An overview of current transformer -

Table of Contents

what is a current transformer

A Current Transformer (CT) is a specialized type of transformer that operates on the principle of electromagnetic induction, primarily used for current measurement and protection in electrical power systems. Its core function is to proportionally step down a high current from the primary side (the high-voltage or high-current circuit) to a much lower, standardized current on the secondary side (typically 5A or 1A), while providing electrical isolation between the two circuits to ensure operational safety. The primary winding of the CT is connected in series with the circuit carrying the current to be measured, and the secondary winding is connected to measuring instruments, protective relays, or other devices. Through a precise turns ratio, the secondary current accurately replicates the magnitude and phase of the primary current, thereby providing a reliable signal source for system monitoring, energy metering, relay protection, and automated control.

Types of Current Transformers

1. By Application

This is the most common classification, determining the CT’s function in the circuit.

Measuring Current Transformers:

Function: To step down high current to a lower value (typically 5A or 1A) for feeding measuring instruments (e.g., ammeters, watt-hour meters).
Characteristics: They require high accuracy within the normal current range. However, during short circuits, the core is designed to saturate, limiting the secondary current to protect the instruments from damage. Accuracy classes include 0.1, 0.2s, 0.5, 1.0, and 3.0.

Protective Current Transformers:

Function: Specifically used for relay protection circuits. They must accurately reflect fault currents during short circuits to ensure protective devices operate correctly.
Characteristics: The core is designed to resist saturation (high linearity) to maintain accuracy even under high fault current multiples. Accuracy classes include 5P, 10P, and TPY (for transient protection).

2. By Insulation Medium

Dry-type Current Transformers:

Use air or conventional insulating materials (like cotton, paper, or resin) for insulation. Suitable for low-voltage (e.g., 0.66kV) applications.

Cast-Resin Current Transformers:

Use epoxy resin or other resin compounds as the main insulation, encapsulating the windings. Commonly found in medium-voltage switchgear (e.g., 10kV to 35kV).

Oil-Immersed Current Transformers:

Use insulating oil for both insulation and cooling. Typically used for high-voltage (above 35kV) outdoor applications where high insulation strength is needed.

Gas-Insulated Current Transformers:

Use Sulfur Hexafluoride (SF6) gas as the main insulation. Often used in high-voltage and extra-high-voltage substations, offering compact size and high safety.

3. By Installation Method

Through-type (Window) Current Transformer:

Has a window for passing a busbar or cable. It has no primary winding of its own; the conductor passing through acts as the primary.

Post-type (Stand) Current Transformer:

Designed to be mounted on a surface and acts as its own support insulator. The primary conductor passes through the center.

Bushing-type Current Transformer:

Mounted on the insulated bushing of a power transformer, circuit breaker, or other apparatus. The bushing itself serves as the primary insulation and conductor.

Bus-bar Type Current Transformer:

Similar to window type, it has no primary winding. The bus-bar itself is used as the primary conductor, passing directly through the CT’s insulating housing.

4. By Operating Principle

Electromagnetic Current Transformer:

The traditional type operating on the principle of electromagnetic induction. This is the most widely used type.

Electronic Current Transformer:

Optical Current Transformer: Uses the Faraday magneto-optic effect to measure current.
Air-core Coil (Rogowski Coil): Has no iron core, offering excellent linearity and a wide measurement range, ideal for measuring transient currents.
Characteristics: They are smaller, lighter, and have a wide frequency response, making them suitable for digital substations and UHV applications. They typically require an accompanying electronic processing unit.

5. By Current Ratio (Transformation Ratio)

Single-ratio Current Transformer: Has one fixed transformation ratio (e.g., 100/5).

Multi-ratio Current Transformer: The primary or secondary winding has taps, allowing for different ratios by changing the connection (e.g., from 50/5 to 200/5).

The working principle of a current transformer

The operating principle of a Current Transformer (CT) is based on the principle of electromagnetic induction. It can be understood as a special type of “transformer.”

Here is a detailed explanation of its core working principle:

1. Core Structure

Primary Winding: During measurement, the primary winding is usually connected directly in series with the circuit where the current needs to be measured. Sometimes, the primary winding is simply a single conductor (busbar) passed through the transformer’s core.
Core: Made of stacked, high-permeability silicon steel sheets (or other magnetic materials) to form a closed magnetic circuit.
Secondary Winding: Wound around the core and connected in series with measuring instruments (like ammeters, watt-hour meters) or protective relays. It typically has many turns.

2. Physical Process

When an alternating current $I_1$ flows through the primary side, it generates an alternating magnetic flux $\Phi$ in the core. This alternating flux links with the turns of the secondary winding and, according to Faraday’s Law of Electromagnetic Induction, induces an electromotive force in the secondary winding. When the secondary winding forms a closed loop (connected to an instrument), a secondary current $I_2$ is produced.

