Accurately grasping the surface pollution class (PC) of roof insulators is of great importance for the safe operation of electric multiple units (EMUs). In this paper, we propose a new method for monitoring the PC of roof insulators in EMUs. In this method, the input voltage and leakage current of insulators are combined into a graph, i.e., a Lissajous figure with more characteristic information. The variation of the Lissajous figure under different factors is then studied. Digital image processing technique was used for morphological analysis of the Lissajous figure. The results show that the eccentricity of the insulator's Lissajous ellipse can be used as an effective monitoring index of the PC of roof insulators.

Roof insulators, pollution class, Lissajous figure