Types and Routine Tests of Current Transformers (CTs)

Current transformers are commonly employed to quantify high-magnitude currents. These transformers reduce the current to be measured, enabling its measurement using a standard range ammeter. A current transformer typically comprises a single or a limited number of primary turns. 

The primary winding, as depicted in the figure below, could be a simple conductor or a busbar encased in a hollow core. The secondary winding, on the other hand, consists of numerous accurately wound turns, specifically designed to achieve a particular turns ratio. Consequently, the current transformer effectively increases the voltage while simultaneously reducing the current.

Construction and Operating Principle

A typical current transformer consists of:

• Primary Winding: Usually a single turn or a few turns, which may be a simple conductor or a busbar passing through a magnetic core. This winding carries the high current to be measured.

• Secondary Winding: Composed of multiple precisely wound turns around the magnetic core. The number of turns is carefully calculated to achieve a specific turns ratio, which determines the transformation of current and voltage.

The CT operates on the principle of electromagnetic induction. It reduces the current proportionally while increasing the voltage, allowing accurate measurement and protection without direct exposure to high currents.

Testing of Current Transformers

CTs undergo rigorous testing to ensure safety, accuracy, and reliability. These tests are categorised into Type Tests and Routine Tests.

1. Type Tests 

These tests verify the transformer’s performance under extreme and specified conditions:

1. Terminal Marking and Polarity Verification ensure the correct identification of terminals and polarity for proper system integration.

2. High Voltage Power Frequency Test on Primary Windings Assesses insulation strength under rated high-frequency voltage conditions.

3. Overvoltage Inter-Turn Test Evaluates the insulation between adjacent turns of the winding under overvoltage stress.

4. Accuracy Class Error Determination Measures the CT’s performance against the defined accuracy class (e.g., 0.2, 0.5, 1.0).

5. Short-Time Current Test Verifies the CT’s ability to withstand short-duration fault currents without damage.

6. Temperature Rise Test Monitors thermal behaviour under rated load to ensure safe operating temperatures.

7. Impulse Voltage Test Simulates lightning or switching surges for CTs used in exposed electrical environments.

8. Special Type Tests (for outdoor CTs)

   • Wet Withstand Voltage Test: Evaluates insulation under wet conditions.

   • Partial Discharge Test: Detects internal insulation defects by measuring discharge activity.

2. Routine Tests (Performed on every manufactured unit)

These tests confirm the quality and consistency of each CT before deployment:

1. Terminal Marking and Polarity Verification confirms correct labelling and polarity for installation.

2. High-Voltage Power Frequency Test on Primary Windings Ensures insulation integrity under rated voltage.

3. High-Voltage Power Frequency Test on Secondary Windings Validates the insulation strength of the secondary circuit.

4. Overvoltage Inter-Turn Test Checks for insulation faults between winding turns.

5. Accuracy Class Error Determination Confirms the CT meets its specified accuracy class.

6. Accuracy Test Verifies the transformation ratio and phase displacement under load conditions.

These tests are critical for ensuring that current transformers operate safely, reliably, and within their specified performance parameters in real-world electrical systems.

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