Electrical Power Systems

or the reference phase a,
E a = (Z g + Z L)I a
...(5.1)
igure 5.2 gives the single-phase
equivalent of a balanced three-phase network
of ig. 5.1.
As the system is balanced, the voltage
and currents in the other phases have the
same magnitude but are shifted in phase by
120°.
Now consider the case where a threephase
transformer forms part of a three-phase
system. Three identical single-phase two
winding transformers may be connected to
form a three-phase transformer. The windings
can be connected in four ways, Y–Y, Y–D,
D–Y and D–D. The phase of Y–Y and D–D
transformer, there is no phase shift between
corresponding quantities on the low and high
voltage windings. However, for D–Y or Y–D
transformers, there is always a phase shift.
igure 5.3 shows a three-phase Y–Y bank.
Schematic representation of this
transformer is shown in ig. 5.4(a) and ig.
5.4(b) shows the single-phase equivalent of
three-phase Y–Y transformer and ig. 5.4(c)
shows the single-line diagram.
Power System Components and Per Unit System 97
ig. 5.3: Three-phase two-winding Y–Y
transformer bank (a) Arrangements of core
and coil (b) Single-line diagram.
ig. 5.4: (a) Schematic representation of three-phase g-g transformer, (b) Single-phase equivalent
of Y–Y transformer, (c) Single-line diagram of transformer.