Study on Multi-factor Aging Characteristics of Motor Stator Coil Insulation

Study on Multi-factor Aging Characteristics of Motor Stator Coil Insulation Xi'an Jiaotong University (Xi'an 710049) Le Bo Zhang Xiaohong Lu Weisheng Jiang Xiongwei Jia Zhidong Xie Hengtai Principle Engineering University (Taiyuan 030024) Song Jiancheng stage, tested the dielectric loss tanW, capacitance, current surge of the bar Non-destructive and destructive parameters such as points - residual breakdown voltage. The correlation between non-destructive parameters and residual breakdown voltage is compared. The characteristic parameters for characterizing the aging state of epoxy mica stator coils are proposed.
0 Introduction The main insulation of large generators (main winding of stator windings) is the most important part of the generator. When the generator is in operation, it is subject to multiple factors such as electricity, heat, mechanical stress and various environmental factors. The main insulation will gradually age, and the long-term running strength will decrease, eventually leading to breakdown. Therefore, the availability and remaining life of the generator depend to a large extent on the condition of the main insulation.
In order to effectively evaluate the aging degree and remaining life of the main insulation, it is necessary to study the aging laws and characteristics of the epoxy mica insulation system.
In this paper, the epoxy mica insulated wire rod of the motor is used as a sample, and the wire rod is aged by multi-factor aging method. The dielectric loss (tanW), capacitance and alternating current tests were performed on the bars of different aging states, and the non-destructive parameters of the bars were obtained. Finally, the residual breakdown voltage of the bars was tested to obtain the characteristic parameters of the aging state of the epoxy mica insulation.
1 Aging mechanism of epoxy mica stator winding insulation At present, the stator winding insulation system is mostly a thermosetting insulation system based on powder mica, epoxy glue as adhesive and glass ribbon reinforcement. In the large generator, the stator winding is insulated and the epoxy glass ribbon is semi-laminated and molded. Since the stator winding is subjected to the combined action of electric, thermal and mechanical stress during the operation of the motor, the winding insulation is subjected to thermo-mechanical stress during the start-stop process. Under these stresses, due to the epoxy and mica and glass The structure and physical properties of the ribbon are different. The aging resistance is worse than that of mica and glass ribbon, which reduces the epoxy performance of the adhesive, destroys the interface structure between epoxy and other materials, leads to insulation delamination, and under the action of operating voltage, air gap As a result of partial discharge, it becomes a dielectric, resulting in a relative thinning of the effective thickness of the insulation and a decrease in the breakdown voltage of the insulation. Eventually the insulation performance fails. Figure 1 is a schematic diagram of motor insulation aging. 2 Sample preparation and aging test scheme 2. Sample preparation The motor coil used in this experiment is a 6 kV, 780 kW coal mill replacement coil of Shaanxi Qinling Power Plant. The main insulation and inter-turn insulation of the coil are made of epoxy-mica insulation conductors and 16 strands of copper conductors. The insulation parameters between the strands are: rated power 780 kW, rated voltage 6 kV, insulation thickness 2, limit temperature 130 °C , Class B insulation, Y wiring. Since the coil volume is too large, it is not suitable for multi-factor aging experiments, so the straight portion of the coil is cut into 30 segments for processing. The treated wire rod is shown in Figure 2.
Strip the end insulation about 2 to expose the copper strands for high voltage wiring. The strands are insulated from each other, and the other ends of the rod are welded with copper sheets. In order to avoid tip discharge at the end, the topic of this paper is the National Natural Science Foundation's key funding project. Project approval number 59837260 smoothes the ends of both ends without burrs.
The two ends of the wire rod are coated with 3 silicon carbide, the middle portion is coated with a low-resistance paint, and the low-resistance paint is separated from the silicon carbide 2 as a protective gap for the three-electrode system. Silicon carbide can evenly end the electric field, preventing end discharge and surface slip. In addition, in the case of electrothermal accelerated aging, the applied voltage is 2.5 times (9.5 kV) of the working voltage, and after the end is coated with silicon carbide, the surface slippage of the sample in the oven can be prevented.
The low-resistance paint is wrapped around the aluminum foil as a measuring pole, and the two sides are wrapped around the aluminum foil as a protective pole. This can make the aluminum foil electrode and the wire rod have good contact, prevent bubble and air gap discharge, and improve the accuracy of the test.
