No abstract

The literature reports various transformer model core studies on local values of the building factor (BF), which consider the core as a 2-D single-package object. For the first time, we report 3-D BF analyses of a three-phase core that exhibits three different widths of packages. The inner BF values were determined by taking advantage of 25 channels of 2.5 mm width through the entire core. BF profiles along the channels were measured by means of a thermistor sensor that was controlled by a 3-D scanning system. For a nominal induction of 1.7 T, the results indicate that the main package of maximum width and thickness behaves similarly to one-package cores, with minimum BF in outer limbs and maximum BF (however, of reduced intensity) in the T joint. On the other hand, outer packages of reduced width and thickness show distinctly reduced BF, especially in the T joints. The results indicate that 2-D studies are not representative of peripheral packages. They show specific performance, especially due to the usual combination of circular limbs with semicircular yokes. The resulting regional off-plane z-flux seems to have a balancing function which reduces the effects of overlaps. However, low BF in the peripheral packages does not indicate good performance. Rather, it indicates poor exploitation of the core material.

In principle, single sheet testers (SSTs) or Epstein testers (ETs) determine iron losses p through the time integral over the field H and the time derivative of the induction B. Both quantities should be determined in equivalent ways, i.e. considering an identical quasi-homogeneously magnetized sample section. Partly, this condition is fulfilled by H-coil methods. However, the so far used very small coils do not meet the demand of equivalence. At least outside of Japan, most labs prefer apparatuses where H is assumed to be proportional to the magnetization current i. But exact proportionality does not exist due to the periphery -the yoke of SST and the corners of ET, respectively. Usually, the periphery is considered through a constant effective magnetic path length lM based on the peak induction Bp. But in fact, lM varies during the period, i.e. it is a function of B(t) and thus also of time. Our novel "PLC method" is characterized by path length consideration. We consider dynamics of length through a "path length function" Λ(B) which is determined for each grade of material in a calibration process by means of a large, rigid H-coil. Considering Λ[B(t)] by software, routine measurements on similar materials can be based on the "corrected" current i(t)/Λ(t). First results are presented for HGO SiFe investigated in an ET. Here, Λ proves to be above 1 for low B and below 1 for very high B. As a final target, application of the method for both ETs and SSTs should yield almost identical loss values that are close to "true" ones.

Domain analyses of modern types of grain-oriented (GO) transformer steel are rare, and they are focussed on single grains under academic magnetization patterns. The present Kerr effect study reveals that grains of industrial, polycrystalline materials react in specific ways according to individual crystallographic orientations. The amount of parameters is very large, especially if additional flux components are given as a further impact. The paper reports a procedure for statistical evaluations of quasi-dynamic Kerr effect images. The instantaneous area ratios of bar domain portion BD, oblique domain portion OD and saturation S are determined with consideration of peak induction BRD in the rolling direction (RD), axis ratio a of rotational magnetization, the angle ψ between B and RD, additional flux Bx in RD or Bz in normal direction. Focus is put on OD(t) which is assumed to be a key function for λ(t), i.e. the course of time of magnetostriction. As the most distinct tendencies, OD increases strongly with increases of a and ψ. OD(t) shows non-sinusoidal course of time with differences of the two halves of cycle. Rotation out of the RD yields sharp onsets of ODs, especially after half-cycle saturation. Additional off-plane flux enhances ODs in a general way.

Loss distributions of transformer cores are affected by z-flux effects if considered for the core surface. Here we propose the application of a thermal sensor which is inserted to the core interior through one of 16 channels of 2 mm width. Measurements were made on a 1-phase core assembled from laser-scribed SiFe. Throughout the core, the local building factor BF proved to increase towards the periphery which can be interpreted by interactions of highest anisotropy and non-linearity. Additional DC superposition yields non-uniform increases of BF at different core positions. Local losses tend to become more balanced -measurement locations with a priori high BF experience lower percentage increases, being more robust with respect to DC components.