, World Congress & Exhibition on Insulators, Arresters & Bushings, Munich, Germany, 18-21 October 2015
[35] X. Liang, S. Li, Y. Gao, Z. Su, J. Zhou, “Improving the outdoor insulation performance of Chinese EHV and UHV AC and DC overhead transmission lines”, IEEE Electrical Insulation Magazine, vol. 36, pp. 7-25, 2020
[36] China Electric Power Industrial Standard DL/T 859, “Artificial pollution tests on composite insulators used on high-voltage AC systems”, 2015 (in Chinese)
[37] National Standards of the People’s Republic of China GB/T 34937, “Insulators for overhead lines-Composite suspension and tension insulators for d.c. systems with a nominal voltage greater than 1 500 V-Definitions, test methods and acceptance criteria”, 2017 (in Chinese)
[38] W. Shaowu, L. Xidong, G. Zhicheng, W. Xun, “Hydrophobicity transfer properties of silicone rubber contaminated by different kinds of pollutants”, 2000 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, Victoria, Canada, 2000, pp. 373-376
[39] X. Liang, S. Wang, J. Fan, and Z. Guan, “Development of composite insulators in China”, IEEE Transactions on Dielectrics and Electrical Insulation, vol. 6, pp. 586-594, 1999
[40] Z. Su, X. Liang, Y. Yin, J. Zhou, P. Li, W. Li, “Important correction factors in HVDC line insulation selection”, 14th ISH, Beijing, China, 2005, p. D-61
[41] X. Liang, S. Li, Y. Gao, Z. Su, J. Zhou, “Improving the outdoor insulation performance of Chinese EHV and UHV AC and DC overhead transmission lines”, IEEE Electrical Insulation Magazine, vol. 36, pp. 7-25, 2020

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Polymeric insulators were first installed about 30 years ago with the hope that they would help reduce the high cost of insulator washing. A relatively high number of mechanical failures due to brittle fracture forced the country’s TSO – Red de Energia del Peru (REP) – to re-evaluate this strategy. Starting in 2009, RTV coated glass insulators began to replace polymeric types on lines in problematic coastal areas, particularly around the city of Chilca, south of Lima.

INMR visited one of the most important of these lines to report on installation of new RTV-coated glass insulators.

500 kV Fénix-Chilca Line (L5011)

The 500 kV L5011 line connects the 580 MW Fénix Generating Substation to the Chilca Substation. Despite having only 23 towers and being only 7.5 km in length, this line is considered particularly important since it operates in an area characterized by heavy marine pollution combined with high humidity, especially during winter. Moreover, failures on this line could result in situations of service unavailability along with high penalties. Only 28 months after it was first commissioned, high discharge activity was already detected leading to a decision to clean the line’s insulators. About 18 months later, high discharge levels were observed once again.

Past investigations conducted to arrive at the optimal frequency to clean coated insulators on this line based on cost and risk found a 12-month cycle to be the best choice. Subsequently, nighttime inspection carried out in March 2019 revealed that half of all insulator strings cleaned 12 months earlier already showed presence of discharge activity (effluvia) that ranged between Grades 2 and 3 (see Fig. 3). This indicated that the strings needed to again be cleaned. Had this same inspection been done during a season with only medium humidity levels, once winter came with its higher humidity, effluvia would likely have reached greater levels, possibly leading to insulator failures.

In mid-February 2024, a decision was made to replace all the coated glass insulators on both outer phases of L5011 and also all the polymeric long rods that had operated in the center phase. This work was completed over an allowed shutdown period of 4 days. The pre-existing coated insulators, which had been in service for only about 4 years, already showed evidence of severe degradation of the coating, particularly in the area near the pin. Similarly, the polymeric types removed from the center phase showed start of erosion processes, particularly at the live end. Polymeric insulators had been selected for this phase due to relatively high glass shatter rates observed in the past. But this strategy has now been abandoned in favor of applying coated glass insulators on all phases.

Engineers at REP explain that there are two different maintenance strategies that can be applied when confronted with a line such as L5011 that experiences particularly heavy pollution that can fall outside the limits found in existing international standards. One is to replace all the insulators every few years without question; the other is to selectively replace insulators only when inspection reveals that they are nearing their end-of-life.