2. 2 aging experimental scheme In order to realistically simulate the operation of the motor coil, the multi-factor aging method is used to carry out the aging experiment of the motor coil, that is, the multi-factor stress simultaneous action route. However, the requirements for aging equipment are not easy to carry out, so the technical route of electricity and heat simultaneously acting, then mechanical vibration and cold and heat cycle alone (see Figure 3).
The maximum withstand temperature of the sample coil is 130 °C, the heat aging temperature is 135 °C, and the voltage applied by electric aging is 2.5 times of the working voltage, that is, the vibration of the motor coil is simulated by mechanical vibration, and the vibration amplitude is 2 Frequency 100 Hz. Place the wire rod in the oven and quickly heat up to 155 °C, then use a fan to force cooling to 20 °C, repeat the cycle to simulate the thermo-mechanical stress generated by the start and stop of the motor.
The aging route is electrothermal aging for 4 d, vibration for 48 h, and then thermal cycling for 50 times. The three types of aging are an aging cycle. After each aging cycle, the wire rod test parameters and the breakdown test are extracted to obtain the state parameters of the wire rods in different aging states. The remaining bars continue to age according to the technical route of Figure 3.
3 AC current measurement 3.1 Measurement principle and method When the AC voltage is applied to the insulation, the current increases with the voltage. The voltage point current corresponding to P and P in Figure 4 is abrupt. These two voltages are often referred to as the first and second current surge point voltages (ie, U and U). The root cause of P and P is the presence of air gaps and defects inside the insulation. P is caused by the initial discharge of the air gap group in the same state (ie, the same air gap size, geometric position, and electric field strength) in the insulator under the test voltage. The occurrence of the second current surge point P due to the expansion of the discharge region. Generally, the air gap content of the unaged epoxy powder mica insulation is small, and the local discharge flashover distance formed by the expansion of the air gap discharge range is very small, and P does not appear or is not obvious. For aged insulation, as the applied voltage continues to rise, a second current surge will occur. A test circuit for the initial voltage of U and the partial discharge or the voltage at which the voltage is initially increased is generally shown in Figure 5. The measurement uses a three-electrode system that shields the surface current and eliminates the effects.
3. 2 AC current measurement results In order to reflect the aging state of the wire rod insulation, the insulation resistance per unit length before U is defined as R / L (L is the insulation resistance R per unit length between the length of the wire rod and the unit length between the U and the insulation resistance R / L, the current increase tendency multiples 0. With the increase of aging time, under the action of various stresses, due to insulation stratification, air gap increase and molecular degradation, the operating current and rated current of the motor bar will increase accordingly, and U And will also change accordingly, and R will also change.
And I versus the residual breakdown voltage. It can be seen that these parameters have a certain correlation with the residual breakdown voltage, wherein U has the largest correlation with the residual breakdown voltage, and the correlation of the remaining parameters is small, but can reflect the trend of residual breakdown voltage. When the main insulation gradually ages, the residual breakdown voltage decreases due to the increase of defects and air gaps, and the insulation resistance, capacitance and current surge voltage and equivalent impedance decrease, but the current increase tendency multiple and the rated current and the operating current increase.
4 Dielectric loss and capacitance measurement 4. 1 Measurement principle and method Dielectric loss tangent is commonly used to assess the aging condition of motor insulation within a certain voltage range. It is generally believed that when insulation is cracked due to thermal, mechanical and electrical effects, inside the insulation Air gaps and delamination are created, and both dielectric loss and discharge increase. This results in a decrease in the initial discharge voltage. Usually ΔtanW and tanW represent the average aging level of insulation [3], and the dielectric loss or capacitance versus voltage curve is shown in Figure 14.
In this paper, a two-electrode system with double-layer shielding structure is adopted for the bridge sample.
Relationship with the residual breakdown voltage of the bar Figure 7 Relationship between the residual breakdown voltage of the bar and the residual breakdown voltage of the bar Figure 9 Relationship between the residual breakdown voltage of the bar and the residual breakdown voltage of the bar Fig. 11 Relationship between P and the residual breakdown voltage of the bar and the residual breakdown voltage of the bar Figure 13 Relationship between the residual breakdown voltage of the bar and the residual breakdown voltage of the bar. For the aged insulation, there is an air gap and stratification inside, when the voltage is lower than When the discharge voltage is started, the dielectric loss and capacitance are basically unchanged. As the voltage increases, the air gap begins to discharge, and the short circuit causes the air gap to increase, the capacitance increases accordingly, and the dielectric loss increases correspondingly due to the increased discharge.
