VP11-01 A novel method for eliminating residual flux in transformers using composite voltage
Creators: Yuzhan Ren
Youhua Wang and Chengcheng Liu
Publisher: name: IEEE
Year: 2025

I Introduction After the transformer is cut out for operation and DC resistance test, the residual flux (RF) can be generated in the transformer core. To date, there is no effective means to distinguish between internal faults of transformers and RF, which can interfere with transformer diagnostic results [1]. In addition, serious inrush current may occur as a result of RF. Therefore, it is necessary to eliminate the RF. The existing demagnetization methods are mainly divided into DC demagnetization method and AC demagnetization method. According to IEEE Std C57.152-2013, the DC demagnetization method is widely used due to its low power requirements and good demagnetization effect. However, reference [2] indicates that there are still non-negligible currents after the last excitation, which may increase the magnetic flux to 10% of the knee point flux after the discharge process, leading to demagnetization failure. This paper aims to study an efficient method for eliminating RF. II Proposed method This paper proposes a demagnetization method using a composite voltage combining DC and AC signals. The applied voltage waveform is shown in Figure 1. The first applied is a DC voltage with periodic polarity changes. Due to the large inherent time constant of transformers, series resistors are used to reduce the demagnetization period. The initial voltage V1 is determined based on the rated current and resistors. A suitable voltage decay rate will be studied for various RF. Once the flux is reduced to a lower level, the applied voltage is changed to a sine signal. When the voltage amplitude is less than the threshold, the demagnetization ends and RF is completely eliminated. III Experimental results and conclusions Experimental tests are conducted on a 20 kVA transformer, as shown in Figure 2. The RF is weakened to within 1% of the knee point flux using the proposed method. By connecting resistors in series, the entire demagnetization process is controlled within 2 s. The maximum power of the proposed method is only 20 W, which is much less than the AC demagnetization method. The experimental results show that the proposed method is more efficient than existing methods.References: [1] S. Zhang et al., “Improved Flux-Controlled VFCV Strategy for Eliminating and Measuring the Residual Flux of Three-Phase Transformers,” IEEE Transactions on Power Delivery, vol. 35, no. 3, pp. 1237–1248, Jun. 2020, doi: https://doi.org/10.1109/TPWRD.2019.2937900. [2] Yasin Zabihinia Gerdroodbari, Mahdi Davarpanah, and Shahrokh Farhangi, “Remanent Flux Negative Effects on Transformer Diagnostic Test Results and a Novel Approach for Its Elimination,” IEEE Transactions on Power Delivery, vol. 33, no. 6, pp. 2938–2945, Dec. 2018, doi: https://doi.org/10.1109/tpwrd.2018.2827179.
I Introduction After the transformer is cut out for operation and DC resistance test, the residual flux (RF) can be generated in the transformer core. To date, there is no effective means to distinguish between internal faults of transformers and RF, which can interfere with transformer diagnostic results [1]. In addition, serious inrush current may occur as a result of RF. Therefore, it is necessary to eliminate the RF. The existing demagnetization methods are mainly divided into DC demagnetization method and AC demagnetization method. According to IEEE Std C57.152-2013, the DC demagnetization method is widely used due to its low power requirements and good demagnetization effect. However, reference [2] indicates that there are still non-negligible currents after the last excitation, which may increase the magnetic flux to 10% of the knee point flux after the discharge process, leading to demagnetization failure. This paper aims to study an efficient method for eliminating RF. II Proposed method This paper proposes a demagnetization method using a composite voltage combining DC and AC signals. The applied voltage waveform is shown in Figure 1. The first applied is a DC voltage with periodic polarity changes. Due to the large inherent time constant of transformers, series resistors are used to reduce the demagnetization period. The initial voltage V1 is determined based on the rated current and resistors. A suitable voltage decay rate will be studied for various RF. Once the flux is reduced to a lower level, the applied voltage is changed to a sine signal. When the voltage amplitude is less than the threshold, the demagnetization ends and RF is completely eliminated. III Experimental results and conclusions Experimental tests are conducted on a 20 kVA transformer, as shown in Figure 2. The RF is weakened to within 1% of the knee point flux using the proposed method. By connecting resistors in series, the entire demagnetization process is controlled within 2 s. The maximum power of the proposed method is only 20 W, which is much less than the AC demagnetization method. The experimental results show that the proposed method is more efficient than existing methods.References: [1] S. Zhang et al., “Improved Flux-Controlled VFCV Strategy for Eliminating and Measuring the Residual Flux of Three-Phase Transformers,” IEEE Transactions on Power Delivery, vol. 35, no. 3, pp. 1237–1248, Jun. 2020, doi: https://doi.org/10.1109/TPWRD.2019.2937900. [2] Yasin Zabihinia Gerdroodbari, Mahdi Davarpanah, and Shahrokh Farhangi, “Remanent Flux Negative Effects on Transformer Diagnostic Test Results and a Novel Approach for Its Elimination,” IEEE Transactions on Power Delivery, vol. 33, no. 6, pp. 2938–2945, Dec. 2018, doi: https://doi.org/10.1109/tpwrd.2018.2827179.