The answer as to which strategy is more effective will depend on the zone in which a line operates. In the coastal zone of L5011, for example, the strategy of inspecting and replacing when needed is deemed more suitable for RTV-coated glass insulators while the strategy of replacing regularly, without need for inspection, has been regarded as more suitable for polymeric types. This is the conservative approach.

RTV coated glass insulators are also applied on REP’s nearby 220 kV lines. As with the 500 kV L5011, these insulators need to be manually cleaned on a regular basis, which can change by line section. For example, due to varying microclimates, even across relatively short distances, some sections are manually cleaned each 4 years while others are cleaned every 6 years. The decision on optimal cleaning cycle will depend largely on level of discharge activity detected during night vision inspection.

One aspect to be considered when it comes to such maintenance operations is that experience has shown that RTV coatings start to degrade from the manual cleaning process. That means that frequency of cleaning only increases over time. When it becomes necessary to clean RTV coated insulators each year is the time that these should be replaced with new units.

The highly corrosive coastal environment around Chilca also impacts 500 kV and 220 kV tower structures. Those with steel components need to be protected again corrosion using specialized treatments that increase costs. Given this, new structures are preferred to be of wood. However, analysis has found that is still not cost-effective to replace existing steel structures with wood.

Although REP has decided against continuing to apply new polymeric insulators on its lines in Chilca, other overhead transmission lines in the area operated by other electricity suppliers continue to feature them, often in tension applications.

The post Applying RTV Coated Glass & Polymeric Insulators Under High Salt Contamination & Humidity appeared first on .

This edited past contribution to INMR focused on air-to-oil bushings using resin-impregnated synthetic (RIS) as the main insulation and applied in power transformer and shunt reactors in systems up to 550 kV.

Nanjing Electric Insulator Co.,Ltd

CHINA

Hitachi Energy Transformer Components and Service

Switzerland

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Bushing Technologies

Bushings for transformers and other applications evolved considerably over the decades, from simple hollow insulators made of porcelain to more elaborate, engineered designs. As bushing voltage levels increased, technologies such as capacitance grading and resin-bonded paper (RBP) bushings were developed.

Later, when RBP bushings reached their limits in terms of partial discharge and dielectric loss, other insulation technologies emerged. Oil-impregnated paper (OIP) technology, although mature and still the most widespread in terms of application, comes with serious risks inherent to use of mineral oil as the insulating fluid:

• Possible loss of insulation media due to ageing of gasket;
• Potential catastrophic failure and fire when operated under abnormal conditions;
• Flammability of oil.

Moreover, under unusual conditions and in the presence of heat as well as the by-products of cellulose degradation, there is risk of physical and chemical reactions between mineral oil and other materials inside the bushing. These have been known to cause corrosive sulfur and copper mobility problems.

Another potential issue inherent to use of OIP bushings is onset of internal partial discharges due to formation of gas bubbles within the oil. Whenever oil and a gas, whether air or nitrogen, is confined in a fixed volume, as the case for a bushing, pressure equilibrium is reached over time at any given temperature. But when temperature changes so too do volume of oil and also gas space. Three variables then come into play. Firstly, gas pressure changes with volume. Second, gas pressure changes with temperature and third, the ability of the oil to absorb gas varies with temperature. As a result, major fluctuations in temperature result in a continuous change in the amount of gas dissolved in the oil. For example, if temperature drops rapidly after being high for some time, the gas cannot escape quickly enough to avoid formation of bubbles in the oil. This phenomenon occurs mostly during transformer testing in the factory but can also take place when the equipment operates under severe cyclic loading combined with rapid cooling, such as in solar and wind generation.

Dry-type resin-impregnated paper (RIP) bushings have been successfully applied worldwide for many years and effectively eliminate issues related to oil reactions, leaks and flammability. This lowers risk of bushing failures that result in explosions or transformer fires, particularly when used in conjunction with composite housings in place of porcelain. Indeed, as RIP technology has become more and more widespread and with demonstrated reliability, utilities in areas that used to be entirely focused on OIP technology, such as the U.S. and IEEE markets, are increasingly adopting dry bushings as the preferred solution. This is especially the case for high and extra high voltage classes ranging from 230 kV and above. However, RIP bushings still present a shortcoming, namely the potential for moisture ingress in the exposed oil side during long-term storage. That risk requires specific storage methods, including use of oil- or gas-filled tanks to protect an RIP bushing’s lower section.

