Hitachi, Ltd.

Yoshitake Kashima

Etsunori Mori

Teruo Fukuda

Fumio Nishikawa

Ryozi Nakatake

A three-phase and three-leg core of a core-type transformer comprising three main legs formed of a plurality of steel sheets stacked in the form similar to a circle in cross section and spaced each other, and upper and lower yokes formed of a plurality of steel sheets stacked in the form similar to a circle in cross section for magnetically connecting the main legs. The steel sheets for forming each main leg are cut diagonally at opposite longitudinal ends thereof, and each yoke is formed of two types of steel sheets, one type being of diagonal cuts disposed at opposite longitudinal ends thereof to provide steel sheets of the trapezoidal shape and the other type being of a diagonal cut disposed at one of opposite longitudinal ends thereof and a right angle cut disposed at the other longitudinal end thereof to provide steel sheets of the trapezoidal shape. The steel sheets for forming the upper and lower yokes have a width greater than the width of the steel sheets for forming the main legs. The opposite longitudinal ends of the steel sheets for forming the center main leg are cut diagonally at an angle less than 45 degrees and joined diagonally and at a right angle to the steel sheets for forming the upper and lower yokes through the entire surfaces. The steel sheets for forming the two outer main legs are cut diagonally at opposite longitudinal ends thereof at 45 degrees and joined diagonally to the steel sheets for forming the upper and lower yokes in an area in which the yoke steel sheets are cut diagonally. This construction is conducive to reduced iron loss and to form the main legs of a small diameter.

Three-phase and three-leg core of core-type transformer

Low, middle and high voltage windings are wound on a magnetic core in this order, the high voltage winding having a high voltage line terminal at its center and neutral terminals at both ends. Two middle voltage partial windings are coaxially wound on the outside of the high voltage winding at upper and lower positions separated by a necessary distance for insulation. One end of each of the middle voltage partial windings are connected to the neutral point of the middle voltage winding, and the other ends thereof are each connected to the respective tap windings of two tap changers. Thus, the middle voltage partial windings and the tap changers constitute a parallel circuit in which a circulating current flows thereby to induce magnetic flux in each of the middle voltage partial windings and tap windings. The middle voltage partial windings and tap windings are so wound that the magnetic coupling therebetween through the induced magnetic flux can be reduced.

Three winding transformer

A multi-layer winding comprising cylindrically-wound conductors and having different diameters are arranged concentrically. The layer windings are connected in series from inside to outside. Each layer winding has a layer insulator arranged on the inside thereof with a flange. Inwardly of the layer insulator and adjacent to the next inward layer winding is formed an oil gap functioning as a cooling-oil path. That oil-gap-equivalent part at and in the vicinity of the ends of the layer winding where the potential is maximum between adjacent layer windings is larger than the other parts thereof. This construction reduces the intensity of the electric field at the ends of each layer winding and eliminates partial discharge, thus improving the reliability and rendering the multi-layer winding compact and light in weight as a whole.

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Hitachi, Japan

Yoshitake Kashima