Evaluating Performance of Polluted Insulators Simplified IEC Approach vs. Statistical Approach
UTILITY PRACTICE & EXPERIENCE
Evaluating Performance of Polluted Insulators Simplified IEC Approach vs. Statistical Approach Simplified design approaches for AC insulator selection are reported in Parts 1, 2 and 3 of IEC/TS 60815 Edition 1.0 200810 “Selection and dimensioning of high-voltage insulators intended for use in polluted conditions”. Finalization of a similar document (IEC 60815-4 Ed. 1.0 Part 4) covering insulators for DC systems is ongoing. The problem is that these specifications only describe a so-called ‘deterministic’ or simplified approach, which unfortunately risks leading to design inaccuracies, especially in severely contaminated service environments and under DC. Inaccurate insulation design under polluted conditions has the potential to greatly increase overall system costs. For example, ‘over-design’ (i.e. specifying extremely long and costly station insulators and apparatus as well as huge towers to accommodate longer insulator sets) can yield excessively high investment costs. On the other hand, ‘under-design’ can result in unacceptably high operating costs by requiring expensive pollution counter-measures such as periodic washing, coating with RTV silicone, etc. It is therefore clearly desirable to minimize the possibility of design inaccuracies. This can be done by following a more comprehensive
Introduction
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The following article, contributed by insulation expert, Igor Gutman, of STRI and respected long-time industry consultant, Alberto Pigini, discusses their view of the simplified approach suggested within the current IEC specifications. They then contrast this with a proposed more accurate methodology. Since this involves statistical analysis to assess compliance with required reliability targets, they refer to it as the ‘statistical approach’. While such an analysis could be made for both marine and industrial type service environments (represented in the laboratory by the salt fog and solid layer pollution test methods respectively), only the second type of environment is considered, usually quantified by ESDD and NSDD. Moreover, reference is made only to AC with the aim of highlighting all the advantages of the statistical approach. Nevertheless, these conclusions are equally applicable to DC, where design under pollution is even more important, since pollution level is the key design criterion for most service environments.
The proposed ‘statistical approach’, by contrast, aims for more accurate design based on detailed information on site conditions as well as on comparative insulator performance. This includes evaluating risk of flashover for the number of insulators exposed to the same pollution/wetting event in that specific service environment.
Both approaches comprise the same basic steps, even if all are not necessarily explicit using the simplified approach: • Determining site pollution severity • Determining reference withstand/ flashover characteristics of the insulator as a function of severity • Determining the insulation (type and dimensions) necessary for the specific contamination conditions.
Q3 2013
Three different design approaches are presented in Part 1 of IEC/ TS 60815 Edition 1.0 2008-10, including some considerations about the statistical approach. However, the dimensioning process detailed in Part 2 (applying to porcelain and glass AC insulators) and Part 3 (applying to polymeric AC insulators) is basically a deterministic one and highly simplified for both ceramic and composite insulators.
approach to design based on measuring site pollution severity as well as insulator strength characteristics.
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Information on both pollution levels and wetting events are needed to optimize insulation design.
Photos: INMR Š
or incomplete. With the simplified approach, pollution severity of a site is therefore often arbitrarily attributed to one class or another based on qualitative considerations, such as past experience.
Estimate of Site Pollution Severity IEC Simplified Approach Compared to IEC TS815 issued in 1986, an important step forward was made in IEC/TS 60815 Edition 1.0 2008-1, i.e. the concept was introduced that actual contamination severity depends not only on the soluble portion of the contaminant (ESDD) but also on the amount of the non-soluble component (NSDD). As illustrated in Fig. 1, the updated approach in IEC 60815-1 defines five discrete site pollution severity (SPS) classes (i.e. very light, light, medium, heavy and very heavy) as well as the specific ESDD and NSDD ranges to which these classes correspond.
