Transformer oil maintenance depends upon transformer design and its durability. Preventive maintenance is inevitable since transformer oil may get contaminated due to moisture and combustible gases. Transformer should be physically inspected once a year while oil leakage and magnetic oil gage reading of the tanks must be observed daily.

Dielectric strength test, moisture content, neutralization test, Dissolved Gas Analysis are studied once year. Interfacial tension tests are conducted to decipher the presence of polar compounds like paint and paper. Color test shows ageing and status of contaminants in oil while tan and delta test is done once in two years to measure presence of water. Oil level indicator and sudden pressure relay are inspected once in five years for their fitness.

Inspection of bushings

Once a week, bushings are checked for their oil levels. They are visually inspected annually and cleaned once in five years.

Transformer types

Transformers are classified based on size (small, medium and large), voltage (electromagnetic, capacitor and optical), step up and step-down transformers, power transformer and distribution transformer and instrument transformer.

Transformer oil filtration-The Hering strategy

Transformer oil filtration is never a universal procedure, at least for Hering. Hering’s century-old expertise which is enhanced by German precision gathers a plethora of information before executing transformer oil filtration for any of its clients.

Hering collects information pertaining to the purpose of the plant, voltage, power rating, oil content, total weight, nature of the oil and its age, flow capacity, temperature increase of the oil in each pass, presence of inlet oil pump, type of vacuum pump station, evacuation of transformer, analysis of residual gas and moisture content, filter measurements and many other intricate details. When such detailed efforts are employed, result oriented transformer oil filtration is ensured. Transformer oil is in contact with all parts of the transformer equipment and its fitness reflects the condition of the transformer, thereby predicting and avoiding any failure.

Oil types and filtration methods

Mineral oil

Mineral oil filtration is conducted by chemical, physical and combined methods. Impurities which react with chemicals are removed by acidic treatment, dehydration which employs calcium compounds and metal hydrides. Such chemical methods remove asphalt, tar and other organic compounds along with water.

Physical methods include dehydration of mineral oil by evaporation where the oil is heated and subjected to vacuum process. Purification equipment uses physical forces such as centrifugal, electrical, magnetic or based on gravity or vibration.

The simplest physical method of transformer oil filtration is sedimentation which is based on employment of gravitational forces. The execution of the sedimentation process depends on the nature of contamination which has drawbacks such as long process time. Besides, the method removes only bigger size impurities which are measured between 50 and 100 microns.

Combined methods employ coagulants like alkyl benzol, alkyl toluol sulfonate, octadecylamine which cause precipitation of colloidal particles while adsorbents like diatomite and bentonite clays selectively adsorb organic and non-organic compounds. Both these methods are effective in removing acids, resins, emulsified and solved waters.

Silica gel is nonetheless in its ability to adsorb impurities in specific surface areas and pore sizes. Other combination methods include ionic exchanges and impurity dissolution like propane deasphalting, selective purification and water wash.

Non mineral oil - As for non-mineral oils, filter presses are employed to separate solids from the liquid phases with the help of many filter plates which form a filter plate package.

New oil - New transformer oil is tested regularly for its appearance, flash point, pour point, viscosity, breakdown voltage, acidity, neutralization value and power and moisture contents. Presence of corrosive sulphur and oxidation tests are part of the testing procedure since they herald the lifespan of the apparatus.

Used oil

Used oil witnesses oxidization where an acid is formed when oil comes in contact with oxygen. The acid later settles as sludge which deteriorates the windings. Testing transformer oil regularly must inevitably be a part of the annual transformer maintenance agenda. The test comprises dielectric breakdown, neutralization number, interfacial tension, colour and moisture content.

Following Dissolved Gas Analysis and PCB analysis, a suitable reclamation procedure is executed with focus on environmental concerns. On a concluding note, transformer maintenance not only elongates lifespan of the apparatus but also adds to its efficiency, thereby increasing profits and return on investments.