36-WG11/Stafford/105

21 December 2001 SITE POLLUTION SEVERITY DA Swift 1.

INTRODUCTION

Ideally, well-established definitions should be provided in IEC Guides. For the revision of 60815, the literature has been studied to determine what is said about “Site Pollution Severity”. Although authoritative publications [1][2][3][4][5] have dealt with this subject, such a definition has – unfortunately -- not been given. This is probably because a succinct explanation is difficult to produce with respect to the pollution flashover performance of high-voltage insulators. Maybe the best way forward is to summarise briefly those main points that are relevant to the approach currently being adopted by IEC TC 36 Working Group 11. The aim being to establish a specification to facilitate the selection and dimensioning of insulators for use at a particular site. The following proposal is offered for discussion. 2.

SALIENT POINTS

Using -- in general -- the phrases and terms given in the various publications, the main points are: (a) The aerodynamic catch from air is the principal process by which the insulators are contaminated [3]. (b) Although wind-borne pollution can be collected in the Directional Deposit Gauge, this value has to be correlated with that collected on an insulator before it can be used for the aforementioned task [1]. (c) NSDD, in addition to ESDD, affects the flashover strength [5]. (d) Obviously the most accurate measurement of ESDD/NSDD, or leakage current, is made on the candidate-insulators that are being considered for installation at that site. However, this is not always possible and so a short string of cap and pin units, or a porcelain longrod , can be employed as a substitute insulator [2]. (e) For the purpose of comparison between different sites, standard insulators should be employed for this substitute insulator [2]. If applied extensively, the results can be used to produce a pollution map [5]. (f) The pollution collected on the insulator is affected not only by the shape, size and material of the insulator but also by its mounting position, i.e. its angle to the vertical and its orientation [2]. (g) An equilibrium deposit is reached when rates of catch and natural purging – due to air flow, rain or leaching during high humidity conditions – are equal. Times to equilibrium may vary from days to years [3]. (h) Measurements must be repeated at intervals frequent enough to find the maximum levels between periods of natural washing [2]. (i) When there is no natural washing, ESDD (and NSDD) can be expressed as a function of logarithm of time [3] – from which a maximum value can be estimated. (j) To a first approximation, a.c. energisation does not affect the amount of pollution collected [1]. In contrast, d.c. energisation substantially increases the pollution-catch [5].

1

3.

COMMENTS

Based on these points, a specification for determining the Site Pollution Severity could be something along the lines of the following: The Site Pollution Severity (SPS) is the maximum value of either ESDD/NSDD, or Leakage Current, recorded over an appreciable period of time – i.e. one or more years -- on a vertically mounted standard insulator comprising either a string of cap and pin units or a porcelain longrod. If rain occurs during this measuring period, the measurements must be repeated at appropriate intervals; SPS is then the largest value recorded during this series of measurements. Note (a): even if the largest values of ESDD and NSDD do not occur at the same time, then SPS is –nonetheless -- taken as the combination of these largest values. Note (b): when there is no natural washing during the measuring period, the maximum value of ESDD/ NSDD can be estimated from the plot of deposit density as a function of the logarithm of time. When using SPS, either to define a test or to dimension candidate-insulator, its value must – of course – be multiplied by appropriate factors to take account of material, shape, size, mounting-angle, orientation of the candidate-insulator and the type of energisation. However, this aspect will be dealt with in other parts of the revised version of 60815 (hopefully, covered in my subsequent documents). To broaden the discussion of this topic, it is worth noting that Pollution Severity Monitors [4] need to be calibrated against the pollution collected by insulators. Although such instruments can also be calibrated in an artificial pollution test – thereby enabling the site’s reading to be expressed in terms of the test’s severity – correlation with the corresponding values for a candidate-insulator is still necessary. This same argument also applies when using flashover statistics for insulators that are not the candidate-ones. That is, a site can be specified in terms of the withstand severity of the standard insulator in the artificial salt-fog test when that insulator has a certain flashover probability at the site for the same test-voltage. However, the Figure of Merit [6] of the candidate-insulator relative to that of the standard one still needs to be known. 4.

REFERENCES

[1] Lambeth PJ: Effect of pollution on high-voltage outdoor insulators, Proc. IEE, vol.118, No.9R, 1971. [2] Cigre Working Group 33-04: The measurement of site pollution severity and its application to insulator dimensioning for a.c. systems, Electra No.64, 1979. [3] Looms JST: Insulators for high voltages, Book, IEE Power Engineering Series No. 7, Peter Peregrinus Ltd, 1988. [4] Cigre Task Force 04.03 of Study Committee 33: Insulator pollution monitoring, Electra No. 152, 1994. [5] Cigre Task Force 33.04.01: Polluted Insulators: A Review of Current Knowledge, Cigre Brochure 158, 2000. [6] Houlgate RG, Lambeth PJ and Roberts WJ: The performance of insulators at extra and ultra high voltages in a coastal environment. CIGRE paper No. 47-15, 1982.

2