Transformer Oil Analysis
Transformer oil or insulating oil is a highly refined mineral or silicone oil that is stable at high temperatures and has excellent
electrical insulating properties. It is used in oil-filled transformers, some types of high voltage capacitors,
fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate,
The fault free operation of power transformers is a factor of major economic importance and safety in power supply utilities and industrial consumers of electricity. In the current economic climate, Industries/Supply Utilites tighten their control on capital spending and make cutbacks in maintenance, an increased awareness is placed on the reliability of the existing electric power supply. Down time is at a premium. Often, the loading is increased on present units , as this will defer purchasing additional plant capacity. Thus the stress on the transformer increases. The net total effect of the thermal , electrical and mechanical stress brought on by increased service needs to be monitored to ensure reliability. Regular sampling and testing of insulation oil taken from transformers is a valuable technique in a preventative maintenance program. If a proactive approach is adopted based on the condition of the transformer oil, the life of the transformer can be extended.
The following are some of the most important analytical tests performed on Transformer oil samples:
Dissolved Gas Analysis (DGA)
A combination of these tests would be chosen dependant upon requirement.
Dissolved Gas Analysis - DGA
Dissolved Gas Analysis is widely accepted as the most reliable tool for the earliest detection of incipient faults in transformers and tap selector units. Hydrocarbon (mineral-based) oils and silicones are used as insulation fluids in transformers because of their high dielectric strength, heat transfer properties and chemical stability. Under normal operating conditions very little decomposition of the dielectric fluid occurs. However, when a thermal or electrical fault develops, dielectric fluid and solid insulation will partially decompose. The low molecular weight decomposition gases include hydrogen, methane, ethane, ethane, ecetylene, carbon monoxide and carbon dioxide. These fault gases are soluble in the dielectric fluid. Analysis of the quantity of each of the fault gases present in the fluid allows identification of fault processes such as corona, sparking, overheating and arcing.
The acidity level of transformer oil is measured to BS2000 Part1:1982 (also IP 1/74 method A).
High acidities accelerate the degradation of the paper insulation and cause corrosion of the steel tanks. Oil Analysis Services normally suggests changing the oil when the acidity approaches 0.3mgKOH/g, but expresses concern when it reaches 0.1mgKOH/g. It is believed that acidity increases exponentially with time.
A measure of the ability of an insulating fluid to withstand electric stress (voltage) without failure. Fluids with high dielectric strength (usually expressed in volts or kilovolts), are good electrical insulators.
An estimation of the fibre content in an oil sample is made by passing polarised light through the oil, clearly showing the fibres and any sediment which is present in the oil. The fibres are classed as being long(greater than 5mm), medium(2 to 5 mm) or short(less than 2 mm), and the number of fibres present is classed as few(1 to 5) or many(more than10). Sediment is classed as being either light, medium or heavy.
The presence of fibres in an oil sample, especially in combination with a high water content, may result in a poor dielectric strength measurement. This is because wet fibres are drawn into the electrical field and cause arcing to occur.
Fibres (& moisture) can easily be picked up during sampling and as such, it is important to ensure that sampling is correctly carried out. By following the sampling technique recommended by us, the oil sample will be more representative of the bulk oil in the transformer and test results will be more reliable. We can supply the sampling devices in order to ensure that samples that are collected are representative.
In the past, there used to be a British Standard which gave a range of colours from 1-10; 1 being white/yellow and 10 being dark brown/black heavily oxidised oil. We use a similar colour range to gauge if there is any sudden deterioration in the oil during the time it is being analysed for different species.
Colour has also been useful in the past as a cross check of the sample identity. For example, a sudden change from dark to light colour can indicate an oil change or incorrect sample designation.
Polychlorinated Biphenyl (PCB) Analysis
In the past, PCB’s were insulating liquids used for their non-flammable properties, mainly in transformers where a fire would be unacceptable, and as the dielectric fluid in capacitors. Unfortunately, the mineral oil used in transformers, switchgear etc. has become cross-contaminated over the years by PCB fluid being placed in dirty mineral oil tanks, by using oil conditioning equipment for both mineral and PCB liquids and by capacitors leaking into mineral oil systems.
PCB’s are fairly non-biodegradable and toxic and do tend to collect in food chains and as a result, legislation has been brought in to prevent widespread contamination. Liquids containing over 50mg/kg(or 50 ppm by weight) must be classed as injurious substances and disposal must be by high temperature incineration, which is expensive. In fact oil suppliers have agreed not to supply oil containing more than 10mg/kg and by the same token, any oil taken away over 10mg/kg is expensive to dispose of even though it may not be as high as 50/mg/kg.
We use capillary column chromatography to determine the PCB concentration in oil. Oil Analysis Services identifies the three main types of PCB i.e. 1242, 1254 and 1260 and reports the total PCB content present.
Metal in Oil Analysis
Metal in oil analysis of transformer oil is used to complement dissolved gas analysis(DGA). When gas in oil analysis indicates the presence of a possible fault, metal in oil analysis will help determine the fault type and pinpoint its location.