3. Current Transformation Relationship

An ideal current transformer follows the principle of magnetomotive force (MMF) balance:

$I_1 \times N_1 = I_2 \times N_2$

Where:

$N_1$ is the number of turns in the primary winding (usually few, sometimes just 1 turn);
$N_2$ is the number of turns in the secondary winding (usually many).

From this, we can derive:

$\frac{I_1}{I_2} = \frac{N_2}{N_1} = K$

Here, K is the turns ratio (or transformation ratio).

Simply put:

Purpose: To proportionally reduce a large current in a power line to a smaller, standardized current (typically 5A or 1A) suitable for measuring instruments and safe operation.
Inverse Relationship: The magnitudes of the primary and secondary currents are inversely proportional to their number of turns. Fewer primary turns and more secondary turns result in a greater reduction of current.

4. Key Characteristic: Secondary Side Must NEVER Be Opened

This is the most significant difference between a current transformer and an ordinary voltage transformer:

Ordinary Transformer (e.g., Voltage Transformer): The primary voltage determines the flux, and the secondary load influences the primary current.
Current Transformer: The primary current $I_1$ is determined by the load on the main circuit and is independent of the secondary side.

If the secondary side becomes open-circuited while current is flowing in the primary:

Flux Surge: The magnetomotive force $I_1 N_1$ produced by the primary current is normally counteracted by the secondary MMF $I_2 N_2$ (MMF balance). If the secondary opens, $I_2 N_2 = 0$ , leaving only the massive magnetizing flux generated by $I_1 N_1$ in the core.
Core Saturation: This huge flux rapidly saturates the core, causing significant hysteresis and eddy current losses. This generates intense heat and can potentially burn the transformer.
High Voltage Danger: The secondary winding has many turns. Under the influence of the large, alternating magnetic flux, a very high, peaked voltage (potentially thousands of volts) is induced. This poses a severe threat to operator safety and can damage the insulation of the equipment.

Working Principle Of A Current Transformer

Summary

A current transformer uses electromagnetic induction to proportionally step down a large primary current to a smaller secondary current based on the turns ratio, for use by instruments and protection devices. The secondary side must ALWAYS remain in a short-circuit state (i.e., connected to low-impedance instruments or shorting blocks), and one end must be reliably grounded to prevent high-voltage hazards.

Applications of Current Transformers (CTs) in Power Transformers

Current transformers (CTs) are vital interfaces between the high-voltage primary system and low-voltage secondary equipment, enabling safe monitoring, protection, and control of power transformers. They step down primary currents to standardized low values (5A or 1A) for instruments and relays.

1. Protection Applications

CTs provide fault current signals to relays, ensuring rapid isolation of faulty equipment.

Differential Protection (Primary Protection): CTs on HV and LV sides compare currents. An internal fault creates imbalance, triggering a trip. Requires high-accuracy, anti-saturation CTs.
Overcurrent Protection (Backup): CTs detect currents above a threshold, activating relays after a time delay.
Ground Fault Protection: Zero-sequence current (from CTs) indicates ground faults, prompting relay action.
Overload Protection: CTs monitor continuous overcurrent, issuing alarms or trips to prevent thermal damage.

2. Metering and Measurement

CTs supply current for operational awareness and commercial transactions.

Load Monitoring: Feeds ammeters for real-time load observation.
Energy Metering: With PTs, provides input to wattmeters and energy meters for power flow, power factor, and billing. Requires high accuracy classes (0.2, 0.5S).

3. Control and Monitoring

Automated control relies on CT signals.

Cooling System Control: CT current triggers cooling fans/pumps based on load to maintain temperature.
OLTC & Condition Monitoring: CT data assists tap changer regulation and feeds fault recorders/monitoring systems for health assessment.

4. Synchronization and Paralleling

CTs provide phase angle and magnitude information to synchronizing relays, ensuring safe connection of a transformer to live busbars.

Summary

Application	Key Function	CT Requirements
Protection	Fault detection (differential, overcurrent, ground)	High accuracy, anti-saturation, reliability
Metering	Load monitoring, energy billing	High precision (0.2/0.5S), linearity
Control	Cooling initiation, tap changer input, monitoring	Good linearity, frequency response
Synchronization	Safe paralleling	Accurate phase & magnitude

CTs are indispensable for safe, reliable, and efficient transformer operation.

Benefits of Using Current Transformers in Power Transformers

Current transformers are indispensable components in power transformer systems, offering numerous advantages that enhance safety, reliability, and operational efficiency. Here are the key benefits:

Personnel and Equipment Safety

Galvanic Isolation: CTs provide complete electrical isolation between the high-voltage primary circuit and the low-voltage secondary equipment (meters, relays). This protects operators and maintenance personnel from dangerous high voltages.
Standardized Low Currents: They convert thousands of amps into safe, standardized values (5A or 1A), allowing the use of standard, low-voltage rated instruments and protective devices.

