594 IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 2, FEBRUARY 2010 Flux Distribution Analysis in Three-Phase Si-Fe Wound Transformer Cores George Loizos , Themistoklis D. Kefalas , Antonios G. Kladas , and Athanassios T. Souflaris Schneider Electric AE, Inofyta, Viotia GR-32011, Greece Faculty of Electrical and Computer Engineering, National Technical University of Athens, Athens GR-15780, Greece This paper shows experimental results of longitudinal flux density and its harmonics at the limb, the yoke and the corner of a three- phase, Si-Fe, five-legged wound transformer core. Results show that the flux density is nonuniform in the cores and there is high level of third harmonic component. Moreover, the lower flux values in the outer cores have been assessed and the simultaneous time variation of the flux in both outer cores has been demonstrated by measurements while the peak magnetic flux density values have been compared to FEM analysis. These findings enable a better understanding of the magnetic behavior of five-legged wound transformer cores and their consideration is expected to achieve respective improvement of the design with respect to core losses and magnetostriction noise. Index Terms—Magnetic cores, magnetic field measurement, magnetic losses, power transformers. I. INTRODUCTION D ISTRIBUTION systems either use stacked or wound core transformers. Most of the past research work has been fo- cused upon the single-phase and the three-phase stacked core type. The effects of core geometry, core material, joint design, and induction on core losses have been investigated by many researchers in the field [1]–[8]. A detailed comparison of lo- calized flux density measurements with finite-element method (FEM) analysis was undertaken in [1] and the discrepancies resulting by inter-laminar flux and rotational magnetization in stacked cores were illustrated. The underestimation of losses in five-limb stacked core transformer with respect to local flux den- sity measurements are underlined in [3] and attributed to joint effects while opposite results are given for three limb trans- former cases. The inhomogeneity of flux density distribution in stacked core transformers has been discussed in [4] men- tioning the package design effects supported by local flux mea- surements. The building factor dependence on joints and par- ticularly on joint angles are examined in [5] through local field measurements. The case of Si-Fe wound core iron loss distribu- tion is investigated in [2] based on local flux measurements. The particular dependence of local field distribution at the joints and iron losses of the cores are analyzed in [8]. Although extensive analysis has been published for the stacked core transformers there is relatively few published work in the case of three-phase, Si-Fe, five-legged wound transformer cores. The added value of this work is the localized flux density measurements at the inte- rior of the wound transformer core; something that has not been presented before according to our knowledge. Measured flux density distributions, flux density waveforms and their harmonics analysis are presented in this paper. Partic- ular investigation on the flux variation in the outer smaller cores with respect to the internal larger ones has been undertaken and the impact on flux distribution has been analyzed. The lower Manuscript received June 20, 2009; revised September 01, 2009; accepted September 01, 2009. Current version published January 20, 2010. Corre- sponding author: G. Loizos (e-mail: georgeloizos@tee.gr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2033022 Fig. 1. Measurements setup of the five-legged wound core transformer. local flux in the outer cores as well as the phase relationship of the flux in these cores has been demonstrated and assessed. The conclusions drawn from this work enable a better under- standing of the magnetic behavior of wound transformer cores in view of improving the design with respect to core losses and magnetostriction noise. II. MEASUREMENTS SETUP The five-legged core wound core transformer, shown in Fig. 1, is made up of four separate cores fastened together to make five legs. The main core dimensions are given in Fig. 2(a). High permeability steel of 0.27 mm thickness with a nominal loss of 0.72 W/kg at 1.5 T/50 Hz was used. The four cores have been annealed to remove harmful stresses induced by manufacturing processes during their production. The primary windings are placed on the three inner legs and no windings are placed on the two outer legs. The three twenty-turn primary coils in delta (D) connection were supplied from a 400 V, 50 Hz three-phase supply via an appropriate variable autotransformer in order to magnetize the cores from 1.0 to 1.8 T. Localized flux density measurements were carried out with one-turn search coils wound around the total width of the core at selected locations and also with two-turn search coils wound around the total width of a layer, inserted at the 3rd, 9th, and 48th layer in each core along the limb (L), yoke (Y), and corner (C) of the cores, as shown in Fig. 1. The 3rd, 9th, and 48th layers are situated at 0.81, 2.43, and 12.96 mm from the core inner edge, respectively. Solderable enameled copper wire 0.1 mm in diameter was used for the search coils. 0018-9464/$26.00 © 2010 IEEE