The iron core of DC Immune Current Transformers (DC Immune CT) serves as a critical component for precise current measurement and protection in power systems, widely deployed in electrical scenarios with DC component interference. In DC Immune applications, magnetic properties directly determine the measurement accuracy, interference immunity, and long-term stability of CT equipment. Suboptimal magnetic performance may lead to misjudgment or failure of the device. Annealing, as the core process in soft magnetic material preparation, plays a decisive role in regulating key soft magnetic properties of DC Immune CT cores, including magnetic permeability, coercivity, and core losses.
A DC-immune CT core is a specially designed current transformer core engineered to suppress the effects of DC bias. It maintains stable magnetic permeability characteristics and current conversion accuracy in AC circuits containing DC components, preventing magnetic saturation issues caused by DC bias.
Traditional CT cores, often made of silicon steel sheets, are prone to magnetic saturation when exposed to DC components in circuits, leading to increased measurement errors or even equipment damage. DC bias-resistant CT cores, through material selection and process optimization, exhibit superior tolerance to DC bias, maintaining stable performance in complex current environments.
DC bias current refers to the intrusion of DC components into the CT core, causing a shift in the core's hysteresis loop and a decrease in magnetic permeability. Excessive DC components can induce magnetic saturation in the core, causing it to lose its normal magnetic conductivity. This prevents the CT from accurately converting current signals, severely impacting the protection and monitoring functions of power systems.
Nanocrystalline and amorphous alloys feature disordered atomic arrangements or nanoscale grain structures, exhibiting low hysteresis losses, high permeability, and significantly reduced sensitivity to DC bias compared to traditional silicon steel materials. These microstructural characteristics enable them to effectively resist magnetic saturation induced by DC bias, making them ideal materials for fabricating DC bias-resistant CT cores .
In soft magnetic material manufacturing, annealing refers to a heat treatment process where the material is heated to a specific temperature, held at that temperature, and cooled at a controlled rate. This process aims to refine the internal microstructure, eliminate residual processing stresses, and optimize magnetic properties to meet the requirements of specific application scenarios.
Stress-relief annealing: Primarily used to eliminate internal stresses generated during processing steps like rolling and cutting, reducing stress interference with magnetic performance;
Nanocrystallization Annealing: For amorphous alloy materials, precisely controlling heating temperature and holding time transforms the amorphous structure into a uniform nanocrystalline structure, enhancing magnetic permeability and saturation resistance;
Magnetic Field Annealing: Applying a magnetically oriented field during annealing guides the alignment of internal magnetic domains, strengthening magnetic properties in specific directions and optimizing the directional magnetic permeability characteristics of CT cores.
Temperature is the core parameter in the annealing process. Precise heating temperature ranges must be set based on material composition (e.g., Fe-Cu-Nb-Si-B nanocrystalline alloys) to avoid excessive grain growth from overheating or failure to achieve structural transformation from insufficient heating. Atmosphere control primarily involves introducing vacuum, hydrogen, or nitrogen to isolate the material from air, preventing oxidation at high temperatures. Hydrogen also serves to reduce surface oxide layers, ensuring core surface quality.
Annealing disrupts the original unstable atomic arrangement within the material through thermal action: For amorphous alloys, it promotes ordered atomic arrangement to form nanoscale grains; For materials with existing grains, it reduces stress concentration between grains and optimizes grain size and distribution. Simultaneously, annealing eliminates microcracks and defects within the material, enhancing structural homogeneity and establishing the microscopic foundation for superior magnetic performance.
Before annealing, atoms in amorphous alloys are randomly arranged without distinct crystalline features. When heated to the nanocrystallization temperature range (typically 500–600°C) and held for a certain duration, atoms gradually aggregate to form nanocrystals with sizes of 10–20 nm. This transformation requires precise control of temperature and time to ensure uniform nanocrystal distribution and prevent excessive grain growth in localized areas, thereby maintaining the material's high magnetic permeability and low loss properties.
During material processing, external forces cause atoms to deviate from equilibrium positions, generating internal stresses. During annealing, atoms absorb energy and rearrange to more stable positions, releasing these stresses. Reduced internal stresses decrease resistance to magnetic domain rotation, facilitating easier domain alignment under magnetic fields. This enhances magnetic permeability and lowers coercivity.
