ABSTRACT
This chapter covers the fundamental basics and theory of operation of instrument transformers and discusses common types of instrument transformers and construction highlights. Though incomplete, the chapter covers the common practices used in industry over the last 30 years. Instrument transformers are divided into two categories: voltage transformers (VTs) and current transformers (CTs). The primary winding of the VT is connected in parallel with the monitored circuit, while the primary winding of the CT is connected in series. The burden of the instrument transformer is considered to be everything connected externally to its terminals, such as monitoring devices, relays, and pilot wiring. For the purpose of establishing a uniform basis of test, a series of standard burdens has been defined for calibrating VTs and CTs. Equipment such as power switchgear and isolated-phase bus, for instance, use instrument transformers within their assembly, but they have test requirements that differ from the instrument-transformer standard.
