Description
Current Transformers (CTs): Full Details
A Current Transformer (CT) is an electrical device used to measure alternating current (AC) in a circuit. It steps down high currents in the primary circuit to a lower, manageable level in the secondary circuit, enabling safe and accurate current measurement by meters or relays.
Types of Current Transformers
Wound CT:
Primary winding directly connected to the circuit.
Used for low-current applications.
Toroidal CT:
A circular core with a secondary winding; the conductor acts as the primary winding.
Ideal for high-current applications and compact spaces.
Bar-Type CT:
The conductor is a straight bar that acts as the primary winding.
Used in high-voltage applications.
Split-Core CT:
The core can open to clamp around a conductor without disconnecting it.
Suitable for retrofitting in existing installations.
Key Features
High Accuracy:
Measures current with minimal error, essential for metering and protection.
Insulated Core:
The magnetic core is insulated to ensure safety and performance.
Wide Range of Current Ratings:
Can handle currents from a few amps to several thousand amps.
Durable Construction:
Designed for long-term reliability under harsh conditions.
Compact Design:
Available in various sizes for easy installation in confined spaces.
Specifications
Parameter Details
Primary Current Rating Typically ranges from 1A to 5000A or more.
Secondary Current Rating Standard ratings like 5A or 1A.
Accuracy Class Classes like 0.2, 0.5, 1.0, 3.0, etc. , define measurement accuracy.
Burden Load in VA (Volt-Amperes) connected to the CT's secondary circuit.
Insulation Level Rated to withstand specified high voltages.
Frequency Typically 50Hz or 60Hz.
Core Material High-permeability materials like silicon steel or ferrite.
Standards Compliance Meets IEC 61869-2, ANSI/IEEE C57.13, or other standards.
Applications
Metering:
Used in energy meters to measure current for billing purposes.
Protection:
Works with protective relays to detect overcurrents, faults, or leakage currents.
Switchgear:
Installed in electrical panels and switchgear for monitoring and control.
Power Distribution Systems:
Ensures accurate current measurement in substations and power grids.
Industrial Equipment:
Used in machines and motors for overload protection.
Advantages
Safety:
Steps down high currents to a safer level for measurement and monitoring.
Accuracy:
High precision for metering and protection.
Versatility:
Suitable for a wide range of current levels and applications.
Durability:
Long-lasting performance in various environmental conditions.
Compact and Lightweight:
Easy to install in existing systems.
Common Terms
Turns Ratio:
The ratio of primary to secondary turns, determining the step-down factor.
Example: 1000:5 (1000A primary stepped down to 5A secondary).
Burden:
The impedance (in VA) connected to the CT’s secondary circuit, affecting accuracy.
Accuracy Class:
Defines how closely the CT measures the actual current. Lower numbers indicate higher accuracy.
Saturation:
The point where the CT core cannot accurately transform the current due to magnetic flux limitations.