Abstracts:Nano-additives can improve the physicochemical and electrical properties of insulating oil by enhancing the microscopic interaction. In this article, the microscale interaction characteristics of cage polysilse- squioxane (POSS)-modified insulating oil under nonuniform electric field and nonuniform temperature field and its influence on molecular diffusion are studied. The results indicate that under nonuniform temperature field, the interaction energy between POSS and insulating oil molecules in the high electric field strength area is higher and has better mutual solubility, and the high-temperature area under nonuniform temperature field cannot promote the mutual solubility between POSS molecules and insulating oil molecules. In addition, displacement vector data show that the increase of electric field strength can promote the displacement of insulating oil and POSS molecules, and the molecular displacement at the boundary of nonuniform electric field is higher. The high-temperature region under the nonuniform temperature field can increase the molecular kinetic energy, but the higher viscosity of the insulating oil molecule prevents heat transmission. Furthermore, the mean square displacement (MSD, nonlinear increase) under the nonuniform temperature field is far less than the MSD (linear increase) under the nonuniform electric field strength. This is mainly because the energy of the system under the nonuniform electric field is always at the same level, but the low-temperature area of system under the nonuniform temperature field continuously absorbs energy from the high-temperature area. This article can provide the microscale interaction of Nano-modified vegetable insulating oil under nonuniform electric field and nonuniform temperature field.

Abstracts:The objective of our work is to investigate partial discharge (PD) behaviors and degradation characteristics in winding insulation of an actual 22 kV cast resin transformer (CRTr) when subjected to bipolar impulse voltages. A differential high-frequency current transformer (HFCT) was employed on the high-voltage coils to measure PD pulses induced by impulse voltages. Additionally, PD measurement at ac voltage was carried out to characterize the aging process resulting from the application of bipolar impulses. Experimental findings indicated that higher-level bipolar impulses could cause large-amplitude PDs in the windings, occurring at the peak value of the applied impulse voltages and increasing with the number of applied impulses. PD inception voltage (PDIV) at ac voltage decreased to the operating voltage, and the discharge amplitude and phasewidth in phase-resolved PD (PRPD) patterns increased due to impulse PD occurrences. Considering these results, it is evident that the bipolar lightning impulses could cause large-amplitude PDs, intensify impulse PD activity, and ultimately lead to dielectric breakdown in the CRTr windings. Space charge accumulation and residual charge deposition resulting from impulse PD play a significant role in the generation and the process of development for impulse PDs in the winding insulation, contributing to an acceleration effect in the degradation process and eventual insulation failure.

Abstracts:The accurate and effective identification of power transformer insulation fault is critical in implementing corrective actions and preventing problem reoccurrence. While the dissolved gas analysis (DGA) forms the basis for fault identification, certain challenges still remain, including the absence of clear theoretical principles, conflict results, and the oversight of multiple faults. This article addresses these issues by employing molecular dynamics (MD) simulations to investigate the decomposition of mineral oil under various insulation fault conditions. Identification is eventually achieved by a clustering-based method with MD results as initial centers. To achieve this, the molecular model of transformer mineral oil is first constructed, and its decomposition mechanism and results are studied under different insulation fault conditions. Afterward, based on the MD results, certain ratios between the decomposed gases are selected and calculated, which are utilized as the initial centers of the clustering. Finally, the fault identification can be achieved by substituting the DGA data into the established clustering classifier. The proposed method is tested with both the IEC TC 10 database and the local DGA dataset. The results show a respective 83.4% and 89% success rate in identifying single or multiple faults, verifying the effectiveness of the proposed method.

