Performance analysis of three-phase five-leg transformers under DC bias using a new frequency-dependent reluctance-based model

Abstract

This paper presents a reluctance-based model considering the frequency-dependent loss nature of the windings for the analysis of three-phase five-leg transformers under grid voltages with direct current (DC) bias. This is very important especially for proper determination of their harmonic current distortion and maximum loading capability (MLC) under DC-biased grid voltage conditions. To figure out the developed model's validity under sinusoidal and DC-biased grid voltage cases, it is comparatively analyzed with the model based on 2D finite element method (FEM). Thus, for the considered transformer type operated under DC bias, the excitation current's harmonic pollution, losses, and reactive power demand parameters are analyzed by using the developed model. Additionally, by regarding these performance parameters, the DC susceptibilities of the considered-type transformer and the single-phase shell-type transformer are comparatively evaluated. Finally, for the studied grid voltage conditions, the effects of two important design considerations as (i) magnetic core material selection and (ii) legs' cross-sectional area sizing on the MLC are investigated. It is concluded from these investigations that under saturation conditions, the transformers, which have the core material with higher permeability or lower reluctance, draw higher excitation current, and have lower MLC ratio when compared to ones having the core material with lower permeability or higher reluctance. However, for unsaturated transformers, which work under DC bias, the case is the opposite to that in saturation conditions. On the other hand, under DC bias conditions, the effect of cross-sectional area sizing on the MLC ratio is much more for the transformer with high permeable magnetic core material with regards to ones with low permeable magnetic core material.