Diagnostic Practices Used For Instrument Transformers

Failures of instrument transformers (ITs) can cause malfunction of system protections, controls and/or metering devices and, in some catastrophic failures, damage to nearby power apparatus, or injury to personnel in the vicinity. The existing IT population in the substations of North American utilities is nearing the end of its service life. Therefore IT failure rates are expected to increase in coming years. Within a project funded by the CEATI LCMSEA Interest Group (composed of 30 utilities - see www.ceatech.ca), a survey of electrical utilities in North America and elsewhere was conducted to identify problems and the most popular practices, diagnostic techniques and tools, and to gather relevant experience with these techniques. The results of the survey are presented in this paper. Over 95% of all ITs use oil-paper insulation. Both explosive and non-explosive failures of ITs are occurring in significant numbers. There is a large variation among different utilities in terms of performing or not performing diagnostic tests, using different techniques, intervals between doing different tests, and their experience with these tests and techniques.