IEC 60422 Mineral Insulating Oils in Electrical Equipment
IEC 60422 Mineral Insulating Oils in Electrical Equipment - Supervision and Maintenance Guidance
10 Handling and storage
CAUTION Safe drum handling and environmental procedures should be adopted according to local regulations. Special attention shall be paid to avoid cross contamination by PCBs.

To ensure satisfactory service, the utmost care in handling the oil is essential. Drums should be clearly marked to indicate whether they are for clean or for dirty oil, and should be reserved for the type indicated. Drums and bulk tankers used for oil awaiting reclamation should not be used for any other product.

Drums should be stored horizontally and placed in such a position that there is a head of oil on the stopper or plug. They should be stored under cover to minimize the ingress of water and to reduce solar thermal cycling due to exposure to sunlight. The use of plastic sheeting is not recommended unless great care is taken to avoid the drums "sweating" with condensation.

During transportation, drums should be in the vertical position for stability and covered to prevent the ingress of water.

In practice, difficulty may be experienced in maintaining the purity of oil when it is transferred from one vessel to another due to the possibility of introducing contamination. Such practice is not recommended without strict adherence to quality control.

It is recognized that storage of oil in damaged drums is not always satisfactory and the transfer of oil from such containers to electrical equipment should be through a suitable treatment plant to remove water and dissolved gases.

In locations with fixed oil-handling equipment, the pipe-work from the clean oil tanks to the electrical apparatus should be kept clean and free from water. Dehydrating breathers should be regularly inspected and maintained. Where portable oil-handling equipment is used, flexible pipe-work and hand pumps should be carefully inspected to ensure that they are free from dirt and water, and should be flushed with clean oil before use. If the clean oil is from drums, it should have been recently tested, and the filling orifices of the drums should be clean.

Hoses used for clean oil and hoses used for dirty oil should be clearly marked and provided with plugs for sealing the ends when not in use. Hoses shall be resistant to oil, as ordinary rubber contains free sulphur, which is corrosive. If wire braided hoses are used the hoses shall be cross-bonded and properly grounded to prevent the build-up of any static charge. For specific problems, reference should be made to the equipment manufacturer's instructions.

11 Treatment
11.1 WARNING
The treatment of used oil has to be carried out with proper care. All countermeasures should be taken to minimize any unreasonable risk to workers, public health and the environment. Experienced and qualified personnel well aware of the associated health and environmental risks associated should always perform oil treatment, strictly in accordance with local regulations. A full risk assessment should always be undertaken before commencing any treatment.

Strict control shall be undertaken in order to avoid cross contamination by PCB.

Strict control shall be undertaken to avoid accidental spills to the environment. Pipes, pumps and hoses shall be carefully inspected for tightness.

As oil treatments are usually carried out under vacuum, special attention shall be paid to avoid emissions to the atmosphere.

Oil treatments produce waste, such as spent filters, oil-contaminated absorbents etc. It is therefore necessary to choose the best available technology to minimize production of waste or spent materials and to dispose of waste strictly according to local regulations.

If the treatment is performed on on-load equipment, strict safety measures shall be taken to avoid risks to the workers. Also, safety measures shall be taken to avoid any damage to the equipment itself.

Due care should be taken when working with hot oil. Workers should use appropriate personal protective equipment according to local regulations and the Risk Assessment.

The properties of the oil after any treatment should be agreed between service provider and customer.

11.2 Reconditioning
11.2.1 General
Two documents exist that give information on reconditioning CIGRÉ Technical Brochure 227, 2003 "Life management techniques for power transformer" and CIGRÉ Technical Brochure 413, 2010 "Insulating Oil Reclamation and Dechlorination".

Reconditioning is a process that eliminates or reduces physical contamination by means of physical processes (filtration, drying, degassing etc.).

Reconditioning is carried out at the user's site, employing physical means only, to remove contaminants from the oil. However, this process does not always result in oil that conforms to Table 3 of this standard.

Reconditioning reduces the particle and water content of the oil. The process may also remove some dissolved gases and other components such as furanic compounds. New datum levels should be established after such a process.

The physical means that are used for removing water and solids from oil include several types of filtration, centrifuging and vacuum dehydration techniques.

If vacuum treatment is not employed it is advisable to limit the temperature to 30 °C. If vacuum treatment is employed, a higher temperature may be advantageous. However, if the vacuum treatment is used, the initial boiling point of the oil being treated should not be exceeded, to avoid undue loss of lighter fractions. If this information is not available, it is recommended that the oil should not be vacuum treated at temperatures over 85 °C.

