Transformers - 4 | Electrical Machines for Electrical Engg. - Electrical Engineering (EE) PDF Download

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THREE PHASE TRANSFORMER
• When three identical units of single-phase transformers are used, the arrangement is commonly called a bank of three transformers or a three- phase transformer bank.
• Three phase current in three primaries produce three-phase fluxes mutually time-displaced by 120°. These fluxes flow through their respective
yokes and then through the central limbs placed together. The resultant flux in the three central limbs must be zero.

Core Type Transformer

Three-phase core-type transformer from three single-phase units
• The reluctance offered to the flux of central limb II is less than the reluctance offered to the outer two fluxes produced in legs I and III. Consequently, the exciting current for the phase winding on the central limb is less than it is for the outer two phase windings. Shell Type Transformer
• A three- phase shell type transformer is obtained if three single-phase shell type cores are placed side by side.

Three-phase shell type transformer with three windings wound in the same direction
• In the magnetic circuits marked 2 and 3, the resultant flux is seen to be equal to the phasor difference of the component fluxes.

• For the same flux density, the cross-sectional areas at 2 and 3 are 86% of the central core area.

parallel Operation of 3-ø Transformers
• The various conditions that must be fulfilled, for the successful parallel operation of 3-phae transformers, are as follows:
• The line voltage rations of the transformers must be the same.
• The transformers should have equal per unit leakage impedances.
• The ratio of equivalent leakage reactance to equivalent resistance should be same for all the transformers.

• The transformers should have the same polarity.
• In addition to these four conditions, two more essential conditions that must be fulfilled for the parallel operation of three-phase transformers are as follows:

• Relative phase displacement: The relative phase displacement between the secondary line voltage of all the transformers must be zero i.e. the transformers to be connected in parallel, must belong to the same group number.

Note:
• Transformers for group numbers 3 and 4 can be successfully operated in parallel.
• Phase Sequence: An improper phase sequence as shown in figure (b) below, would give zero voltage across switch S₁ and line voltages across switch S₂ and S₃. Consequently the parallel operations is not possible.