Abstract
This paper gives a background to the importance of monitoring the insulating oil quality in service. Mineral insulating oil production, functional requirements of insulating oil and types of oils are covered. Examples of oil condition monitoring are given. Monitoring and maintaining oil quality are aiming at an overall management of in service oil.
Introduction
Reliable operation of electrical equipment is of great importance to power generation, transmission and distribution companies. Maintenance guides and code of practice are prepared and used by utilities for safe operation and to minimise failure. Mineral insulating oil is used for insulation and cooling in the oil field electrical equipment. For reliable operation of these equipments the condition of insulating oil should be monitored by regular sampling and analysis of the samples. Corrective actions are usually taken based on the analysis results.
Insulating Oil Production
Refining crude oil produces mineral insulating oils. Refining is the collective term for the processes involved in changing the crude oil into oil with the required properties for a particular application. The choice of crude oil and refining technique governs the properties of the final product. Insulating oil can be classified as paraffinic or naphthenic, which is mostly related to thecrude oil used at the refinery. Paraffinic oils have poor low temperature properties and low solvent power towards oxidation products, especially sludge. This may lead to precipitation of sludge on the windings and blockages in the ducts. Conversely, oil oxidation products are soluble in naphthenic oil.
The resistance of oil to oxidation is a crucial factor in its service life and it depends on the presence of antioxidants. Some oils will contain these naturally and they can be enhanced by the refining processes, typically hydrogenation. Oils that are deficient in natural antioxidants can be improved by the addition of oxidation inhibitors. Both inhibited and uninhibited oils are supplied to a specification, usually IEC 60296 [1] or individual customer specifications. Once oil has been delivered to the equipment, it should be classified as used. To check the quality of used oil and its suitability for further use different standards are employed. The most common standard for evaluating used oil is IEC 60422 [2]. However, different national and user specifications are available. Oils, which fail to meet the required specifications, should be reclaimed or disposed of accordingly.
Functional Requirement of Insulating oil
The functional requirements of insulating oil are insulation, heat transfer, and arc quenching.
To meet the insulation function, the oil must have high dielectric strength and low dielectric dissipation factor to withstand the electric stresses imposed in service.
To meet the heat transfer and a cooling function, the oil must have viscosity and pour point that are sufficiently low to ensure that oil circulation is not impaired at the most extreme low temperature conditions for the equipment.
To meet the arc quenching function, the oil requires a combination of high dielectric strength and low viscosity to provide sufficient insulation and cooling to ensure the arcing is extinguished.
Chemical and Physical Requirements of Insulating Oil
The more general requirements are, in many cases, related to and inter dependant with the three main functional requirements, and the oil needs to have the following general properties shown in Table 1.
Type of Oils
Of the various fluid options available including synthetic oils, mineral oil has, for the vast majority of situations, proven to be the most cost effective choice, which meets the above functional requirements. (Synthetic oils are considerably more expensive and may have inferior insulating properties, and are only considered for lower voltages where fire retardant or other properties are essential).
There are two broad types of mineral oil, namely paraffinic and naphthenic.
Paraffinic oil is derived from crude oil containing substantial quantities of naturally occurring n-paraffins (wax). Paraffinic oil has a relatively high pour point and may require the inclusion of additives to reduce the pour point.
Naphthenic oil is derived from crude oil containing a very low level or none of naturally occurring n-paraffins (wax). Naphthenic oil has a low pour point and requires no additives to reduce the pour point. Naphthenic oil provides better viscosity characteristics and longer life expectancy, and sludge is soluble and thus does not deposit out on windings, blocking cooling ducts and reducing cooling efficiency.
Oil condition monitoring
One of the first signs of deterioration is the change in oil clarity and colour. The functional deterioration is monitored by analysis, which should be done by experienced personnel based on Risk Management and Life Cycle Analysis. Characteristic values, evaluation of trends and rate of variation, and normal (typical values) are to be considered.
PCB contamination is to be considered according to local regulations. Specific PCB analysis is to be undertaken if suspected. PCB prevention has to be taken to avoid exposure to environment and people.
The frequency of examination of oil in service must be set in line with the specific situation. A general rule does not apply. The optimum interval 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 check interval can be every 1-4 year. Details are outlined in IEC 60422 [2]. Economical factors and reliability requirements have to be compromised.