Accurate Monitoring and Billing

Precise Measurement: High-accuracy CTs (Classes 0.2, 0.5S) enable exact load current measurement, ensuring reliable operational data.
Revenue Metering: They facilitate accurate energy accounting and fair billing between utilities and customers by providing precise current signals to watt-hour meters.

Enhanced Protection and Fault Mitigation

Rapid Fault Detection: CTs enable high-speed protection schemes like differential protection, which can detect internal transformer faults in milliseconds and initiate tripping before catastrophic damage occurs.
Selective Coordination: They provide the current signals needed for overcurrent relays to operate selectively, isolating only the faulty section while keeping the rest of the system in service.
Minimized Damage: By enabling quick fault clearance, CTs help reduce the magnitude and duration of fault currents, minimizing thermal and mechanical stress on the transformer windings.

Operational Efficiency and Control

Automated Cooling Control: CT signals automatically trigger cooling fans and oil pumps based on real-time load current, optimizing energy consumption and extending equipment life.
Load Management: Continuous current monitoring allows operators to balance loads effectively and prevent overloading, ensuring the transformer operates within its design limits.

System Flexibility and Standardization

Instrument Standardization: Regardless of the primary current (hundreds or thousands of amps), secondary equipment can be standardized for 5A or 1A inputs, simplifying design, inventory, and maintenance.
Scalability: CTs with multiple taps or ratios provide flexibility for future system expansion or changes in load conditions without replacing the entire transformer.

Troubleshooting and Diagnostics

Fault Analysis: CTs feed data to fault recorders and disturbance monitoring systems. Analyzing fault current waveforms helps engineers determine the cause and location of faults, improving system reliability over time.
Condition Monitoring: Continuous current data helps assess transformer health, detect anomalies (like winding imbalances), and plan predictive maintenance.

Summary of Benefits

Benefit Category	Key Advantages
Safety	Isolation from HV, protection against electric shock
Protection	Fast fault detection, minimized damage, selective tripping
Measurement	Accurate load monitoring, precise revenue metering
Control	Automated cooling, optimized loading, improved efficiency
Standardization	Uniform secondary equipment, simplified maintenance
Diagnostics	Fault recording, condition assessment, predictive maintenance

In summary, current transformers are not merely accessories but fundamental components that make modern power transformer systems safe, intelligent, and economically viable.

How to select a current transformer used in a transformer

Selecting the right CT involves matching its electrical characteristics to the system requirements while ensuring physical compatibility.

Primary Current (Iₚ)

Objective: Find the rated primary current.
Method: Calculate the maximum continuous load current of the transformer. The CT primary rating should be slightly higher than this value to avoid saturation during normal operation. Common ratings are standardized (e.g., 100A, 200A, 400A, etc.).

Secondary Current (Iₛ)

Standard Ratings: Choose between 1A or 5A.
Decision Factor: 1A is generally preferred for long cable runs between the CT and the meter/relay to minimize power losses. 5A is a traditional standard and is suitable for shorter distances.

Accuracy Class

For Metering: Choose Class 0.2, 0.5, or 1.0. The lower the number, the higher the precision for billing and monitoring.
For Protection: Choose Class 5P10 or 5P20.
- The “5P” means a composite error of 1% at rated current.
- The number (10 or 20) is the Accuracy Limit Factor (ALF) , indicating the CT can maintain accuracy up to 10 or 20 times the rated current during a fault.

Burden (Rated Output)

Objective: Ensure the CT can power the connected devices.
Method: Calculate the total impedance of the secondary wiring and all connected devices (relays, meters).
Specification: Select a CT with a VA (Volt-Amp) rating that exceeds this calculated burden (e.g., 5VA, 10VA, 15VA). If the burden is too high, the CT will saturate and become inaccurate.

Voltage & Insulation

System Voltage: The CT’s insulation voltage must match the line-to-line voltage of the primary circuit (e.g., 0.72kV, 12kV, 24kV).
Type: For medium voltage, epoxy-resin cast is standard for indoor switchgear.

Selection Summary Table

Parameter	Key Considerations	Common Specifications
Primary Current	Maximum load current + margin	e.g., 100A, 400A, 800A
Secondary Current	Distance to devices; compatibility	1A (long distance) or 5A (standard)
Application	Billing accuracy vs. Fault detection	0.2 / 0.5 (Metering) or 5P10 / 5P20 (Protection)
Burden (VA)	Total load of relays, meters, and wire resistance	e.g., 5VA, 10VA, 15VA
System Voltage	Operating voltage of the panel	e.g., 0.72kV, 12kV, 24kV
Physical Type	Space constraints; Busbar or Cable	Window (Toroidal) , Wound, or Bar-type

Quick Rule of Thumb

Size: Match the primary current to the transformer’s full load.
Purpose: Use Class 0.5 for meters; use 5P10 for protection relays.
Load: Add up the wire length and device loads to confirm the VA rating.