4. 2 Dielectric loss and capacitance measurement results The change of capacitance and dielectric loss with voltage is defined as Δ= and the parameter when it is rated voltage. Among them, the theoretical constant is usually compared with different bars. The results of measuring the dielectric loss and capacitance of a motor bar with multiple different multi-factor aging stages are shown in the relationship with the residual breakdown voltage. Figure 16 Relationship between tanW and residual breakdown voltage versus residual breakdown voltage Figure 20Δ / and remaining Relationship of breakdown voltage 5 Test results The correlation coefficient between each parameter and the remaining breakdown voltage was analyzed.
Among them, the correlation with the residual breakdown voltage is the largest, reaching 72, which is a strong correlation, which is consistent with the conclusion obtained by the asphalt mica insulation system. The AC breakdown voltage of asphalt mica insulation is close to that of Pi2. The AC current test has been adopted by many countries in 16.23, and the slope is 2.23, which is close to the result of insulation with asphalt mica.
Therefore, for the epoxy mica insulation system, U can also be used as a characteristic parameter for evaluating the insulation aging state.
The correlation coefficient of the parameter, the correlation coefficient of ΔtanW and the residual breakdown voltage, is a weak correlation. The correlation coefficient of the remaining parameters is less than 20, and it can be considered that there is no correlation with the remaining breakdown voltage.
In the previous research on asphalt mica insulation system, dielectric loss data such as tanW and ΔtanW have strong correlation with residual breakdown voltage. Although they are not sensitive to local insulation aging coils, they can better reflect asphalt. The overall aging level of mica insulation. Various electrical parameters are related to environmental factors such as temperature, humidity and air pressure, especially temperature and humidity. For example, when the humidity changes slightly, the dielectric loss and capacitance will increase dramatically. The aging test has a long cycle and a large time span, and environmental factors are changed during the test.
6 Conclusions and recommendations Multi-factor aging mode can simulate the operation of the motor more realistically and accelerate the aging of the wire rod insulation.
For the motor stator coil of epoxy mica insulation, U has a good correlation with the residual breakdown voltage and can be used as a characteristic parameter to characterize the aging state of the stator coil. The correlation between other non-destructive parameters and residual breakdown voltage is to be studied.
The measurement of dielectric loss and capacitance is susceptible to environmental factors (especially humidity) and should be kept constant during testing.
Due to the influence of environmental conditions, there are errors in the measurement of dielectric loss and capacitance. Therefore, whether dielectric loss (tanW) and dielectric loss increment (ΔtanW) are characteristic parameters of aging of epoxy mica insulated wire rods are to be verified by experiments.
e. There are fewer test samples in this paper, and the correlation between the dielectric loss data and the residual breakdown voltage needs to be verified.
f. It is recommended that the subsequent aging test be carried out in a constant temperature and wet box to keep the environmental conditions unchanged.
1 Xiaojinghu Zhengzhi (Day). High-voltage asynchronous motor aging diagnostic technology. Country 4 Wang Shaozhen and so on. Operating characteristics and testing of large generator insulation. Beijing: Water Conservancy and Electric Power Press, 1992 Dr. Lebo, Ph.D., engaged in large-scale generator main insulation aging and life evaluation. Dr. Jia Zhidong, graduate student, engaged in large-scale generator main insulation aging law and life evaluation 3 Conclusion 3 same shell bus three-phase voltage Taking the values ​​( 1, 0, 0), ( 0, 1 , 0) and ( 0, 0, 1) respectively, the electric field coupling coefficient function curve of the output voltage obtained when the capacitive sensor is rotated to the different positions around the center of the bus bar housing The calculated values ​​are in good agreement, and the measurement method is correct and feasible.
1 Shi Baozhuang and so on. Measurement of fast transient overvoltages in GIS. High voltage technology, 3 Zou Jianhua and so on. The principle of VFT phase voltage measurement of GIS three identical shell busbars is divided into 4 Zou Jianhua et al. Study of electric field coupling coefficient and sensor placement position in three-phase V FT voltage measurement. High-voltage electrical appliances, 1993, (5): 3 Zou Jianhua, born in 1964, Ph.D., associate professor, engaged in high-voltage and high-current measurement and control technology research.

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