Current Transformer (CT)’s type, Working principle & Phasor Diagram

Definition:

A Current Transformer (CT) is an electrical device used to measure alternating current (AC) in high-voltage circuits. It works by producing a scaled-down replica of the current flowing through a primary conductor, allowing for safe monitoring and measurement.

Key Features:

• Operation: The Current Transformer (CT) operates on the principle of electromagnetic induction, where the primary current generates a magnetic field that induces a current in the secondary winding of the transformer.
• Transformation Ratio: A Current Transformer (CT) are characterized by their transformation ratio, which indicates the relationship between the primary current and the secondary current. For example, a CT with a ratio of 100:5 means that 100 A in the primary circuit results in 5 A in the secondary circuit.
• Applications: Current Transformer (CT) are commonly used in power systems for metering and protection, allowing for the safe measurement of high currents without direct connection.

Overall, Current Transformer (CT) are essential for the accurate monitoring of electrical systems, ensuring safety and efficiency in power distribution and management.

Types of current transformer

Current transformers (CT) come in various types, each designed for specific applications and requirements. Here are the main types of current transformers:

1. Wound Current Transformer

• Description: These Current Transformer (CT) have a primary winding that is wound around a core, and the conductor carrying the current to be measured is passed through this winding.
• Applications: Often used in high-current applications where the primary current is known and manageable.

2. Bar-Type Current Transformer

• Description: This type has a solid conductor (bar) as the primary winding. The bar passes through the Current Transformer (CT) , and the secondary winding is wrapped around it.
• Applications: Common in industrial applications for monitoring large currents.

3. Split-Core Current Transformer

• Description: Features a core that can be opened, allowing it to be clamped around an existing conductor without interrupting the circuit.
• Applications: Used in retrofitting existing systems for monitoring without having to disconnect the primary circuit.

4. Toroidal Current Transformer

• Description: A circular, donut-shaped core with the conductor passing through the center. The secondary winding is placed around the core.
• Applications: Often used for protection and metering in power distribution systems.

5. Instrument Transformer

• Description: These are specifically designed for use in measuring instruments and protective relays. They can be either wound or toroidal.
• Applications: Used in metering applications, such as electricity meters and protective relay systems.

6. Protection Current Transformer

• Description: Designed to provide accurate current measurements under fault conditions, with high accuracy and stability.
• Applications: Used in protective relaying systems to detect over currents and faults.

7. Resistive Current Transformer

• Description: Measures current by generating a small voltage proportional to the current flowing through a known resistance.
• Applications: Generally used for specific measurement applications, often in laboratory settings.

Each type of Current Transformer (CT) has unique characteristics suited for different operational needs and environments, making them essential for electrical measurement and protection systems.

Working principle of CTs

Current transformers (CTs) operate on the principle of electromagnetic induction. Here’s a breakdown of how they work:

1. Basic Concept: CTs are designed to step down high currents to a lower, manageable level for measurement and protection purposes. They provide a safe means of monitoring current in high-voltage systems.
2. Structure: A typical CT consists of a primary winding (which is essentially a conductor through which the high current flows) and a secondary winding (which produces a smaller, proportional current). The primary winding can be a single conductor or multiple turns around a core.
3. Operation:
   • When current flows through the primary winding, it creates a magnetic field around the conductor.
   • This magnetic field induces a current in the secondary winding due to Faraday’s law of electromagnetic induction.
   • The relationship between the primary current (Ip) and the secondary current (Is) is defined by the turns ratio of the windings. For example, if the primary has 100 turns and the secondary has 1 turn, the secondary current will be 1/100th of the primary current.
4. Accuracy and Ratings: CTs are rated based on their transformation ratio and burden (the load on the secondary). They are designed to accurately measure current over a specified range and can have different classes of accuracy for various applications.
5. Applications: CTs are widely used in electrical metering, protection systems, and control equipment. They help in monitoring load currents, providing data for energy management, and protecting circuits from overloads.

A Current Transformer (CT) function by transforming high current levels into lower, manageable levels through electromagnetic induction, allowing for safe monitoring and measurement in electrical systems.

Phasor Diagram of Current Transformer

A phasor diagram for a current transformer (CT) visually represents the relationship between primary and secondary currents and voltages in the system. Here’s an explanation of the key components typically included in such a diagram:

Components of the Phasor Diagram

1. Primary Current (Ip):

   • Represented as a vector (phasor) originating from the origin and typically shown at an angle of 0 degrees. This is the current flowing through the primary winding of the CT.

2. Secondary Current (Is):

   • This vector is proportional to the primary current but scaled down based on the transformation ratio of the CT. It is usually shown at the same angle as the primary current, assuming the CT is ideal and the burden is properly matched.

3. Burden Voltage (Vb):

   • The voltage across the secondary winding due to the load (burden) connected to the Current Transformer (CT). This vector is also drawn at an angle depending on the type of load (resistive or reactive). For a purely resistive load, it will be in phase with the secondary current. For a reactive load, it may lead or lag depending on whether the load is inductive or capacitive.

4. Phase Relationships:

   • In an ideal Current Transformer (CT) , the secondary current will be in phase with the primary current. However, in real applications, factors like burden impedance can introduce phase shifts. The phasor diagram can show these shifts if the burden is inductive or capacitive.

Phasor Diagram Representation

The main flux is taken as a reference. The primary and secondary induced voltages are lagging behind the main flux by 90º. The magnitude of the primary and secondary voltages depends on the number of turns on the windings. The excitation current induces by the components of magnetising and working current.

The secondary current lags behinds the secondary induced voltage by an angle θº. The secondary current relocates to the primary side by reversing the secondary current and multiply by the turn ratio. The current flows through the primary is the sum of the exciting current I₀ and the product of the turn ratio and secondary current K_t I_s.

• Axes: The horizontal axis usually represents the real part (in-phase component), while the vertical axis represents the imaginary part (out-of-phase component).

where, I_s – secondary current
E_s – secondary induced voltage
I_p -primary current
E_p – primary induced voltage
K_t – turn ratio, number of secondary turn/number of primary turn
I₀ – excitation current
I_m – magnetising current
I_w – working component
Φ_s – main flux

In a simple case with an ideal CT:

• Ip (Primary Current) = 100 A at 0°
• Is (Secondary Current) = 1 A at 0° (assuming a turns ratio of 100:1)
• Vb (Burden Voltage) = 5 V at 0° (for a resistive load)

Summary

The phasor diagram for A Current Transformer (CT) helps visualize the relationship between the primary and secondary currents and the effect of the burden. It provides insights into the operational characteristics of the CT and is crucial for understanding how it will perform in a given electrical system.