Instrument transformers (a collective term for current transformers and voltage transformers) generally refer to specialized transformers designed for instrumentation purposes.
An instrument transformer is a specialized type of transformer serving two primary functions: first, to extend the measurement range of AC electrical instruments; and second, to isolate high-voltage and high-current circuits, stepping them down to lower voltage and current levels to serve as input signals for relay protection systems, automation devices, and control circuits.
Instrument transformers are utilized within power systems to serve as signal sources for circuits involved in measurement, control, indication, and relay protection. They effectively insulate instruments, relays, and similar devices from the high-voltage and high-current circuits being monitored. Furthermore, they allow for the use of instruments and relays with significantly smaller physical dimensions-and standardized specifications-compared to those required for direct measurement of high-voltage, high-current circuits. Instrument transformers are primarily employed when measuring high voltages and high currents; they are also referred to as instrument mutual inductors.
The operating principle of an instrument transformer is identical to that of a power transformer; however, it is specifically engineered to interface with electrical measuring instruments and to extend the measurement ranges of ammeters or voltmeters. In this configuration, the primary winding of the transformer is connected to the current or voltage being measured, while the measuring instrument is connected to the transformer's secondary winding circuit. Consequently, the low current or voltage measured by the instrument becomes directly proportional to the actual current or voltage being measured in the primary circuit.
The benefits of using instrument transformers are as follows:
1. The secondary winding of an instrument transformer provides a reduced voltage level; this lowers the insulation requirements for the measuring instruments and enhances the safety of the measurement operations.
2. Instrument transformers enable the electrical isolation of the power supply lines from the measuring instruments.
3. Connecting wires can be used to route the measurement instruments away from the instrument transformers to other locations, thereby facilitating remote measurement.
4. They allow for the standardization of secondary voltages or currents (typically 110 V or 5 A), which facilitates the interchangeability of measuring instruments.
