Toward understanding the relationship between the microstructure and propagation behavior of water trees

This study investigates the microstructural changes of water trees in different stages of propagation. It is proposed that microstructural changes within a tree influence its macroscopic propagation. Five groups of XLPE cable samples with pinhole defects were subjected to an accelerated water tree aging experiment for different aging times. The morphologies and sizes of water trees in different aging periods were observed by optical and scanning electron microscopes. The microscopic observations show that the propagation rates of water trees can change during different aging stages. In the initial stage, the propagation rate of water trees can reach 0.46 μm/hour. However, in the stagnation stage, the propagation rate decreases considerably to as low as 0.01 μm/hour. In the late stage, water trees with irregular shapes can form, and the propagation rate of the longest water tree branches can rise to 0.33 μm/hour. In addition, the shape of the microvoids within the water trees can change. In the later period of the stagnation stage, microvoids with irregular shapes (not round shape) can form by the connection between microvoids with different sizes. The simulation results of the electric field indicate that the microstructural change within the water trees is the fundamental reason for the changes of the propagation rates. In different propagation stages, the changes in the density, the sizes, and the shape of the microvoids can lead to the changes of water distribution and electric field distribution in water tree regions, which further results in different propagation rates.