Three Phase Transformer

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Three Phase Transformer

Generation, transmission and distrbution of electric energy is invariably done through the use of three-phase* systems because of its several advantages over single-phase systems. As
such, a large number of three-phase transformers are inducted in a 3-phase energy system for stepping-up or stepping-down the voltage as required. For 3-phase up or down

transformation, three units of 1-phase transformers or one unit ot 3-phase transformer may
be used. When three identical units of 1-phase transformers are used, Fig. 1.75 (a) the arrangement is usually called a bank of three transformers or a 3-phase transformer bank.
A single 3-phase transformer unit may employ 3-phase core-type construction, Fig. 1.75 (b) or 3-phase shell-type construction (not shown). A single-unit 3 phase core-type transformer uses a three-limbed core, one limb for each phase winding as shown in Fig. 1.75 (b). Actually, each
limb has the l.v. winding placed adjacent to the laminated steel core and then h.v. winding is placed over the L.v. winding. Appropriate insulation is placed in between the core and L.v. winding and also in between the two windings.

A 3-phase core-type transformer costs about 15% less than a bank of three 1-phase transformers Also, a single unit occupies less space than a bank.

Three-phase Transformer Connections

Three-phase transformers may have the following four standard connections:

(a) star-delta (Y-∆)

(b) delta-star (∆-Y)

(c) delta-delta (∆-∆)

(d) star-star (Y-Y)