current transformer (CT)

The current transformer (CT) is  type of transformer used to mak measure alternating current (AC). To  produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. 

Current transformer 

A current transformer (CT) is a type of transformer used to measure alternating current (AC). It produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. Here’s how a current transformer works:

Current transformer 

Basic Principle

1.Primary Winding: 

The primary winding of a current transformer is connected in series with the line carrying the current to be measured. This winding usually consists of one or very few turns of thick wire.

2. Secondary Winding: 

The secondary winding consists of many turns of finer wire. This winding is connected to a measuring device, such as an ammeter or the input of a protective relay.

3. Magnetic Core:

 The primary and secondary windings are wound on a magnetic core, which provides a low reluctance path for the magnetic flux.

Working Mechanism

1.Current Flow: 

When current flows through the primary winding, it generates a magnetic field around the conductor.

2. Magnetic Induction:

 The magnetic field induces a current in the secondary winding proportional to the current in the primary winding. The ratio of the primary to secondary currents is the inverse of the turns ratio of the transformer.

3. Load Connection:

 The secondary winding is connected to a load, usually an ammeter or other measuring instrument. The induced current in the secondary winding is scaled down from the primary current by the ratio of the number of turns in the primary and secondary windings.

Key Points

Current Ratio:

 The current ratio (primary to secondary current) is determined by the turns ratio of the transformer. For example, if the primary winding has 1 turn and the secondary has 100 turns, the current ratio is 100:1.

Safety:

 CTs are used for safely measuring high currents. The secondary current is much lower and safer to handle, making it easier to connect to measuring instruments.

Accuracy:

 CTs are designed to provide accurate current measurements over a wide range of currents. They are crucial for protection systems in electrical power distribution.

Burden: 

The load connected to the secondary winding is referred to as the burden. The CT must be designed to work accurately with the expected burden, which includes the impedance of the connected instruments and wiring.

 Applications

Metering: 

Used in power meters to measure electrical consumption in residential, commercial, and industrial settings.

-Protection:

 Used in protective relays in substations to detect fault conditions and initiate circuit breaker operations.

Monitoring:

 Used in various monitoring devices to provide data on current flow for analysis and system management.

Example

Imagine a CT with a primary winding of 1 turn and a secondary winding of 100 turns. If 1000 A flows through the primary winding, the secondary winding will carry 10 A, assuming a perfect transformer with no losses. The ammeter connected to the secondary winding will thus measure 10 A, representing 1000 A in the primary circuit.

Understanding CTs is crucial for engineers and technicians involved in electrical power systems, ensuring accurate measurement and safe handling of high currents.