This paper presents analysis and find out a suitable statistical model for modelling failures of medium voltage overhead lines in Vietnam grid. Practical operation data is collected and filtering out outlier in order to support modelling process. Application of the non-homogeneous Poisson point process is selected to the studyof the rates of occurrence of failures when they are time dependent, and the times between failuresareneitherindependent nor identically distributed. Matlab software is utilized in all calculation for maximum likelihood estimation. Result of study is an appropriate approach for modelling failuresof Vietnam distribution overhead lines and typical parameters for those common distribution lines. Result of study is also a ground work for other research on reliability analysis, optimal preventive maintenance, asset management and planning of medium distribution gird.

Keywords: Non-Homogeneous Poisson Process (NHPP); medium voltage distribution line; rates of occurrence of failures(ROCOF).