High energy faults not only degrade the transformer’s insulation(oil, paper, wood etc) but can generate metal particles that will disperse in the oil. These particles will be distributed throughout the transformer, mainly due to oil circulation. Certain transformer components produce specific metal particles. These metal particles will be found alone or in differing combinations and concentrations. The type of particles will help to narrow the list of components involved in the fault.
Metals that may be found in transformer oil are: aluminium, copper, iron, lead, silver, tin and zinc. For example, copper would be found in the windings and also any bronze or brass components. Lead is found in soldered joints, connectors and other peripheral components. Iron is located in the transformer core and tank, whereas aluminium can be found in the windings, corona shields and ceramic bushings. Lugs, bolts, connectors and some peripheral components may also contain tin, silver and zinc.
Metal in oil analysis can be performed using different methods. Atomic absorption spectroscopy(AA) and inductive coupled plasma spectrometry(ICP) are two examples of the techniques used to measure metal concentrations in oil. Typically the metal particles contained in the sample will be burned at high temperatures to produce free metal atoms. The presence of these atoms in a flame(AA) or in a plasma(ICP) can be quantified by measuring the absorption(AA) or emission(ICP) of discrete frequencies in the radiation spectrum by the free metal atoms against a known standard. We use an inductively Coupled Plasma Emission Spectrometer(ICP) to measure the metal concentrations in oil.
There are no established threshold levels for metal in oil, but as data is being generated and cases documented, metal in oil analysis is becoming another proven tool used to detect transformer faults well before they escalate into major problems.
One metal in oil analysis report is not enough to get a true picture of the units health. Subsequent tests should be performed to trend the results and look for developing problems. Results are provided in history table form and can be represented graphically.
Furfuraldehyde Analysis (FFA)
Provided the oil inside a transformer has been well maintained and there is no serious defect, the life of a transformer often depends upon the state of the paper insulation on the windings. It has been shown that in oil, as the paper degrades and becomes weaker, furfuraldehyde is one of the many degradation products. It has also been shown that a linear relationship exists between the logarithm of the mass of furfuraldehyde produced and the resulting degree of polymerisation(DP) or strength of the paper. When the DP falls to approximately 250, the paper insulation is very brittle and the transformer can be considered to have reached the end of its service life. Therefore by measuring the furfuraldehyde concentration in the oil, the remaining service life of the transformer can be estimated.
Water In Oil Content (Moisture content)
The water content of an oil sample is measured using an automatic Mitsubishi moisture meter, which is based on the Karl Fischer titration and coulometric end-point. All laboratories use these instruments now to measure dissolved water and probably over 90% use the Mitsubishi model. The instrument is calibrated electronically but is checked with the aid of standard water in methanol solutions. The water content is given in mg/kg(ppm).
For an oil sample originating from a 33kV transformer, the water content is normally considered to be a little high when it reaches 25mg/lg(ppm) and too high when it reaches 30mg/kg)ppm). These limits vary depending upon the source of the oil sample.
Moisture, in combination with the fibre in main tanks and selectors and carbon in diverters, dramatically reduces the electric strength of the oil to very unsafe levels which can result in plant failure.
In tests conducted on a population of 100 primary transformers ranging from 132kV down to 33kV all with on load tap changers, the loss due to failure of selectors and main tank windings averaged about one per year. During the following period of 8 years in which moisture was monitored on an annual basis and controlled, with 30 ppm set as the value at which units were llovaced, the loss was reduced by approximately 90%, average moisture content being about 18 ppm.
Establishing the importance of moisture did not negate the issue of DGA. DGA was adopted as a very useful partner to moisture analysis and testing was carried out on an annual basis. This enabled fault trends to be monitored and remedial action to be taken before fault damage and supply loss occurred. It also allowed capital replacement programs for the plant to be more accurately determined.
The resistivity of a liquid is a measure of its electrical insulating properties under conditions comparable to those of the test.
High resistivity reflects low content of free ions and ion-forming particles and normally indicates a low concentration of conductive contaminants.
These characteristics are very sensitive to the presence in the oil of soluble contaminants and ageing products.
Resistivity is normally carried out at ambient temperature but useful additional information can be obtained if the test is carried out at ambient and a higher temperature such as 90degrees celcius.
Unsatisfactory results at both temperatures indicate a greater extent of contamination than a poor value at the lower temperature only, and the oil is therefore less likely to be restored to a satisfactory level by drying and low temperature filtration.
Inter Facial Tension (IFT) - ASTM D971
A very sensitive method for detecting oil soluble polar contaminants such as acids and sludges formed from the oxidation of transformer oils. The more the contaminants in the oil, the lower the IFT. New oil specification is 40 dynes/cm minimum.
Dielectric Dissipation Factor (DDF)
DDF is the tangent of the loss angle. The loss angle is the angle by which the phase difference between applied voltage and the resulting current deviates from Pi/2 rad when the dielectric of the capacitor consists exclusively of the insulating material. A rising dissipation factor is an indication of oil ageing or oil contamination. The dissipation factor is strongly influenced by polar components and is therefore a very sensitive parameter.
Compiled by Rami E. Kremesti M.Sc.