Check measurements shall be carried out on the basis of the following criteria, which apply to transformer oil:
Management of in service oil
Monitoring and maintaining oil quality is essential in ensuring the reliable operation of oil-filled electrical equipment. Electrical power industries have established code of practice in many countries. The frequency of testing and action based on the test results are the most important factor. Large power companies, normally, find uneconomical the full application of the recommendations to distribution transformers and accept a higher failure risk. Conversely, the industry users whose activities depend on the reliability of their power supply may wish to institute more frequent and stricter controls of oil quality as a means of guarding against power failures.
In general the oil in service can be placed in four different groups as follows:
Where a test result is outside recommended limits it is important to check the results with the previous one. If appropriate, a fresh sample should be obtained for confirmation before any other action is taken. If rapid deterioration is observed more frequent tests should be instituted and appropriate remedial action should be taken. In general several characteristics will have to be unfavourable in order to justify action.
References
This paper gives a background to the importance of monitoring the insulating oil quality in service. Mineral insulating oil production, functional requirements of insulating oil and types of oils are covered. Examples of oil condition monitoring are given. Monitoring and maintaining oil quality are aiming at an overall management of in service oil.
Introduction
Reliable operation of electrical equipment is of great importance to power generation, transmission and distribution companies. Maintenance guides and code of practice are prepared and used by utilities for safe operation and to minimise failure. Mineral insulating oil is used for insulation and cooling in the oil field electrical equipment. For reliable operation of these equipments the condition of insulating oil should be monitored by regular sampling and analysis of the samples. Corrective actions are usually taken based on the analysis results.
Insulating Oil Production
Refining crude oil produces mineral insulating oils. Refining is the collective term for the processes involved in changing the crude oil into oil with the required properties for a particular application. The choice of crude oil and refining technique governs the properties of the final product. Insulating oil can be classified as paraffinic or naphthenic, which is mostly related to thecrude oil used at the refinery. Paraffinic oils have poor low temperature properties and low solvent power towards oxidation products, especially sludge. This may lead to precipitation of sludge on the windings and blockages in the ducts. Conversely, oil oxidation products are soluble in naphthenic oil.
The resistance of oil to oxidation is a crucial factor in its service life and it depends on the presence of antioxidants. Some oils will contain these naturally and they can be enhanced by the refining processes, typically hydrogenation. Oils that are deficient in natural antioxidants can be improved by the addition of oxidation inhibitors. Both inhibited and uninhibited oils are supplied to a specification, usually IEC 60296 [1] or individual customer specifications. Once oil has been delivered to the equipment, it should be classified as used. To check the quality of used oil and its suitability for further use different standards are employed. The most common standard for evaluating used oil is IEC 60422 [2]. However, different national and user specifications are available. Oils, which fail to meet the required specifications, should be reclaimed or disposed of accordingly.
Functional Requirement of Insulating oil
The functional requirements of insulating oil are insulation, heat transfer, and arc quenching.
To meet the insulation function, the oil must have high dielectric strength and low dielectric dissipation factor to withstand the electric stresses imposed in service.
To meet the heat transfer and a cooling function, the oil must have viscosity and pour point that are sufficiently low to ensure that oil circulation is not impaired at the most extreme low temperature conditions for the equipment.
To meet the arc quenching function, the oil requires a combination of high dielectric strength and low viscosity to provide sufficient insulation and cooling to ensure the arcing is extinguished.
Chemical and Physical Requirements of Insulating Oil
The more general requirements are, in many cases, related to and inter dependant with the three main functional requirements, and the oil needs to have the following general properties shown in Table 1.
Type of Oils
Of the various fluid options available including synthetic oils, mineral oil has, for the vast majority of situations, proven to be the most cost effective choice, which meets the above functional requirements. (Synthetic oils are considerably more expensive and may have inferior insulating properties, and are only considered for lower voltages where fire retardant or other properties are essential).
There are two broad types of mineral oil, namely paraffinic and naphthenic.
Paraffinic oil is derived from crude oil containing substantial quantities of naturally occurring n-paraffins (wax). Paraffinic oil has a relatively high pour point and may require the inclusion of additives to reduce the pour point.
Naphthenic oil is derived from crude oil containing a very low level or none of naturally occurring n-paraffins (wax). Naphthenic oil has a low pour point and requires no additives to reduce the pour point. Naphthenic oil provides better viscosity characteristics and longer life expectancy, and sludge is soluble and thus does not deposit out on windings, blocking cooling ducts and reducing cooling efficiency.