Current Transformers Series Features & Specifications

This series of current transformers (CTs) is engineered for generator matching, power system protection and metering applications. With versatile configurations, these CTs deliver reliable performance, easy installation and strong environmental adaptability, fully compatible with power transformers, circuit breakers, switchgears and cable terminations.

Core Features by Model

Board Mounted Bushing-Type Generator CT (BGCT)

Purpose-built for mounting over generator high-voltage terminal bushings, and compatible with Isolated Phase Bus (IPB) at higher system voltage levels. Its open-frame design supports custom mounting footprints, and the lightweight structure serves as a cost-effective alternative to traditional cast units, with simplified installation and maintenance.

Encapsulated Bushing-Type Generator CT (MGCT)

Slip-over design for direct mounting on generator high-voltage bushings, also suitable for high-voltage IPB enclosures. The fully encapsulated coil delivers excellent dust, moisture and corrosion resistance, ensuring stable and long-term operation in harsh, hazardous industrial environments.

Outdoor Mounted Slip-Over Bushing CT (SBCT)

Self-contained unit for external mounting on high-voltage bushings of transformers, circuit breakers and cable terminations. No modification to primary equipment is required, enabling fast and simple installation. It is a reliable, cost-effective solution for system protection upgrades or additional metering points in outdoor high-voltage scenarios.

Special Application Outdoor Slip-Over Bushing CT (eSP)

A custom variant of our outdoor slip-over BCT series, developed specifically for SP/SPS power circuit breakers. Field-proven since 1980 for breakers up to 72kV, it is the ideal solution to add extra CTs for breakers originally supplied with only one CT per pole, with no need for existing equipment modification.

Internally Mounted Dry-Type Bushing CT (DBCT)

Dry-type insulation design optimized for built-in installation in indoor high-voltage equipment. It can be mounted on dead-tank circuit breaker bushings, and is also suitable for weather-tight cabinets or MC switchgear compartments. Properly installed, it delivers stable performance at higher system voltage levels, ideal for oil-free, maintenance-free equipment matching.

Internally Mounted Oil-Type Bushing CT (OBCT)

Engineered for built-in installation in oil-immersed high-voltage equipment. It can be mounted on high-voltage bushing ground collars, or inside the tank of oil-insulated transformers, circuit breakers or voltage regulators. When installed correctly, it adapts seamlessly to higher voltage levels, with excellent compatibility with oil-immersed primary equipment.

Universal Optional Features & Customization Capabilities

Winding & Ratio Flexibility: Available in Single Ratio (SR), Dual Ratio (DR) and Multi Ratio (MR) winding configurations. Custom ratios, accuracy classes, and primary current ratings over 8000A are supported, with designs adaptable to match existing CT characteristics for retrofit or new projects.
Core Performance Optimization: Gapped cores for precise remanence control and optimized transient response are available, as well as air core linear couplers for specialized measurement and protection applications.
Mounting & Mechanical Customization: Pre-assembled multi-core stacks are available to reduce on-site installation time. Custom conduit openings (threaded up to 1.5″ NPT, non-threaded up to 52mm) are supported, along with special solutions like multi-unit installation on a single pole.
Standards & Environmental Compliance: All units can be designed in accordance with IEC and CSA international standards, with customization for 50Hz/60Hz grid frequencies. The oil-type model supports a temperature class rating up to 155°C for high-temperature operating environments.
Flexible Delivery Options: Windings can be supplied with special test taps as required; standalone coils with terminals or flexible leads are also available to meet diverse manufacturing and on-site maintenance needs.

Conclusion

As an indispensable core component in electrical power systems, the current transformer (CT) leverages the principle of electromagnetic induction to step down high currents to standardized low values and achieve galvanic isolation between high and low-voltage circuits. Its diverse classifications cater to various application scenarios such as metering, protection and control, with the cardinal rule of never opening the secondary side being critical to safeguarding equipment and personnel safety. When paired with power transformers, CTs deliver reliable current signals for equipment protection, accurate metering and intelligent control, boasting multiple advantages including enhanced safety, high precision and operational efficiency. Proper CT selection requires matching parameters like system current, accuracy class and insulation rating to actual requirements, while a full range of customizable CT series can meet the installation and performance needs of different working conditions. In essence, CTs serve as a vital interface between the high and low-voltage sides of power systems and form the foundation for the safe, stable and intelligent operation of electrical equipment, providing essential support for the efficient operation and development of modern power systems.

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