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Voltage transformers (also called potential transformers) are another type of instrument transformer. They are used by the electricity supply industry to accurately measure high voltages for metering and protective relay purposes. They are designed to present negligible load to the voltage being measured and to have a precise turns ratio to accurately step down dangerously high voltages so that metering and protective relay equipment can be operated at a lower, and safer, potential.
This safer voltage is typically 66 to
The transformer winding connection points are typically labled as H1, H2 (sometimes H0 if it is internally grounded) and X1, X2, and sometimes an X3 may be present. X3 allows the tapping of 120 volts and 69 volts from the same winding. In other applications a Y winding (Y1, Y2, Y3) may also be available on the same voltage transformer, and is identical to the X winding but electrically isolated. The H windings (sometimes called high side or primary) are connected to the high voltage and the X and Y windings (sometimes called low side or secondary) to the metering or protective relay. The high side (primary) may be connected phase to ground or phase to phase. The low side (secondary) is usually phase to ground.
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