Oil condition monitoring
One of the first signs of deterioration is the change in oil clarity and colour. The functional deterioration is monitored by analysis, which should be done by experienced personnel based on Risk Management and Life Cycle Analysis. Characteristic values, evaluation of trends and rate of variation, and normal (typical values) are to be considered.
PCB contamination is to be considered according to local regulations. Specific PCB analysis is to be undertaken if suspected. PCB prevention has to be taken to avoid exposure to environment and people.
The frequency of examination of oil in service must be set in line with the specific situation. A general rule does not apply. The optimum interval 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 check interval can be every 1-4 year. Details are outlined in IEC 60422 [2]. Economical factors and reliability requirements have to be compromised.
Check measurements shall be carried out on the basis of the following criteria, which apply to transformer oil:
- Oil characteristics may be checked periodically.
- The frequency of examination may be increased where any of the significant properties indicate that the oil is in fair or poor condition, or when trend analysis indicates significant changes
- 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 complimentary testing such as interfacial tension.
| Property | Required level | Function |
| Breakdown voltage | High | To provide dielectric strength to prevent breakdown of the oil under electrical stress |
| Dielectric dissipation factor | Low | To ensure that the dielectric losses are small and that the oil thus provides satisfactory insulating properties |
| Viscosity | Low | To ensure that the oil flows well under all (particularly low) temperature conditions thus providing the necessary cooling and, where appropriate, arc quenching properties |
| Pour point | Low | To ensure that the oil flows satisfactorily under low temperature conditions |
| Flash point | High | To eliminate the risks of ignition of vapours above the oil during maintenance or in service |
| Density | Low | To ensure that ice cannot float on the oil surface at very low temperatures and cause internal flashover |
| Particle size and count/ | Low | The presence of such contaminants, especially in the presence of moisture, can considerably reduce Fibre content the electrical strength |
| Acidity/Sulphur content | Low | To eliminate the risks of sludge formation and corrosion |
| Moisture | Low | The electrical strength of the oil will be impaired and moisture will be absorbed into any paper insulation, reducing insulation life and increasing the risk of dielectric breakdown |
| Oxidation stability | Good | To reduce oxidation processes in service which degrade the oil, producing acids and sludge. These can reduce the effectiveness of cooling and cause general internal deterioration and eventual failure |
| Polychlorinated biphenyl (PCB) | Low | To meet the requirements of environmental legislation |
| Furan | Low | The effectiveness of condition monitoring by trend assessment of furan content is impaired |
| Polycyclic aromatics (PCA) | Low | To meet the requirements of health and safety legislation |
| Interfacial Tension (IFT) | High | To ensure absence of polar compounds in the oil and suitability of the oil for use as insulating material |
| Naphthenic carbon in | over 40% | To ensure produced sludge is soluble in the oil, since sludge is soluble in naphthenic oil and insoluble paraffinic oil. This will avoid precipitation of sludge on winding or any other part of the transformer |
Management of in service oil
Monitoring and maintaining oil quality is essential in ensuring the reliable operation of oil-filled electrical equipment. Electrical power industries have established code of practice in many countries. The frequency of testing and action based on the test results are the most important factor. Large power companies, normally, find uneconomical the full application of the recommendations to distribution transformers and accept a higher failure risk. Conversely, the industry users whose activities depend on the reliability of their power supply may wish to institute more frequent and stricter controls of oil quality as a means of guarding against power failures.
In general the oil in service can be placed in four different groups as follows:
- This group of oil is in satisfactory condition for continued use
- This group of oil require only reconditioning for further service
- This group of oil are in poor condition that can be restored to satisfactory condition only after reclamation
- This group of oil are in such condition that it is technically advisable to dispose of them
Where a test result is outside recommended limits it is important to check the results with the previous one. If appropriate, a fresh sample should be obtained for confirmation before any other action is taken. If rapid deterioration is observed more frequent tests should be instituted and appropriate remedial action should be taken. In general several characteristics will have to be unfavourable in order to justify action.
References
- IEC60296, Specification for Unused Mineral Insulating Oils for Transformers and Switchgear, 1982.
- IEC 60422, Supervision and Maintenance Guide for Mineral Insulating Oils in Electrical Equipment. 1989.