More recently, the crepe paper used in winding the RIP core has been replaced by a polyester synthetic material. This has resulted in development of what has become known as the resin-impregnated synthetic or RIS type bushing. These are based on the same established condenser grading technology and manufacturing processes as RIP but use of a non-hygroscopic synthetic material eliminates risk of moisture ingress. At the same time, other bushing insulation characteristics improve as well.

RIS Bushing Construction

The construction of an RIS type bushing is similar to other technologies with available drawlead or bottom connection options. The active part, or condenser core, consists of a thin non-woven synthetic fabric wound tightly around an aluminum tube or solid copper rod. During winding, condenser electrodes made of thin aluminum foil are introduced at specific locations and intervals to provide electric field grading in the bushing’s radial and axial directions. The synthetic fabric, manufactured to specific tolerances, has homogeneous properties, consistent thickness and smooth surface finish that together allow smooth insertion of aluminum foil during the winding process. The result is superior placing of concentric layers and more uniform electric field distribution.

Synthetic Insulation Properties

The synthetic insulating material used in RIS bushing condenser cores is non-hygroscopic. This ensures that virtually no ambient moisture is absorbed either during raw material storage in the factory or later within the finished bushing condenser core.

Fig. 2 compares moisture content, in percentage of weight, after prolonged storage at 50% relative humidity and 23ºC for a range of different insulating materials. As can be seen, cellulose gains 6.24% in weight due to moisture absorption whereas synthetic materials remain at only 0.23% to 0.32% weight gain. In fact, samples of resin-impregnated insulation stored under water for a long-term yielded similar low water absorption, as shown in Fig. 3.

At the same time, from a dielectric point of view, resin-impregnated synthetic material has stable properties that are comparable in permittivity and power factor to the crepe paper used in RIP bushings. The curves in Fig. 4 show power factor and relative permittivity for these materials as a function of temperature. This similarity in dielectric properties allows for easy interchangeability of RIP/RIS bushing designs in terms of dimensions as well as electrical ratings.

RIS Bushings

Since introduction, RIS style bushings have been installed and placed in service around the world. For example, some types of RIS bushings were introduced into the IEEE market in 2016, mainly at 230 kV, 345 kV and 550 kV system voltages. Annex 1 shows the typical technical specification for a 550 kV 3000-amp RIS replacement bushing according to IEEE C57.19.01 ratings. The distinctive slender construction of these bushings and use of standard components such as insulators, flanges, etc. allows easy customization of an RIS transformer bushing to dimensionally match whatever legacy bushings are being replaced.

The main advantages of RIS bushings, equipped with composite insulator housings, include:

• Construction free of oil and paper;
• Flame-resistant properties;
• Insulation thermal class of 120ºC;
• Lower sensitivity to humidity ingress during long-term storage;
• More controlled winding and also better electric field profiles;
• No dangerous fragments if housing damaged by internal or external factors;
• Low risk of damage due to improper handling and vandalism due to high impact strength and shock resistance;
• Lower weight, simplifying handling and reducing stress on transformer tank;
• Suitability for even very low ambient temperatures;
• Seals designed as O-rings in self-contained channels and made of temperature-resistant elastomers;
• Voltage tap and/or test tap according to IEEE requirements;
• Easier transport and storage in any position;
• Availability per IEEE voltage classes and current ratings up to 550 kV;
• Complying and exceeding applicable IEEE bushing standards and tested accordingly.

Routine Testing

According to IEEE C57.19.00, routine tests are intended to verify quality and uniformity of workmanship and materials used in manufacturing transformer bushings. Every unit manufactured to this standard must undergo a full such test as part of the production process. Since condenser bushings made of synthetic materials are yet to be covered by any IEEE standards, acceptance criteria for some routine tests are still not defined. Manufacturers are therefore required to demonstrate ratings by what they consider equivalent or more adequate test limits. Suitable limits for power factor and partial discharge testing in the case of RIS bushings are discussed below:

Bushing Power Factor

Power factor is the measure of internal dielectric losses within the bushing insulation, relative to its capacitive reactive power, expressed in percentage. Fig. 5 shows the phasor representation of the resistive and total currents flowing through the condenser core, with Cos Ø being the ‘power factor’ of the insulation. In an ideal capacitor, the active current, I_act, would be near zero (i.e. no losses) and the ideal power factor would also approach zero as the phase angle, Ø, nears 90º. In practical terms, a power factor of 0.005 (0.5%) or lower is considered healthy.

