18-06-2013, 01:17 PM
Three-Phase Transformers
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All major power generation, transmission and distribution systems are three-phase ac systems.
These systems need three phase transformers. There are two different ways to construct threephase
transformers.
Advantages of three-phase transformers with a common core are:
Lighter
Smaller in size and less foot print
Cheaper
Slightly more efficient
Three-phase transformer connections
Regardless of the construction type (three-separate cores or single three-phase core), primary and
secondary windings can be independently connected in either a Wye (Y) or a Delta (Δ). This
gives four possible connections.
Wye-Wye (Y-Y) connection
Wye-Delta (Y-Δ) connection
Delta-Wye (Δ-Y) connection
Delta-Delta (Δ-Δ) connection
Analysis of Three-Phase Transformers
Circuit computations involving three-phase transformers under balanced conditions can be made
by dealing with only one transformer or one phase, and recognizing the that conditions in the
other phases are the same except for the phase displacements. It is convenient to carryout
computations using a single phase basis (usually know as per phase basis). In dealing with Δ-Y,
Y-Δ and Δ-Δ transformers, all impedances on Δ sides can be transformed to equivalent Y
connected impedances.