What is CT and PT Transformer? An In-Depth Explanation

Transformers are critical components of the electrical power system. They transfer electrical energy between circuits, and without them, we would not be able to transmit electricity over long distances. Current transformers (CTs) and potential transformers (PTs) are two types of transformers that are essential for accurate metering, protection, and control of the power system. In this article, we will take a detailed look at CT and PT transformers, their working principles, and their applications.

Table of Contents

1. Introduction
2. Basic Principles of Transformers
3. Current Transformers (CT)
   • Types of CTs
   • Construction of CTs
   • Working Principle of CTs
   • CT Ratio and Burden
   • Applications of CTs
4. Potential Transformers (PT)
   • Types of PTs
   • Construction of PTs
   • Working Principle of PTs
   • PT Ratio and Burden
   • Applications of PTs
5. Comparison of CTs and PTs
6. Conclusion
7. FAQs

Introduction

In simple terms, transformers are electrical devices that transfer energy between two circuits by electromagnetic induction. They consist of a magnetic core and two or more coils of wire. When an alternating current flows through one of the coils, it generates a magnetic field that induces a voltage in the other coils.

CTs and PTs are two types of transformers that are used in the power system to measure current and voltage respectively. They are used in a wide range of applications, including metering, protection, and control.

Basic Principles of Transformers

Before we dive into the details of CTs and PTs, it’s important to understand the basic principles of transformers. Transformers operate on the principle of electromagnetic induction. When an alternating current flows through a coil of wire, it generates a magnetic field that expands and collapses with the changes in the current.

If a second coil of wire is placed close to the first coil, the expanding and collapsing magnetic field induces an electrical voltage in the second coil. This voltage is proportional to the rate of change of the magnetic field and the number of turns in the second coil.

By varying the number of turns in the second coil, the voltage induced in the second coil can be stepped up or down relative to the voltage in the first coil. This is the basic principle of a transformer.

Current Transformers (CT)

A current transformer, as the name suggests, is used to measure electrical current. It is also known as a “instrument transformer” because it steps down the current flowing in the power system to a level that can be safely measured by instruments such as ammeters.

Types of CTs

There are two main types of CTs – wound and toroidal. Wound CTs consist of a primary winding that is connected in series with the power system, and a secondary winding that is connected to the measuring instrument. Toroidal CTs, on the other hand, have a split core that can be easily clamped around the conductor carrying the current.

Construction of CTs

A typical CT consists of a primary winding, a secondary winding, and a magnetic core. The primary winding is connected in series with the power system, and the secondary winding is connected to the measuring instrument. The magnetic core is made of a high-permeability material such as laminated silicon steel.

Working Principle of CTs

When current flows through the primary winding of a CT, it generates a magnetic field that induces a voltage in the secondary winding. The voltage induced in the secondary winding is proportional to the current flowing in the primary winding.

CT Ratio and Burden

The CT ratio is the ratio of the primary current to the secondary current. The CT ratio determines the level at which the current is stepped down by the CT. For example, a CT with a ratio of 1000:5 means that for every 1000 amps of current flowing in the primary winding, 5 amps of current will flow in the secondary winding.

The burden of a CT is the maximum current that the measuring instrument connected to the secondary winding can safely handle without causing damage. The burden of a CT is typically expressed in VA (volt-ampere) or in ohms.

Applications of CTs

CTs are used in a variety of applications in the power system, including:

• Metering: CTs are used to measure the current flowing in the power system for billing and monitoring purposes.
• Protection: CTs are used to detect overcurrents and short circuits in the power system and to isolate faulty equipment.
• Control: CTs are used in control circuits to monitor the current flowing in the power system and to activate protective devices.

Potential Transformers (PT)

A potential transformer, also known as a voltage transformer, is used to measure electrical voltage. It steps down the high voltage in the power system to a level that can be safely measured by instruments such as voltmeters.

Types of PTs

There are two main types of PTs – electromagnetic and capacitor. Electromagnetic PTs consist of a primary winding that is connected in parallel with the power system, and a secondary winding that is connected to the measuring instrument. Capacitor PTs, on the other hand, use a capacitor to step down the voltage in the power system.

Construction of PTs

A typical PT consists of a primary winding, a secondary winding, and a magnetic core. The primary winding is connected in parallel with the power system, and the secondary winding is connected to the measuring instrument. The magnetic core is made of a high-permeability material such as laminated silicon steel.

Working Principle of PTs

When voltage is applied to the primary winding of a PT, it generates a magnetic field that induces a voltage in the secondary winding. The voltage induced in the secondary winding is proportional to the voltage applied to the primary winding.

PT Ratio and Burden

The PT ratio is the ratio of the primary voltage to the secondary voltage. For example, a PT with a ratio of 10,000:120 means that for every 10,000 volts of voltage in the power system, 120 volts will be measured by the secondary winding.

The burden of a PT is the maximum load that the measuring instrument connected to the secondary winding can safely handle without causing damage. The burden of a PT is typically expressed in VA (volt-ampere) or in ohms.

Applications of PTs

PTs are used in a variety of applications in the power system, including:

• Metering: PTs are used to measure the voltage in the power system for billing and monitoring purposes.
• Protection: PTs are used in protection circuits to monitor the voltage in the power system and to activate protective devices.
• Control: PTs are used in control circuits to monitor the voltage in the power system and to activate protective devices.

Comparison of CTs and PTs

CTs and PTs are both essential components of the power system, but they have some key differences. CTs measure current, while PTs measure voltage. CTs step down the current to a safe level for measurement, while PTs step down the voltage to a safe level for measurement. The CT ratio is the ratio of the primary current to the secondary current, while the PT ratio is the ratio of the primary voltage to the secondary voltage.

Conclusion

CTs and PTs are critical components of the power system. They are used in a wide range of application including metering, protection, and control. CTs are used to measure current, while PTs are used to measure voltage. Both devices step down the current or voltage to a safe level for measurement and have a ratio and burden that determines their accuracy and maximum load capacity.

Understanding the role of CTs and PTs in the power system is essential for electrical engineers and technicians working in the power industry. Proper selection, installation, and maintenance of CTs and PTs can ensure accurate and reliable measurement and protection of the power system.

FAQs

1. What is the difference between a CT and a PT?

• CTs measure current, while PTs measure voltage. CTs step down the current to a safe level for measurement, while PTs step down the voltage to a safe level for measurement.

2. How is the ratio of a CT or PT determined?

• The ratio of a CT or PT is determined by the number of turns in the primary winding compared to the number of turns in the secondary winding.

3. What is the burden of a CT or PT?

• The burden of a CT or PT is the maximum current or voltage that the measuring instrument connected to the secondary winding can safely handle without causing damage.

4. What are the applications of CTs and PTs in the power system?

• CTs and PTs are used for metering, protection, and control in the power system.

5. How can CTs and PTs be properly maintained?

• CTs and PTs should be regularly tested for accuracy and calibrated as necessary. The insulation should also be inspected for damage and replaced if necessary.