Effects of Asymmetric Coupling on Winding AC Resistance in Medium-Frequency High-Power Transformer

Abstract

Medium-frequency high-power (MFHP) isolated dc/dc converter has been extensively employed in various applications, including the high-voltage dc, distributed generation, high-speed railway, more electric ship, and more electric aircraft. The MFHP transformer is one of the most important components of the dc/dc converter. In MFHP application, the circulating current often occurs in the parallel secondary windings for transformer with foil-wound winding, together with primary-series secondary-parallel (PSSP) structure. It is because of the existence of asymmetric mutual inductance, described as asymmetric coupling between the primary and the secondary winding, that the absolutely symmetry cannot be achieved. Thus, a knee frequency exists, which in turn leads to significantly increased ac resistance if the working frequency of transformer is set above it. To predict the knee frequency caused by asymmetric coupling in windings, an analytical model together with a scaled-down experimental transformer to obtain the asymmetrical coupling coefficients is developed to predict the knee frequency for real-scaled transformer. The accuracy of developed analytical model is validated by the measured result. Then, a prototype 980 kVA megawatt-level MFHP transformer having PSSP structure is been built based on the analytical and experimental optimized knee frequency.Index Terms-AC resistance, circulating current, knee frequency, medium frequency high power (MFHP) transformer.

Parasitics of Orthocyclic Windings in Inductors and Transformers

“…Orc., respectively. The analytical result with (17) and with (19) but without equivalent permittivity and diameter compensation equations (22,23) are also presented as Anal. Norm.…”

“…Secondly, the winding curvature also leads to the two dimensional effect, and the analytical solution in the cylindrical coordinates is used to modify Dowell's equation with correction factors [20]. Thirdly, the analytical model is also improved by coefficients considering the thermal effect [21], asymmetric coupling [22], phase shift between winding currents [23], etc. For the Litz wire, there are analytical models derived from both Cartesian and cylindirical coordinates systems [4,24,25].…”

Transformatörlerin Ortalama Sargı Dirençlerinin Gerçek Zamanlı Ölçümü

“…biçiminde yazılabilir. Transformatör kayıpları üzerine yapılan bazı çalışmalar stray kayıplarının bir sonucu olan R stray direncinin ve dolayısıyla R ac direncinin transformatör sargılarından akan akımın harmonik içeriğine bağlı olarak değiştiğini göstermektedir [12,13]. Öyle ki, (5) eşitliğinde gösterilen sargı akımının etkin değerinin aynı olması koşuluyla harmonikli akımın neden olacağı yük kayıpları harmoniksiz durumdaki kayıplardan daha fazla olacaktır.…”

Design Analysis of High Frequency Linear Transformer with Ārtap Framework

“…There are many papers, which presents loss prediction and the design of magnetic components [1,2,4,7,[9][10][11][12][13][14][15][16][17][18][19][20][21]. Most of the design methodologies are still based on the Dowell formulas, which has certain limitations [1,9,11,22] and most of the conventional converter designs applies sandwich windings to increase the coupling of the transformer [20].…”