Oil analysis; transformer oil tests

Oil analysis

Why do we need an oil analysis? What are the oil quality standards? What kind of equipment should be used to restore the performance of used oil?

Power transformers are key components of power plants, substations and other power grid facilities. The importance of transformers can hardly be overstated, as they transform electricity that drives the modern world of business and commerce.

Transformers, depending on their structure and applications, can work in various conditions and loads. It is very important to ensure their reliability and continuous uninterrupted service.

Reasons of transformer failure

Even under constant supervision, it is not always possible to avoid unforeseen failures and damage to transformer equipment. At least 80% of all known reasons of transformer failure are related to deterioration of insulating oil in transformers.

Transformer oil deteriorates in the presence of oxygen and moisture.

If the oil opens a channel for oxygen and moisture to reach solid insulation, it causes prematurely aging pf the solid insulation and breaks the complete electrical insulation system.

Other causes of transformer failures are faults in bushings, cooling systems, core and windings, as well as faults resulting from overloading and overheating, or those due to mechanical and electrical stress, short circuits and arcs.

And yet, the most serious consequences come from contamination of transformer oil and accumulation of sludge in the windings and in the solid insulation. Such contamination leads to premature mechanical failure of insulation, often resulting in noisy substations, damage to insulating surfaces, partial discharge, electrical breakthroughs and blackouts.

Oil contamination can be caused by malfunctions of external components or design errors that can adversely affect the oil. For example, damage to an oil pump can lead to contamination of transformer oil with metal particles.

Even a broken gasket can let air and water into the transformer and be the reason for premature aging of oil and solid insulation.

Transformer oil analysis to control its contamination level

Regular sampling and testing of transformer oil determine the amount of contaminants in oil and possible transformer failures. Oil sampling, oil analysis and testing should be performed every 1 to 3 years, depending on the transformer capacity.

If oil test results reveal problems, maintenance and removal of impurities from insulating oil is required.

The tests are evaluated according to industry standards.

Transformer oil is tested for dielectric strength, color, content of gas, water, solids, acids and alkalis.

Transformer oil tests

Flash point

An important oil test is flash point measurement. When oil is contaminated, its dielectric strength decreases along with the flash point temperature. Especially when oil has volatile substances, the amount of combustible gases also increases, and with the low dielectric strength of oil the risk of ignition and explosion also grows.

In order to determine the flash point of transformer oil, it is poured into a closed vessel and heated. The temperature at which oil vapors and air ignite by an electric spark is called the flash point. The flash point and the dissolved gas analysis are the diagnostic tools able to determine internal damage to the transformer.

Pour point

Another temperature-related parameter of oil is the pour point. It is the ability of the oil to flow at low temperatures. If oil cannot flow, it will not be able to transfer heat and prevent damage of solid insulation by harmful impurities. Non-flowing oil can damage the oil pump and other components of power transformer.

Neutralization number

One of the most important parts of oil analysis is neutralization number test. It is an analysis of the alkali number and acid number, which is defined by how much sodium or potassium hydroxide (KOH) is needed (in milligrams), to neutralize the acid compounds contained in 1 g of oil, reported  as milligrams KOH per one gram of the sample. This acid number is an indication of oxidation and aging of the oil. After the acid number exceeds 0.08, the oil is cleaned and purified to remove harmful impurities.

Other tests of transformer oil only add to the above. Their outcomes are not regulated by the European Standard. Such parameters are density of the oil, determined by densimeter, and static and dynamic viscosity, measured with a viscometer.

These oil tests evaluate the current oil characteristics and the condition of solid insulation. The results help the operator to identify and prepare the maintenance program for cleaning and recovery of transformer oil.


Maintenance of transformer oil and oil change have become more automated with comprehensive oil purification systems. A part of oil purification is filtration that removes mechanical impurities and resinous substances by passing the oil through porous screens (filters). Filters provide fine and coarse filtration. Modern filters are made from metal and plastic mesh, felts, fabrics, paper, non-woven materials and ceramics.

Filtration is enhanced by installing more filters and adding second filtration stage. High quality filtration also removes moisture, dust, fibers and other oil aging products.

CMM-R transformer oil regeneration plant

Automated filtration equipment for transformer oils purification effectively removes solid particles, free and dissolved water and dissolved gas.

Equipment for filtration, purification and regeneration of transformer oils must meet certain performance criteria:

  1.       Purify and filter the oil in one cycle;
  2.       Substantially prolong the service life of oil and transformer;
  3.       Mobile, versatile in application with different equipment, indoors or outdoors;
  4.       Easy to maintain and operate.

Effective maintenance helps prevent transformer failures and prolong the life of expensive electrical equipment.