Transformer Oil Monitoring & Maintenance

 

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Transformer oil in the transformer has two primary functions:

  • To create an acceptable level of insulation in conjunction with paper covering of conductors and other solid insulation. 
  • To provide a cooling medium capable of extracting the heat without deterioration of the insulation medium

Insulation oil in transformer is a pure hydrocarbon (mineral) oil obtained by the distillation and refining of petroleum. Its properties as a non sludging and lo oxidation medium can be improved by addition of inhibitors which will delay or reduce the degradation by oxidation.

The major requirements of the transformer oil are:

  • Oxidation stability
  • Viscosity
  • Flash point
  • Pour Point
  • Corrosive sulphur content
  • Essential breakdown voltage 
  • Dissipation factor

Calculation of the heat transfer from the core or windings to the oil necessities a knowledge of the various physical properties of the oil such as thermal conductivity, specific heat, density and coefficient of expansion.

 

Transformer Oil Maintenance:

Prolonged operation at temperature greater than 75oC causes the oxidation and sludging . Each increase of 6K in temperature will double the ageing rate of the oil impregnated paper and pressboards. The ageing rate is accelerated by high moisture content. Thermal and electrical discharge deterioration of the transformer oil causes fracture of the paraffinic oil molecule, releasing hydrogen and hydrocarbon ions which combine together to form hydrocarbon gases which dissolve in the transformer oil. Combination of hydrogen and the simpler hydrocarbon gases such as methane (CH4), ethane (C2H6), ethylene (C2H4) and acetylene (C2H2) will produce ranges of ratios which can be interpreted as indicating a range of oil temperatures. Acetylene for instance can be produced by sparking or heating at temperatures greater than 700oC and will indicate internal sparking/arcing. High levels of hydrogen will indicate ionic bombardment caused by the partial discharge.

 

Highly sophisticated gas chromotographs have been developed to measure very small quantities of dissolved gases enabling transformer to be monitored during their factory tests and also to indicate incipient faults immediately after the commissioning.

 

On-site monitoring systems have been developed which can produce a complete analysis of the transformer along with data log enabling transformers with known problems to be monitored and also time required to fault can also be determined.

 

Deterioration of the oil-impregnated cellulose at operating temperatures causes fracture of the cellulose chain thereby releasing moisture, carbon monoxide and carbon dioxide which could be used to indicate the reduction of the mechanical strength of the paper insulation. However, the detection of carbon monoxide by gas chromatograph employing thermistor detectors worsens the statistical repeatability of the results. The solid thermal decomposition products in particular furfuraldehyde, will dissolve in the transformer oil and can be detected by high performance liquid chromatograph at very high levels of sensitivity. A level of approximately 10 mg/lit furfuraldehyde in oil would indicate complete embitterment of all conductor paper in the transformer