Revision of IEC 62217-2012 and IEC TS 60815-1/-2/-3:2008 started from 2017 under IEC TC 36 MT19 and WG11. This edited contribution to INMR by LIANG Xidong of Tsinghua University in Beijing introduces the main revisions and related explanations so as to better understand the revisions of these two important documents in the field of outdoor insulators.
Background
IEC 62217 and IEC TS 60815 series are under revision since 2017. IEC 62217 is the standard on the general definitions, test methods and acceptance criteria of polymeric high voltage insulators. It provides the general technical requirements for different types of polymeric insulators. IEC TS 60815 is the series technical specifications on the selection and dimensioning of ceramic/glass and polymeric insulators intended for use in polluted conditions. Current version of them are IEC 62217-2012, IEC TS 60815-1/-2/-3:2008 and IEC TS 60815-4:2016.
The revision of IEC 62217-2012 and IEC TS 60815-1/-2/-3:2008 started from 2017 under IEC TC 36 MT19 and WG11. Now it is nearly finished. The revision of above relevant documents is introduced in this paper in two separate parts. The selection and dimensioning guide for d.c. insulators specified in IEC TS 60815-4 is not under revision this time.
Part 1. Revision of IEC/TS 60815-1/-2/-3:2008
1.1. Introduction
For outdoor insulators, the deposition of environment pollution is an unavoidable problem they have to face for tens of years during their lifetime. Insulators’ pollution withstand voltage decreases rapidly under wet condition when pollutant accumulated onto its surface. Therefor the proper selection and dimensioning of insulator become one of the key questions for the insulation co-ordination of high voltage power systems exposed to polluted conditions. Under design will result in the increase of flashover risk for high voltage system, and over design will result in the obvious increase of tower dimension or high voltage equipment dimension.
The purpose of IEC TS 60815 series is to provide a guide for the proper selection and dimensioning of overhead line and substation insulators. The first edition of IEC TR 60815 was published in 1986 [1]. Its revision started in 1997 and lasting for more than 10 years. The goal of the revision in 1997 was to make the technical document applicable to both a.c. and d.c. systems, as well as to insulators made from different materials, containing ceramic, glass and polymeric insulators. The second edition of IEC TS 60815-1/-2/-3:2008 are the current version under revision [2,3,4]. The revision of the second edition launched in 2017. The technical structure remains the same.
The difficulties of the revision come from several aspects. The first one is the proper evaluation of pollution environment. Natural pollution and wet situation differ from place to place. The second one is about power system operation experiences and maintenance regulations which differ from one utility to the other. The third one is about the tradition and habit of insulator selection and application, which also differ from one utility to the other. All these differences increased the difficulties of the revision work. Moreover, IEC TS 60815 series is not a design code for line and substation project. It is a guide in general. Sometimes it provides suggested result and sometimes it only provides a method for users to find their solution. It is not easy for readers with different background to correctly understand the technical document, and to understand the way to describe and organize the contents.
1.2 Main Steps & Basic Approaches in Insulator Selection & Dimensioning
The proper selection and dimensioning of outdoor insulators include two main steps: (1) Evaluation of environment pollution type and severity, and (2) Selection of insulator profile and dimensioning of insulator’s unified specific creepage distance (USCD).
Pollution and wet conditions are different from one place to the other in a big area. Insulators with different profiles accumulate different site pollution severity (SPS) even they are located at the same place. Pollution withstand voltage for different insulators are different even if they are artificially polluted to the same pollution level. These three key issues make the proper insulator selection and dimensioning a very complex and very difficult work. Therefore, the evaluation of site pollution severity (SPS) is normally using classified results rather than exact ones.
As to the first step, the evaluation of pollution condition is performed only from the judgement of pollution source and distance, wind direction and wetting frequency in IEC TR 60815-1986 [1]. It is obviously not a satisfactory method for insulator’s pollution evaluation. In IEC TS 60815-1:2008, direct pollution measurement from reference insulators is required [2].
Pollution measurement is quite a big topic. Before the starting of pollution measurement, there are three approaches proposed in IEC TS 60815-1:2008 [2]. Approach 1 is based on experiences. If previous experiences on insulator selection and dimensioning are satisfied, this experience can be followed and no pollution measurement is required. Approach 2 and 3 are based on pollution measurement. The artificial pollution test will be performed on selected and dimensioned candidate insulator if budget and time are allowed in approach 2. Nevertheless, there is no confirmation through artificial pollution test in approach 3.
In many cases, users will find that they have some experience but not sufficient to follow approach 1, and they took some measurement but not sufficient to follow approach 2 and 3. Then a combination of both approaches seems necessary and helpful.