Abstracts:The movement and aggregation of cellulose particles reduce the insulation performance of the transformer, thereby increasing the risk of oil-paper insulation failure. In this study, an experimental platform is constructed to synchronously record the particle motion images and voltage, allowing for the observation of the typical motion mode of the particle between electrodes, namely, the back-and-forth motion mode, under alternating current (ac) voltage. The connection between the cellulose particle’s movement and the ac voltage phase in this motion mode is subsequently examined, delving into the trajectory of particle motion, which is captured through the implementation of image processing technology. The results indicate that the cellulose particle does not always move in the direction of the electric field lines. Within one cycle of the power frequency voltage waveform, the particle exhibits two states: staying on the electrode surface and moving in the oil gap, with each state occupying approximately half of the time. In addition, the cellulose particle consistently moves within the oil gap during the second and fourth quadrants of the voltage cycle, while it remains on the electrodes during the first and third quadrants. Subsequently, an analysis model is conducted to simulate the particle’s trajectory as it moves between the electrodes, coupled with the calculations and analysis of the variation in the velocity and the primary forces exerted on the particle. The results show that the particle tends to move toward the more concentrated area of the electric field, and the closer it is to the center of the spherical electrode, the denser its motion trajectory is. This study provides a theoretical basis for the subsequent insulation hazard assessment of cellulose impurities.

Abstracts:This article presents a trend analysis of long-term ac partial discharge (PD) characteristics till dielectric breakdown by accelerated degradation test in an aged 22 kV cast resin transformer (CRTr). The long-term ac PD characteristics till dielectric breakdown showed that the maximum PD charge ${Q}_{\max }$ and the average PD charge ${Q}_{\text {mean}}$ increased sharply during the cumulative charging time ${t}_{\text {c}}$ , which was between 100 and 200 h before dielectric breakdown. These results suggest that defects could expand and propagate due to PD degradation and penetrate the insulating paper, resulting in large PD as a precursor to dielectric breakdown. With a voltage acceleration factor ${\text {AF}}_{\mathrm {\textit {v}}}$ and a frequency acceleration factor ${\text {AF}}_{\mathrm {\textit {f}}}$ in the accelerated degradation test considered, the time at which this sign was observed was well before dielectric breakdown. Therefore, this sign can be expected as one of the indicators of equipment replacement. The time sequence of ac PD characteristics just before dielectric breakdown was successfully measured. The result indicates the possibility that a fatal dielectric breakdown can be prevented owing to a fast interruption utilizing a circuit breaker. Furthermore, a breakdown site in the winding of the aged 22 kV CRTr could be located by the thermography, i.e., high electric field sections between the top block layers of the high-voltage side winding.

Abstracts:Bubbles are typical defects in the oil-paper insulation system of transformers. The partial discharge (PD) caused by them significantly reduces the insulation performance of oil-paper insulation, posing a great threat to the insulation of transformers. However, there is currently limited research on the evolution process of bubble PD and the deterioration characteristics of oil-immersed pressboard (OIPB) caused by bubble PD. This article first constructs a bubble PD test platform, using air bubbles in stagnant oil as the research object. In the research object, the shape, volume, and gas composition inside the bubbles will change with the variation of voltage and PD, which can fully reflect the accumulative effect of long-term PD in bubbles. Then, this article conducts experimental research on the evolution and breakdown process of bubble PD. The results show that the evolution of PD inside bubbles may greatly change the phase-resolved PD (PRPD) spectrum, and this article proposes a possible explanation for the evolution of bubble PD: the gas composition inside the bubble dominates the evolution of PD. Finally, based on the inhibitory or promoting effects of different fault gases on the evolution of PD, this article establishes a connection between bubble PD and the deterioration of OIPB. Scanning electron microscopy (SEM) is used to analyze the microstructure of OIPB in different damage states, and the micromechanism of bubble deformation and insulation failure is discussed. The work helps to understand the PD characteristics of bubble defects and the mechanism of causing insulation failure in OIPB, providing a reference for risk assessment of transformers.

