In standard practice ,the core of three - phase transformers has three legs.
All these cores are made of high-grade silicon sheet steel with high permeability and low loss, and are built up in layers to fit closely within the circular coil .
When cold - rolled grain -oriented silicon sheet steel is used the core construction is employed to take the best advantage of the excellent characteristics of the steel.

In power transformers the standard coil is a concentric arrangement with a high -voltage outer winding , a low -voltage inner winding and a tap leading from the outer coil .
Since transformers vary in voltage and current, coils must be selected on the basis of electrical and mechanical suitability to each class of transformer.
Diala,´s coils come in four standard types:
Disk - Type continuous
The disk type continuous winding coil is interspaced with oil ducts and is strong both electrically and mechanically.
When there are two or more parallel conductors for large currents , transposition is made at a large number of points , so that there will be no cross current between the parallel conductors.
High Series - Capacitance
Transformers for high voltages employ the high series capacitance winding. The series static capacitance of the coil is increased to such an extent by this special winding that the initial potential distribution of the incoming surge voltage (determined by the ratio of the series static capacitance to ground static capacitance ) becomes almost linear . This reduces internal potential oscillation to a minimum and greatly improves the impulse voltage characteristics and dielectric strength of the coil . It also permits reduction of oil duct size, for an economical design with a good space factor . This type of coil is wound continuously except for several joints which are unavoidable , and is as mechanically strong as the disk-type continuous winding mentioned above .
Helical
Transformers for heavy current are generally low in voltage but require an increased number of parallel conductors .This makes it necessary to use a helical winding because this helical winding has the same number of transpositions as there are parallel conductors, it can withstand the tremenendous mechanical force produced by short-circuit currents and its cylindrical shape prevents deformation. The coil is also able to withstand vertical forces because it is secured at top and bottom by pressure plates and spacers.
Cylindrical
Cylindrical winding is used for medium-and small-size transformers. There are two configurations , one that has a perfect-circle cross-section, the other with a squared circle . Both feature simple construction, compact size, and economy . Vertical oil ducts assure maximum cooling effect. The parallel conductors are completely transposed as in the other three types.

Diala large power transformers use rectangular tanks, made of rolled steel.
Tanks to be subjected to vacuum oil filling in the factory or field are reinforced to assure sufficient strength against full vacuum, and undergo strict pressure and leakage tests to check their strength and oil-tightness .Covers are normally welded to tanks , but they may be bolted if so requested.

1.

Power transformers of Normal Cooling [ONAN] in group of connections DYN11 with
transmission ratio of ( 33KV/11.5KV) with various capacities (5,10,16,31.5)MVA.

2.

Power transformers of enforced and normal cooling type [ONAF] in various of connections with transmission ratio of (132/33/11.5)KV and capacity 63 MVA in two groups of connections YnDyn and YnYnd.

Advantage Transformers high Efficiency, and quality of performance and electrical properties as follows: