Failure Analysis of COS used in Distribution System

Abstract

Abstract – Failure cases of COS and cause of these were investigated. The fractured porcelain body had a large part in distribution system of KEPCO recently. Quality of the material of porcelain and cement was examined and manufacturing processes were checked. There was no notable difference in microstructure of the porcelains compared with a sound one but the failures was concentrated on which sulfur based compound used to stick steel on the porcelain body instead of the cement. The mechanical strength of sulfur compound varies depending on melting temperature. The compound started to melt around 135℃ and fired with blue flame above 160℃ because it contains sulfur. The mechanical strength of the solidified compound is in inverse proportion to the melting temperature. To guarantee the characteristics of the compound the melting temperature should be controlled. Keywords : Cut-out switch, COS, Fault analysis, Fractured porcelain, Sulfur compound 1. Introduction The electric utility system is usually divided into three segments that are generation, transmission and distribution. The distribution system consists of a much wider variety of components, loads and interconnections than does other segments. A primary distribution feeder is connected with various equipments such as fuses, distribution transformers, reclosers and switches. The system is exposed to many interruption possibilities, the most important of which are those due to primary overhead or underground cable failure or transformer failure. The feeder utilize cut-out switch (COS) to restrict the interruption caused by failures of overhead transformer and secondary loads. Fig. 1 shows the COS that is composed of porcelain body, steel connectors and fuse cartridge. The fuse cartridge has fuse links its inside that consists of short fusible element section to sense the overcurrent. Attached to this there is a long conductor, commonly called a fuse leader or fuse link tail which connects to the rest of the fuse hardware. It begins to substitute the porcelain body with polymer but the great part of COS having porcelain body. Fuses are the most efficient and simple protection devices. COS is the typical expulsion fuse that the most basic and effective type of overcurrent device presently being used by the utility industry. That is also one of the most reliable devices that they can provide their function for over 20 years with essentially no maintenance. Its main function is current interruption. What they actually do is setup a high dielectric environment that prevents the arc from re-establishing when the fault current passes through a current zero.