NOTE Processing inhibited mineral oil under vacuum and at elevated temperatures may cause partial loss of oxidation inhibitors. The common inhibitors, 2,6-di-tert-butyl-paracresol and 2,6-di-tert-butyl-phenol, are more volatile than mineral insulating oil. The selectivity for removal of water and air in preference to loss of inhibitor and oil is improved by use of a low processing temperature.

If it is desirable to reduce particles or free water, cold treatment at atmospheric pressure may be appropriate.

Filters efficiently remove solid impurities, but are generally capable of removing only small quantities of free water. Where relatively large quantities of free water are present, most of it can, and should, be removed before filtration of the oil.

Equipment used for filtering oils subject to the risk of contamination by carbon (e.g. from tapchangers) should not be used for other oils because of the risk of cross-contamination.

To prevent loss of additives, the conditions that have been found satisfactory for most inhibited mineral oil processing are shown in Table 7.

Centrifugal separators are, in general, satisfactory for removing free water from oil and can deal also with any finely divided solid impurities.

If oil is purified whilst hot, its viscosity is reduced and the throughput with certain types of purifier is greater. On the other hand, sludge and free water are more soluble in hot oil than in cold. Particles and free water are, therefore, more effectively removed by cold treatment. Dissolved and free water and dissolved gases are effectively removed by hot vacuum treatment.

If the oil contains solid matter, it is advisable to pass it through some type of filter before processing it under vacuum.

11.2.2 Reconditioning equipment
11.2.2.1 Filters
Filtering equipment usually forces oil under pressure through absorbing material such as paper or other filter media. Filters of this type are normally used to remove contaminants in suspension. It should be noted that the nominal micron ratings, commonly used to characterise these filters, are based on gravimetric tests and applying efficiency, based on weight, which takes no regard of particle size. (The filter medium should be capable of removing particles larger than 10 µm although local regulations may prescribe a lower value e.g. 5 µm). Such equipment does not de-gas the oil.

The ability of a filter to remove water is dependent upon the dryness and quantity of the filter medium. When filtering oil that contains water, the water content of the filter medium rapidly comes into equilibrium with the water content of the oil. A continuous indication of the water content of the outgoing oil is useful to monitor the efficiency of the process.

Care should be taken to ensure that paper filters are of the correct grade so that they do not shed fibres.

During service, filters become contaminated with used oil and solid contaminants, therefore the disposal of filters should be carried out strictly in accordance with local regulations. Special consideration will have to be given to filters likely to be contaminated with PCB.

11.2.2.2 Centrifuges
In general, a centrifuge can handle a much greater concentration of contaminants than can a conventional filter but cannot remove some of the solid contaminants as completely as a filter.

Consequently, the centrifuge is generally found in use for rough bulk cleaning where large amounts of contaminated oil have to be handled.

Often the output of the centrifuge is put through a filter for the final clean-up.

11.2.2.3 Vacuum dehydrators
The vacuum dehydrator is an efficient means of reducing the gas and water content of a mineral insulating oil to very low values. (The use of a vacuum dehydrator to remove excessive water from paper insulation systems using oil circulation is not an efficient process. Special techniques may need to be considered.)

There are two types of vacuum dehydrator; both function at elevated temperature. In one method, the treatment is accomplished by spraying the oil into a vacuum chamber; in the other, the oil flows in thin layers over a series of baffles inside a vacuum chamber. In both types, the objective is to expose a maximum surface and minimum thickness of oil to the vacuum.

In addition to removing water, vacuum dehydration will de-gas the oil and may remove some of the more volatile acids and some of the 2-furfural.

11.2.3 Application to electrical equipment
11.2.3.1 Direct reconditioning
The oil is passed through a purifier and then stored in suitable clean containers. When the electrical equipment is to be refilled the oil is passed through the purifier again, and then directly into the equipment. This method can be used for switchgear. It is also suitable, too, for the smaller transformers, but care is needed to ensure that the core, the windings, the interior of the tank and other oil-containing compartments are thoroughly cleaned. The oil-containing compartments of all equipment should also be well cleaned, by means of oil from the purifier.

11.2.3.2 Reconditioning by circulation
The oil is circulated through the purifier, being taken from the bottom of the tank of the electrical equipment and re-delivered to the top. The return delivery should be made smoothly and horizontally at or near the top oil level to avoid, as far as possible, mixing cleaned oil with oil that has not yet passed through the purifier. The circulation method is particularly useful for removing suspended contaminants, but not all adhering contaminants will necessarily be removed.

