—Insulating oil replacement is one of the most commonly used measures to solve oil deterioration in field transformers. However, the effects of oil replacement upon transformer condition assessment are still unknown. This article presents a simulated oil replacement in the process of an accelerated thermal aging test. The accelerated thermal aging experiment was conducted at 130°C for 80 days; insulating oil was replaced on the 25th day of aging. Several common condition assessment methods, including degree of polymerization of the paper, oil acidity, dielectric strength of the oil–paper insulation, moisture estimation, and furfural analysis, were compared before and after oil replacement. Results showed that oil replacement is effective in solving the degradation of insulating oil but not in restraining the aging of paper. The variation of dielectric strength of insulation paper caused by oil replacement could be neglected. Moisture estimation of insulation paper was closely related to the aging degree of oil–paper insulation but not obviously influenced by oil replacement. Furfural analysis would be seriously affected by oil replacement because furfural in oil was eliminated during the oil-drainage process, so the effect of oil replacement should be corrected when estimating transformer aging condition with furfural analysis method.

The furfural concentration in oil is a well-accepted chemical marker indicating the aging condition of power transformers. However, recent researches suggest that the furfural concentration in oil is affected by moisture, because moisture influences the furfural partitioning ratio in oil-paper insulation system. In this study, the correlation of moisture and furfural partitioning ratio was investigated. The accelerated thermal aging test and moisture absorption test were conducted to prepare oil-paper samples with different furfural and moisture concentrations. For samples with different moisture concentrations, the furfural partitioning ratios between oil and paper were compared. Results showed that samples with high humidity exhibited a high furfural mass fraction in oil. This finding implied that high humidity promoted the furfural diffusion from paper to oil. Further, the interference of moisture on aging assessment of insulation paper was analyzed. Results showed that the increase of moisture concentration in oil-paper system would significantly affect the accuracy of insulation paper aging assessment. A corrected equation for aging assessment of insulation paper with different moisture concentrations was established. Verification result showed that the equation could effectively correct the interference of moisture and enhance the accuracy of the aging assessment of insulation paper.

Estimating the moisture content of insulation paper is important in transformer condition evaluation, the results of which are affected by thermal aging. This study investigates the independent effects of aged oil and aged paper on moisture equilibrium charts to determine which of the two primarily causes the inaccuracy in moisture estimation of aged transformers. A thermal aging experiment was conducted. At five aging degrees, four kinds of oil–paper insulation systems were formed: new oil–new paper, aged oil–new paper, new oil–aged paper, and aged oil–aged paper. Moisture equilibrium curves were drawn for each insulation system. The relative moisture content of oil and that of paper were calculated. Results showed that the moisture content of insulation paper decreased with aging, which led to the migration of the moisture equilibrium curves and further influenced the moisture estimation results. A comparative analysis showed that the migration of the moisture equilibrium curves was mainly caused by aged paper at the early and middle aging stages and by the synergistic effects of aged paper and aged oil at the late aging stage. The effects of aging on moisture distribution were verified using a retired transformer.

In order to solve the existing noise reduction measures in substation is not comprehensive, and the lack of substation on-site interference measures, the monitoring system of partial discharge electromagnetic interference in ultrahigh voltage (UHV) substation environment is studied. First of all, the ultrahigh frequency (UFH) interference information in substation is analyzed. Then signal suppression measures are adopted according to the interference signals in the substation. Finally, the UFH monitoring system is established based on the random pulse interference signal elimination method based on fuzzy clustering analysis. The results show that the electromagnetic interference monitoring system of partial discharge in the environment of UHV substation studied in this research can significantly improve the detection and localization of partial discharge, and can have more choices in determining the frequency band, which is helpful to improve the accuracy of time delay measurement so as to improve the accuracy of localization. The result of this research provides a good guidance for solving electromagnetic interference problems in UHV substations in China.

Oil-impregnated paper (OIP) capacitive bushings are critical insulation components of power transformers, and internal defects may induce partial discharge (PD) and further lead to insulation breakdown, threatening equipment safety and power supply reliability. This study investigates the electric field distribution and insulation response characteristics of four typical defects in a 35 kV OIP bushing, including serrated cuts at the edge of the capacitor foil, metallic tips between capacitor screens, internal creepage along the lower porcelain insulator, and poor grounding of the end screen. First, full-scale three-dimensional models of normal and defective bushings are established to analyse the effects of different defects on electric field distribution and dielectric response parameters. Corresponding defective specimens are then fabricated, and dielectric parameter measurements and partial discharge tests are performed to verify the simulation results. The simulation results indicate that the maximum electric field strengths at the defect locations of the serrated foil, metallic tip, and internal creepage defects reach 8.92 kV/mm, 2.82 kV/mm, and 12.19 kV/mm, respectively, while their influences on dielectric response parameters are limited. In contrast, the loss of end-screen grounding increases the end-screen potential to approximately 26 kV and causes significant variations in dielectric parameters. The experimental results are highly consistent with the simulation analysis. This study reveals the electric field distortion mechanisms and response differences associated with typical bushing defects, providing theoretical support for defect mechanism analysis and insulation condition assessment.

In this study, a phase-generated carrier (PGC) demodulation algorithm combined with the extended Kalman filter (EKF) algorithm based on an electro-optic modulator (EOM) is proposed, which can achieve nonlinear distortion (such as modulation depth drift and carrier phase delay) suppression for high-frequency phase carrier modulation. The improved algorithm is implemented on a field-programmable gate array (FPGA) hardware platform. The experimental results by the PGC-EKF method show that total harmonic distortion (THD) decreases from −32.61 to −54.51 dB, and SINAD increases from 32.59 to 47.86 dB, compared to the traditional PGC-Arctan method. This indicates that the PGC-EKF demodulation algorithm proposed in this paper can be widely used in many important fields such as hydrophone, transformer, and ultrasound signal detection.