IEC 60422 Mineral Insulating Oils in Electrical Equipment
IEC 60422 Mineral Insulating Oils in Electrical Equipment - Supervision and Maintenance Guidance
6 Sampling of oil from equipment
It is essential that every effort be made to ensure that samples are representative of the insulating oil in equipment. Experience indicates that oil is sometimes rejected unjustifiably because inadequate care has been taken whilst sampling. Careless sampling procedures or contamination in the sample container will lead to erroneous conclusions concerning quality and incur waste of time, effort and expense involved in obtaining, transporting and testing the sample.
Whenever possible, sampling from equipment shall be at normal operating conditions or very shortly after de-energization.
Sampling should be performed by an experienced person, who has received adequate training, in accordance with IEC 60475.
Where available, manufacturer's instructions should be followed.
7 Categories of equipment
In order to take account of different user requirements, equipment has been placed in various categories as shown in Table 2 below.
8 Evaluation of mineral insulating oil in new equipment
A substantial proportion of electrical equipment is supplied to the final user already filled with mineral oil. In such cases, as the oil has already come into contact with insulating and other materials, it can no longer be considered as "unused oil" as defined in IEC 60296. Therefore its properties shall be regarded as those applicable to oil in service, even though the electrical equipment itself may not have been energized.
Oil properties should be appropriate to the category and functions of the equipment (see Table 3).
The extent of the changes in properties may vary with the type of equipment due to the different types of material and ratios of liquid-to-solid insulation, and should be within the limits of Table 3. Properties not included in Table 3 (with the exception of oxidation stability for which no in service limits have been established) should be within the limits of IEC 60296.
As the characteristics of oil in new equipment are an integral part of that equipment design, the user may request these characteristics to be better than the minimum standards suggested in Table 3, which are based on the experience of many years of operating practice.
9 Evaluation of oil in service
9.1 General
Insulating oil in service is subjected to heat, oxygen, water and other catalysts, all of which are detrimental to the properties of the oil. In order to maintain the quality of the oil in service, regular sampling and analysis should be performed.
Often the first sign of oil deterioration may be obtained by direct observation of the oil clarity and colour through the sight glass of the conservator. From an environmental point of view, this simple and easy inspection can also be used to monitor leakage and spills of oil.
The interpretation of results, in terms of the functional deterioration of the oil, should be performed by experienced personnel based on the following elements of risk management and life cycle management:
• characteristic values for the type and family of oil and equipment, developed by statistical methods;
• evaluation of trends and the rate of variation of the values for a given oil property;
• normal, or typical values, for "fair" or "poor" for the appropriate type and family of equipment.
In the case of oil contaminated with PCBs, environmental impact is a critical factor to consider, as are local regulations. If it is suspected that oil has become contaminated with PCBs specific analyses should be undertaken and interpretation of the results should be used in risk assessment to take into account prevention and mitigation of potential damage to the environment and to avoid unreasonable risks for staff and the public.
9.2 Frequency of examination of oils in service
It is impossible to lay down a general rule for the frequency of examination of oils in service which will be applicable to all possible situations that might be encountered.
The optimum frequency will depend on the type, function, voltage, power, construction and service conditions of the equipment, as well as the condition of the oil as determined in the previous analysis. A compromise will often have to be found between economic factors and reliability requirements.
Much greater difficulties exist in deciding frequency of testing and permissible oil deterioration levels which are acceptable for all applications of insulating oil in relation to differences in operating policies, reliability requirements and types of electrical system. For example, large power companies may find the full application of these recommendations to distribution transformers uneconomical. Conversely, the industrial user, whose activities depend on the reliability of his power supply, may wish to institute more frequent and stricter controls of oil quality as a means of guarding against power failures.
By way of a guide, a suggested frequency of tests suitable for different types of equipment is given in Table 4. However, some equipment is designed having systems that are designed to control exposure of the oil to atmosphere. Where such systems are maintained in good condition, less frequent testing may be appropriate based on life cycle analysis (LCA) and/or life cycle management (LCM) and risk assessment (RA).
Generally, check measurements should be carried out on the basis of the following criteria, which apply particularly to transformer oils:
a) Characteristics may be checked periodically, at intervals as suggested in Table 4, unless otherwise defined.
b) The frequency of examination may be increased where any of the significant properties indicates that the oil is in fair or poor condition, or when trend analysis indicates significant changes.
c) The oxidation of the oil will accelerate with increased temperature and in the presence of oxygen and water. Therefore heavily loaded transformers may need more frequent oil-sampling and complementary testing such as interfacial tension.
d) The testing frequency should be established by means of a cost/benefit evaluation based on life cycle analysis and risk assessment. For some owners this approach may indicate different testing frequencies from those indicated in Table 4. For instance, some electrical utilities may prefer not to perform this programme on to this type of transformer and small industries may prefer to include this type of transformer even in a higher category.
