Metering Transformers

n electric power installations metering transformers are used extensively for monitoring voltages and currents. A POTENTIAL transformer, sometimes called PT, is used to monitor a power line voltage. A CURRENT transformers, sometimes called CT, is used to monitor a power line current delivered to a load. Transformers are also used in many other non-metering applications.

An electrical transformer is a device for stepping down, stepping up or isolating AC voltages or AC currents. A transformer usually consists of a primary coil and one or more secondary coils wound around a common core. The core is generally iron or directional silicon steel for low frequencies. Ferrite cores are used in high frequency applications, and air cores are used in very high frequency transformers. Recent technology using air cores for low and medium frequency resulted with the development of current transformers known as Rogowsky probes.

When an alternating (AC) voltage is applied to the primary winding of a potential transformer, an alternating magnetic field is generated that is sensed by the secondary coil. The secondary coil then generates an AC voltage whose waveform is the same as the waveform of the primary voltage. The amplitude of the AC voltage generated by the secondary coil depends on the ratio of primary to secondary turns, often known as the “turns ratio”. It also depends on the core material, the driving frequency and coupling.

Step-down Voltage transformers are used to reduce a high voltage to a lower lever. In the electric power industry such Potential Transformers have an output that is generally 110 Volts, 120 Volts, 240 Volts or 480 Volts. AYA potential transformers are available for use on power line voltages as high as 36,500 Volts AC. They are used to scale down the line-to-neutral voltage of a Wye system or the line-to-line voltage of a Delta system to the rated input scale of a monitoring meter, which is typically 120 V in the United States.

Potential...