BACKGROUND TO IEC 815
Introduction IEC 815 is a pollution guide which was published by the IEC in 1986 based on principles established during the preceding decade. These principles concerned initially porcelain post insulators and bushings and were later extended to cover porcelain and glass long rod and cap and pin suspension insulators. Much of the basic information in the guide concerning pollution types and levels is also applicable to other, more recent, insulator technologies; but basically the guide is only really applicable to the above insulator types. A common request is for the guide to be applied to composite suspension insulators. In such a case the guide needs to be used with extreme caution since many of the parameters are meaningless or illadapted to the profiles which can be produced by modern technology. This problem has been recognised by the IEC and currently work is under way both in IEC and CIGRE to revise IEC 815 to cover present insulator technology and materials. In the following we shall clarify the applicability of IEC 815 to composite insulators, report on the philosophy of revision as it currently stands in IEC.
The concepts, methods and scope of IEC 815
Firstly, IEC 815 is published as an IEC Guide and as such is intended to give guidance rather than strict rules. The aim of the guide is stated as being " .. to give, on the basis of experience in service and numerous test results in naturally and artificially polluted conditions, simple general rules that should assist in choosing the insulator which should give satisfactory performance under polluted conditions." Furthermore, the list of insulators covered by the guide is limited to ceramic and glass post, bushing, long rod and cap and pin suspension insulators for a.c. lines. Insulators with semiconducting glaze or covered with any insulating material (RTV coatings ?) are specifically excluded. Given these limitations, the guide then defines pollution severity levels in a general manner, giving four levels from "Light" to "Very Heavy" on the basis of the environment (situation, industry, climate etc.). This classification has been widely adopted and is used for site classification throughout the industry. These pollution severity levels are then equated with a "minimum specific leakage distance" (in mm/kV phase-to-phase) for each level on the basis of past experience. At this point the guide already admits a shortcoming, in that "Some insulators specially shaped for particular kinds of pollution may not satisfy these conditions even though they perform satisfactorily in service". A typical example of this is flat-profile insulators originally designed for desert conditions which also show excellent behaviour in marine/industrial pollution. Having established minimum specific leakage distances for each pollution level, the guide then proposes some profile parameters and influencing factors which should be taken into account. Once again, these guidelines are based on service experience and laboratory tests available in the 1970's and are therefore limited to the traditional insulators cited above. The guide states clearly "The following parameters are not intended to limit the future development of insulator design. They simply recommend certain limits....which must be interpreted flexibly in order to provide a high probability of satisfactory performance in service."
The parameters which characterise the profile are then listed and guidance on the influence of position and diameter is given. In the case of the diameter, a weighting factor for the specific leakage distance of post and hollow insulators is given, starting at 300 mm average diameter. It should also be noted that although the position of the insulator is considered negligible in the main text of the guide, the appendix D, which gives the values of the profile parameters, specifically states that they are related to insulators in a vertical position. Finally guidance is given on determination of creepage distance and pollution severity, followed by annexes on greasing and washing, artificial tests and on how to calculate the profile parameters.
Current work on IEC 815 and changes in concept
The official position today As shown above, IEC 815 does not apply to composite insulators, nor can it be applied for d.c. applications. This has been recognised by IEC TC 36 and a request was made to CIGRE WG 33.04 for information. The work is being currently undertaken by a Task Force in WG 33.04 (TF33.04.01 Convenor Don Swift, University of Natal) and an ad-hoc working group of TC 36 officers was also set up (AHWG 815 - H. Kaerner, A. Porrino - TC 36, G. Villa - SC 36A, C. Lumb SC 36B and W. Lord SC 36C). This ad-hoc group prepared the draft New Work Proposal for the revision of IEC 815 which was circulated in July 1997. The work so far in CIGRE has shown many shortcomings of IEC 815, notably that the specific leakage distance concept can only be a first rough approach - needing much modification according to the type of pollution, the insulator profile, the climatic conditions etc. Moreover, the application of IEC 815, as it stands, to composite insulators was declared to be unrealistic. As a result, a letter was addressed to IEC TC 36, this is noted in the minutes of the meeting held in Durban as follows: "... Dr. F. Rizk, Convenor of CIGRE 33.04 sent a letter to the Secretary of TC 36, informing that WG 33-04 is making a review of current knowledge and preparing a pollution application guide .... The letter ends with the following remark: 'We cannot stress too strongly the non applicability of IEC 815 to composite insulators and feel that it should be brought to the notice of the delegates at your next TC 36 meeting'...". The TF 33.04.01 is currently preparing a "review of current knowledge", to be followed by a pollution guide which is intended to aid in applying this knowledge. The AHWG 815 has worked in parallel to define the structure of the future revised IEC 815. Once the results of the CIGRE work are available, they will be transmitted to the official IEC working group which is currently being set up to carry out the actual revision. (At the TC 36 London meeting in May 1997, it was decided to start this work, a new work item proposal was circulated and the work has been approved).