As for bushings using other insulation technologies, RIS bushings are factory tested for C1 main insulation capacitance and power factor at 10 kV at normalized ambient temperature of 20ºC. These values are stamped in the nameplate for reference and condition assessment of the bushing in the field. Typical C1 power factor values for a new RIS bushing range in the order of 0.25% to 0.4%, depending on bushing voltage class. Field-testing RIS bushings at temperatures other than the normalized 20ºC requires application of correction factors, available in the instruction manual or from the manufacturer, as shown in Fig. 6. Deviations in C1 capacitance or power factor are indications of possible ageing or damage within the insulation and therefore must be investigated.

Correction factor, K, is the reciprocal of the relative Cos Ø_rel: from the correction curve in Fig. 6: K = 1/Cos Ø_rel. For example, C1 field-testing of an RIS bushing, with nameplate power factor of 0.26%, at cold ambient temperature yielded:

Mean temperature of bushing based on ambient and transformer oil temperature:

T_bushing = 10°C

Measured C1 Cos Ø from insulation analyzer:

Cos Ø @10°C = 0.32%

Relative Cos Ø for 10ºC, from curve:

Cos Ø_rel = 1.2

Correction factor: K = 1/Cos Ø_rel = 0.83

Corrected C1 power factor to normalized ambient of 20ºC: Cos Ø = K * 0.32 = 0.26%, matching nameplate value.

Partial Discharge (PD)

Partial discharges occur whenever there is discontinuity in electric field intensity that exceeds the dielectric withstand of a small portion of an insulating material. It is therefore critical that factory acceptance testing detects any PD anomaly for inception and extinction at the prescribed voltage level. Dry type RBP bushings have intrinsically higher partial discharge levels since tiny voids and cracks are normally formed during manufacture of their condenser cores. In fact, PD-related damage is a common cause of failure for this type of bushing since self-healing of the insulation, as the case for air or OIP insulation, is not possible.

By contrast, modern resin-impregnated RIS dry-type condenser cores for transformer bushings undergo more stringent impregnation and drying. This process is carried out under vacuum and strictly controlled temperature and duration, ensuring no formation of voids or cracks. The result is bushings with no detectable PD up to the highest system voltages. Validation of the manufacturing process is done during routine testing of the complete bushing. Here, PD is measured at increasing AC test voltage steps up to the one-minute power frequency withstand test level, U_P, while simultaneously also measuring main insulation C1 capacitance and power factor. Only external test laboratory background PD should be detected and 1 pC internal PD is set as acceptance criteria during the bushing test at 10 kV; 0.5V_L-G; 1.05V_L-G; 1.5V_L-G; 1.82V_L-G; 2.0V_L-G; U_P and decreasing back to 10 kV through the same voltage steps. This special factory test confirms that PD inception voltages for RIS bushings are never reached, even at the highest possible bushing operating voltages or temporary system overvoltages. It also verifies that there are no deviations in C1 capacitance and power factor values at various test voltages.

Impulse Testing

According to IEEE C57.19.00-2004, Section 7, any impulse testing of transformer bushings is considered a design test and not a routine test. This means that impulse tests are not required during factory acceptance tests. For end-users, it is considered best practice to include impulse tests in the transformer bushing technical specifications they issue in order to ensure that the manufacturer selected has implemented some form of impulse testing during the routine test plan. For example, the impulse test sequence included in IEC 60137-2017, Section 9.3 should be requested for all bushings rated above 69 kV voltage class. The impulse test sequence should include, as a minimum:

• one full lightning impulse of negative polarity at 105 % of rated withstand voltage followed by;
• two chopped lightning impulses of negative polarity at 115 % of rated withstand voltage, followed by;
• four full lightning impulses of negative polarity applied at 105 % of rated withstand voltage.

Full AC routine tests should be performed before and then repeated after the impulse test sequence. This practice is particularly important for replacement bushings, where these would not be subjected to any impulse tests during the transformer factory test.

Bushing Replacement

During routine maintenance and inspection of power transformers, several critical aspects are checked in regard to the bushings. Condition of the bushings is assessed based on results of this inspection or through an on-line bushing sensor. Main reasons why a bushing needs to be replaced include:

• Increase in power factor, capacitance or PD above manufacturer recommended values;
• Loss of oil or identified oil leak;
• Presence of combustible gasses in the bushing oil;
• Damaged bushing.

