During the operation of the generator, under the action of various stresses, the stator insulation gradually deteriorates or ages, eventually leading to breakdown of the insulation. The unplanned downtime of the unit caused serious damage to the reliability of the entire power system.

At present, most of the large generator stator insulation uses epoxy mica insulation system. During the operation of the generator, the stator insulation is prone to microscopic defects such as delamination, shelling or cracking. It is useful to verify microscopic defects and the macroscopic parameters it causes, whether it is the aging mechanism of the laboratory materials, or the insulation state diagnosis and residual life assessment applied to the generator site.

The wire changer was subjected to multi-factor accelerated aging.

In polymer matrix composites, due to the structure of the interface phase and the structure of the polymer matrix and filler, its effect on the dynamic mechanical properties of the composite can be achieved by changing the glass transition (a-transformation) of the polymer and hindering the polymer. The side chain rotation of the macromolecular chain or the local motion of the main chain is indirectly expressed. The storage modulus and dynamic viscosity of the composite material can also be used to characterize the bond strength of the interface. Therefore, dynamic mechanical analysis is an effective means to study the structure, molecular motion, interface state and properties of composite materials.

When analyzing the main insulation of the generator, the epoxy resin is the matrix, the mica and the glass fiber are the fillers. The material can be regarded as the composite elastic medium of “epoxy resin-interface-mica” or “epoxy resin-interface-glass fiber”. The strain of the strain medium near the inner point under the stress and the stress received are in accordance with Hooke's law, and thus the wave equation of the acoustic wave in the elastic medium can be obtained: the sine operator.

Due to the continuous improvement of the performance of ultrasonic transducers and the continuous development of computer technology and information science, it has injected vitality into the development of traditional ultrasonic testing, and provided practical guarantee for its rapid and effective evaluation of motor insulation performance.

3),. The experimentally measured speed of sound is actually the average velocity of the ultrasonic waves passing through the upper and lower insulation layers and the copper conductors.

During the aging of the wire rod, the change of the copper wire is negligible with respect to the aging of the insulating layer, and the change of the sound speed can be considered mainly from the aging of the insulation.

In addition, the factors affecting the speed of sound measurement are mainly the contact between the ultrasonic probe and the sample. Because ultrasound travels much faster in air than in solid media. If the probe and sample are charged with glycerin or water as a coupling agent to achieve the transmission of acoustic energy, the sensitivity of the detection is improved. Silicone grease is used as a coupling agent in this paper.

The stator bars are rectangular copper strands wrapped with epoxy mica insulation.

From the acoustic point of view, copper and insulation are actually inhomogeneous media. It is very difficult to study this structure, so the attenuation caused by this structure should be reduced. The higher the frequency of the ultrasonic wave, the greater the attenuation in propagation. The probe with a working frequency of 500 kHz is selected.

4 Experimental results and analysis In polymer-based composites, in general, the properties of the interface to the composite can be characterized by the storage modulus and dynamic viscosity of the composite. See the experimental data of the storage modulus of the insulation at room temperature. It can be seen from the figure that during the insulation aging process, the storage modulus at room temperature is continuously decreasing.

See the apparent activation energy (AAEGT) of the glass transition calculated from the mechanical loss temperature spectrum. The AAEGT curve in the figure indicates that the AAEGT of the wire rod increases after aging. AAEGT represents the energy to be overcome during the glass transition of the material. The higher the glass transition temperature (the conclusion of the dynamic viscosity temperature spectrum), the larger the AAEGT, the greater the energy required to supply the transition.

Insulation becomes black, layered insulation expands, and delamination (c) aging AAEGT characterizes the change of the material itself. It is precisely because of the change of the material that the storage modulus and dynamic viscosity change, and the bonding performance of the insulating interface becomes worse. Microscopic defects such as stratification continue to appear. It is a photograph of the cross section of the bar, from which the change in interface performance can be seen. (b), (c) The insulation near the copper strands in the figure is obviously blackened, and the insulation begins to expand; (d) The delamination of the insulation in the figure is clearly visible, and the insulation expansion is severe.

There is a close relationship between the properties of the medium (such as the density of the solid, the modulus of elasticity, the bond strength) and the amount of ultrasound (such as the speed of sound) that describes the acoustic properties of the medium. When the insulation aging occurs microscopic defects such as delamination, the insulation must contain a certain air gap. Ultrasonic waves travel in gas at an order of magnitude less than in solids, so the presence of air gaps reduces the ultrasonic propagation velocity. Therefore, the ultrasonic sound velocity which changes with the insulation of the wire rod is closely related to the insulation state of the wire rod. See 0. It can be seen from the curve that the ultrasonic sound velocity changes very much with the aging time, and the sound velocity decreases as the aging time increases. At the end of aging, the amplitude of sound velocity decreases, and the dispersion also increases.

The reason for the analysis may be that the bonding performance of the interface is declining during the aging process of the wire rod insulation, and the microscopic defects such as delamination cause the speed of sound to decrease; the form of the damage is different in the late stage of the insulation aging until the insulation failure, such as the closeness of the wire rod with severe heat aging. Insulation of copper strands, â–² test data statistics median 0 ultrasonic sound velocity with insulation aging time changes its aging is high temperature carbonization of insulating materials, and mechanical vibration and thermal cycle aging often lead to severe stratification of insulation. In these cases, even if the aging time is the same, the difference in ultrasonic propagation characteristics may be large, so the dispersion in the late stage of insulation aging is relatively large.

The analysis of dynamic mechanical parameters and the results of ultrasonic sound velocity test show that the decrease of interfacial adhesion performance during insulation aging is the cause of the decline of insulation performance.

5 Conclusions The dynamic mechanical analysis of the stator bar insulation proves that the apparent activation energy of the glass transition in the insulation aging process is large, the storage modulus and dynamic viscosity are decreasing, and the interface bonding performance is deteriorated. It provides an experimental basis for the application of ultrasonic waves in generator stator insulation diagnosis.

In this paper, ultrasonic technology is applied to the non-destructive testing of generator stator bar insulation. The ultrasonic sound velocity is significantly reduced with the aging time of the epoxy mica insulation, especially in the late stage of aging, the magnitude of the decrease is large, and the dispersion is also increased. It is proved that the ultrasonic sound velocity can be used to characterize the aging state of the stator bars of the generator.

The physical meaning reflected by the relationship between the ultrasonic sound velocity and the insulation state--the microscopic defects caused by the insulation aging, consistent with the results of the dynamic mechanical analysis, all prove that the decrease of the insulation performance is due to the decrease of the bonding performance of the insulation interface.

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