For continuous or spiral windings which feature multiple continuously transposed conductors (CTC) wound in parallel, the transposition structure is commonly used to suppress circulation between conductors. This introduces a local asymmetry in transformer windings. Fault analyses have shown that building an asymmetric structure in transformers increases deformation risk in these windings. Researchers have yet to fully investigate this asymmetric structure. This study was conducted to observe the influence of transposition structure in a 110 kV transformer. The conductor's relative position forms three patterns during the transposition process; two parameters are established to describe the position-changing process. A finite-element method (FEM) model is built to investigate the magnetic field and electromagnetic force distributions. The results indicate that the transposition structure distorts the magnetic field distribution. The maximum distortion factor of the axial component of magnetic flux density (B _z ) along the axial direction caused by the transposition structure is 14.6%. The transposition structure only changes the radial change slope of B _z . The gap caused by the transposition process aggravates imbalance in the ampere-turns distribution, increasing the amplitude of the radial component of magnetic flux density (B _r ) at the middle height of the transformer low voltage (LV) winding. The maximum amplitude of B _r at the transposition structure increases by 513%. The Lorentz force over the CTCs remains constant from before to after the transposition process, though its linear density over the CTCs is unevenly distributed throughout. The axial component F _z varies from -31 to 10 kN/m, while the radial component F _r varies from -146 to -51 kN/m. The results show that the initial stage of the CTC height rising segment is a weak point and should be strengthened in the manufacturing process.

When an external short circuit occurs, the axial electromagnetic force increases dozens of times. Under the effect of the axial short-circuit electromagnetic force, the transformer windings vibrate violently. The spatial distribution of the disks is constantly changing during vibration, which can change the temporal and spatial distributions of the leakage magnetic field and the axial electromagnetic force. This process is called the strong coupling phenomenon of the structural field and the leakage magnetic field. In this article, a strong coupled magnetic-structural model is proposed by using the analysis method. The strong coupling equations, where the independent variables are the axial position of the disks, are obtained. The vibration process with strong coupling phenomenon considered can be obtained. The accuracy of the strong coupling model is verified by the iterative method. By comparing the results calculated by the strong coupling model with those calculated by the weak coupling model, the influence of the strong coupling phenomenon on the vibration process is obtained. Due to the influence of the strong coupling phenomenon, the winding vibration intensity increases and the maximum displacement of the disks can be doubled, which implied that the strong coupling phenomenon cannot be ignored when investigating the short-circuit strength of power transformers.

Since 2018, there have been seven incidents of explosions and fires in ultra‐high voltage transformers in China, which are caused by internal arc discharge in transformer oil. Various pressure measurement experiments have been conducted to explore the laws of discharge pressure. However, differences in experimental conditions, discharge energy, and current peak values have led to discrepancies in the discharge power and pressure laws in these experiments. Thus, it is crucial to investigate the relationship between arc discharge power and pressure peak in transformer oil. An arc discharge test platform was established to measure pressure waveforms based on the RLC circuit. Additionally, a shadow optical path observation system was implemented to record the development process of discharge arc, bubble pulsation, and shock waves. The study reveals the positive relationship between the average power and the peak value of the shock wave; the former refers to the breakdown and expansion stage of the arc channel, and the latter is generated by the discharge, respectively. Overall, the pressure generation mechanism of arc discharge in transformer oil and the relationship between arc discharge power and peak value of discharge pressure are revealed.

Bubbles, as typical defects of oil–paper insulation systems, pose a great threat to the insulation state. In this article, the characteristics and mechanisms of the partial discharge (PD) inception, evolution, and induced breakdown of bubbles embedded between oil-impregnated paper (OIP) layers are investigated. The results show that the PD inception has a significant statistical time delay due to the lack of free electrons. The PD inception time delay (PDTD) follows an exponential distribution and leads to the large dispersion of the PD inception voltage (PDIV), which follows a Weibull distribution. The changes in PD behavior [charge, repetition rate, pulse current waveform, and phase-resolved PD (PRPD)] over time all show two separable stages (stages 1 and 2). The continuous recording of bubble area and gas composition under PD reveals that the evolution of bubble PD is attributed to the change in gas composition rather than the change in surface conditions as believed by researchers. The OIP breakdown induced by bubble PD indicates two breakdown modes, fast and slow, which correspond to direct-induced breakdown in stage 1 and cumulative-damage breakdown in stage 2. The former occurs because of the severe distortion of the electric field by the space charge at the head of the discharge channel inside the bubble, whereas the latter occurs because of the gradual deterioration of the OIP due to the continuous PD action. This study increases our understanding of the mechanisms of bubble discharge and of oil–paper insulation deterioration, and it provides guidance for fault diagnoses.

Under temperature and moisture combined, bubbles will be formed in oil-impregnated paper/pressboard (OIP/OIPB). Although many studies have been done to understand the thermo-induced bubble phenomenon in OIPB, the formation mechanism is not, let alone its effect on discharge characteristics and insulation performance of OIPB. In this article, the phenomena and mechanisms of thermally induced bubbles, their effects and mechanisms on the partial discharge (PD), and breakdown characteristics of OIPB are investigated. The results show that the bubble inception temperature (BIT) decreases significantly with the increase of the moisture of OIPB. There is a growth process of bubbles from small to large, and the heating temperature and moisture content of OIPB affect the gas component inside the bubble. Thermal bubbles significantly reduce the PD inception voltage (PDIV) and breakdown voltage (BV), and the PD evolution and dissolved gas components in the oil show significant differences in the presence/absence of bubbles. Interestingly, PDIV and BV start to decrease in a temperature interval before BIT, and we infer that many microbubbles and gaseous channels exist inside OIPB before observable bubbles are formed on the surface. Considering the fibrous porous structure of OIPB and combining it with the bubble nucleation theory, we propose a new model for interpreting bubble formation and insulation degradation. The model suggests that bubbles (vapors) form first in microcavities and holes inside the OIPB and then spill over to the surface to form observable bubbles. The microcavities and pores in the OIPB are filled with gas (vapor) as the main cause of PD inception and insulation degradation. This study contributes to understanding the mechanisms of bubble formation in oil–paper insulation as well as provides a reference for risk assessment during overload operation and diagnosis of bubble-induced related faults of transformers.

Partial discharge (PD) induced by bubble will reduce the insulation performance, lead to insulation failure and seriously affect the safe operation of power transformer. Studying the influence of PD in internal bubble on oil paper insulation and understanding the insulation failure mechanism can provide guidance for engineering practice. In this paper, a bubble PD defect model is designed to obtain the mechanism of PD degrading insulation. The inception and breakdown voltage of PD decrease seriously with the increase of bubble area and height. Combined with the surface state of breakdown paper observed by electron microscope, it is concluded that PD can induce the breakdown of oil paper insulation. Then, through the comparative analysis of bubble area and PD parameters, a positive feedback model for insulation degradation is obtained, representing that bubble defects will deteriorate rapidly in exponential form under the effect of PD. Finally, combined with the electron microscope and energy spectrum analysis, the deterioration process of insulating paper is divided into four stages, obtaining the micro deterioration and failure mechanism of insulating paper. This study contributes to understanding the mechanism of insulation failure by bubble, as well as provides a reference for risk assessment of transformers.