1.3 Site Pollution Severity Class: Technical Link Between IEC TS 60815-1 & IEC TS 60815-2/-3
IEC TS 60815 series is a whole. The technical content of above mentioned two main steps are distributed into three documents, IEC TS 60815-1/-2 and -3. The evaluation of environment pollution type and severity are described in IEC TS 60815-1. The general principle of insulator selection is also included in IEC TS 60815-1. The detailed selection and dimensioning guide on insulator’s unified specific creepage distance (USCD) are introduced in IEC TS 60815-2 for ceramic and glass insulators, and in IEC TS 60815-3 for polymer insulators, both for a.c. systems. The key technical link between IEC TS 60815-1 and IEC TS 60815-2/-3 is the pollution class obtained based on the measurement of pollution accumulated on reference insulators. Fig. 1 shows this link and the relation between IEC TS 60815-1 and IEC TS 60815-2/-3.
Environment pollution type and severity are evaluated by different methods, mainly through the measurement of pollution accumulated onto the reference insulators, to obtain the pollution class. This is the work in IEC TS60815-1.
Then, the unified specific creepage distance of reference insulator (RUSCD) is suggested according to worldwide experience under a.c. condition, shown in Fig. 2. This figure is important for insulator dimensioning as well as for IEC TS 60815 series. It comes from tens of years worldwide experiences and it is familiar to worldwide utilities.
The shed profile of the candidate insulator is selected according to the pollution type. Then the USCD of the candidate insulator is dimensioned after several corrections of RUSCD, say shed profile correction, diameter correction, altitude correction and multiple string correction. Artificial pollution test is performed to confirm the result of insulator selection and dimensioning in approach 2. This is the work in IEC TS 60815-2/-3, shown as in Fig.1.
The problem from pollution presents quite a big diversity. Fig. 2 is the normal knowledge and experience for worldwide utilities when dealing with the pollution problem. It could be recognized as another type of definition on insulator’s pollution severity in a certain sense. For example, the pollution severity will be recognized as class c if an insulator works well for many years under the USCD of 34.7mm/kV, and with some flashovers under the USCD of 27.8mm/kV. This is why the relation between ESDD/NSDD and SPS class in IEC TS 60815-1:2008, shown as Fig. 3 in this paper, is followed with a description that it could be overlaid when sufficient local or national information is available. The same meaning was kept in this revision. This is the same meaning that approach 1 is at the first position, and operation experience is with the highest priority when determining pollution class, selecting and dimensioning insulators. Unfortunately, this figure appears in IEC TS 60815-2 and -3, a little bit later in IEC TS 60815 series, so its importance may be ignored.
Table 5 in IEC TS 60815-1:2008 on the description of typical environments is helpful for the overall evaluation of pollution class after pollution measurement while its importance may also be ignored. In this revision, the table is slightly revised and renumbered.
1.4 SPS, SPS Value & SPS Class
Site pollution severity (SPS) was introduced in IEC TS 60815-1:2008. The concept of SPS actually contains two meanings, namely SPS value and SPS class. SPS value is the measured results from reference insulators, while SPS class is obtained after overall judgement sometimes. For type A pollution, SPS value includes both ESDD and NSDD. However, in the 2008 version, SPS value and SPS class were not distinguished, which is inconvenient in actual use. In this revision, a clear distinction was made between SPS value and SPS class.
After the measurement of SPS value from reference insulator, i.e., ESDD/NSDD for type A pollution, SES for type B pollution, SPS was classified into Class a, b, c, d, and Class e from very light, light, medium, heavy and very heavy in the 2008 version. Some users worried that USCD of Class e (say 53.7mm/kV in figure 2) could not prevent flashover in some extremely polluted conditions. Then a new name of pollution class, extremely heavy Class f, is added in this revision at the top-right corner of Fig. 4. Class f is only for some special situation of extremely heavy pollution when the USCD of Class e cannot meet the requirements. The corresponding USCD value is not assigned to Class f in this revision.
In Fig. 4, ESDD/NSDD were measured on non-energized reference insulators. The pollution accumulation of non-energized reference insulator is closer to that on energized one under a.c. voltage (of course they are different), when compared to the situation energized under d.c. voltage. Therefore, the title of figure 4 was marked with under a.c. voltage. It is only to remind readers that the pollution accumulation under d.c. voltage will be significantly different from the situation of non-energized insulators.
Reference insulators for type B pollution, both cap-and-pin and long rod insulators were added in this revision. The content of pollution measurement by directional dust deposit gauge (DDDG) was updated in this revision.
1.5 Pollution Measurement & Pollution Map
In IEC/TS 60815-2008, the SPS is determined by direct measurement on polluted reference insulators. It sounds perfect compared to the rough judgement from pollution environment. However, it must be noted that the less measurement points may result in false SPS description. On the other hand, many measurement points need a huge workload. Only when the measurement is carried out at the right place and right time, and the measurement is sufficient and the results are accurate, the measurement results can be used to determine the SPS class. Improper measurement of pollution will lead to improper pollution class judgement.
