instrument transformer

instrument transformer [′in·strə·mənt tranz‚fȯr·mər]

(electricity)

A transformer that transfers primary current, voltage, or phase values to the secondary circuit with sufficient accuracy to permit connecting an instrument to the secondary rather than the primary; used so only low currents or low voltages are brought to the instrument.

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Instrument transformer

A device that serves as an input source of currents and voltages from an electric power system to instruments, relays, meters, and control devices. The basic design is that of a transformer with the primary winding connected to the power system, and the secondary winding to the sensing and measuring equipment. Data from these devices are necessary for the operation, control, and protection of the power system. The primary reason for setting up the instrument-transformer interface is to provide analog information at low voltage levels, insulated from the higher system voltages. The range of use is from 480 V through the maxima of the 765–1000-kV power systems. See Electric power systems, Transformer

Current transformers are connected in series with the power conductor. In many cases this conductor serves as the one-turn primary. The principal types are the window type, where the power conductor is passed through a hole in the center of the current transformer; the bar type, where the power conductor is fastened to a one-turn bar which is part of the current transformer; and the bushing type, a toroidal core and winding that is slipped over the insulating bushings of circuit breakers, transformers, and so forth.

Voltage transformers and coupling capacitor voltage transformers are connected in parallel from one conductor to another or to ground. The coupling capacitor voltage transformer is widely used at the higher system voltages of 115 kV and above. It is a voltage transformer tapped across part of a capacitor unit connected from the conductor to ground.