Instrument transformers are used in the measurement of voltage, current, energy, and power in ac circuits. The main reasons why instrument transformers are employed in measurements are:
In power systems, the levels of currents and voltages handled are very high, and therefore direct measurements with conventional instruments is not possible without compromising the operator safety, the cost and size of the instrument. In such a case, instrument transformers can be effectively employed to step down the voltage and current within range of the existing measuring instruments of moderate size.
Instrument transformers are either current transformers (CT) or voltage/potential transformers (PT).
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Current transformers (CTs) are used to extend current ranges of instruments.
Voltage transformers are used to increase voltage ranges.
Instrument transformers have their primary winding connected to the powerline and secondary windings to the measuring instrument. In this way, the measuring instruments are isolated from the high power lines.
Related: Power Measurement in AC Circuits (Single-phase & Polyphase Systems)
Instrument transformers are used when measuring current and voltage of large alternators, motors, transformers, buses and other power transmission equipment. Voltages in such cases may range from 11,000 V to even 330, 000 V. Instruments transformers are used to step down this high voltages to safe levels for measurement so that it doesn’t pose great danger to operating personnel.
Shunts and multipliers used for extension of instrument ranges are appropriate for dc circuits and to some extend, for low power, low accuracy ac circuits. Instrument transformers have certain unique features as compared to shunts and multipliers as listed below:
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