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The rise in hot spot temperature is a critical indicator of transformer winding defects, making its monitoring essential. Traditional temperature measurement methods, however, have certain constraints in obtaining hot spot information due to the complex structure of transformers. This article thus proposes a nondestructive testing for measuring the transformer winding hot spot temperature based on ultrasonic thermometry, without altering the transformer’s internal structure. It also explores the potential application of ultrasonic temperature measurement technology inside transformers. The propagation pattern of ultrasound within a transformer is simulated, and the effect of hot spot faults at varying positions and temperatures on the spatiotemporal distribution of ultrasonic propagation is examined. The range of effective acoustic signals has been determined through comparison. An artificial intelligence-based hot spot temperature inversion method is proposed, leading to the establishment of an effective diagnostic technique for hot spot temperature within the hot spot region, accomplishing a preliminary inversion of hot spot faults. This research offers a novel approach to measuring the internal temperature of transformers.

Power transformers play a critical role in electric energy transmission, while making the assessment of their aging condition highly significant. By measuring the dielectric response data in the hot-spot region of transformer oil-paper insulation, their aging condition can be effectively analyzed. However, the occurrence of non-Debye phenomena during time-domain dielectric response measurements severely impacts the accuracy of aging condition analysis. In response, this article constructs a time-domain fractional element (TFE) model to analyze these non-Debye deviations. The model combines the principles of fractional-order calculus and extended Debye (ED) model, providing an explanation for the occurrence of errors. Then the genetic algorithm (GA) has been applied to optimize the polarization and depolarization current (PDC) data in hot-spot region to obtain TFE model parameters. The final results show that the model can well verify the occurrence of wavefront exponent peaks and long power-law tail phenomena and enhance the fitting accuracy in PDC curve. Meanwhile, the relationship model of parameters with variations in test temperature and aging degree is established, enabling more accurate analysis for the aging condition of hot-spot region insulation in transformer when compared with the extended Debye (ED) model.

To study the change characteristics of space charge in oil-paper insulation under a slightly nonuniform electric field and a uniform electric field, the existing space charge measuring device was improved, and sphere- and column-plate electrode models were prepared. Oil-paper insulation samples at different aging stages were obtained via electrothermal aging. The real-time waveforms of the space charge distribution in the samples at different aging stages were measured in the process of polarization with a direct-current (dc) voltage and depolarization with a dc voltage, and the effects of different electric-field types on the charge distribution and motion characteristics of oil-paper insulation samples at different aging stages were examined. The results indicated that the existence of a slightly nonuniform electric field accelerated the change in the charge-density peak at the interface and increased the fluctuation range of the charge-density curve in the process of applying a dc voltage. With the aggravation of electrothermal aging, the amount of charge injected into the sample increased, and the slightly nonuniform electric field intensified this process, affecting the accumulation and dissipation of space charge in the sample. Electrothermal aging increased the trap distribution density and energy-level depth. However, for an oil-paper insulation sample at the same aging stage, compared with the uniform electric field, the slightly nonuniform electric field had no obvious effect on the trap energy-level depth, but it increased the amount of trapped charge and changed the trap density distribution in the sample. The results of this study provide a theoretical basis for examining the influence of slightly nonuniform electric fields on the space charge distribution characteristics of oil-paper insulation subjected to ac/dc combined electrothermal aging.