Several specification and suggestions about proper pollution measurement on reference insulators were specified and suggested in the 2008 version and this revision. But the setting of proper measurement location was not mentioned. The drawing of pollution map is very helpful for proper pollution classification after pollution measurement. Unfortunately, pollution distribution map is not available for current revision. CIGRE just launched a joint working group CIGRE JWG C4A3B2B4.75 to deal with the pollution map in 2023, which will conduct a systematic review of pollution measurement and pollution map drawing. Hope the result could be used in next revision of IEC TS 60815 series.
The State Grid Corporation of China (SGCC) carried out a large amount of pollution measurement, obtained huge measurement data and relevant experience (more than 23,390 measurement points set within the area of SGCC), and drawn the pollution map covering entire service area. And this pollution map is updated regularly. Some important experience of proper pollution classification through pollution map is worth to be introduced shortly here. More information could be found in the paper published at IEEE Electrical Insulation Magazine in 2020 with the title of “Improving the Outdoor Insulation Performance of Chinese EHV and UHV AC and DC Overhead Transmission Lines”. Experiences of China over 50 years are summarized in this paper [5].
Fig. 5 shows the example of measurement result, Fig. 5a, and overall evaluated local pollution map, Fig. 5b. The dispersion of SPS measurement value is inevitable, not because of measurement errors, but resulted from the randomness caused by very local environments around reference insulators. In practice, insulators used within a certain region quipped with the same or similar creepage distance. It means that only one pollution class is recognized in one region. It is thus not practical to determine SPS class directly for each single measurement point. Instead, SPS class should be determined by considering measured SPS values, type of pollution, wetting characteristics, and operation experiences, taking the operation experiences as the priority factor to determine the proper pollution class for this area [5].
1.6 Correction from RUSCD to USCD of Selected Candidate Insulator
After the determination of pollution class, RUSCD can be determined based on the reference insulator according to the SPS class from IEC TS 60815-2/-3, as shown above in figure 1 and 2. According to the type of pollution, a suitable candidate insulator can be selected considering the profile. Then the correction from the RUSCD of the reference insulator to the USCD of the candidate insulator is needed considering four factors: profile, altitude, diameter, and the number of insulators in parallel, as shown in Equation 1.
Ka, Kd, Ks and Kp represent altitude correction factor, diameter correction factor, shed profile correction factor and correction factor of insulator strings in parallel respectively.
It should be noted that the reason for the correction still comes from the pollution measurement. The measurement is based on the reference insulator. However, the candidate insulator is different from the reference insulator, so correction is required. In this revision, Ks and Kp are newly introduced, Ka is slightly modified and there are still many discussions for Kd.
When the profile is obviously different, the USCD may be different from RUSCD. The correction factors are still under study. Kp is 1.1 when there are many insulators in parallel, e.g., transmission line sections with more than 100 strings and is assumed to be polluted to the same degree and affected by the same pollution/wetting event. Kp is 1.0 when there are few insulators in parallel, e.g., station insulators and apparatuses.
After the correction, confirmation by testing is required according to Approach 2, but there is no confirmation by testing for Approach 3.
1.7 HTM & Non-HTM
This revision makes a clear distinction between the definition and scope of non-HTM and HTM. In heavily polluted areas, HTM insulators present improvement of pollution withstand voltage. In general, HTM insulators can be used with shorter creepage distance compared with non-HTM insulators in the same area. As to the quantitative expression of shorted proportion for HTM insulators, it took great number of discussions within WG11, for example, the words of “up to 30% reduction”.
1.8 Other Items in This Revision
For pollution measurement, the DDDG measurement method was also revised. The statistical method is largely updated. Profile suitability on ceramic and glass insulators was simplified from five levels to three levels.
Part 2: Revision of IEC 62217-2012
2.1 Introduction
The 1st and 2nd editions of IEC 62217 were published in 2005 and 2012, respectively [6,7]. The revision of the 2nd edition was started in 2017 as well. It specifies the general technical requirement for all polymeric high voltage insulators to better ensure the long-term performance of insulators. It is also expected to provide specification for insulator design, material selection, manufacturing process and design test.
In this revision, the scope has been expanded to make it applicable to AC and DC systems, to transmission lines and power stations, to indoor and outdoor use. Some test items have been improved as well.
IEC 62217 is revised to strengthen the screening of unqualified insulators through design tests for materials and manufacturing, and to help to control long-term performance through targeted tests. The recognized modifications of technical requirements were involved in this revision while others were not involved.
