VP12-01: Measurement and Modeling of the Residual Flux in the Single-Phase Three-Limb Transformer

I Introduction Residual flux density (Br) causes an inrush current when transformers are re-energized, leading to winding deformation, current imbalance and failure of the protection function [1]. The precise measurement of Br is crucial to suppressing the generation of the inrush current. Existing measurement methods have rarely been studied for the widely used single-phase three-limb transformers. In this paper, a novel method for measuring and modeling the Br of single-phase three-limb transformers is proposed, which can measure the distribution of the Br in various parts of the transformer core. II Methodology and modeling The structure of the magnetic domains in the core varies with different Br. When the same voltage excitation is applied to the transformer winding, the response current waveform will be influenced by the Br in the transformer core. The response current waveform is shown in Fig. 1. Compared to the response current without Br, the greater deviations in waveforms, the larger Br in the core. Based on the difference between response current waveforms, the Br can be determined by the Pearson correlation coefficient. The relationship between Br and the response current is established by using the field-circuit coupling method. Unlike the core-type transformer, the single-phase three-limb transformer has two magnetic circuits. For the response current, the Br in the core and the yoke are co-determinants. In order to improve the accuracy of simulating calculation, the local DC magnetization curve at the Br is measured and adopted. The material properties of the core column and yoke are properly set according to the corresponding Br. Then the fitted model for calculating Br is obtained based on simulation results. III Conclusion The experiment is conducted on the 10 kV, 250 kVA single-phase three-limb power transformer, as shown in Fig. 2. The maximum measurement error does not exceed 6.02%, proving the validity and accuracy of the proposed method. The method is suitable for all single-phase transformers, providing valuable information for suppressing the inrush current.References: [1]Y. Z. Gerdroodbari, M. Davarpanah, and S. 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 (2018)