During the natural aging process of the oil and insulating parts, especially in the case of thermal or electrical failures, cracked gases form, and are dissolved in the oil. The rate of decomposition and the type of gases change during defective operation, which could be a result of thermal overloading and/or electrical faults. Based on the quantity or type of fault gases, the gas increase rates and the proportions between the gases, the type of failure can be deduced.

Partial discharges with lower energy mainly lead to the formation of hydrogen and methane, as well as small quantities of ethane. Thermal overheating results in the pyrolysis of hydrocarbons. In temperatures between 300-700°C ethylene and propylene accompanied by larger quantities of CO and CO2 prevail. Over 700°C mainly ethylene, propylene and hydrogen form, and above 1000°C acetylene are also formed. Electrical discharges (arcs and spark discharges) cause separation of hydrogen and acetylene, as well as methane and ethylene. By thermal-oxidative cellulose degradation, larger quantities of CO and CO2 are formed.

The color and appearance of transformer oil are useful for comparative evaluation: a quick color darkening or dark oil is a sign of oil ageing. The analyzed oil color is assigned a number ranging from 1 to 8, whereby the level of discoloration is indicated by means of the rising color number.

By analyzing the appearance, undesirable by-products can be detected. Cloudiness or sediment indicate free water, insoluble sludge or dirt particles. If these are present, then also the breakdown voltage and/or the loss factor and eventually also other oil values are out of the norm and measures must be taken.

The breakdown voltage indicates how well insulating oil can withstand an electrical load and is therefore decisive for the operational efficiency of a transformer. The breakdown voltage is measured according to the standard IEC 60156. The initial measures to be taken in case of a drop below the insulation voltage limit values of the relative transformer type, depend on the values of the other characteristic oil values.

Water formation is caused by the ageing of cellulose- insulating material (paper, pressboard, laminated wood), ageing of oil and infiltration of humidity from the environment by means of badly maintained air dryers and/or defective sealing systems.

There is an equilibrium between the water content in oil and the moisture content in the solid insulation, it reduces the breakdown voltage and accelerates the ageing process. This equilibrium though, is temperature and time-dependent.

As there is never an even temperature in an operating transformer, oil temperature in the tank is much higher at the top than at the bottom, and the same counts for the temperature distribution in the windings - the moisture distribution curves are only a rough estimate. In the moisture distribution curves the water content in the oil can theoretically be distinguished from the moisture content in the cellulose, based on the temperature. This is however valid only for insulation paper and not for pressboard and laminated wood. A precondition is constant temperature and a set equilibrium.

Through progressive ageing (Oxidation) of the oil, polar decomposition products develop. These deteriorate the dielectric properties of the oil. The result of far advanced oil ageing is sludge formation. Sludge greatly compromises the windings because it leads to the formation of sediments, which prevent heat removal. This heat accumulation in turn causes the winding paper to age intensely. A timely diagnosis of the incipient acid formation is therefore important, so that timely counteractive measures can be initiated.

The dielectric dissipation factor of an insulating material is the tangent of the dielectric loss angle.

The dielectric loss angle, is the angle in which the phase difference between the applied voltage and consequential current diverges from π/2 rad, when the capacitor's dielectric comprises only of 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.

In addition to the neutralization value and the dissipation factor, interfacial tension is another indicator of sludge formation in the transformer. This test measures the concentration of polar molecules in oil, which form during the ageing process. The higher the concentration, the lower the interfacial tension and the likelier it is for the insulating oil to form sludge.

Inhibitors are age protecting agents which delay the decomposition of the insulating oil. The IEC 60296 accredited inhibitor DBPC (Di-tertiary-butyl-para-cresol), with a weight percentage of 0.25 ± 0.040 , is used. Inhibited oils are preferred for transformers > 200 MVA, for heavily loaded transformers such as traction transformers, for oven transformers or at special customer request.
When the accelerated oil ageing process is caused by a transformer malfunction, the original inhibitor content (0,3), can be restored by adding a calculated amount of DBPC in powder form, once the failure has been fixed. Even in the case of normal DBPC- deterioration, the oil can be re-inhibited, although an oil change would be less expensive. This decision can be made only based on the results of the other oil value tests: if these are normal then a re-inhibition can be successful.

The decomposition of paper is caused by the processes of hydrolysis, pyrolysis and oxidation. The degree of polymerization (DP-value) of paper was defined by the standard IEC 60450 and it counts the number of polymerized glucose rings. During the paper decomposition, the DP-value is reduced and the tensile strength decreases. New cellulose has a DP-value of 1000-1100, aged cellulose on the other hand has a value of only 150-200, which means the end of the transformer's lifespan.

As it is not possible to take paper samples during running operation, the condition of the solid insulation is estimated based on the cellulose decomposition products (2- Furfural). For this purpose a regular trend analysis is necessary.

An exact link between the furan content and DP-value cannot be deduced at present. However it is possible to make a trend analysis: based on the change of the furan content, information about thermal conduct of the solid insulation over the years can be obtained.