Amorphous Metal Cores for High-Efficiency Instrument Transformers

Last Updated: March 2025

Instrument transformers play a critical role in electrical systems, providing accurate measurement and protection functions that ensure system integrity and safety. As grid infrastructure becomes increasingly complex and energy efficiency standards more stringent, the selection of core materials for instrument transformers has gained renewed attention. Among the advanced materials available today, amorphous metal cores have emerged as a superior option for high-efficiency instrument transformers. This article explores why amorphous metals are transforming instrument transformer design and performance.

Amorphous material

Understanding Instrument Transformers and Their Requirements

Instrument transformers serve two primary functions in electrical systems:

1. Current Transformers (CTs): Convert high primary currents to standardised low secondary currents for measurement and protection devices.

2. Voltage Transformers (VTs/PTs): Transform high system voltages to standardised low voltages for metering and protection.

For these specialised transformers, core material selection is particularly critical due to their unique requirements:

• Exceptional accuracy: Minimum deviation in transformation ratio and phase angle

• High linearity: Consistent performance across operating ranges

• Low magnetising current: Minimal burden on the measured circuit

• Excellent temperature stability: Consistent performance regardless of ambient conditions

• Minimal hysteresis: Reduced measurement errors from magnetic memory effects

• High permeability: Efficient operation at low excitation levels

Traditional instrument transformers have typically used high-grade grain-oriented electrical steel (GOES) or nickel-iron alloys. However, amorphous metal cores are increasingly preferred for applications demanding the highest efficiency and accuracy.

What Are Amorphous Metal Cores?

Unlike conventional crystalline materials, amorphous metals (sometimes called metallic glasses) possess a non-crystalline atomic structure similar to glass. This unique structure is achieved through rapid cooling of the molten alloy at rates exceeding one million degrees Celsius per second, preventing atoms from arranging into the ordered crystalline patterns found in conventional metals.

Key Composition and Manufacturing

Most amorphous metals used in transformer applications are iron-based alloys containing:

• Iron (Fe): 80-85%

• Boron (B): 8-12%

• Silicon (Si): 7-10%

• Additional elements like carbon and phosphorus in small amounts

These materials are typically produced in thin ribbons (20-30μm thickness) using rapid solidification techniques such as melt spinning, where molten metal is ejected onto a rapidly rotating copper wheel that facilitates the necessary cooling rate.

Advantages of Amorphous Cores for Instrument

Transformers

1. Superior Magnetic Properties

High Initial Permeability

• Amorphous cores achieve initial permeability values of 10,000-30,000 compared to 1,500-2,000 for conventional GOES

• This results in more efficient magnetic coupling at the low flux densities typical in instrument transformer applications

• Enables more accurate measurements, especially at lower current levels

Exceptionally Low Coercivity

• Typical coercivity values of 2-4 A/m versus 6-10 A/m for conventional GOES

• Reduces hysteresis effects that can cause measurement errors

• Enables more accurate reproduction of waveforms in protection applications

Low Core Losses

• Core losses can be 70-80% lower than conventional GOES

• Reduces measurement errors due to heat generation

• Enables smaller, more efficient designs with reduced cooling requirements

2. Improved Linearity and Accuracy

The performance of instrument transformers is often quantified by accuracy class ratings that specify maximum permitted errors. Amorphous cores enable:

• Achievement of higher accuracy classes (0.1, 0.2S) with less core material

• More consistent performance across the full range of operation

• Reduced variation in ratio and phase errors across temperature fluctuations

• Better performance under harmonic-rich conditions in modern power systems

3. Enhanced Frequency Response

Modern power systems increasingly contain non-linear loads and distributed generation that introduce harmonics and transients. Amorphous cores offer:

• Wider bandwidth for accurate measurement of harmonic content

• Better reproduction of transient signals for protective relaying

• More consistent performance across varying frequencies

• Reduced measurement errors in systems with power quality issues

4. Reduced Size and Weight

The superior magnetic properties of amorphous metals allow:

• Up to 30-40% reduction in core size for equivalent performance

• Lighter weight transformers for easier installation and reduced structural requirements

• More compact designs for space-constrained applications

• Reduced transportation costs and environmental impact

  
  

Application Areas for Amorphous Cores Instrument Transformers

Transformers

High-Precision Metering Applications

Revenue-Grade Metering

• Grid interchange points where financial settlements depend on measurement accuracy

