Dielectric Strength of Transformer Insulating Oil

Dielectric Strength of Transformer Oil

CMM in ELKIMA  transformer was successfully tested

The Dielectric Strength of transformer oil is very important since it is essential in maintaining the reliable operation of power transformers.  The dielectric strength of transformer oil is mainly determined by the presence of acids, water, and other contaminates.  It is therefore, important to keep the transformer oil as free from such contaminates as possible.  The dielectric strength of the oil will decrease with time and based on the service conditions where the transformer is located.

The Dielectric strength of oil is extremely sensitive to hydration or introduction of water and moisture.  Under the action of the electric field of the emulsified oil, droplets of water are drawn to places where the field strength is particularly high.   This process begins the process of oil breakdown.  There can be an even more dramatic reduction of dielectric strength when the oil becomes contaminated by other impurities other than water such as acids and particulate matter. Some particulate matter, in the form of fibers, can become arranged in the form of lines known as “lines of force.”  Under the influence of the field forces, the wet fibers are not only drawn into places where the field is stronger, but become arranged in a way that greatly facilitates the breakdown of the transformer oil.

The Dielectric strength of transformer oil is also known as the breakdown voltage of the transformer oil.  “Breakdown Voltage” (BDV) is measured by observing the voltage that is required to jump a spark between two electrodes immersed in the oil separated by a specific gap or distance.  Also known as the sparking strength, the higher the voltage required to jump the spark, the higher the BDV will be in the oil.  The lower the voltage required to jump the spark, the BDV will be lower indicating  the presence of moisture content and other conducting substances in the oil.

Clean oil with a low moisture content gives higher BDV results than oil with high moisture content and other conducting impurities.  A rating of 30 kV is the minimum breakdown voltage at which transformer oil can be safely used in a power transformer.

Dielectric strength of oils is also determined by the dielectric loss tangent or “Tan Delta.”  As stated above, dielectric strength of transformer oil is mainly determined by the presence of acids, water, and other contaminates in the oil.   Another important factor in oil performance, is the low temperature “pour point.”  A low temperature pour point of -45°C and below is necessary for allowing oil to flow at extremely cold temperatures.

Operational properties of transformer oils are checked by electrical insulating and physico- chemical characteristics :

• Determination of the electrical strength of oil;
• Determination of the loss tangent oil;
• Determination of the oil’s moisture content. The method is based on separation of hydrogen by reacting water in oil located in hydride oxygen;
• Determination of the oil’s gas content.  Performed using an absorption meter.  Gas content is determined by measuring changes in the residual pressure in the vessel after pouring it into a test oil sample;
• Determination of mechanical impurities.  Quantitative solids content consists of passing dissolved in gasoline samples of transformer oil through ashless filter paper;

Dielectric Loss Tangent

Characteristics of transformer oils and methods of maintaining the quality characteristics.  Dielectric loss tangent (tgδ)- index of oil quality, is sensitive to the presence of various contaminants in the oil (colloidal formations soluble organometallic compounds, and various products of aging oil and solid insulation).

Increasing the dielectric loss in insulating oils occurs due to:

• Asphaltic-resinous substances (form a colloidal solution in oil)
• Soaps
• Water (excluding moisture condition true solution)
• Acids (only affected under heating, not at room temperature)
• Dissolving paints in the transformer oil, that usually lead to sharp increase of acid number.

Colloidal materials may be:

• Components of the windings and varnish and old oil sludge;
• Soaps , resulting from the interaction of acidic products of aging transformer oils with transformer metals;
• Muddy-acidic products not contained in the structure of the metal, for example:
• acids, sparingly soluble in oil, tar, asphalt and other products of oxidation.

Dielectric losses for fresh oils characterize the quality and the degree of purification at the factory where the oil was blended, and in operation, it indicates the degree of pollution and oil aging.

The presence of water in the oil increases dielectric loss and attaches U-shaped forms in dependence on tgδ temperature (temperature at least 50°C).

There is a threshold concentration of water in the oil for a given temperature and relative air humidity above which tgδ greatly increases.

Test for Puncture

To determine dielectric strength, transformer oil is periodically tested by means of an oil punching apparatus.  To test the oil sample, transformer oil is poured into a porcelain vessel in which two “disk electrodes,” one of 8mm diameter and one of 25mm diameter, are mounted.  The distance between the electrodes is set at 2.5 mm.  A vessel is filled with oil and set in an oil punch. The oil is allowed to settle for 20 minutes to let any air to escape from the test sample.  Next, voltage to the electrodes is gradually gradually increased 1 – 2 kV in the second before the onset of breakdown. When testing transformer oil one makes 6 breakdowns at 10 minute intervals. The first testing is considered the trial and its result is ignored.  For the value of the breakdown voltage, an average is taken from the five of subsequent breakdowns.

Transformer Maintenance

The best way to maintain the dielectric strength of transformer insulating oil is to initiate a well planned and executed preventive maintenance program utilizing the GlobeCore Service Life Extension Program that incorporates the GlobeCore Process to regenerate transformer oil.  With the use of an “Oil Analysis” program, the regeneration process can be initiated before the acid number of the transformer oil rises above 0.08.  Above that benchmark number, the contaminates in the oil become aggressive against the insulating paper thereby shortening the service life of the transformer.

Its time to get with the Process, The GlobeCore Process!

Dielectric Strength of Transformer Oil in pdf