Dielectric constant and dielectric loss of insulating materials

1.Definition of dielectric constant
Dielectric constant（K）Can be vividly depicted as the reaction of the atomic layer within the polymer chain when polarization occurs[Treating polarization as a molecular chain with an external electric field“epigenetic”Response (such as movement or displacement)].KThe value is not related to the movement of polarized polymers, but to their capacitance. It is a measurement of the ability of insulating materials to store electrical energy. As one of the results of polar functional group movement, electrical energy is stored. The dielectric constant is the ratio of stored electrical energy in insulating polymers to vacuum stored electrical energy of the same size.

  The dielectric constant is closely related to the polymer structure. A small amount of dipole regions in polymer insulation materials (such as polyethylene) can store tiny amounts of electrical energy,KThe value is also very small. When there is a polar bond in a polymer (note: in a compound molecule, covalent bonds formed by different types of atoms, due to the different abilities of two atoms to attract electrons, the shared electron will inevitably be biased towards the atom with stronger electron attraction ability. Therefore, the atom with weaker electron attraction ability is relatively positively charged. Such covalent bonds are called polar covalent bonds, abbreviated as polar bonds),KThe value will increase. Containing dipoles (note: if the electronegativity of several atoms within a molecule varies greatly, atoms with higher electronegativity will attract electrons closer to themselves, making the occupied area more negative; in addition, atoms with lower electronegativity will make the area more positive. This forms thePolymers of electric dipoles (such as ethylene copolymers containing vinyl acetate or ethyl acrylate) are more effective than polyethylene (orXLPE）It can store more electrical energy and has a higher dielectric constant value. Polymers with significant polarity have higher dielectric constants; For example, polyamide materials[Nylon or Heparen (chlorosulfonated polyethylene)].KThe value varies with frequency, at a frequency of50~60HzThe time value is relatively small. The dielectric constant is a ratio (the ratio of polymer to vacuum stored charge of equal size), without a unit, just a dimensionless value.

  2.Testing Procedure and Effects of Dielectric ConstantKFactors of Value

The dielectric constant testing program can refer toASTM-D150andIEC60260Essentially, the test object is an insulating material sample placed between two metal flat electrodes and subjected to a certain voltage. The sample size is specified in the relevant procedure, and the sample must be flat[Should be larger than the diameter of the measuring circular electrode50mm（2in）]The second voltage is applied between the two electrodes without the sample. The ratio of the above two measured values is the dielectric constant. The dielectric constant is the ratio of the capacitance of a dielectric material to the capacitance of air of the same size. Due to the higher dielectric constant value of insulating materials compared to vacuum, regardless of the type of polymer insulation,KAll values are greater than1The dielectric constant of polymers at room temperature is generally2~10The lower the value, the smaller the corresponding dielectric loss. The dielectric constant is related to temperature and also to the testing frequency (with weaker correlation than the former). The testing frequency is generally within60~1000Hzbetween. The dielectric constants of various common insulation materials are shown in the table6-1Polyethylene with different densities (crystallinity)KThere are slight differences in values; Density increases,KThe value slightly increases, but the difference is small.

  Based on the above principle, as polyolefin insulated cables age and oxidize (such as forming water trees)KThe value will slightly increase.

  3.dielectric loss

  From the discussion in the previous section, it can be seen that cable insulation is similar to capacitance. Under an alternating electric field, most capacitors will lose some electrical energy. The ratio of energy consumed by an insulator to energy stored in each cycle is called the loss factor, or the tangent of the loss angle when the insulation system is subjected to a sinusoidal alternating electric field. This energy loss is converted into heat, so the loss factor can be used as a parameter to measure insulation efficiency.

  Loss can also be represented by power factor, which is the sine value of the loss angle. The power factor and loss factor are different, but<0.1At that time, the values of the two were very close. Low loss insulation is similar, usually less than0.1Therefore, the numerical difference between the two factors can be ignored. As the loss increases, the difference between and becomes larger.