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Transformer Riddles No.93 - Capacitance and DF test
I want to know what should be the limits of C & DF for normal threshold, Specially when previous results are not available to compare with.
Author : Talha
 
#1
Sat, April 6th, 2013 - 08:30
The power factor is an indication of the amount of energy that ls lost as heat to the oil. When pure oil acts as a dielectric, very little energy is lost to the capacitance charging.
Contaminants will increase the energv absorbed by the oil and wasted as heat. The power factor ls a function of the phasor angle (the angular displacement) between au AC potential applied to the oil and the resulting current. The test is performed by passing a current through a test cell of known gap, and “sing a calibrated capacitance or resistance bridge to separate and compare the reactive and resistance portions of the current passing through the oil.
According to ASTM D-924 Dissipation Factor (or Power Factor) is a measure of the dielectric losses in an electrical insulating liquid when used in an alternating electric field and of the energy dissipated as heat. A low dissipation factor or power factor indicates low ac dielectric losses. Dissipation factor or power factor may be useful as a means of quality control, and as an indication of changes in quality resulting from contamination and deterioration in service or as a result of handling.
The loss characteristic is commonly measured in terms of dissipation factor (tangent of the loss angle) or of power factor (sine of the loss angle) and may be expressed as a decimal value or as a percentage. For decimal values up to 0.05, dissipation factor and power factor values are equal to each other within about one part in one thousand. In general, since the dissipation factor or power factor of insulating oils in good condition have decimal values below 0.005, the two measurements (terms) may be considered interchangeable.
The exact relationship between dissipation factor (D) and power factor (PF) is given by the following equations:



Relative Permittivity (Dielectric Constant)—Insulating liquids are used in general either to insulate components of an electrical network from each other and from ground, alone or in combination with solid insulating materials, or to function as the dielectric of a capacitor. For the first use, a low value of relative permittivity is often desirable in order to have the capacitance be as small as possible, consistent with acceptable chemical and heat transfer properties. However, an intermediate value of relative permittivity may sometimes be advantageous in achieving a better voltage distribution of a-c electric fields between the liquid and solid insulating materials with which the liquid may be in series. When used as the dielectric in a capacitor, it is desirable to have a higher value of relative permittivity so the physical size of the capacitor may be as small as possible.
 
Author : Hamid - From: Iran
 
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