HT CURRENT TRANSFORMER

• Primary Winding: Connected in series with the main circuit carrying the current to be measured. Usually has 1 or a few turns.

• Magnetic Core: Provides a low-reluctance path for magnetic flux.

• Secondary Winding: Has many turns of fine wire. Connected to meters, relays, or other instruments.

Working Principle

• When current flows in the primary conductor, it creates a magnetic flux in the core.

• This induces a proportional current in the secondary winding according to the turns ratio.

• The secondary current is a scaled-down version of the primary current, maintaining the same waveform.

Typical Ratio Example

If a CT has a ratio of 100:5, it means:

• Primary current = 100 A

• Secondary current = 5 A
  So, when 200 A flows in the primary, the CT secondary will supply 10 A.

Applications

1. Measuring Instruments Ammeter, wattmeter, power analyzer.

2. Protection Overcurrent relays, earth fault relays, differential protection.

3. Metering Energy meters for billing in substations/industries.

4. Isolation Provides electrical isolation between high-voltage power circuit and measuring instruments.

Key Points to Remember

• Secondary must never be left open-circuited when energized (dangerous high voltage may appear).

• Accuracy depends on burden (load on CT secondary) and class of CT (e.g., Class 0.5, 1.0 for metering; 5P10, 10P20 for protection).

• CTs are usually rated in terms of Primary current / Secondary current (e.g., 200/5 A).