During nanocrystallization annealing, excessive grain growth can be suppressed by controlling heating rates (e.g., 5–10°C/min), soak times (typically 30–120 min), and cooling rates. A uniform nanocrystalline grain structure increases the material's specific surface area, reduces barriers to magnetic domain wall movement, and prevents local magnetic property variations caused by uneven grain sizes, ensuring consistent magnetic performance across the entire core.
Vacuum: Completely isolates oxygen, preventing high-temperature oxidation of the material; suitable for oxidation-sensitive nanocrystalline alloys.
Hydrogen: Not only prevents oxidation but also reduces existing oxide films on the material surface, enhancing magnetic permeability. However, hydrogen safety and purity control must be strictly managed;
Nitrogen: Cost-effective with weak oxidizing properties, serving as a standard protective atmosphere for conventional annealing to meet oxidation prevention requirements for general soft magnetic materials.
Surface oxidation forms an oxide layer with significantly lower permeability than the base material, increasing magnetic circuit losses. Oxidation may also induce new stresses and defects within the material, disrupting microstructural uniformity. Therefore, effectively preventing oxidation during annealing is crucial for ensuring stable magnetic properties and extending the service life of DC Immune CT cores .
This section constitutes the core content of the article, analyzing the mechanism by which annealing influences magnetic properties in a technical yet accessible manner.
Annealing reduces resistance to magnetic domain rotation and migration by eliminating internal processing stresses and microdefects. When exposed to a magnetic field, domains align more readily along the field direction, significantly enhancing magnetic permeability. For DC Immune CT cores, high permeability translates to superior sensitivity to weak current signals. This enhances measurement accuracy and precision, ensuring reliable signal capture even in low-current or complex current environments.
Coercivity measures a material's resistance to demagnetization and is directly correlated with hysteresis loss. By controlling annealing processes—such as extending holding times or implementing slow cooling—the internal grain structure can be optimized. This reduces pinning points on magnetic domain walls, allowing domains to reverse more readily during magnetic field changes and thereby lowering coercivity. Reducing coercivity directly decreases hysteresis loss, enhancing the energy conversion efficiency of DC Immune CT equipment. It also ensures stable performance under long-term DC bias conditions, preventing overheating caused by excessive losses.
The relationship between annealing temperature and core losses follows a “decrease-then-increase” pattern: At lower temperature ranges, as temperature rises, internal material stresses gradually dissipate and the grain structure optimizes, leading to reduced core losses (including hysteresis and eddy current losses). However, when temperatures exceed the optimal annealing point, grain coarsening occurs, increasing eddy current losses and causing core losses to rise. Furthermore, a well-designed annealing process enhances the stability of the internal material structure, reduces the impact of temperature variations on magnetic properties, improves the performance consistency of DC Immune CT cores across different operating temperatures, and minimizes measurement errors caused by temperature fluctuations.
The uniform nanocrystalline grain structure of annealed cores effectively suppresses excessive domain displacement: under DC bias, interactions between nanocrystals limit magnetic saturation, maintaining high permeability. Simultaneously, specific microstructures formed during annealing (e.g., uniform grain boundary distribution) enhance the material's “resistance” to DC components, reducing the impact of DC bias on the hysteresis loop. This improves the saturation resistance of DC Immune CT cores, ensuring reliable operation in DC interference environments.
Amorphous strip accumulates significant internal stresses during rolling, necessitating stress relief annealing: heat the strip to 300–400°C, hold for 60–120 minutes, then cool at 5–10°C/min. This process effectively releases internal stresses, preventing magnetic property degradation caused by stress release after subsequent core forming, and lays the foundation for subsequent nanocrystallization treatment.
FINEMET-type nanocrystalline alloys are commonly used materials for DC bias-resistant CT cores. Their nanocrystallization annealing requires strict parameter control: heating to 520–560°C, holding for 30–90 minutes, with a heating rate of 2–5°C/min and a cooling rate of 10–20°C/min. This process transforms the amorphous alloy into a uniform nanocrystalline structure with grain sizes around 10 nm, significantly enhancing the material's magnetic permeability and saturation resistance to meet DC Immune CT application requirements.