2.2 Housing Material
Test samples of housing material are encouraged to be taken from insulators first. If this is not possible in some cases, it is then allowed to prepare samples with the same chemical composition and cure process.
Previous versions of the IEC 62217 did not include tests on hydrophobicity. This revision introduces tests on hydrophobicity and distinguishes the hydrophobic transfer test methods of HTM and non-HTM. It should be noted that the hydrophobic transfer test is to distinguish HTM and non-HTM instead of ranking the performance of different HTM. Based on the successful experiences of different countries, two methods of hydrophobicity transfer were introduced as in IEC TR 62039-2021 [8]. Method A is to cover a layer of foil as a frame, and brush slurry in the window of the mask. Method B is to pretreat the HTM surface with dry kieselguhr first, and then apply the pollution solution to the samples. After the test, the average contact angle needs to be greater than 90 degrees, and the minimum is greater than 80 degrees, to be considered as passed.
Tracking and erosion is a key performance of polymeric insulator housing materials, but the DC tracking and erosion test still needs to be further studied.
2.3 Core Material
In previous version, the water diffusion test on core material was expressed as with or without housing. In this revision, water diffusion test was split into two parts, with and without housing. Test samples are also encouraged to be cut from insulator first if applicable.
The leakage current criterion of water diffusion test was 1 mA r.m.s. without relation to rod diameter in previous version, which is obvious unreasonable. In this revision the leakage current criterion is modified into diameter dependent ones. The criteria are given separately according to the diameter, as follows:
• For solid cores (without housing) with diameter D in mm,
I≤ 100μA(r.m.s), when 0＜D < 50mm
I≤ 200μA(r.m.s), when 50 ≤ D < 90mm
I≤ 500μA(r.m.s), when 90 ≤ D < 200mm,
I≤1000μA(r.m.s), when 200 ≤ D < 400mm.
For solid cores with diameter larger than 400 mm, experience is currently not available.
• For hollow cores (without housing), I ≤ 500μA (r.m.s)
• For resin cast test specimens and for core material which is not manufactured by using anhydride hardener, I ≤ 1000 μA (r.m.s)
• For solid cores (with housing) with diameter D in mm,
I≤ 100μA(r.m.s), when 0＜D < 50mm
I≤ 200μA(r.m.s), when 50 ≤ D < 90mm
For solid cores with diameter larger than 90 mm, experience is currently not available.
If the water diffusion test on solid core with housing is passed, no additional test for solid core without housing is required for the diameter less than 90 mm.
For suspension insulators, a stress corrosion test has been added. The specific test steps and criteria are the same as in IEC/TR 62039-2021.
2.4 Electric Field Control Around End Fittings
Electric field control around end fittings should be given more attention in the case of polymeric insulators. In the Appendix, it is pointed out that three electric field thresholds need to be considered, namely:
(1) electric field on the grading/corona ring and end fitting, proven by test and simulation;
(2) electric field along the housing surface, proven by test and simulation;
(3) electric field at the so-called triple point at the sealant, where air insulation and housing insulation meet the metallic end fitting working also as the seal. See Fig. 6 as an example, proven by simulation.
3. Summary & Outlook
The revision of IEC 62217 and IEC TS 60815 series last for 6 years and experienced 3 years’ online meetings. Hopefully the final version could be finished in 2024. Too much work remains to be done. Hope the quality of insulator could be improved. Hope insulators could be properly selected and dimensioned. Hope insulators operation experience could be better. The strong support of experts in IEC WG 11 and MT 19 is greatly appreciated.
References:
[1] IEC TR 60815-1986, Guide for the selection of insulators in repect of polluted conditions
[2] IEC TS 60815-1:2008, Selection and dimensioning of high-voltage insulators intended for use in polluted conditions – Part 1: Definitions, information and general principles
[3] IEC TS 60815-2:2008, Selection and dimensioning of high-voltage insulators intended for use in polluted conditions –Part 2: Ceramic and glass insulators for a.c. systems
[4] IEC TS 60815-3:2008, Selection and dimensioning of high-voltage insulators intended for use in polluted conditions –Part 3: Polymer insulators for a.c. systems
[5] Xidong Liang, Shaohua Li, Yanfeng Gao, Zhiyi Su, and Jun Zhou, Improving the Outdoor Insulation Performance of Chinese EHV and UHV AC and DC Overhead Transmission Lines, IEEE Electrical Insulation Magazine 2020 Vol. 36 No. 4 7-25
[6] IEC 62217-2005, Polymeric insulators for indoor and outdoor use with a nominal voltage >1 000 V – General definitions, test methods and acceptance criteria
[7] IEC 62217-2012, Polymeric HV insulators for indoor and outdoor use – General definitions, test methods and acceptance criteria
[8] IEC TR 62039-2021, Selection guidelines for polymeric materials for outdoor use under HV stress
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