Instrument Transformer Fundamentals
Voltage Transformers for Measurement
Define primary and secondary rated voltages, connection, accuracy and burden for the measurement circuit.
A measurement voltage transformer supplies a scaled voltage to meters or protective relays. Specify the instrument circuit separately from the power transformer's load rating.
Distinguish measurement and power duties
An instrument voltage transformer reproduces the primary voltage within its specified accuracy limits for an instrument circuit. A distribution transformer supplies the energy demanded by downstream loads. State the intended duty first so the supplier can select the right equipment class.
| Equipment duty | Purpose | Main specification focus |
|---|---|---|
| Measurement voltage transformer | Scaled signal for a meter or relay | Ratio, accuracy, burden and insulation |
| Distribution transformer | Supplies energy to downstream loads | Capacity, load, primary and secondary voltage |
| Circuit interface | Connects the instrument circuit | Connection, grounding and approved protection |
Read the ratio on the correct voltage basis
An illustrative 12,000:120 V ratio is 100:1. Under the ideal ratio, 11,400 V on the primary gives 114 V on the secondary. Confirm whether each rated voltage is line-to-line or phase-to-neutral, and use the specified connection instead of assuming a conversion between them.
Example — ideal voltage transformation
Primary : secondary = 12,000 : 120 = 100 : 1
11,400 V primary → 114 V secondary
Use rated voltage basis and the specified winding connection. Actual accuracy and operating limits are defined by the equipment specification.
Define the measurement interface
List the connected meters and relays, their burden, lead arrangement, required accuracy and insulation level. Include the protection and grounding arrangement on the approved drawing. Review fuse selection and secondary-circuit protection against the equipment instructions.
Drawing and protection record
Record primary and secondary voltage basis, phase connection, terminal identification, burden, accuracy requirement, insulation level, grounding and protective devices on the approved measurement-circuit drawing.
Frequently Asked Questions
What is another name for this device?
Potential transformer is commonly used for an instrument voltage transformer. Match terminology to the equipment specification.
Does the ratio define the complete specification?
Add accuracy, burden, insulation, frequency, connection and installation details.
Prepare a voltage measurement specification
Share the primary and secondary voltage basis, connection, accuracy, burden, insulation, frequency, instrument list and approved protection drawing.