Insulation overheating is a major cause of transformer failures, which makes it important to quickly and accurately predict the thermal distribution characteristics related to the transformer’s fine structure. In this work, a hybrid model of CFD-based spline interpolation (CFD-SI model) is proposed for the real-time prediction of transformer thermal characteristics. Taking a 750 kV oil-immersed power transformer as an example, a CFD model is established from a complete Conjugate Heat Transfer model and validated simultaneously by 24-hour experiments and the empirical formulation. A group of 30 CFD simulations is employed to establish a hybrid model. Validation results of 8 randomly selected cases demonstrate that this hybrid model can accurately describe heat flow distributions and predict hot-spot locations with a computational speed of 5570 times faster than the CFD model. Furthermore, a series of 42 predictions of hot spot temperatures under various scenarios are performed using this hybrid model to characterize insulation aging due to overheating, which enabled continuous management over its lifetime.

Power systems grapple with the challenges of high load rates and intermittent new energy sources integration. Transformers, as vital equipment, employ oil/pressboard (oil/PB) insulation. Uneven moisture distribution in this insulation can jeopardise safety thresholds, necessitating precise moisture assessment for grid stability. A novel mathematical model, adsorption–desorption and porous media moisture transfer (ADP‐MoT), is presented. This model incorporates adsorption and desorption processes within the porous pressboard, enabling a description of the dynamic moisture transfer between the oil and pressboard. Using this mathematical model, simulations for moisture dynamics were performed on a 750‐kV transformer across four typical days. The results indicate that temperature fluctuations are the primary driving factor for moisture migration at the oil/PB interface. Convection and diffusion contribute to moisture movement towards cooler regions. Fluid properties and structural characteristics induce a distinctive streamline‐shaped moisture flow within horizontal oil channels, with localised moisture accumulation in specific areas. Moreover, the analysis of 96 transient results uncovers potential free‐state moisture formation during severe conditions, underscoring the importance of monitoring the pressboard at winding bases during high load fluctuations. In conclusion, this study significantly contributes to scientifically identifying and addressing risks tied to new energy sources integration in power systems.

Free water degrades the insulating properties of oil and poses a risk to electrical equipment, highlighting the urgent need to elucidate the mechanism of its formation. In this work, we developed an oil-water mixture model based on molecular dynamics (MD) simulations to explore the nucleation and growth of water in insulating oil. The water cluster trajectories were analyzed to quantitatively explore the kinetics and thermodynamics of this process. Key findings indicate that lower pressures induced by local high-frequency vibrations not only weaken hydrogen bonding and reduce the solubility of water in oil, but also reduce the free energy required for nucleation and decrease the critical nucleation radius, thus facilitating nucleation. Meanwhile, it increases water molecule mobility, promoting the growth of pre-formed nuclei. Moreover, while higher initial water concentrations improve solubility and slow diffusion, increased molecular weight boosts collision chances among water molecules, facilitating nucleation and cluster growth. Importantly, surpassing a certain water concentration threshold triggers secondary nucleation, leading to more free water formation. Furthermore, a mathematical model and a predictive framework were developed to statistically describe and predict the dynamics of free water under varying scenarios, and validated through experiments.

Bubbles in oil-paper insulation migrate and may finally accumulate, posing enormous insulation risks. Accurate simulation of bubble migration and accumulation is of vital importance. The cross-scale nature of micrometer-millimeter bubble movement in meter-scale oil-paper insulation structures makes macroscopic simulation methods often ignore the microscopic details of bubbles, which makes calculation accuracy hard to guarantee. In this study, bubble dynamics in oil-paper insulation were analyzed by the Euler-Lagrange approach, in which the distributed interaction on the bubble interface was simplified to lumped interfacial forces. The lumped bubble-oil drag force and bubble-paper friction were standardized measured, and effectively modeled combining the microscopic behaviors of bubbles. The accuracy of the Euler-Lagrange method with modeled interfacial forces was finally verified by experiment. Results showed that the deformation of a steady-rising bubble in oil intensified as bubble size increased, which had an enhanced effect on the drag force. The bubble-paper contact state and paper friction were affected by bubble-paper contact pressure and bubble motion inertia. The bubble slid in close contact with the paper when contact pressure dominated while slid without contact when bubble inertia dominated. Both the drag force and paper friction were effectively modeled considering these bubble microscopic behaviors. The agreement between simulation results and experimental results of bubble trajectories and accumulation behaviors in a horizontal oil channel was satisfactory, verifying the accuracy of the simulation method with modeled interfacial forces. This method can be further coupled with multi-field simulation realizing the precise bubble distribution analysis in oil-immersed power equipment.

To ensure the safe and efficient operation of electrical equipment, it is essential to study the micro-dynamic mechanisms and thermodynamic characteristics of the insulating oil emulsification process at the nanoscale. In this work, the behavior of water in insulating oil under various temperatures was visualized through a dynamic oil-water model using molecular dynamics (MD) simulations and in-situ observation. Quantitative analysis revealed temperature’s influence on water droplet dynamics, exposing key details in nucleation and growth. The research findings indicate that the electrostatic shielding effect is critical in the nucleation process, and strong hydrogen bonding among water molecules promotes stable aggregate formation. Higher temperatures decrease the electrostatic shielding by the oil-water mixture and reduce molecular orderliness, decrease distances, and promote molecular collisions and nucleation. Furthermore, under sustained supersaturation, higher temperatures accelerate water molecules motion, boosting free water growth and facilitating the transition of water molecules from a free state to a dissolved state. The magnitude of nucleation rate was generally on the order of 10 ³⁵ m ^-3 ·s ^-1 and the growth rate was 0.3-1.8 m·s ^-1 in the simulated parameters range. These findings aid in understanding water issues in electrical equipment, improving design and operation for enhanced reliability and efficiency.

Microbubbles in oil-paper insulation may coalesce and evolve in size during migration and accumulation, forming more harmful millimeter bubbles to the insulation. There is little knowledge about the coalescence occurrence sites and coalescence-induced size evolution of bubbles in oil-paper insulation. In this study, coalescence-induced bubble size evolution is experimentally observed to occur at bubble accumulation sites on the paper surface. The process and size growth characteristics of mesoscopic coalescence between bubbles are numerically analyzed based on the microscopic mechanism of film drainage and rupture. The macroscopic coalescence pattern and size distribution evolution characteristics of accumulating bubble clusters are summarized, and the evolution of the maximum bubble size is modeled and verified. Results show that the coalescence time of two bubbles increases with their Laplace radius and decreases with temperature, and bubble size growth rate per unit coalescence time reaches the maximum when two bubbles are equal-sized. In an accumulating bubble cluster, each bubble preferentially coalesces with the smallest bubble in contact, and the maximum bubble size in the cluster is the pioneer of size evolution. Its evolution satisfies the theoretical most efficient bubble size evolution function derived from the ideal path where each coalescence is equal-sized bubble coalescence. The model of coalescence-induced maximum bubble size evolution is proven to accurately calculate the growth process of the maximum bubble in accumulating bubble clusters and can be further coupled with bubble dynamics simulation realizing the prediction of millimeter high-risk bubble formation at bubble accumulation sites in oil-immersed power equipment.