Instrument Transformers

Instrument transformers serve an important role in the protective circuits of industrial and commercial electric power systems. They are auxiliary devices that are used in the measurement of current and voltage. They reproduce in their secondary circuits, in a definite and known proportion suitable for use in protective devices, the current or voltage of their primary circuits with the phase relations substantially preserved. They are used to protect personnel and apparatus from high voltages and to permit the use of reasonable and standardized insulation levels and current-carrying capacities in the protective device circuits. The high degree of accuracy required for meters and instruments is not essential for protective purposes. Since system protection and coordination is the subject, the aspects of high accuracy for metering are not covered. The two general types are 1) current transformers, used in current measurement; and 2) potential transformers, used in voltage measurement.