RELATIVE PERMITTIVITY (DIELECTRIC CONSTANT)
There is a relationship between the dissipation factor, the power factor, and the permittivity or dielectric constant. They all relate to the dielectric losses in an insulating fluid when used in an alternating electric field. The permittivity is represented as a complex quantity in the following manner: e* = e‘ – j e“ ; where e* is the complex permittivity, e‘ is the real or measured permittivity, and e“ is the imaginary permittivity. In the presence of an alternating field there is created a capacitance current and a resistive current that are 90o out of phase with respect to each other. The vector sum of these two currents represents the total current and the angle between the capacitance current vector and the resulant total current vector is defined as the loss angle, d. The ratio of the imaginary to the real part of the permittivity is equal to tan d ; i.e. tan d = e” / e’. The factor tan d is defined as the dissipation factor, D, and represents the dielectric loss in the insulating fluid. The power factor, P, is defined as sin d. The relationship between D and P is the following: D2 – F2 = D2 F2 , thus if you know one value you can calculate the other. Furthermore for small values of d, tan d ~ sin d, thus for values of tan d up to 0.05 the power factor and the dissipation factor are the same within one part in a thousand.
The details of the entire procedure are given in the ASTM D 924 standard and are only briefly mentioned here. The measurements are made in specially designed cells that are machined to precise dimensions. The measurements are done at precise temperatures, usually 25 and 100 oC, thus the cells have to be kept at a constant temperature. The actual measurement is one of comparing the capacitance of the cell filled with the insulating fluid sample in a sensitive electronic bridge circuit. The result is usually expressed as a percentage for the dissipation factor or power factor.
The dielectric loss factor relates to the inability of molecules in the insulating fluid to reorient themselves with an alternating electric field. This ability is dependent on the temperature of the sample, the size of the molecules involved, and their polarity. It is also dependent on the frequency of the alternating field. The dissipation factor and the permittivity are both affected by the molecular size, composition, and relative orientation of functional groups within the molecules. In general within a series of similar molecules, the permittivity will increase as the molecular weight increases. The above described factors are electrical characteristics of the insulating fluid and can be used to monitor the quality of the oil with regard to deterioration in use and for the presence of contaminents.
The IEEE has suggested guidelines for Power Factors depending on the type of oil and the unit it is being used in (IEEE C57,106-1991). Some representative values are given below:
|Type of Oil/Unit||Power Factor|
|@ 25 oC||@ 100 oC|
|Shipment of New Oil from Refinery||max. 0.05%||max. 0.3%|
|New Oil Received in New Equipment|
|< or = 69 kV
69 – 288 kV
> 345 kV
|New Oil for Circuit Breakers||max. 0.05%||max. 0.30%|
|Suggested Limits for Oil used
in Circuit Breakers
|max. 1.0%||Not Spec.|