Inaccurate insulation design under polluted conditions has the potential to greatly increase overall system costs. However, in many practical situations, information on ESDD and NSDD levels is either not available
Statistical Approach According to the statistical approach, site severity must be accurately determined before final design, e.g. by installing experimental test stations with nonenergized or energized insulators having geometries similar to those selected for representative site conditions. Based on numerous such measurements, it then becomes possible to define contamination severity having a 2% probability of being exceeded (ESDD2%, NSDD2%) as well as its standard deviation using a log normal statistical distribution. Typical values for standard deviation in terms of Ln(ESDD) have been found to fall between 0.4 and 0.8 and an average of 0.6 is typically used. Other parameters collected in the field could also help in arriving at more accurate dimensioning of insulation. Among these are the chemical characteristics of pollutants and their local distribution (e.g. by determining the ratio of contamination on the top versus bottom
Fig. 1: Dependence of pollution severity on ESDD and NSDD for cap & pin insulators.
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Photos: INMR Š
Under-design of insulators in severe service environments can result in unacceptably high operating costs by requiring ongoing, expensive pollution counter-measures.
surfaces of cap & pin insulators as well as along the string length). Such contamination parameters are sometimes not available for the specific insulator selected under energized conditions. In these cases, a reasonably precise estimate of ESDD2% can still be made by translating results obtained on the reference string to the actual insulators using a series of coefficients based on experience. It must be pointed out, however, that these coefficients are subject to uncertainty. Therefore, as far as possible, reference to measurements carried out on energized insulators of
the specific type being selected are preferable.
insulator withstand characteristics are not given but rather used to define required unified specific Furthermore, it should be creepage distance (USCD) values. emphasized that any contamination The basic assumption in Part 2 of layer by itself will not lead to IEC/TS 60815 Edition 1.0 2008flashover unless it is somehow 10 is that withstand of all insulators wetted. Therefore, it is also necessary can be expressed in terms of USCD, to establish the number of critical originating from a reference curve wetting events each year (Nt) and (shown in Fig. 2) and where, this aspect is not covered using the USCD = A * ESDDa (mm/kV, mg/cm2) EQ1 simplified approach.
Estimation of Insulator Performance
averaging the specific creepage distance values for cap & pin insulators when tested according to the standard solid layer methodology with NSDD close to 0.1 mg/cm2.
IEC Simplified Approach In the simplified approach, specific
Photo: INMR Š
Measuring ESDD and NSDD levels near polluted cement plant in South Africa, using directional dust deposit gauge and reference string.
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Fig. 2: Reference USCD versus pollution classes.
Deposition of contamination is usually different along top and bottom surfaces of insulators as well as along string length.
Fig. 3: Results of pollution tests with solid layer method on cap & pin insulators with standard and anti-fog profiles.
Based on results, as per Fig. 3, the average curve (close to that assumed as a reference in IEC 60815 Parts 2 and 3) can be approximated by the following parameters A=50 and a=0.22, i.e.
following final indication is given in Part 2 of IEC 60815 for ceramic insulators (the continuous curve in Fig. 2):
USCD = 50 * ESDD0.22 (mm/kV, mg/cm2) EQ2
where Kad=1 when Da is less than 300 mm; Kad=0.0005 Da + 0.85 when Da is equal to or larger than 300 mm.
