Insulating Oil Analysis

Just like lube oil analysis mentioned elsewhere a lot of value can be derived from transformer oil testing.

The tests for dissolved gases and furfuraldehyde usually indicate the condition of the transformer whilst most of the other tests e.g. acidity, electric strength, fibres reflect the quality of the oil. Since it is not desirable to have any water present, that test can also indicate whether the dryers are functioning correctly.

Service Categories

  • Transformer Oil Condition Tests
  • Machine Condition Tests

For oil quality the control of moisture is most important (maximum of 30mg/l or ppm). It is moisture in combination with fibre in main tanks and selectors and carbon in divertors that so dramatically reduces the electric strength of the oil to very unsafe levels that can result in plant failure.

Water Content

High water content will reduce the insulating properties of the oil, which may result in dielectric breakdown. It can cause breakdown of cellulose based paper insulations in the windings, together with accelerating corrosion.

Acidity

High acidity can result in corrosion and varnish deposits, together with degradation of oil and paper in the windings. Consequently, sludging can occur within the oil, thus reducing heat dissipation qualities of oil and causing overheating. Reduced insulating properties and an increase in water content normally go hand in hand with high acidity.

Electric Strength

Low electric strength indicates oil is no longer capable of performing the vital function of insulating under high electrical fields.  Poor electric strength is often linked with high water and fibre content.

Colour

An indication of level of oxidation and degradation of oil.

Fibres

Under high electric fields the behaviour of fibres can cause a dramatic reduction of insulating qualities of oil. Although invisible to the naked eye fibres are often introduced via poor maintenance techniques.

Polychlorinated Biphenyls (PCBs)

PCBs are insulating liquids used for their non-flammable properties, mainly in transformers, whether a fire would be unacceptable and as the dielectric in capacitates. Unfortunately the mineral oil used in transformers has become cross contaminated over the years. As PCBs are fairly non-biodegradable and tend to collect in food chains legislation was brought in to prevent wide-spread contamination. Liquids containing over 50 mg/kg must be classed as injurious substance and disposed of via costly high temperature incineration. It is therefore essential those responsible for plant know their level of contamination and have strict controls on oil movements to site.

These tests offer information on condition of the transformer itself and help identify developing faults.

Dissolved Gas Analysis (DGA) and Buchholz Gas Analysis

This is the single most important test performed on transformer oil and is used to determine the concentration of certain gases such as nitrogen, oxygen, carbon monoxide, carbon dioxide, hydrogen, methane, ethane, ethylene and acetylene.

The concentration and relative ratios of these gases can be used to diagnose certain operational problems and incipient faults in transformers, which may or may not be associated with a change in physical or chemical property of the insulating oil.

For example, high level of carbon monoxide relative to other gases may indicate thermal breakdown of cellulose paper while hydrogen in conjunction with methane may indicate a corona discharge. Acetylene is considered a significant gas generated as it is formed in breakdown of oil at temperatures in excess of 700oC and can indicate a serious high temperature overheating fault.

 

FFA (Furfuraldehyde)

This is a measure of the degradation of cellulose paper in the windings i.e. as the paper ages its degree of polymerisation (DP) reduces and thus so does its strength as it becomes more brittle. The DP of paper can directly related to the concentration of furan derivatives in the oil which are formed as a direct result of the breakdown of the polymeric structure of cellulose paper. New paper has a DP rating of 1250 while at 250 the paper is sufficiently brittle to fall away from the windings. It is therefore possible to use FFA to estimate the used life of the transformer as well as which will be dependent on factors such stresses on plant, overheating etc. FFA formation can also indicate low temperature overheating.

Metals in Oil

Elemental data helps to pinpoint the origins of faults for example iron and copper can be indicative of arcing between a copper contact and iron core.

Detecting Ingress of SF6 into Insulating Oil

Detection of SF6 in transformer oil is a new service offered by OAS.

"Sulphur Hexafluoride or SF6 is used in the electrical industry as a gaseous dielectric medium for high-voltage sulfur hexafluoride circuit breakers, switchgear, and other electrical equipment, often replacing oil-filled circuit breakers (OCBs) that can contain harmful polychlorinated biphenyls (PCBs). SF6 gas under pressure is used as an insulator in gas insulated switchgear (GIS) because it has a much higher dielectric strength than air or dry nitrogen. The high dielectric strength is a result of the gas's high electronegativity and density. This property makes it possible to significantly reduce the size of electrical gear. This makes GIS more suitable for certain purposes such as indoor placement, as opposed to air-insulated electrical gear, which takes up considerably more room."

Where seals between the SF6 gas-filled and oil-filled circuits begin to fail, the gas may ingress into the oil. At Oil Analysis Services we are able to help detect ingress of SF6 into transformer oil via FTIR spectroscopy. Please get in touch if you would like a quotation.

Swansea Tribology Services
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