Experience has shown that it is generally necessary to pass the total volume of oil through the purifier not less than three times, and equipment having an appropriate capacity should be chosen with this in mind. The final number of cycles will depend on the degree of contamination, and it is essential that the process be continued until a sample taken from the bottom of the electrical equipment after the oil has been allowed to settle for a few hours, passes the breakdown voltage test.

It is recommended that the circulation should be performed with the electrical equipment disconnected from the power source. In all cases the oil should be allowed to stand for some time in accordance with the manufacturer’s instructions before the equipment is re-energized.

WARNING It is the practice in some countries to perform this process with the transformer energized, but this shall only be done after full risk assessment has been carried out.

Another technique is sometimes used for transformers, in which oil is continuously circulated during normal service through an adsorbent, such as molecular sieve, thus keeping both oil and windings dry and removing many oil oxidation products. This is a specialized method not further considered in this guide.

11.2.3.3 Sealed instrument transformers
In order to avoid the risk of introducing air into the transformer, which may lead to premature failure, oil reconditioning shall be done strictly in accordance with the manufacturer's instructions pertinent at the time of reconditioning.

11.3 Reclaiming
11.3.1 General
This is a process that eliminates or reduces soluble and insoluble polar contaminants from the oil by chemical and physical processing. Reclamation processes require special competence, equipment and experience. The resulting product should be evaluated on critical parameters to achieve information about process efficiency and to be able to estimate remaining lifetime.

This process may result in oil, which originally conformed to IEC 60296, being restored to an acceptable standard. Reclamation of oils of moderate to high acidity will usually result in oils with a lower oxidation resistance than the original new oil.

Before performing a reclamation process, a laboratory feasibility test is recommended.

There are two types of oil reclaiming: percolation and contact.

11.3.2 Reclaiming by percolation
The full process consists of three consecutive steps.
1) The oil, being taken from the bottom of the electrical equipment, is heated to a given temperature and circulated through a filter (to eliminate the particles and suspended solids) being re-delivered to the top.

2) It is then circulated through one or more cartridges containing fuller's earth or other suitable material, to eliminate soluble polar contaminants.

3) The oil is finally circulated through a reconditioning device (vacuum dehydrator or centrifuge) to eliminate water and gases.

Fuller's earth is an active material containing both internal and external polar active sites, which allow the non-polar components of the oil to pass through without retention but which retains the polar contaminants or degradation compounds dissolved in the oil.

Several different clays are available that have proven suitable for these purposes. The most widely used are of the sepiolite, bentonite, attapulgite or montmorillonite type of which fuller's earth is the most commonly used. They are constituted of silicate anions [Si2O5]n condensed with octahedral layers of the type X(OH)2 where X may be magnesium, aluminium, etc.

Normally, fuller's earth is treated to increase its specific surface area and the concentration and polarity of its Lewis acids. Fuller's earth can be used alone or in combination with other chemicals like trisodium phosphate, activated charcoal and sodium silicate.

The retention of contaminants by adsorbent active sites is, generally, improved by temperature, thus the process normally takes place at 60 °C to 80 °C.

Experience has shown that it is usually necessary to pass the total volume of oil through the adsorbent not less than three times; therefore, equipment of appropriate capacity should be chosen for this purpose. The final number of cycles will depend on the degree of initial contamination and the desired final level for properties.

In the case of highly contaminated equipment, it is usual to transfer all the oil to a suitable clean container, reclaim a small portion of the oil and use it to thoroughly wash the electrical equipment, especially the windings. This portion of the oil is disposed of according to local regulations, and the remaining oil is then reclaimed as described above.

It is important to bear in mind that a small portion of the oil, less than 5 %, remains absorbed by the adsorbent, thus some unused oil shall be used for topping-up the equipment at the end of the process.

During service the adsorbent becomes contaminated with used oil and solid contaminants, therefore the disposal, or re-activation, of the substance should be carried out strictly in accordance with local regulations. Special consideration will have to be given to adsorbent likely to be contaminated with PCB.

NOTE Some on-line reclaiming and procedures using sorbents or combinations of sorbents and pre-treatment steps have been shown to efficiently remove also corrosive sulphur compounds from oil.

11.3.3 Reclaiming by contact
This process consists of stirring the contaminated oil, in the presence of fuller's earth, in a suitable container. It is not an appropriate system for industrial applications as it needs very long outage periods for the electrical equipment, but may be useful for the recycling of large amounts of waste oils.

Normally this process is used in the laboratory to investigate the feasibility of a reclamation process in a given oil and to estimate the final levels of the properties that can be reached by reclamation in the field.