This simplified approach assumes that, for all insulators, the same α coefficient of about 0.22 can be applied and that reference curves are just shifted in the case of station insulators, assuming different A values (i.e. to account for their larger diameters). This is assumed for a wide range of parameters characterizing insulator geometry. In particular, based only on laboratory tests, the USCD values for ceramic insulators of large diameter would be obtained by multiplying the values in Fig. 2 and the Kad value from Fig. 3 (i.e. the dotted upper curve in Fig. 4). However, to also take into account the fact that, in service, large insulators contaminate less than those with small diameters, the
USCDD = USCD * Kad EQ3
In the case of composite insulators, Part 3 of IEC 60815 gives basically the same indications as for ceramic insulators while suggesting that USCD can be different and indeed that a different USCD may be used based on available service experience. The same is said in regard to influence of insulator diameter. Statistical Approach In reality, dependence of an insulator’s pollution performance on its geometry and material is much greater than assumed in the simplified approach. The parameters of EQ 1 therefore will depend on
both an insulator’s material and the efficiency of its profile. Just focusing on cap & pin insulators, for example, the results per Fig. 3 indicate that, even assuming the same slope (α =0.22), the upper and lower curves can be approximated by assuming A values of 65 and 40 respectively (see Fig. 5) with resulting variations in required USCD of over 50%. Considering a wider range of experimental results for cap & pin insulators (as per CIGRE brochure 361 issued by WGC4.03.03 “Outdoor insulation in polluted conditions: Guidelines for selection and dimensioning. Part 1: the AC Case) an even larger spread of results is found, with α varying from 0.12 to 0.37 and A values ranging from 37 to 86. The accuracy of such an extrapolation on the basis of the average curve for cap & pin insulators is even lower when applied to station insulators.
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Photo: INMR ©
Similarly, the indications in Part 3 of IEC 60815 applying to composite insulators are less accurate and one can even say substantially ‘qualitative’. For example, results of recent tests on composite long rod line insulators with the modified solid layer test procedure are reported in Fig. 6 and indicate that the margin for line composite insulators with respect to cap & pin insulators is greater than 30% at high pollution severities.
No matter the extent of contamination layer, wetting is needed to trigger flashover.
Furthermore the results in CIGRE WG C4.303 brochure “Artificial Pollution Test for Polymer Insulators, results of Round Robin Test” (to be issued in 2014) are lower than those suggested by service experience and expressed by the following equation (i.e. the continuous line in Fig.6): USCD = 37 * SDD0.16 (mm/kV, mg/cm2) EQ4
Fig. 4: Influence of average diameter on USCD for ceramic insulators.
Fig. 5: Range of results from pollution tests performed by solid layer method on cap & pin insulators with standard and anti-fog profiles.
It has to be considered, however, that EQ4 is intended to give a general indication for composite insulators. The significantly better performance of composite line insulators compared to cap & pin insulators can at least partially be attributed to size (i.e. their diameter is much less). All these considerations point to the importance to selecting insulator design based on actual performance, as derived from laboratory tests. Usually such tests are made contaminating the insulators with a standardized slurry consisting of the amount of salt needed to obtain the required SDD value and also a quantity of kaolin leading to NSDD values of about 0.1 mg/ cm2. Theoretically, to obtain results that are directly applicable to the specific insulator design in the specific environment where it will be installed, tests should be made with a contaminant composition as close as possible to the actual one in service (e.g. respecting the actual NSDD/ESDD ratio). However, since this is not usually the case, the equivalent laboratory SDD value can be evaluated by correcting the ESDD value by a coefficient Kn, with the
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following equation used to obtain an approximation of Kn, assuming that the same indications hold for AC and DC: Kn = (NSDD/NSDDo)0.35 EQ 5
where NSDDo is 0.1 mg/cm2 and for NSDD≼0.02 mg/cm2.
Insulation Selection for Specific Pollution Level Simplified IEC approach The simplified approach in IEC 60815 Parts 2 and 3 is essentially a deterministic approach. It arrives at a definition of pollution classes which should assure correct application of the insulators, but without giving any indication of the expected risk of flashover and thus about resulting system reliability.
Fig. 6: Comparison of results on line composite insulators and reference curves.
Statistical Approach The statistical approach, by contrast, uses specialized software to arrive at indications of the expected risk of flashovers, assuming sufficiently accurate data input concerning pollution severity and insulator performance characteristics. A schematic representation of the parameters necessary for such a risk evaluation is shown in Fig. 7.
