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Transformer Condition Monitoring

MINERAL INSULATING OIL AS AN AID TO TRANSFORMER CONDITION MONITORING

 

 Condition monitoring is a powerful and reliable diagnostic tool to protect large, expensive transformers and can be applied at a reasonable cost to all sizes and types of oil insulated plant 

 

 

ROUTINE ANALYSIS

 

Reliable Performance of mineral insulating oil in an insulating system depends upon certain basic oil characteristics, which can affect the overall performance of the electrical equipment.

 

Resistivity & Dissipation Factor

 

These tests are sensitive to the presence of soluble polar contaminants, and products of oil ageing. The resistivity of an oil is a measure of its resistance to the flow of direct current, whereas the dissipation factor (DDF) test procedure uses alternating current, but the two properties are related though not with such precision as would permit really accurate inter-conversion by means of a formula. Both properties are very temperature dependent, DDF increasing and resistivity falling with rise in temperature, and both, like the electric strength, are effected by physical condition, but unlike electric strength both are greatly influenced by even minor chemical changes such as those effected by oxidation.

A transformer oil should have low DDF and a high resistivity.

 

Acidity

 

Acidity increases as a result of oxidative ageing and is used as a guide in determining when oil should be changed, the British Standard is 0.50 maximum (mg KOH/g).

 

Water Content

 

Water can originate from the atmosphere or be produced by the deterioration of insulating materials, and in general remains in solution.  The solubility of water in oil increases with temperature and acidity.  In a transformer the total mass of water is in the cellulose insulation system and the oil.  The bulk of the water being present in the cellulose.  Small changes in temperature significantly change the water content of the oil but only slightly that of the cellulose.  In the case of transformers therefore the oil temperature at sampling is critical.

 

Electric Strength

 

Electric strength is a measure of the ability of oil to withstand electric stress.  Clean dry oil is confirmed with a high electric strength.  High water, solid particles and fibres in combination reduce electric strength values significantly.

 

Colour

 

Not a critical property, but gives a guide to the general condition of the oil.

 

 

 

Appearance

 

Haziness may indicate the presence of free water, insoluble sludge, carbon or other contaminates.

 

Odour

 

An acid odour confirms the oil to be at an advanced stage of ageing.  Hydrocarbon gases require the analysis of Dissolved gases.

 

Sediment

 

Visual examination, looking for the presence of oil oxidation products, colloidal carbon, bitumen and general debris.  The presence of sediment may deteriorate the electrical properties of the oil.

 

Fibres

 

Visual examination through polarised light.  The excessive presence of fibres and moisture, significantly lower the electric strength.

 

 

 

 

Dissolved Gas Analysis

 

Monitoring and maintaining Insulating Oil quality and the integrity of a transformer are essential in ensuring reliable plant operation. Large oil-filled electrical equipment, such as transformers and reactors, is a critical element of an electrical power system. The early detection of incipient faults in transformers will greatly reduce unplanned power outages and improve the reliability of the power and service supplied to customers.

 

Damages to the equipment will be restricted when incipient faults are detected and timely action is taken. Early detection limits the amount of adjacent damages and confines the area requiring repair and maintenance. The cost of a power transformer is high, but monitoring the performance of the transformer via the oil is inexpensive compared to the costs of a failure in a transformer and the costs of interruption in power supplies.

Polychlorinated Biphenyl (P.C.B) Analysis

 

A group of synthetic hydrocarbons which had good thermal stability and electrical characteristics.

 

However their chemical stability and resistance to bio degeneration has given cause for concern, in terms of environmental pollution.

 

Increased concern resulted in the total ban of its use by an International agreement in 1986.

 

Common handling facilities have lead to widespread contamination of mineral insulating oil.

 

Currently governed by E.C regulations and a limit set of 50mg/Kg maximum in Insulating Oil.

 

Furans Analysis

 

Factors limiting the lifetime of a transformer is the thermal degradation and ageing of the cellulose insulation system.

 

Thermal degradation of the cellulose is accompanied by the production of oil soluble by-products. These products being Carbon Monoxide and Dioxide, water and Furanic Compounds.

 

Oxides and water are analysed and reported under Routine and Dissolved Gas Analysis, trending of concentrations of furanic compounds in oil provide early indications of insulation degradation.

 

Kraft paper, in conjunction with insulating oil, is used to insulate the conductors. It is used in preference to other types of paper because of its high purity. Kraft paper consists of approximately 90% cellulose, 6% lignin and the remainder made up of hemicelluloses. The paper gets its strength from the inter-molecular hydrogen bonds between cellulose chains, which also produce a semi-crystalline structure.

 

Following factory drying, the mechanical strength of the paper would have an initial value of 1000. This figure being the Degree of Polymerisation (D.P.). To measure the D.P. requires a sample of the paper from the conductors. This is not a practical option for a transformer in service.

 

Several Laboratories have carried out studies relating D.P. to Furans in Insulating Oil. A Furans concentration of <0.1 mg/Kg relating to 1000 D.P.

 

Interfacial Tension

 

The interfacial tension test measures the strength of the interface between oil and water.

 

The interfacial tension depends on the polar groups in the oils, whilst tan delta (90 degrees, 50 Hz) tells something about the content of ionisable contaminants.

 

 

Corrosive Sulphur

 

Sulphuris present in refined oil as sulphur-contaminating molecules.  The amount depends on oil refining processes, degree of refining and crude oil type.

 

Due to poor refining or contamination, reactive compounds giving corrosion at normal operating temperatures may be present.  They are detected by standard tests as ISO 5662 or DIN 51353, which are included in all major specifications.

 

At high temperatures, >300 ˚c, sulphur-containing oil molecules may decompose and react with metal surfaces to form metal sulphides.  Such reactions may take place in switching equipment and will impact the conductivity of contacts.

  

Monitoring and Maintaining Insulating Oil quality and the integrity of a transformer are essential in ensuring reliable plant operation. Large oil-filled electrical equipment, such as transformers and reactors, are a critical element of an electrical power system.  The early detection of incipient faults in transformers will greatly reduce unplanned power outages and improve reliability of the power and service supplied to customers.

 

The cost of a power transformer is high, but monitoring the performance of the transformer via oil is inexpensive, compared to the cost of failures and interruption in power supplies.

 

A transformer oil carries information about the condition of the transformer.

 

Analyzing the oil in service can therefore give early warnings to its physical condition, integrity of the core and windings, and the condition of the cellulose insulation system.

 

To avoid serious problems this data can be used as a basis for corrective measures under controlled and planned conditions.