Other factors to consider in determining whether or not to replace a bushing are:

• Inadequate bushing specification or technology for the application;
• Older style bushing that do not allow for installation of remote monitoring.

Further special issues to consider include:

• Outdated/changed technical specification or obsolete standards;
• Unusual operating conditions and renewable energy/special applications;
• Demanding load profiles under abrupt changes in operating and ambient temperatures;
• Bushing air-side termination with rigid bus connection.

Selecting Replacement Bushings

Thousands of transformer bushings are replaced every year. RIS bushings are compatible for replacing legacy bushings from most manufacturers and their manufacturing process allows for customization of dimensions as well as for special designs. When replacing bushings connected via draw lead cables, RIS bushings can be provided with customized adapter bolts or studs to match existing draw lead studs with no or only minimal changes to the draw lead cable. When replacing bushings in high seismic zones or near coastal region where pollution and salinity are severe, bushings equipped with composite insulators and hydrophobic silicone sheds should be used. These insulators are ideal for such service locations since they are mechanically stronger than porcelain and also provide better performance under heavy pollution.

Key Parameters to Consider

Fig. 7 provides critical dimensions, as per IEEE C57.19.01-2017 Section 4.

• Suitable bushing technology;
• Current rating equal or greater than existing bushing;
• Turret, tank cover and CT opening allowance;
• Static shielding requirements and tank clearances;
• Lower length “L” dimension;
• Lower end diameter ‘Dmax’;
• Bottom connection configuration A, R;
• Flange mounting BCD and number of holes;
• Air side height and air clearance requirements;
• Mounting angle to vertical;
• Top terminal configuration, A, R.

Meeting the above critical dimensions as well as verifying internal transformer tank electrical clearances and shielding provisions will help minimize risk of bushing incompatibility.

In addition to improvements in bushing technology, on-line sensors also contribute to prevention of transformer failures. For example, Siemens has an upcoming sensor system measuring capacitive leakage current and providing comparison/sum of values to provide basic information on health of a bushing. Benefits include reducing unplanned outages as well as cost savings achieved through planning replacement or mitigation measures.

Conclusions

Resin-impregnated synthetic style bushings represent an exciting technological innovation and these oil-free, paper-free dry transformer bushings are now available for system voltages up to 550 kV. The flame-resistant insulating body of each RIS condenser bushing is made of a special synthetic impregnated with epoxy resin under vacuum. The design of these new bushings is based on mature RIP condenser core technology, proven in field for more than 60 years. While paper is a good electrical insulation material, it is also hygroscopic and absorbs moisture from its environment. Humidity negatively impacts power factor and ageing of bushings but proper drying of paper is time and labor-intensive process during manufacture. Following an intensive development program, the special paper used in RIP bushings can now be replaced with a synthetic web with more homogeneous properties and minimal moisture absorption that substantially reduces or even eliminates such disadvantages.

RIS bushings are characterized by stable dielectric properties, attributable in part to the major reduction in risk of moisture absorption at exposed active surfaces, e.g. the oil end of transformer bushings. In addition, their higher temperature class gives bushings of this series greater thermal reserves. Since RIS bushings are not oil-filled, there are no constraints with regards to the storage position. The standard RIS bushing are designed for installation in a position 0-30° from the vertical and in many cases horizontal installations at 90º from vertical are possible.

KERI High Power & High Voltage Laboratories

South Korea

STRI

Sweden

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References   
[1] C57.19.00-2004 IEEE Standard General Requirements and Test Procedures for Power Apparatus Bushings
[2] CIGRE Transformer Reliability Survey WG A2.37 Interim Report April 2012
[3] CIGRE Transformer bushing reliability A2-755 2019
[4] Bushings for power transformers, K. Ellis. AuthorHouse, 2011.
[5] Section 4.1.2, CIGRE Transformer bushing reliability A2-755 2019
[6] Bubble evolutions in bushings, ABB publication 1ZBC000001C2704, 2011.
 [7] Parts and Service News For the Power Transformer and Circuit Breaker Maintenance Community, March 1998, Special No. 1
 [8] C57.19.01-2017: IEEE Standard for performance characteristics and dimensions for power transformer and reactor bushings
 [9] IEC 60137-2017: Insulated bushings for alternating voltages above 1000V

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