11.3.4 Renewal of additives
As oil reclamation is performed after the oil its ageing, it is inevitable that the inhibitors (natural or added ones) in the oil are at least partly spent. It is therefore recommended that the additives be replaced in the reclaimed oil after the reclaiming process and before the equipment is re-energized. The most widely used additives are 2,6-di-tert-butyl-paracresol (DBPC) and 2,6-di-tert-butyl-phenol (DBP). Metal passivators will also be reduced or removed due to their polar nature.

11.4 Decontamination of oils containing PCBs
11.4.1 General
As keeping in service transformers containing PCB-contaminated oil may be permitted by some local regulations, these devices shall not always be considered waste. Should the oil became accidentally contaminated, there are several processes and techniques available for either on-site and off-site decontamination of PCB contaminated oils. These processes are based on chemical reactions between PCBs and the reagent to remove the chlorine present. All PCB decontamination methods, either off-site or on-site, have to be applied by skilled companies complying fully with local regulations.

Off-site decontamination techniques are limited by considerations for the safe transportation of contaminated equipment and liquid to an authorized oil processing facility and are the subject of local regulations.

11.4.2 Dehalogenation processes using sodium and lithium derivatives
These processes are typically applied in batch and use reagents based on metallic sodium, sodium hydride, lithium hydride and additives, for the dehalogenation of PCB in the oil. This type of process is typically run under pressure and medium to high temperature (150 °C to 300 °C). This temperature is higher than the flash point of the oil (140 °C to 150 °C) and therefore introduces subsequent safety risks.

WARNING Proper measures shall be taken to minimize the risk of fire or explosion, especially in the presence of wet oil.

11.4.3 Dehalogenation processes using polyethylene glycol and potassium hydroxide (KPEG)
This process, developed to overcome the problems associated with the use of metallic sodium, uses a liquid reagent based on polyethylene glycol (PEG) and an alkaline metal hydroxide such as potassium hydroxide (KOH). This type of process, which is run at temperatures of 130 °C to 150 °C, has a limited efficiency on some types of contaminants (e.g. Aroclor 1242).

11.4.4 Dehalogenation in continuous mode by closed circuit process
This process uses a solid reagent consisting of a high molecular weight glycol mixture, a mixture of bases and a radical promoter or other catalyst for chemical conversion of organic chlorine to inert salts, on a high surface area particulate support.

This process normally runs at 80 °C to 100 °C and has the capability to decontaminate equipment on-site, through continuous circulation of the oil in a closed system (without draining the oil or using auxiliary tanks), using the solvent capability of the oil for continuous extraction of PCB from solid materials inside the equipment.

NOTE Some dehalogenation processes have been shown to efficiently remove some corrosive sulphur compounds from oil.

12 Replacement of oil in electrical equipment
12.1 Replacement of oil in transformers rated below 72,5 kV and in switchgear and associated equipment
A small extra quantity of oil is needed to rinse the interior of the tank and the immersed parts. It is essential that the tank and the surfaces of conductors and insulators be cleaned effectively and subsequently be kept free from fibres. Such fibres are readily introduced by the use of unsatisfactory cleaning materials during plant maintenance; in practice the only efficient and permissible materials are synthetic. It is also essential that the tank and other surfaces be kept free from water.

A final pressure flush with clean oil of known quality has proved beneficial in the removal of fibres and other extraneous material.

It should be noted that possibly up to 10% of the original oil might remain adsorbed in the solid insulation and its contaminants may take some time to migrate into the new oil.

Application of a vacuum extraction procedure has proved valuable in removing other contaminants, provided the equipment can withstand the vacuum.

There should be as little aeration as possible during the filling of tanks and, as far as possible, the end of the delivery pipe should be held below the surface of the oil in order to avoid splashing; alternatively, the tanks should be filled from the bottom.

There should be a standing period of not less than 12 h to allow de-aeration before commissioning transformers (1 h may be adequate for switchgear and 1 h to 2 h for power transformers both with a nominal system voltage of less than 16 kV).

12.2 Replacement of oil in transformers rated 72,5 kV and above
Reference should be made to the equipment manufacturer.

12.3 Replacement of oil in electrical equipment contaminated with PCB
Reference should be made to the equipment manufacturer and local regulations. A full Risk Assessment should be carried out.

13 Passivation
Metal passivator is added as a stock solution, dissolved in insulating oil. Such stock solution is commercially available, but some service providers prepare the stock solution on-site, with oil from the actual unit. The stock solution can be added via an oil processing unit or other suitable equipment.

This treatment is recommended for unused and in-service insulating oil. For in-service oils that fall in fair or poor condition for acidity (Table 5), a case-by-case study is recommended.