Specific points In order to briefly explain where IEC 815 fails when applied to composite insulators, a summary of the parameters or concepts which are missing or are not covered by the existing guide is necessary. This summary cannot cover all the current work on IEC 815 (the present CIGRE draft of the review of knowledge is 112 pages long), but the following contains some of the primary points given in the draft review of current knowledge by CIGRE TF 33.04.01.
• Scope of the guide/source of knowledge As stated above, IEC 815 was prepared on the basis of knowledge acquired up to the 1970's on traditional insulators, many of them of design dating from the 1950's. Much of this knowledge was of the form "This type of insulator works satisfactorily in this environment", often there was no experience in other environments or at higher pollution levels and generally no appreciation of how much flashover margin the insulator actually had. Furthermore, none of the information concerned composite insulators which were only just beginning to make an appearance. Therefore, the application of this knowledge to the vastly different profiles and materials of modern composite insulators needs extreme caution. • Pollution severity levels The pollution severity levels defined in IEC 815 are useful for defining an environment, however they do not take into account the interaction of the insulator with the environment. It is now recognised that many factors such as self-cleaning, pollution solubility (fast or slow), wetting event frequency and duration, insulator profile, surface state etc. can vastly modify the actual amount of pollution deposited on insulators and the resulting influence of this pollution layer on the withstand voltage of the insulator. Since the pollution layer is generally harmless when dry and the amount of wetting, and its effect, depend on many factors, it becomes necessary to make a "risk of flashover" approach which takes these factors into account. It is therefore impossible to characterise a pollution environment based on the surrounding conditions alone. • Specific leakage distance The specific leakage distance concept can still be a useful tool for comparison of insulators or sites. However its use alone as a guarantee of performance is open to doubt, notably for the reasons stated in the previous paragraph. However, it would appear that the concept is generally very pessimistic so it can be used as a "rule of thumb" without major risks. For example, it has been shown that a composite insulator profile, when compared in artificial pollution tests with a traditional cap and pin insulator, will give a 15% to 30% higher withstand voltage even when hydrophilic. The aspect of hydrophobicity further influences the specific leakage distance concept. Hydrophobic composite insulators present an even higher margin (40% to 50%) with respect to the traditional leakage distances suggested by IEC 815. The evaluation of hydrophobicity loss and recovery, and the resulting influence on the pollution withstand level of composite insulators, is a complex subject which has already generated much discussion and is being taken into account by TF 33.04.01. • Profile parameters The profile parameters given in IEC 815 are, as previously stated, based on traditional insulators. Certain parameters are based on common sense (shed clearance, ratio between arcing distance and leakage distance), some however are based on experience with insulators of much larger diameters than composite suspension insulators. For example, the minimum difference in diameter between alternating sheds was established for post insulators to avoid bridging between sheds under rain where the large insulator diameter led to the collection of a large quantity of water. (The quantity of water dripping off the shed per unit length of the perimeter rises linearly with the average diameter of the insulator). We can suspect that this parameter was only introduced to ensure satisfactory rain performance on larger diameter insulators and was not pollution related. These points are all being studied by the CIGRE Task Force. Solutions or rules may not necessarily be possible with the current state of knowledge, but it is hoped that sufficient guidance can be given to help evaluate the suitability of different insulator types, profiles and materials for given pollution conditions. _____