What Are Voltage Transformers?

Voltage transformers are instrument transformers that step high primary voltages down to a standardized, lower secondary voltage suitable for use by metering equipment, protection relays, and control systems, while maintaining an accurate proportional relationship between the two. Also called potential transformers in North American usage, they are installed at substations and switchyards wherever high-voltage circuits must be monitored without connecting instruments directly to the energized conductors. The standard secondary voltage in most applications is 110 V or 120 V, regardless of the primary voltage, which may range from a few kilovolts to several hundred kilovolts.

Voltage transformers belong to the broader category of instrument transformers, which also includes current transformers. Their design and performance requirements are governed by IEEE C57.13, the principal North American standard, which defines accuracy classes, burden ratings, test procedures, and thermal ratings for both types. Adherence to this standard ensures interoperability between transformers from different manufacturers and with the metering and relay equipment connected to their secondaries.

Electromagnetic Design and Principle

An electromagnetic voltage transformer operates on the same inductive coupling principle as a power transformer: a primary winding carries the high voltage and establishes a time-varying magnetic flux in a laminated iron core, and a secondary winding coupled to that core delivers a proportionally lower voltage to the burden. The turns ratio determines the voltage step-down ratio, with small deviations from the ideal ratio arising from winding resistance, leakage reactance, and core magnetizing current. To minimize these errors, electromagnetic voltage transformers use high-permeability grain-oriented silicon steel cores and carefully wound high-voltage primary coils insulated with impregnated paper or cast resin. The secondary burden, the load in volt-amps presented by connected instruments and relays, must remain within the rated burden for the accuracy class to be maintained.

Accuracy Classes and Metering Grades

Voltage transformers are classified by the maximum permissible ratio error and phase displacement over a specified voltage range and burden range. IEEE C57.13 defines accuracy classes 0.3, 0.6, and 1.2 for metering applications, where the number expresses the maximum allowable percent ratio error. Protective relaying applications use broader accuracy classes because relay operations typically require accuracy only under fault conditions when voltages deviate significantly from nominal. Temperature affects accuracy through changes in winding resistance and core losses, and the standard specifies that accuracy class must be maintained over a defined operating temperature range and a voltage range from 90 to 110 percent of rated value. Calibration of precision revenue-grade metering circuits traces the ratio accuracy of the installed voltage transformer back to national standards maintained at facilities such as the National Institute of Standards and Technology.

Capacitive Voltage Transformers

At transmission voltages above approximately 115 kV, electromagnetic voltage transformers become physically large and costly because the primary insulation must withstand the full line-to-ground voltage. Capacitive voltage transformers (CVTs) address this by using a capacitor voltage divider to step down the voltage to an intermediate level before applying it to a small electromagnetic transformer. The intermediate voltage, typically a few kilovolts, is then transformed to the standard 110 V secondary. CVTs also provide a convenient coupling point for power-line carrier communications used in protection and telecontrol signaling, making them dual-purpose apparatus at many high-voltage substations. The accuracy characteristics of CVTs depend on the tuning of a ferroresonance suppression circuit that damps oscillations inherent in the capacitor-inductor combination, and this behavior is covered in the IEA PVPS report on reactive power and measurement in modern power systems.

Applications

Voltage transformers have applications throughout the power industry, including:

• Revenue metering at transmission and distribution substations, where accurate voltage signals are combined with current transformer signals to measure energy
• Distance protection relay systems, where the voltage signal determines the apparent impedance seen by the relay
• Synchronizing and auto-reclosing schemes, where the phase and magnitude of bus and line voltages must be compared
• Power quality monitoring, where recorded secondary voltage waveforms support flicker and harmonic analysis