Features and Applications of Instrument Transformers

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:

• To extend (multiply) the range of the measuring instrument.
• To isolate the measuring instrument from a high-voltage line.

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).

Current Transformers (CTs)

Current transformers (CTs) are used to extend current ranges of instruments.

Potential Transformers (PTs) /Voltage Transformers (VTs)

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.

Advantages of Instrument Transformers over Shunts and Multipliers

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:

• Shunts cannot be used for circuits involving large currents otherwise the power loss in the shunt itself will become excessively high.
• Using shunts for extension of range on ammeters in ac circuits will require careful designing of the reactance and resistance proportions for the shunt and the meter. Any deviation from the designed time constants of the shunt and the meter may lead to errors in measurement. This problem is not present with current transformers being used with ammeter.
• Multipliers, due to leakage current can introduce errors in measurement, and can also result in unnecessary heating due to power loss.
• Measuring circuits involving shunts or multipliers, being not electrically isolated from the power circuit, are not only not safe for the operator, but also insulation requirements are exceedingly high in high-voltage measurement applications.
• High voltages can be stepped down by voltage/potential transformers to a moderate level that can be measured by standard instruments without posing much danger to the operator and also not requiring too much insulation for the measuring instrument.
• Clamp-on type or split-core type current transformers (CTs) can be used to measure current effectively without the need for breaking the main circuit to insert the CT primary winding.
• Single range moderate size instruments can be used to cover a wide range of measurement, when used with a suitable multi-range current transformer or voltage transformer.
• Lastly, instrument transformers can help in reducing the overall cost, since various instruments including metering, relaying, diagnostic and indicating instruments can all be connected to the same instrument transformer

You may also read: Performance Characteristics of Digital Measurement Instruments