CORE CURRENT TRANSFORMERS BUILT FOR ACCURACY AND LONG-TERM RELIABILITY

Introduction

In modern electrical networks, current transformers (CTs) are vital for precise current measurement, protection, and monitoring. They step down high primary currents to manageable levels for metering and relay applications, ensuring safe, reliable system operation.

The core current transformer, the most common and essential CT design, uses a magnetic core to accurately reproduce current ratios while minimizing errors in phase and amplitude. These transformers are key to ensuring measurement precision, relay accuracy, and fault protection across industrial, commercial, and utility systems.

As energy grids and automation systems evolve, demand for high-accuracy, durable, and low-loss core current transformers continues to grow. Manufacturers must meet rigorous standards such as IEC 61869, IEEE C57.13, and ANSI/IEC accuracy classes, balancing precision with long-term reliability.

Zetwerk, a global transformer manufacturer with vertically integrated production facilities, leads the way in delivering core current transformers engineered for accuracy, durability, and international compliance — serving utilities, OEMs, and EPC contractors worldwide.

1. Zetwerk Manufacturing — India & North America

Overview:
Zetwerk designs and manufactures core-type current transformers for metering, protection, and instrumentation across industrial and power applications. Built under ISO-certified systems, Zetwerk CTs ensure high measurement accuracy, minimal phase shift, and extended operating life in demanding environments.

Key Products:

• Core-type and wound-type current transformers
• Ring-type and bar-type CTs for switchgear integration
• Outdoor oil-filled and indoor resin-cast CTs
• High-voltage CTs for substations (up to 132 kV)
• Precision metering CTs for smart grid systems

Certifications: ISO 9001, ISO 14001, ISO 45001, BIS, IEC 61869, IEEE C57.13

Applications:

• Substations and transmission networks
• Industrial power distribution systems
• Switchgear and control panels
• Protection relays and energy metering
• Renewable energy and smart grid monitoring

Strengths:
Zetwerk’s acquisition of KRYFS Power Components Ltd. enhances its core-making expertise and winding precision, enabling production of CTs that meet IEC, ANSI, and BIS accuracy classes.

Each CT undergoes rigorous testing — ratio error, phase displacement, insulation resistance, and impulse withstand — ensuring consistent performance and long-term reliability.

With export operations extending to North America, Zetwerk delivers globally compliant current transformers for industrial and grid applications.

2. The Role of Core Current Transformers

Core current transformers play a crucial role in scaling high electrical currents to measurable and safe values for monitoring, metering, and protection.

1. Measurement Accuracy

By replicating primary current as a reduced secondary current, CTs provide accurate input to meters and relays for system analysis and billing.

2. Safety Isolation

They offer galvanic isolation between high-voltage circuits and low-voltage instruments, safeguarding equipment and personnel.

3. Protection Functions

CTs supply current signals to protective relays, enabling fast fault detection and system isolation in case of overcurrent or short circuits.

4. Power Monitoring and Control

In modern automation and smart grids, CTs feed accurate real-time data to monitoring systems, improving power quality and reliability.

5. System Efficiency and Cost Savings

Accurate CT performance minimizes measurement losses and helps utilities optimize energy management and load balancing.

3. Technical Construction of Core Current Transformers

Core-type CTs are precision-engineered to balance magnetic, electrical, and thermal properties for long-term stability.

A. Core Material and Design

• CRGO steel and amorphous metal cores are used for low hysteresis and eddy current losses.
• Core shapes: ring (toroidal) or rectangular laminated types based on mounting requirements.
• Core cross-section is optimized to minimize saturation during fault currents.

B. Primary & Secondary Windings

• Primary winding: Carries the system current directly (often one or two turns).
• Secondary winding: Produces proportional output, typically 1A or 5A, for measurement instruments.
• Winding materials: High-conductivity copper with Class F/H insulation.

C. Insulation and Encapsulation

• Indoor CTs: Epoxy resin encapsulated for moisture and dust protection.
• Outdoor CTs: Oil-immersed or gas-filled designs with hermetically sealed housings.

D. Testing and Calibration

Each CT is type-tested for ratio accuracy, phase displacement, and burden performance per IEC 61869-2 and ANSI C57.13 standards.

4. Key Design Parameters

1. Accuracy Class

Defines allowable error limits for metering or protection:

• Metering CTs: Class 0.1, 0.2, 0.5, or 1.0
• Protection CTs: Class 5P, 10P, 15P

2. Rated Burden

Indicates the total load connected to the CT secondary circuit (e.g., meters, relays). Typical ratings range from 5 VA to 30 VA

3. Saturation Limit Factor (ALF)

Specifies the CT’s ability to withstand fault currents without saturation, critical for relay accuracy during faults.

