AC Circuits and Power Systems in Practice

8Voltage Transformers

Voltage transformers (VTs), or potential transformers (PTs) as they are also called, are used to provide a scaled representation of the potentials on MV or HV busses, one that may conveniently be handled by protection relays, energy meters and general instrumentation equipment. Depending on the intended application, VTs may either be connected to system phase voltages or across line voltages. Therefore a VT’s primary rating must be appropriate to both the bus potential and its intended method of connection.

Because a VT’s secondary voltage is ideally a scaled replica of the bus voltage to which it is connected, it is important that the device does not introduce errors of its own into the measurement. Accordingly, manufacturers limit the burden that may be connected to the secondary winding(s), so as to reduce the potential dropped across the transformer’s short‐circuit impedance. Therefore the rated burden current is generally well below the thermal current limit of a VT. Despite this, magnitude and phase errors are introduced into the secondary voltages, but provided they are kept sufficiently small, their effects can be accepted. These errors are more of a problem in metering than in protection applications, since they directly affect the accuracy of the metered data. Limitations on the errors introduced are imposed by the various metering class designations.

Voltage transformer secondary potentials generally take one of several standardised voltages, ...