Experience with 15-500 kV Instrument Transformers and Bushings

Instrument transformers are used in metering and protective relaying applications for which accuracy and transient performance are important functional requirements. Some magnetic voltage transformers are also used to discharge capacitor banks or other devices that can store a trapped charge. High voltage bushings are associated with oil filled power transformers and reactors. The reliability of instrument transformers and bushings is often determined by the insulation system, both initially and in the long term. This paper reports the more significant failures and troubles that have been experienced by one utility for instrument transformers and bushings rated 15-500 kV. Corrective measures have included modifications to the design and application as well as transport practices.