Abstracts:This study investigates the influence of accelerated thermal aging and degradation on the breakdown strength of natural ester oil (NEO-FR3) compared to mineral oil (MO) under dc voltage stresses. The base oils undergo accelerated thermal stresses overextended aging periods of 90, 200, and 500 h. The primary focus is to understand the breakdown phenomena influenced by various oil properties. To ensure the reliability of the results obtained, statistical hypothesis testing is performed to evaluate the repeatability by analyzing the statistical patterns in direct current (dc) breakdown voltage (DCBDV) values derived from the experimental data. The experimental data are validated and modeled by employing a three-parameter Weibull distribution. The study reveals that natural ester oil (FR3) exhibits minimal variation in breakdown voltage (BDV) under thermal aging and maintains higher BDV than MO under both polarities. However, increasing the aging duration decreases the breakdown strength for both oil samples. The majority of the data conforms to Weibull distributions, confirming the reliability of the findings. Moreover, the study explores dc conductivity through frequency domain spectroscopy (FDS), investigating the effects of thermal aging on conductivity and overall dielectric strength. The analysis uncovers distinct differences in the aging characteristics of MO and NEO-FR3. It is seen that FR3 exhibits a significant rise in dc conductivity over time, while its BDV does not decrease proportionally. On the other hand, MO is less affected by changes in conductivity but is more prone to BDV, especially when subjected to negative polarities.

Abstracts:This article presents a 2-D axisymmetric numerical model of streamer propagation in transformer oils, constructed based on the drift-diffusion control equation of charged carriers. This model is energized by submicrosecond positive pulsed voltage to simulate streamer behaviors in various voltage levels, gaps, and oils. The simulation results indicate that a smaller gap and a higher voltage level result in faster streamer propagation. The average velocity of streamers varies in 2–400 km/s, but in most cases, this value is concentrated in 11–95 km/s. The field strengths for streamers in naphthenic oil and natural ester range in 3.5– $6.2\times 10^{{8}}$ and 1.7– $4.2\times 10^{{8}}$ V/m, respectively, whereas the space charge densities are 500–2200 and 200–900 C/m3, respectively. Streamers in both oils rapidly accelerate around the needle electrodes. However, the streamer in natural ester experiences deceleration as it moves away from the needle electrodes at lower voltages, whereas at higher voltages, its velocity is either comparable to or exceeds that of the naphthenic oil. In the process of streamer propagation, the field strength and space charge density at the streamer head are mainly related to the liquid nature. The increase of the applied voltage can improve the average propagation speed of streamer, but the change of the real-time propagation speed is mainly determined by the propagation stage where the streamer is. Furthermore, by summarizing the simulation results in different cases, a theoretical model of streamer propagation, i.e., competition between the Laplacian electric field and the space charge effect, is proposed from the perspective of the behavioral mechanism of space charge. With the propagation of streamer, the extent to which the space charge effect can offset the diminishing Laplacian electric field, becomes crucial in determining the streamer’s acceleration potential. In regions far from the needle electrode, the enhanced space charge effect emerges as the overriding factor, propelling the streamer’s acceleration.

Abstracts:Due to its exceptional thermal conductivity and enhanced insulating properties, boron nitride (BN) is increasingly employed as a filler in room-temperature-vulcanized (RTV) silicone rubber (SR) coatings, serving as external insulation for oil-immersed ceramic-based porcelain transformer bushings. However, the effectiveness of these RTV SR coatings may significantly degrade when exposed to transformer mineral oil. Hence, this study examines the impact of mineral oil on SR doped with BN nanoparticles. SR nanocomposites with varying concentrations of wt.% 1, 3, 5, and 7 n BN are synthesized and thermal aged in mineral oil at 80 °C for 200 h. Experimental results reveal a substantial degradation of the SR matrix post-aging, attributed to the infiltration of mineral oil. Fourier transform infrared (FTIR) spectroscopy, mass analysis, and 3-D microscopy results reveal the breaking of the side methyl groups and the main chain of SR (Si-O-Si), along with the detachment of filler particles, resulting in increased surface roughness. Tracking and erosion of the test specimens evaluated through inclined plane tracking and erosion tests (IPT) as per IEC 60 587 shows a reduction in tracking and erosion resistance for all test specimens after aging. Further, the surface temperature distribution measured through infrared thermography during IPT demonstrates that all doped specimens experienced a lower surface temperature than the unfilled specimen. The thermal stability further investigated through thermogravimetric analysis reveals that 5 wt.% composite exhibits higher thermal stability post-aging. The experimental results reveal that the nanocomposite with 5 wt.% BN exhibits superior resistance to degradation caused by mineral oil.