Examples of Applications 1. Comparison of Cap & Pin / Composite Insulators with Performance Conforming to Average Curves (EQ2 and EQ4) An example of this type of evaluation is made for a string used for a 420 kV system, considering two insulation options: ceramic cap & pin and composite. The comparison is made assuming a typical creepage factor of 3.2 for ceramic and 3.6 for composite insulators, ESDD of 0.3 mg/cm2, NSDD of 0.1 mg/cm2, 100 insulator strings in parallel on the hypothetical line section of about 10 km length, exposed to the same heavy contamination conditions and with 10 critical wetting events per year. The example assumes that the performance of ceramic and composite insulators is described by EQ2 and EQ4 respectively.
Figure 7: Schematic representation of parameters necessary for flashover risk evaluation.
Fig. 8: Number of flashovers per year versus insulator length.
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Design According to Simplified Approach The simplified approach does not formally directly cover such service conditions, which can be characterized as ‘extreme’ pollution (i.e. beyond ‘very heavy’ and shown as the grey zone in Fig. 1). However, assuming that EQ2 could still be applied, a USCD value of about 55 mm/kV would be suggested.
Figure 9: Required specific creepage distances for AC lines in extreme environment.
Design According to IEC Simplified Approach The site pollution severity (SPS) in this case can be classified as ‘heavy’ (see Fig.1) and a USCD of 43.3 applies (see Fig.2). Consequently the following insulator lengths would be specified: cap & pin: 3.32 m; composite insulators: 2.95 m. But no indication can be given about expected system reliability using these chosen insulators lengths installed on the hypothetical line section. Design According to Statistical Approach Estimated numbers of flashovers per year are reported in Fig. 8 as a function of insulator length and it is evident that there is a dependence of required line performance on insulator length. Furthermore, the number of flashovers changes significantly depending on the number of insulators considered subjected to the same pollution conditions as well as on the number of pollution events. The insulator length selected using the simplified approach corresponds to a number of flashovers per year of about 0.3 for the cap & pin insulators (i.e. Mean Time Between Flashover MTBF 3 years), confirming that the simplified approach gives reasonable results
if the assumption is made that the insulator performance conforms to the curve assumed in IEC 60815 Part 2. The insulator length selected for composite insulators according to the simplified approach corresponds to a much lower number of flashovers (an MTBF of about 0.002, or about 500 years). Thus, it is clear that application of insulators selected according to simplified approach would in reality lead to two orders of magnitude difference in reliability of the complete OHL section. 2. Design for Extreme Service Conditions & for Insulators Characterized by Specific Performance The extreme desert and marine environment affecting a transmission line operating in eastern Saudi Arabia, for example, have been described by the following parameters: •
•
• •
Design According to Statistical Approach The statistical approach is applied using the specific insulator performance as determined by laboratory tests on the specific insulators adopted for this line and expressed by the following equation: USCD = 65 * SDD0.29 (mm/kV, mg/cm2) EQ6
Estimated number of flashovers per year as a function of specific creepage distance is shown in Fig. 9. In order to have a low flashover risk per year, unified specific creepage distances of 80-90 mm/kV would be required, as actually adopted and proven necessary by service experience in this environment. It is evident from this example that reference to specific insulator performance characteristics is very important and that the application of the statistical approach is essential whenever there are extreme pollution conditions in AC. The same applies for the DC case in all contamination environments, where pollution conditions generally govern design.
Conclusions
In general, the statistical approach to insulator specification permits an optimized and more rational design ESDD=0.5 mg/cm2, than does the simplified approach. 2 NSDD=2.5 mg/cm Moreover, the application of this Equivalent SDD may be evaluated statistical approach is essential in all as approx. 1.54 mg/cm2, according cases of extreme pollution conditions in AC. Moreover, because pollution is to EQ 5 the main dimensioning parameter in DC, the statistical approach is again 30 wetting events per year the preferred approach to insulator design in this case. 750 strings of ceramic cap & pin insulators in parallel
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