Transformer Oil

Dissolved gas analysis (DGA)

It may seem like a bold statement but dissolved gas-in-oil analysis (DGA) is the most powerful and cost-effective tool in improving Transformer reliability.
DGA detects a wide range of problems in the early stages and then throughout the life cycle. It is a great asset management tool for expensive apparatus such as transformers and has also been applied to load (energized) tap changers, oil circuit breakers and other oil-filled devices. It has been applied to a variety of dielectric liquids beyond mineral oils, including natural and synthetic esters, silicone and high temperature hydrocarbons.
Dissolved gas analysis (DGA) has been an industry standard for the detection and determination of faults in transformers for over 30 years

Below is Case Study of a Windfarm Step Up Transformer (WTG) diagnosed with a Discharges of high Energy fault (D2) , where urgent Remedial action was taken to avoid in service failure .If this Discharge Fault (D2) was left undetected catastrophic failure would have resulted in an significant Electrical Outage- with associated production losses affecting Share holder value.

Actual finding of the Internal Inspection.

Insulating liquid.

Transformers may use one of three classes of insulating liquid for their non-solid insulation and as a cooling medium; dielectric liquids,
The various dielectric liquids, often loosely referred to as oils, are almost universally used in high voltage power transformers.

The dielectric liquid is used for heat transfer and as an insulation medium, as well as for impregnating
paper-based, solid insulation, and for diagnostics purposes, being responsible for providing information for assessment of normal / abnormal behaviours.

Monitoring and maintaining dielectric liquid quality are essential to ensure the reliable operation and service life of a transformer. Like most materials, dielectric liquid deteriorates. It diminishes both in quality and its ability to withstand electrical stress and transfer heat from the core and coils to the environment. Invariably, dielectric liquid condition deteriorates though contamination or degradation (ageing), and, depending on the type of dielectric liquid, forming acids, and other polar compounds.

Water Content (ppm)
Excessive moisture is one of the primary causes of low insulating liquid dielectric breakdown strength. High water content may be detrimental to the transformer under a variety of conditions.

Moisture analysis-why is it important to determine the moisture content in transformer oil?

• To determine if there are any leaks in the transformer
• Increase in moisture accelerates insulating paper degradation
• To determine if decrease in insulating strength is due to high water content in the transformer oil
• Paper and oil degradation
Water, in minute quantities, is harmful in power equipment because it is attracted to the places of greatest electrical stress and this is where it is the most dangerous. Water accelerates the deterioration of both the insulating oil and the paper insulation, liberating more water in the process.
The dielectric breakdown voltage of insulating material is a function of the water content. The water migrates between the solid and liquid insulation in a transformer with changes in temperature.

Dielectric strength(kV).

The dielectric strength is the measure of the insulating oils ability to withstand electrical stress without failure.
The test involves applying an ac voltage at a controlled rate to two electrodes immersed in the insulating fluid. The gap is a specified distance. When the current arcs across this gap the voltage recorded at that instant is the dielectric strength breakdown strength of the insulating liquid.
Contaminants such as water, sediment and conducting particles reduce the dielectric strength of insulating oil. Combination of these tends to reduce the dielectric strength to a greater degree.

Acidity or Neutralisation Number (NN).

Insulating oil may contain acidic constituents that are present as additives or as degradation products formed during service, such as oxidation products.
Acidic products of oxidation within the oil will increase rapidly once present. These contaminants have a very aggressive effect on the materials of the transformer. The acidity value of the oil sample will indicate the oil life remaining before it contributes to the deterioration within the transformer. If the acidity increases above a certain level the oil requires replacement /regeneration to prevent deterioration of both current carrying and insulating components within the transformer.

Interfacial tension (IFT).

The Interfacial Tension (IFT) measures the tension at the interface between two liquids (oil and water) which do not mix. The test is sensitive to the presence of oil decay products and soluble polar contaminants from solid insulating materials. Interfacial tension measurements on electrical insulating oils provide a sensitive means of detecting small amounts of soluble polar contaminants and products of oxidation. A high value for new mineral insulating oil indicates the absence of undesirable polar contaminants. The test is frequently applied to service-aged oils as an indication of the degree of deterioration.

QUALITY INDEX SYSTEM(OQIN) or Myers Oil Quality Index

Dividing the Interfacial Tension (IFT) by the Neutralisation Number (NN) provides a numerical value that is an excellent means of evaluating oil condition. This value is known as the Oil Quality Index (OQIN) or Myers Index Number (MIN). The four functions of insulating oil are to provide cooling, insulation, protection against chemical attack and prevention of sludge build-up.

Good Oil in which these functions are efficiently provided. Proposition A provides all the required function; a drop in IFT to 27.0 may signal the beginning of sludge in solution. Marginal Oils is not providing proper cooling and winding protection. Organic acids are beginning to coat winding insulation; sludge in insulation voids is highly probable.

Bad Oils, sludge have already been deposited in and on transformer parts in almost 100 percent of these units. Insulation damage and reduced cooling efficiency with higher operating temperatures characterize the Very Bad and Extremely Bad categories.

“Disaster City” the concern should be how much life remains in the transformer, not just the oil condition.

Once the oil colour changes from the yellows into amber’s and browns, the oil has degraded to the point where the insulation system has been affected. Radical colour changes may be caused by: Electrical problem, Pot head or bushing compounds, uncured varnishes or polymers, new oil in a dirty unit. The situation where NN and IFT were bad, but the colour was light may indicate contamination from sources other than oxidation i.e. a refining problem.

Although a low IFT with a low NN is an unusual situation, it does occur because of contamination such as solid insulation materials, compounds from leaky pot heads or bushings, or from a source outside the transformer.

