Narrow Results

The degree of polymerization (DP) has been regarded as an important symbol of mechanical strength, reflecting the aging condition of transformer insulation paper. In this article, a new concept called fracture degree is proposed on the basis of DP. First, nine cellulose I_β crystal models with different fracture degrees were built. Then relevant mechanical parameters and hydrogen bond numbers were calculated by molecular dynamics (MD) simulation. Results showed that during the aging process of insulation paper with fracture of cellulose chain, the elastic constant C₃₃ produces appreciable impact on the Young's modulus (E). With the decrease of DP and increase of fracture degree, the Young's modulus step decreases. To the 50% and 100% fracture degree models respectively, the relationship between their different degrees of polymerization and Young's modulus is subjected to similar exponential distributions. With the increase of the fracture degree, the average hydrogen bond number drops, and the change rules apply to the Young's modulus. Since hydrogen bond is the main factor of mechanical strength, it can be inferred that the fracture degree influences mechanical strength seriously.

Molecular dynamics (MD) simulation is applied to investigate the structural characteristics of Al₂O₃–SiO₂ system with the Al₂O₃ content at 60 mol.% (Mullite — 3Al₂O${}_3\cdot$2SiO₂). MD models containing 5250 atoms in cubic box with periodic boundary conditions are constructed at 3500 K in 0–100 GPa pressure range. The Born–Mayer–Huggins potential is applied in this study. Based on the cluster analyzes and MD data visualization technique, the structural characteristics of Mullite melt, such as the short-range and intermediate-range order, the local environments of oxygen and network structure will be clarified. Under compression, the Al–O bonds are broken, leading to incorporation of Al atoms into Si–O subnet through both nonbridging oxygen (NBO) and bridging oxygen (BO), forming –Al–O–Si-network (Al–O–Si, Si–O–Al₂, Si₂–O–Al, Si–O–Al₃, Si₂–O–Al₂ and Si₃–O–Al). The degree of polymerization (DOP) of the silica network and alumina network increase with pressure and the DOP of SiOx-network is lower than that of AlO${}_x$-network under compression. The statistics of corner, edge and face-sharing bonds between two adjacent TO${}_x$ units (T is Si, Al; x = 3–7) as well as characteristics of all type OT${}_y$ linkages (y = 2–6) are investigated in detail. Structural and compositional heterogeneities are clarified via analysis of structure and size distribution of TO${}_x$-clusters.

To comparatively study the insulation ageing life of vegetable insulating oil-paperboard and mineral oil-paperboard, we conducted accelerated thermal ageing experiments at 170°C. Then according to the temperature rise of vegetable insulating oil transformer, we conducted accelerated thermal ageing experiments at 150°C for vegetable insulating oil-paperboard and at 140°C for mineral oil-paperboard. The appearance, polymerization degree, and SEM microstructure of the paperboard after different ageing experiments were comparative analyzed. The results show that after the oil-paperboard system is accelerated ageing for 1 000 h at 170°C, that is equivalent to 20 years natural ageing, the structure of paperboard in vegetable insulating oil is damaged severely, which indicates that the lifetime of transformer are in the late stage; while the structure of paperboard in mineral oil maintain complete, and the polymerization degree is still above 500, which indicate that the lifetime of transformer are in the middle stage. The accelerated ageing rate of the vegetable insulating oil-paperboard system at 150°C is slower than that of the mineral oil-paperboard system, which indicates that the lifetime of the vegetable insulating oil-paperboard is longer than that of the mineral oil-paperboard.