End Winding Analysis In Order To Minimize The Circulating Current In A Large Synchronous Generator

1174 words - 5 pages

Abstract- This paper deals with the circulating current distribution calculation and its resultant ohmic losses in a large synchronous generator. Circulating currents are produced by the induced voltage differences between parallel strands in a bar that cause high ohmic losses in high power synchronous generators. In this paper, three different cases are assumed for the armature end winding structure, and a 3-D finite element model is adopted to calculate the effectiveness and resultant losses of these cases. As mentioned later, by the use of an appropriate transposition in slot area, both main and leakage fluxes inside the machine are the same for all transposed strands bundle in active part and consequently the induced voltage difference in each strand in a bundle is negligible. However, the currents flowing in the end winding of the generator produce leakage flux in the slot region that causes circulating currents in strands, and consequently electrical power losses in the machine.
Index terms— Synchronous generator, slot region, end winding, circulating current, finite element method.

I. INTRODUCTION
Circulating currents are produced by the induced voltage differences between parallel strands in a bar due to different linkage magnetic flux. In large synchronous generator with high power density, the Roebel bars are used for the stator windings in which strands in a bundle are completely transposed in slot region by different arrangement.
By the use of this procedure, the overall physical position of each strand in a bundle is the same. Therefore, the both main and leakage fluxes are the same for each strand in each bundle, and consequently the induced voltage difference in each strand is negligible. Anyway, the currents flowing in the end winding of the generator produce leakage flux in the slot region. The resultant leakage flux produces voltage differences between strands that cause circulating current in the strands. The flowing of circulating currents in the stator bars result in copper losses and consequently overheating in the generator.
Because of significant effects on power losses, and also on the total efficiency of generator, minimization of the circulating currents is very important. This needs the accurate calculation of strands current distribution.
Many studies are done on presenting various methods to compute the circulating current distribution in transposed stator winding [1-5].
However in above papers various arrangements of end winding of stator are not considered, whereas leakage flux produce by currents flowing in the end winding has most effects on the strands’ circulating current in the transposed stator winding. In this paper, the effects of bundle transposition in slots are examined in various degrees of strands transposition in a large synchronous generator. In slot region a 2-D finite element model is adopted to calculate the effects of 90, 180, 360, and 540 degree bundle transposition strands...

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