Dissipation Factor (Tan D) or Power Factor.

This is a measure of the dielectric losses in an electrical insulating liquid when used in an alternating electric field and of the energy dissipated as heat. A low dissipation factor or power factor indicates low ac dielectric losses. Dissipation factor or power factor may be useful as a means of quality control, and as an indication of changes in quality resulting from contamination and deterioration in service or as a result of handling

Corrosive sulphur

The presence of potentially corrosive sulphur compounds in the transformer oil under appropriate thermal conditions and winding materials (bare or un-enamelled copper) can lead to the development of copper sulphide. The presence of corrosive sulphur compounds in the transformer oil can lead to the decrease of dielectric integrity of the active part, by the formation of metallic sulphides in operating transformers. Depending on the type of sulphur compounds in the oil, different thermal conditions and construction metals present, silver, copper, and/or winding paper will be affected by sulphides deposition.

Polychlorinated biphenyl (PCB) analysis

Polychlorinated biphenyls PCBs, (or askarels as they are often known), are highly regulated therefore insulating liquids that may contain PCBs should be tested to ensure proper handling and disposal.
Transformer owners have a responsibility for identifying PCB contaminated installations and for actions required to dispose of contaminated oil and equipment.
PCBs are Persistent Organic Pollutants (POP’s) governed by UNEP (United Nations Environmental Program) according to the outcome of the Basel Convention that was held in March 1999 and the treaty that was ratified in Sweden in May 2001, with South Africa being a signatory.

Oxidation Inhibitors

The presence of inhibitors in the oil will increase the useful life of the oil with respect to oxidative degradation in the presence of oxygen. As the oil is exposed to this type of oxidative degradation, the oil will be protected as long as there is inhibitor present. However, as the process proceeds the inhibitor will be used up and when it is gone the oil will degrade at a much higher rate. Thus, the determination of the amount of inhibitor present can be used to estimate the useful life of the oil. It can also be used to determine whether or not new oil has been properly inhibited prior to its use. As the inhibitor is used up its concentration can be monitored, and additional inhibitor added as needed to maintain a proper concentration in the unit.

Particle count

Particles in insulating oil in electrical equipment may have numerous possible sources. The equipment itself may contain particles from manufacturing and the oil may contain particles from storage and handling if not properly filtered. Metal wear and the ageing of oil and solid materials may produce particles during the service life of equipment. Localized overheating over 500°C may form carbon particles. The carbon particles produced in the on-load tapchanger
diverter switch may migrate by leakage into the bulk oil compartment to contaminate the oil-immersed parts of the transformer. A typical source of metallic particles is wear of bearings of the pumps.

Metals in Oil

Metals-In-Oil Metals such as copper, iron, zinc, and lead can be detected and can be indicators of incipient-fault conditions, potential bearing wear from pumps or other wear metals from vibration of components.

Sediment and sludge

This test distinguishes between sediment and sludge. Sediment is insoluble material present in the oil.

Sediment includes:

insoluble oxidation or degradation products of solid or liquid insulating materials; solid products arising from the conditions of service of the equipment; carbon and metal particles, metallic oxides and sulphides;

Evaluation of Transformer Solid Insulation

Direct Evaluation

The mechanical properties of insulating paper can be established by direct measurement of its tensile strength or degree of polymerization (DP). These properties are used to evaluate the end of reliable life of paper insulation. It is generally suggested that DP values of 150-250 represent the lower limits for end-of-life criteria for paper insulation; for values below 150, the paper is without mechanical strength. Direct measurement of these properties is not practical for in-service transformers
Analysis of paper insulation for its DP value requires removal of a few strips of paper from suspect sites. This procedure can conveniently be carried out during transformer repairs. The results of these tests will be a deciding factor in rebuilding or scrapping a transformer.
Note: Since it is usually not practical (and often dangerous to the transformer) to obtain a paper sample from a de-energised, in-service transformer

In Direct Evaluation-Furan Analysis

By measuring the quantity and types of furans present in a transformer oil sample, the paper insulation overall DP can be inferred with a high degree of confidence. The types and concentration of furans in an oil sample can also indicate abnormal stress in a transformer, whether intense, short duration overheating or prolonged, general overheating. Furan analysis can be used to confirm Dissolved Gas Analysis where carbon monoxide present indicates problems with solid insulation

Furanic compounds in the oil:

It has been shown that the amount of 2-furaldehyde in oil (usually the most prominent component of paper decomposition) is directly related to the DP of the paper inside the transformer.
Paper in a transformer does not age uniformly and variations are expected with temperature, moisture distribution, oxygen levels and other operating conditions. The levels of 2-furaldehyde in oil relate to the average deterioration of the insulating paper. Consequently, the extent of paper deterioration resulting from a “hot spot” will be greater than indicated by levels of 2-furaldehyde in the oil.

If the DP value is below 200 (see table below for DP values and their meanings):

The mechanical strength of the paper is indicated by the estimated degree of polymerisation (DP). New paper starts with a DP of 1000 or more, as the paper starts to age or is damaged by poor fluid management or operational events, so the DP reduces. A DP of 200 indicates end of life.

When the estimated DP is less than 350 some areas of the paper may be seriously compromised. A transformer with such a low DP can continue to operate normally providing it does not experience any external events. In a situation where the load is suddenly changed, or the transformer is subject to mechanical shock or there is a through fault on the system this transformer has a higher risk of failing.
It is strongly advised that exposure to fault risk should be managed carefully and plans for end of life developed.

It should be noted that aggressive oil reconditioning when the mechanical strength of the paper is so low can do more harm than good. Oil reconditioning can remove the evidence of paper degradation but the degradation itself is irreversible.