• Utility customer billing, particularly for large industrial customers

• Renewable energy generation output measurement for compensation

Power Quality Monitoring

• Critical infrastructure where power quality must be continuously assessed

• Manufacturing facilities with sensitive equipment

• Data centers requiring precise power monitoring

Protection Applications

Differential Protection Systems

• Transformer differential protection requiring precise matching between CTs

• Busbar protection where multiple CTs must perform consistently

• Line differential protection where transient response is critical

Distance Protection

• Transmission line protection requiring accurate impedance calculation

• Applications with complex ground return paths

• Systems requiring precise fault location capabilities

Special Applications

Low-Power Instrument Transformers (LPITs)

• Modern digital substations using IEC 61850 process bus architecture

• Applications requiring direct digital output from instrument transformers

• Smart grid applications with increased communication requirements

DC-Tolerant Current Transformers

• HVDC transmission systems

• Solar PV interconnections with potential DC offset

• Battery storage integration points

Implementation Considerations

While amorphous cores offer significant advantages, several considerations must be addressed during implementation:

Design Adaptations

Core Assembly Techniques

• Amorphous metal's ribbon form and brittleness require specialized winding and assembly techniques

• Annealing processes must be carefully controlled to optimize magnetic properties

• Proper core insulation is essential to prevent inter-ribbon short circuits

Mechanical Protection

• Additional protective measures are needed due to the brittle nature of amorphous materials

• Cores must be protected from mechanical stress during manufacturing and transportation

• Special mounting considerations may be necessary to prevent performance degradation

Economic Considerations

Cost-Benefit Analysis

• Higher material costs (typically 1.5-2× conventional GOES) must be balanced against performance benefits

• Reduced size and weight can offset higher material costs through savings in other components

• Enhanced accuracy and efficiency provide lifecycle value that justifies initial investment

Total Ownership Cost

• Lower losses reduce measurement errors that could result in revenue losses

• Improved accuracy may reduce the need for redundant measurements

• Enhanced reliability can reduce maintenance requirements and extend service life

  
  

Case Study: Substation Metering Upgrade

A major utility recently upgraded the metering CTs at a critical substation interconnection point using amorphous core technology. The results demonstrated several key benefits:

Before Implementation:

• 0.5 accuracy class CTs with conventional GOES cores

• Estimated measurement uncertainty of approximately ±0.5%

• Significant variation in performance across seasons due to temperature effects

• Limited ability to accurately measure harmonic content

After Implementation:

• 0.2S accuracy class CTs with amorphous cores

• Measurement uncertainty reduced to approximately ±0.2%

• Consistent performance across temperature variations

• Improved harmonic measurement capabilities

• 15% reduction in CT physical size, simplifying installation

The utility reported that the investment in amorphous core CTs paid for itself within 18 months through improved billing accuracy alone, with additional savings from reduced maintenance requirements and enhanced power quality monitoring capabilities.

Future Developments

Research and development in amorphous metals continue to advance the technology:

New Alloy Formulations

• Development of alloys with higher saturation flux density

• Research into compositions with improved temperature stability

• Explore cobalt-based formulations for specialised applications

Advanced Manufacturing Techniques

• Improved production methods for thinner ribbons

• Enhanced annealing processes for optimised magnetic properties

• Development of automated core assembly techniques

Integrated Solutions

• Combinations of amorphous and nanocrystalline materials for optimised performance

• Development of integrated sensors for smart grid applications

• Integration with digital signal processing for enhanced measurement capabilities

Conclusion

Amorphous metal cores represent a significant advancement for instrument transformer technology, offering substantial improvements in accuracy, efficiency, and overall performance. While these materials command a premium price compared to conventional core options, the performance benefits and lifecycle advantages make them an increasingly attractive choice for modern grid applications where measurement precision is paramount.

As power systems continue to evolve with increased renewable integration, distributed generation, and complex load profiles, the superior performance characteristics of amorphous-core instrument transformers will become increasingly valuable. For applications requiring the highest levels of measurement accuracy, especially under challenging conditions, amorphous metal cores have established themselves as the preferred solution for forward-looking system designs.

References:

1. IEEE Std C57.13-2016: "IEEE Standard Requirements for Instrument Transformers"

2. IEC 61869 series: "Instrument transformers" standards

3. Johnson, M. et al. (2023). "Performance Comparison of Amorphous and Crystalline Core Materials in High-Accuracy Instrument Transformers." IEEE Transactions on Power Delivery, 38(2).

4. Williams, P. (2024). "Advances in Metallic Glass Applications for Electrical Measurement Systems." Journal of Materials Engineering and Performance, 33(4).

5. European Commission. (2023). "Metering Requirements for Cross-Border Energy Trading - Technical Implementation Guidelines."

Are you considering amorphous metal cores for your instrument transformer applications? Contact our technical consulting team for assistance with material selection and design optimis

ation.