Magnetic field annealing requires applying a DC or AC magnetic field of 0.1–1 T during the annealing process, with the magnetic field direction aligned with the core's magnetic permeability direction. During heating and holding phases, the magnetic field guides internal magnetic domains to align along the field direction, inducing magnetic anisotropy. This treatment further enhances the DC Immune CT core's magnetic permeability in specific orientations, reduces hysteresis losses, and optimizes device performance in directional current measurement.
Annealing parameters vary across materials. Beyond the two core materials mentioned above, conventional soft magnetic materials typically require annealing temperatures of 400–600°C, holding times of 30–180 minutes, and cooling rates of 5–20°C/min. In actual production, parameters require fine-tuning based on material composition, core dimensions, and performance requirements to ensure process stability and product consistency.
Excessively high annealing temperatures or prolonged holding times cause excessive grain growth (e.g., nanocrystalline grain size exceeding 50 nm). This reduces grain boundary area and increases domain wall movement resistance, leading to significant permeability degradation. Simultaneously, coarse grains increase eddy current losses, degrading the core's overall magnetic properties and failing to meet the accuracy requirements of DC Immune CT equipment.
Inadequate annealing temperature or insufficient holding time fails to fully eliminate internal stresses and amorphous structures, leaving substantial residual stresses and unstable microstructures within the material. Under these conditions, the core is prone to magnetic saturation under DC bias, increasing measurement errors. Residual stresses may also cause structural deformation during prolonged use, shortening equipment lifespan.
Improper annealing atmosphere control (e.g., insufficient vacuum, low nitrogen purity) allows air ingress into the furnace, causing surface oxidation. Furthermore, impurities like oil residue and dust within the annealing furnace may adhere to the material surface, causing contamination. Oxidation and contamination disrupt the material's microstructure, reducing magnetic permeability and increasing core losses. They may also impair the core's assembly compatibility with other components, leading to equipment failures.
Improper annealing may also result in uneven annealing effects within the same batch or across different regions of the same core, such as localized overheating or insufficient holding time, leading to variations in the core's magnetic properties. Such variations cause unstable measurement accuracy in DC Immune CT equipment, inconsistent product performance between batches, increased subsequent quality inspection and after-sales costs, and damage to corporate brand reputation.
B-H Curve Testing: By measuring magnetic flux density (B) across varying magnetic field strengths (H) and plotting the B-H curve, key parameters like magnetic permeability, coercivity, and saturation flux density are visually represented to assess magnetic performance quality.
DC Biasing Test: Simulates real-world DC biasing conditions to measure magnetic permeability changes and saturation behavior under varying DC bias currents, verifying resistance to DC biasing.
Loss Measurement: Specialized instruments measure hysteresis and eddy current losses at specific frequencies and magnetic field strengths to evaluate energy conversion efficiency, ensuring compliance with low-loss equipment requirements.
X-ray Diffraction (XRD): Analyzes diffraction patterns to determine material crystal structure (e.g., nanocrystalline phases, grain size), verifying successful transformation from amorphous to nanocrystalline structure during annealing.
Transmission Electron Microscopy (TEM): Directly observes the microstructure within the material, such as nanocrystal size and distribution, to determine grain uniformity and detect defects like oxidation or contamination, providing a basis for process optimization.
Post-annealing testing must cover multiple samples from the same batch to ensure consistency in magnetic properties and microstructure. Additionally, sampling tests across different batches are required to validate process reproducibility, guaranteeing stable product performance during long-term production. Consistency and reproducibility form the foundation for mass production and application of DC Immune CT devices, reducing equipment failure risks and enhancing customer trust.
Post-annealing test results directly determine the performance of DC Immune CT cores in residual current devices (RCDs): Low permeability and poor DC bias resistance may cause RCDs to inaccurately detect leakage currents, leading to delayed or failed protection. Excessive core losses can induce device overheating, shortening service life. Therefore, test data must be correlated with actual device performance requirements to ensure core quality meets safety standards for RCDs .
Annealing significantly influences the magnetic properties of DC bias-resistant CT cores by regulating their microstructure, notably affecting magnetic permeability, coercivity, core losses, and DC bias resistance. It is the core process determining magnetic performance. Precise annealing process control (e.g., temperature, atmosphere, duration) effectively enhances core performance, while improper annealing leads to magnetic property degradation.