30.1 : Instrument Transformers

Instrument transformers, comprising voltage transformers (VTs) and current transformers (CTs), play crucial roles in power substations by providing isolated replicas of current or voltage for measurement and protection purposes. Voltage transformers reduce the primary voltage to levels suitable for relay operation and measurement, while current transformers scale down the primary current. The primary winding of a current transformer often consists of a single turn, achieved by threading the power system's primary conductor through its core, although some applications may require multiple primary turns.

For protection purposes, voltage transformers are ideally designed to produce a scaled-down, in-phase representation of the primary voltage. The secondary of a voltage transformer should connect to a high-impedance voltage-sensing device, ensuring the entire secondary voltage appears across the sensing device. Conversely, a current transformer's secondary should connect to a low-impedance current-sensing device, allowing the entire secondary current to flow through the sensing device.

In practical applications, voltage transformers may experience voltage drops and phase-angle differences between the primary and secondary sides. Similarly, in current transformers, some current may be divided between the low-impedance sensing device and the transformer's shunt excitation impedance. The performance of a current transformer largely depends on its ability to accurately reproduce the primary current without significant errors or saturation effects.

These transformers are essential for the accurate operation of protective relays and measurement devices, ensuring the safe and reliable operation of power systems. By scaling down high voltages and currents, instrument transformers enable the monitoring and control of electrical parameters within manageable ranges, maintaining system stability and protection.