Understanding Instrument Transformers in Electrical Circuits

Instrument transformers are special types of transformers used in electrical systems to measure high alternating currents and voltages. They are essential for the safe and accurate operation of metering and protection equipment in power systems.

There are two main types of instrument transformers:

• Current Transformers (CTs): Used to step down high currents to a safe, measurable level.
• Potential Transformers (PTs) or Voltage Transformers (VTs): Used to step down high voltages to a safe, measurable level.

How Transformers Work

The fundamental principle behind the operation of any transformer, including instrument transformers, is electromagnetic induction. A changing magnetic field in one coil (the primary) induces a voltage and current in another coil (the secondary) placed near it.

For electromagnetic induction to occur continuously and effectively in a transformer, the magnetic flux linking the coils must change over time. This changing flux is produced by a changing current in the primary winding.

Instrument Transformers and AC Circuits

Let's consider why instrument transformers are used in specific types of circuits:

• Alternating Current (AC) Circuits: In an AC circuit, the current and voltage vary sinusoidally with time. This continuously changing current in the primary winding of a transformer produces a continuously changing magnetic flux in the core. This changing flux then induces a voltage in the secondary winding, allowing the transformer to function correctly (stepping up or down voltage/current).
• Direct Current (DC) Circuits: In a DC circuit, the current is constant over time (unless there is a sudden change like switching). A constant DC current produces a constant magnetic flux. Since there is no change in magnetic flux, according to Faraday's law of induction, no voltage is induced in the secondary winding after the initial transient period when the DC supply is switched on or off. Therefore, a standard transformer cannot continuously transfer power or signal from the primary to the secondary in a steady-state DC circuit.

Because instrument transformers rely on the principle of electromagnetic induction which requires a changing magnetic flux, they can only operate correctly in circuits where the current and voltage are continuously changing, i.e., AC circuits.

Therefore, instrument transformers are used exclusively in AC only circuits for measurement and protection purposes.