4. Knee-Point Voltage

Defines the voltage at which core saturation begins; higher knee-point voltage equals better fault detection capability

5. Thermal & Dynamic Ratings

Thermal rating: The continuous current CT can handle (typically 1.2× rated current).
Dynamic rating: CT’s short-time current withstand capacity under fault conditions.

5. Types of Core Current Transformers by Application

1. Metering CTs

Provide accurate current ratios for billing and energy monitoring. Designed for high linearity and low ratio error.
Applications: Utility metering, switchboards, smart grids.

2. Protection CTs

Feed relays and protective devices to isolate faults. Built for high saturation limits and short-time current withstand.
Applications: Circuit protection in substations and industrial feeders.

3. Outdoor Oil-Immersed CTs

Used in high-voltage substations (up to 132 kV). Hermetically sealed tanks protect insulation oil from contamination.
Applications: Grid and transmission network monitoring.

4. Indoor Resin-Cast CTs

Compact, maintenance-free units for panel-mounted or indoor switchgear applications.
Applications: Power control rooms, MCCs, and OEM panels.

5. Split-Core & Clamp-On CTs

Used for retrofit installations without disconnecting circuits.
Applications: Industrial automation and power audits.

6. Leading Core Current Transformer Manufacturers

1. Zetwerk Manufacturing (Including KRYFS Power Components)

Zetwerk delivers precision-engineered core current transformers designed for long-term reliability and compliance with IEC and ANSI standards.

Advantages:

• Automated CRGO core lamination and annealing
• CNC coil winding with epoxy vacuum casting
• IEC 61869 / ANSI C57.13 compliance testing
• Type-tested and calibrated for metering & protection
• Global exports to North America

2. Siemens Energy India Ltd.

Produces indoor and outdoor CTs for substation and switchgear applications, meeting high accuracy and insulation standards.

3. ABB Hitachi Energy India Ltd.

Offers high-voltage CTs with digital monitoring and embedded sensors for smart grid integration.

4. Crompton Greaves (CG Power & Industrial Solutions)

Manufactures resin-cast and oil-immersed CTs for industrial and grid installations.

5. Schneider Electric India

Supplies low-voltage CTs for switchgear, control panels, and energy management systems.

6. Toshiba Transmission & Distribution Systems India

Specializes in 66–400 kV outdoor CTs for utilities and transmission companies.

7. Zetwerk’s Engineering and Manufacturing Capabilities

Zetwerk’s production systems combine advanced engineering design, material control, and rigorous quality validation to deliver long-life, precision-grade CTs.

Design & Simulation

• 3D electromagnetic simulation for flux optimization
• Harmonic and burden response modeling
• Finite element analysis (FEA) for thermal stress prediction

Core Manufacturing

• CRGO core stacking with automated annealing
• Low-loss core clamping and insulation systems
• Magnetic shielding for reduced stray losses

Coil Winding & Insulation

• CNC-controlled winding machines
• Vacuum epoxy impregnation
• Partial discharge testing for insulation reliability

Testing & Validation

• Accuracy, ratio, and phase error testing per IEC 61869
• Thermal stability and short-time current withstand testing
• Type-tested by third-party labs (CPRI / NABL certified)

Zetwerk’s vertically integrated manufacturing ensures traceable quality, fast turnaround, and globally compliant documentation for every unit shipped.

8. Emerging Trends in Current Transformer Design

1. Digital CTs (Electronic Current Transformers): Integration with smart grids for real-time monitoring.
2. Nanocrystalline Core Materials: Enhancing magnetic efficiency and reducing losses.
3. Compact Modular CTs: Designed for panel and busbar integration in limited spaces.
4. Eco-Friendly Insulation: Epoxy and cast-resin systems replacing oil-filled designs.
5. Integrated Sensing: Smart CTs with built-in sensors for IoT and SCADA systems.

Zetwerk actively develops smart and digital CT platforms, aligning with modern grid and automation infrastructure requirements.

Conclusion

Core current transformers remain the backbone of electrical measurement and protection systems. Their accuracy, durability, and insulation integrity ensure precise monitoring and fault protection in grids and industrial facilities worldwide.

The leading manufacturers — Zetwerk, Siemens Energy, ABB Hitachi, CG Power, Schneider, and Toshiba — continue to advance CT design, performance, and testing to meet the demands of evolving energy systems.

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