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| A Characterization Method for Main Insulation Degradation of Generator Stator Bars Based on Polarization Current and De-Trapping Current |
| Wang Siyu1, Zhou Kai1, Zhu Guangya1, Yang Shuai2, Zhang Yue2 |
1. College of Electrical Engineering Sichuan University Chengdu 610065 China;
2. Dongfang Electric Machinery Co. Ltd Dongfang Electric Group Deyang 618000 China |
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Abstract The main insulation of large generator stator bars is inevitably subjected to thermal stress during operation. The internal epoxy resin will undergo physical phenomena such as post-curing and degradation. Degradation will directly reduce the dielectric performance of the entire main insulation system, thereby affecting the safe and stable operation of the entire generator set. The existing research mainly focused on the dielectric response method. The evaluations of the insulation performance of stator bars are mainly from a macro perspective They cannot delve into the micro field. A trap charge density and trap depth detection method was adopted to evaluate the post-curing and degradation status of the insulation. The method was based on polarization current and de-trapping current. This method can characterize the degradation state and mechanism of generator main insulation under thermal stress.
The samples of epoxy-mica flakes were prepared. The extraction method of the de-trapping current was analyzed. The related theory of characterizing the trap charge density and trap depth with the de-trapping current was defined. The samples were injected with trapped charge using a needle plate electrode. The influence of the polarization and depolarization current (PDC) method testing field strength, charge injection time, and charge injection voltage on the de-trapping current were discussed. The low polarization field strength could make it difficult for trap charges to de-trap. The higher polarization field strength might introduce space charge limiting current. It may affect the de-trapping current. The increase in injection time leads to an increase in trapping charge. Traps with depth between deep and shallow traps trap more charge. The equivalent trap depth was shifting in a moderate direction. The low injection voltage could make it difficult to inject charges, while the high voltage could cause charges to accumulate in deep traps. The higher voltage might damage the sample. Finally, it was determined that the polarization field strength was 4 kV/mm, the charge injection time was 15 minutes, and the injection voltage was 10 kV.
Subsequently, the samples were subjected to thermal aging at 135℃ for 0 d, 5 d, 10 d, 20 d, and 30 d. Then the aged samples were subjected to de-trapping current testing. Through the evolution process of trap charge density and trap depth, the mechanisms of insulation post-curing in the early stage of thermal aging and insulation degradation in the later stage of thermal aging were clarified. Finally, the breakdown field strength test, differential scanning calorimetry (DSC) test, and partial discharge test were carried out. The overall dielectric strength change rule, molecular chain defect evolution mechanism, and air gap defect evolution mechanism during the post-curing and degradation process of insulation were characterized. The evolution of exothermic and endothermic peaks existed in DSC testing. The post-curing and degradation processes of the epoxy resin molecular chains inside the main insulation were clarified. The evolution law of the size of the internal air gap and the degree of air gap accumulation at the interface of the main insulation were clarified through partial discharge testing.
The feasibility of characterizing the degree of degradation of epoxy-mica through de-trapping current was proved in this article.
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Received: 27 November 2023
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2025, Vol. 40 
