A Comprehensive Analysis of Nanocrystalline Core
A critical component of a CT is its magnetic core, which significantly influences its performance, accuracy, and efficiency. Accuracy is the most essential property of a current transformer. Traditionally, Cold-Rolled Grain-Oriented (CRGO) silicon steel has been the material of choice for these cores. Nowadays, a superior alternative is available on the market—nanocrystalline core material. In this article, we will discuss the fundamental properties of nanocrystalline materials, their application in CT cores, and a comparative analysis with CRGO cores.
Fundamentals of Magnetic Core Materials
Let’s discuss the fundamental properties that influence the performance of a magnetic core. Understanding these properties will allow us to analyze the advantages of nanocrystalline core materials easily.
Permeability: Indicates how easily the material can be magnetized.
Saturation Magnetization: The maximum magnetic flux density a material can attain before reaching saturation.
Coercivity: When the external magnetizing force is removed, a magnetic material should ideally return to its non-magnetic or demagnetized state. Coercivity refers to the material’s resistance to demagnetization.
Core Losses: Energy dissipated as heat within the core during magnetization, primarily due to hysteresis and eddy current losses.
Nanocrystalline Materials: Chemical and Physical Properties
Nanocrystalline materials are composed of crystals. The size of grains in nanocrystalline materials is typically less than 100 nanometers in size. This fine grain structure imparts unique magnetic properties, making them suitable for high-performance applications. Commonly, this material is an iron-based alloy. This alloy is made by adding elements like silicon, boron, niobium, and copper in iron. The alloy (i.e. Fe-Si-B-Nb-Cu) is melted at a high temperature (~1300-1400°C). The molten metal is ejected through a fine nozzle onto a rapidly rotating copper wheel. The cooling rate is extremely high (~106 Kelvin/second), which prevents large crystal growth. The rapid solidification with controlled annealing method ensures nanocrystalline structure.
CRGO Core
CRGO, i.e., Cold Rolled Grain Oriented steel, is a silicon-iron alloy. This alloy has a grain-oriented structure. The silicon steel undergoes a rolling process so that the grains align or orient in the rolling direction. We know that hysteresis loss occurs due to the orientation and reorientation of randomly oriented iron alloy grains along the direction of the applied magnetic field. Since the grains in CRGO are already aligned, hysteresis loss is reduced. Cold Rolled Grain Oriented steel saturates at a high flux density (~2.0 T), allowing it to handle high power. However, it has higher eddy current losses due to its relatively larger grain size (~100 microns). Additionally, the larger grain size results in lower permeability compared to amorphous and nanocrystalline materials. Because of this, the application of CRGO is limited in high-frequency performance.
Nanocrystalline Core and its Advantages
Due to its ultra-fine grains, the nanocrystalline core offers much higher permeability. This is because ultra-fine grains can easily orient themselves in response to an external magnetic field. This property also reduces the magnetizing current required in current transformers, thereby minimizing errors and improving the accuracy of CTs. The smaller grain size means easy orientation of grain. Also for lack of grain boundaries, hysteresis loss is significantly reduced. Eddy current loss is also lower because the electrical resistivity of nanocrystalline materials is higher (~100-120 μΩ⋅cm vs ~45 μΩ⋅cm for CRGO).
$$ Since,\; eddy\; current\; power\; loss \propto \frac{Voltage^2}{R} $$
We have already mentioned that due to the fine grain size, which allows easy orientation, the B-H curve becomes narrower. As a result, nanocrystalline cores work efficiently up to MHz frequencies, whereas CRGO is limited to 50/60 Hz applications. This makes nanocrystalline cores a very good option for high-frequency transformers, inductors, and power electronics.
Magnetostriction causes mechanical vibrations and noise in transformers. It occurs due to the continuous alignment alteration of grains, which leads to continuous increase and decrease in the overall size of the core in response to twice the applied frequency of the magnetic field. Additionally, due to the easy orientation of grains, nanocrystalline cores exhibit much lower magnetostriction than CRGO, thereby reducing audible noise.
Disadvantage of Nanocrystalline Iron Alloy
The main disadvantage of nanocrystalline material is its lower flux density, which is not as high as CRGO (~2.0 T). However, it is still sufficient for high-frequency applications. Although in 50 to 60 Hz extra high voltage applications like grid transformers, still CRGO is preferred due to its higher flux density. Two other major disadvantages of nanocrystalline iron alloys are:
Mechanical Fragility: Due to their nanocrystalline structure, these materials are more brittle and require careful handling.
Higher Cost: The manufacturing process, which involves rapid solidification and annealing, makes nanocrystalline materials more expensive than CRGO materials.