Understanding Current Transformers (CTs)

Understanding Current Transformers (CTs): The Silent Sentinels of Power Systems

by Ohm Engineering Works | Apr 08, 2025 | Electric Company

In the world of electrical engineering, precision, safety, and control are non-negotiable—especially when dealing with high-voltage currents that flow through power systems. One unsung hero that enables this safe operation is the Current Transformer (CT). Often overlooked, CTs are essential instruments that help measure and monitor massive currents without exposing personnel or equipment to dangerous voltages.

Let’s unravel the world of CTs in a professional yet easy-to-understand way, especially for those who aren't electrical engineers.

What is a Current Transformer (CT)?

A Current Transformer (CT) is a type of instrument transformer. Its primary role is to take a high-voltage current—the kind that powers industries and grids—and step it down to a smaller, safer level. This smaller current is then sent to devices like ammeters, energy meters, and protection relays, which cannot handle the original high current directly.

Think of a CT as a translator between two worlds:

The primary side is connected to the high current in the power system.

The secondary side delivers a proportionally smaller current for measurement or protection.

This setup enables electrical systems to be monitored, controlled, and protected without direct exposure to high voltage, ensuring safety and system reliability.

Why Do We Need CTs?

1. Safety – Measuring high currents directly is hazardous.

2. Accuracy – CTs provide consistent, proportional readings.

3. Isolation – They isolate measurement tools from high-voltage circuits.

4. Protection – They supply input to relays that trip circuits during faults.

Classification of Current Transformers

CTs aren’t one-size-fits-all. Depending on the application, construction, or insulation type, they are classified into various types. Here’s a breakdown:

1. Based on Application

a. Measuring CTs

These are designed for accuracy. They help monitor load currents and send data to meters for energy billing or system analysis. Suitable for normal load conditions.

b. Protection CTs

These CTs are built for rugged performance. They operate during abnormal or fault conditions and send signals to protection relays, helping trip circuit breakers to prevent damage.

c. Dual Purpose CTs

Engineered for both measuring and protection. These offer multi-functionality and are common in compact systems where space and efficiency are key.

2. Based on Core Type / Construction

a. Wound Type CT

The primary winding is physically wound on the core. Used when both primary and secondary windings need insulation and accurate ratio.

b. Bar Type CT

The primary conductor is a solid bar or busbar running through the CT core. These are robust, simple, and economical, ideal for high-current applications.

c. Ring Type CT

Also called toroidal CTs. There’s no primary winding; instead, the conductor passes through the CT’s center. Common in switchgear and cable-mounted installations.

d. Core Balance Current Transformer (CBCT)

Used for earth fault protection. All phase conductors pass through the CT core. If there's no fault, the net current is zero. Any imbalance indicates leakage.

e. Special Protection CTs

These are custom-designed CTs used in very sensitive or unique protective schemes, such as differential protection in transformers or motors.

3. Based on Tank Design

a. Dead Tank CT

The core and windings are enclosed in a grounded metallic tank. These are safer because the live parts are enclosed, and they're ideal for grounded substations.

b. Live Tank CT

The core is in a high-voltage potential tank, often mounted on an insulator. More compact and lightweight but requires more care during maintenance.

4. Based on Insulation Type

a. Oil-Insulated CT

Uses insulating oil to cool and insulate the internal parts. Common in outdoor, high-voltage environments.

b. Dry Type CT

Air or solid insulation instead of oil. Used in indoor panels or clean environments.

c. SF₆ Gas-Insulated CT

Uses sulfur hexa fluoride gas, known for excellent insulation and arc-quenching. Ideal for compact GIS substations.

d. Air Support Type

These CTs use air insulation and are usually found in low to medium voltage applications where space isn't a constraint.

5. Based on Structure & Performance

a. Single Ratio, Multiple Core CT

Has a fixed ratio but multiple cores—one for metering, another for protection, etc. Keeps functions independent and improves reliability.

b. Multiple Ratio, Multiple Core CT

Can adjust the transformation ratio as needed. Ideal where load conditions vary or in smart grid applications.

Conclusion: Choosing the Right CT Matters

Current Transformers are more than just passive electrical components. They’re watchdogs—measuring, monitoring, and protecting power systems every second. Whether you're an electrical professional or a facility manager, understanding the types and applications of CTs can help ensure the safety, reliability, and efficiency of your operations.

Next time you walk past a transformer yard or open a switchgear panel, remember—behind every accurate reading and protective action, there’s likely a CT working silently in the background.