DP-01 Analysis and Design of Current Transformers for Low-Frequency Transmission Systems Using Hybrid Magnetic Materials
Creators: Minrui Xu
Zhixin Li
Shufeng Lu
Tianchao Huang
Guangchen Ma
Jiajia Liu
Hang Zhou and Jiaxin Yuan
Publisher: name: IEEE
Year: 2025

With the advancement of fractional frequency transmission, offshore wind power will be transmitted by low power frequencies around 10~30Hz. However, traditional current transformers face saturation distortion issues at low frequencies. Utilizing materials with high magnetic saturation intensity could add measurement errors. This paper proposes a transformer structure that maintains accuracy at low frequencies. This structure comprises two iron cores: one made of conventional silicon steel with an air gap, and the other made of nanocrystalline alloy material without air gap. These iron cores are concentrically placed, with the coils connected in series. According to the equivalent circuit, when the nanocrystalline alloy is saturated, its permeability becomes smaller, and the silicon steel core will share secondary induced voltage. In this case, transformers with hybrid materials will have better accuracy under low-frequency occasions. Fig. 1 illustrates the structure of the proposed transformer. The transformer body consists of two concentrically placed iron cores. The inner core is silicon steel, while the outer core is nanocrystalline alloy. The silicon steel core includes an air gap with a width of δ. The secondary windings with N2 turns are connected in series. Fig. 2 compares the measurement results between the proposed and traditional transformers under 10Hz. The black waveform represents the primary side current, while the orange one denotes the secondary side of the proposed transformer. The blue waveform corresponds to the secondary side waveforms of conventional single-core silicon steel transformers. The secondary side current has been adjusted according to the turn ratio (N2/1). It is evident that the traditional transformer exhibits distortion due to low-frequency saturation, whereas the new transformer maintains high measurement accuracy. Data analysis indicates that the ratio error of the new transformer is about 0.14% at 10Hz, which means it achieves an accuracy level of 0.5.References: Sehloff, David, and Line A. Roald. "Low frequency AC transmission upgrades with optimal frequency selection." IEEE Transactions on Power Systems, Vol. 37.2, p.1437-1448 (2021).
With the advancement of fractional frequency transmission, offshore wind power will be transmitted by low power frequencies around 10~30Hz. However, traditional current transformers face saturation distortion issues at low frequencies. Utilizing materials with high magnetic saturation intensity could add measurement errors. This paper proposes a transformer structure that maintains accuracy at low frequencies. This structure comprises two iron cores: one made of conventional silicon steel with an air gap, and the other made of nanocrystalline alloy material without air gap. These iron cores are concentrically placed, with the coils connected in series. According to the equivalent circuit, when the nanocrystalline alloy is saturated, its permeability becomes smaller, and the silicon steel core will share secondary induced voltage. In this case, transformers with hybrid materials will have better accuracy under low-frequency occasions. Fig. 1 illustrates the structure of the proposed transformer. The transformer body consists of two concentrically placed iron cores. The inner core is silicon steel, while the outer core is nanocrystalline alloy. The silicon steel core includes an air gap with a width of δ. The secondary windings with N2 turns are connected in series. Fig. 2 compares the measurement results between the proposed and traditional transformers under 10Hz. The black waveform represents the primary side current, while the orange one denotes the secondary side of the proposed transformer. The blue waveform corresponds to the secondary side waveforms of conventional single-core silicon steel transformers. The secondary side current has been adjusted according to the turn ratio (N2/1). It is evident that the traditional transformer exhibits distortion due to low-frequency saturation, whereas the new transformer maintains high measurement accuracy. Data analysis indicates that the ratio error of the new transformer is about 0.14% at 10Hz, which means it achieves an accuracy level of 0.5.References: Sehloff, David, and Line A. Roald. "Low frequency AC transmission upgrades with optimal frequency selection." IEEE Transactions on Power Systems, Vol. 37.2, p.1437-1448 (2021).