Study on the Intermittent Temperature Loss Spectrum of Multi-factor Aging Process for Large Generator Main Insulation Ma Xiaoqin, Lu Weisheng, Le Bo, Xie Heng (Xi'an Jiaotong University National Key for Electrical Insulation of Power Equipment.
In the low temperature region, the thermal motion of polar molecules is very weak, in a frozen state, the dielectric loss and tanS are small, the conductance loss is negligible compared with the relaxation loss, and the relaxation loss is proportional to g (that is, proportional to the dielectric loss with temperature). The exponential curve increases; when the temperature rises, the molecular thermal motion increases, and after neglecting the conductance loss, the tanS maximum occurs near (1)r=ere1; as the temperature continues to rise, the molecule is completely released and the relaxation time decreases. Polar molecules The orientation can be increased and decreased in time; in the high temperature region, the molecular thermal motion is increased, which hinders the orientation of the dipole molecules in the direction of the electric field, and the electrical conductivity increases sharply with the increase of the conductivity index, and the corresponding dielectric loss and tanS It increases exponentially with increasing temperature.
If the conductance loss is large, the relaxation maximum value of tanS will not be significant. When the conductivity of the medium is large, the maximum value of tanS does not appear.
3 Test and Analysis 31 Dielectric Loss Measurement Part of the stator bar used in this paper is taken from a 16-year water-cooled steam turbine generator with a rated power of 300 MW and a rated voltage of 18 kV. The main insulation of the stator is epoxy. Powder mica F-class insulation, these wires are electrically, thermally and mechanically combined with three-factor accelerated aging. In order to accurately measure the dielectric loss tangent of the bar, this test uses a three-electrode measurement system to shield the surface current and evenly measure the end electric field of the pole.
Medium: Cx and Rx are sample capacitance and resistance respectively; temperature control heating box; Q is shielding box; CN is standard capacitor, R3 and R4 are resistance proportional arms, used to balance capacitance Cx and Cn; capacitor C4 To balance the loss tangent of the sample. Adjust R3 and C4 to make the bridge in equilibrium, that is, zero the zero.
32 test results and analysis keep up with the field changes such that the relaxation polarization loss atanleS with the temperature of the wrist due to the major insulation of the book2 large motor at 0.2Un dielectric loss is mainly caused by conductance loss, relaxation polarization loss and interlayer polarization loss This parameter mainly reflects the characteristics of the epoxy-mica insulation material itself.
With the change of temperature, to understand the change of insulation material with aging time.
And the temperature spectrum of the dielectric loss tangent tanS of the unoperated spare bar at 0.2Un and the dielectric loss tangent tanS of the run sample bar at 0.2Un under different aging cycles. It can be seen that for the spare wire rod that has not been operated, the peak value of the dielectric loss tangent is very obvious, and the peak temperature is about 70C to 80 C, and the relationship between tanS and T is completely matched. It shows that the dielectric loss of the bar is mainly due to the relaxation polarization loss. As can be seen from (a) to (c), a tan peak appears in almost every aging cycle. In the joint three-factor aging, the reason may be that the temperature corresponding to the peak has exceeded the measured temperature range, or the peak may have disappeared.
The main insulation of large generators is epoxy resin, which is a polar polymer. Below the glass transition temperature, the macromolecules are in a rigid state of strong bonding, the molecular thermal motion is weak, and it is basically in a frozen state. The conductance loss is negligible compared with the relaxation loss, and the relaxation loss is directly proportional to the dielectric loss tangent. It increases exponentially with temperature; when the temperature rises to the glass transition temperature, the molecular thermal motion increases, the relaxation time decreases, and the relaxation polarization loss changes with the electric field due to the orientation of the polar molecules. The height is gradually decreasing. This results in the appearance of a peak of the 0 curve of the tanST with different aging cycles. It can be seen from the above analysis that the ctffT curve bSh continuously increases k, and the peak of the nST curve is also closely related to the peak of the high temperature squarebook3 line and the glass transition temperature of the main insulation. As the main insulation ages, its glass transition temperature is also moving, that is, Tm will increase with the aging time. Not only that, the epoxy bond undergoes chemical changes during the aging process, and infrared spectroscopy shows that it has undergone hydrolysis reaction. 14. Due to the continuous generation of small molecules and ions in the main insulation, the conductance loss is continuously increased. With the aging of the main insulation of the generator, the development of delamination defects in the insulator, the interlayer polarization loss is also added, which will cause the maximum value of tanS caused by the relaxation polarization loss to be no longer obvious. Even disappearing, this is why the peak of the tan curve tends to be more and more gradual. These factors make the temperature Tm corresponding to the peak of the tan time curve gradually increase with the aging, even beyond the measurement range or the case where the curve has no obvious peak.
As can be seen from the above analysis, a large generator insulating dielectric loss tangent tanS temperature spectrum corresponding to the peak temperature Tm mainly reflects the performance of the insulating material, which changes with aging time reflects the multi-factor insulation aging process changes in the intrinsic material, i.e. an insulating material in the microscopic changes during aging. In our experiment, we found, the same batch of bars, the measured parameter values ​​of dispersion is small, has good representation, which variation with time of the aging also consistent; in contrast, the measured dispersion medium and the amount of loss tangent is large, aging variation of each stage is not obvious. Further, it was found through experiments and analyzes, the dielectric loss tangent of a glass transition temperature measured by dynamic mechanical experiments peak temperature tanS temperature spectrum corresponds closely related and non-destructive measurement of bars, so you can replace destructive dynamic mechanical experiments, to better changes main insulation material in multi-factor aging process. In the late stage of aging, the peak of the tan curve becomes slow or even disappears due to the addition of the conduction loss and the interlayer polarization loss, which makes the temperature unmeasurable. At this time, it is necessary to combine other parameters to evaluate the aging state of the main insulation.
4 Conclusions By studying the variation of the dielectric loss tangent tanS temperature spectrum of the main insulation of large generators with the aging time under the combined three factors, it is found that the temperature Tm corresponding to the peak of the tan T curve increases with the aging time. The peaks also tend to be flat, which is the result of aging of the insulating material. Therefore, Tm can be used as a characteristic parameter reflecting the aging of the main insulation.
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