Wye Connections
In a Wye connection, the line voltage is √3 times of phase voltage. Line voltage is
the voltage between two phases in three phase circuit and phase voltage is the
voltage between one phase to the neutral line. And the current is same for both
line and phase. It is shown as expression below
Delta Connection
In delta connection, there are three wires alone and no neutral terminal is taken.
Normally delta connection is preferred for short distance due to the problem of
unbalanced current in the circuit. The figure is shown below for delta connection.
In the load station, ground can be used as neutral path if required.
In delta connection, the line voltage is same with that of phase voltage. And the
line current is √3 times of phase current. It is shown as expression below
Why No Neutral Wire for Delta Connection?
The three phases in a delta connection can provide a voltage difference between
them. At some points in the cycle, current is flowing out of Phase A and into
Phase B while Phase C is at zero. At other points, current flows out of Phase B and
into Phase C, while Phase A is zero. At other points again, Phase C sources current
which flows into Phase A and Phase B.
One distinct advantage of a Δ-connected system is its lack of a neutral wire. With
a Y-connected system, a neutral wire was needed in case one of the phase loads
were to fail open (or be turned off), in order to keep the phase voltages at the
load from changing. This is not necessary (or even possible!) in a Δ-connected
circuit. With each load phase element directly connected across a respective
source phase winding, the phase voltage will be constant regardless of open
failures in the load elements. The only consequence of a source winding failing
open for a Δ-connected source is increased phase current in the remaining
windings.
One quick check of this is to use Kirchhoff’s Voltage Law to see if the three
voltages around the loop add up to zero. If they do, then there will be no voltage
available to push current around and around that loop, and consequently there
will be no circulating current. Starting with the top winding and progressing
counter-clockwise, our KVL expression looks something like this:
Delta Connections with a Neutral
A high-leg delta supply is sometimes used where one winding of a deltaconnected transformer feeding the load is center-tapped and that center tap is
grounded and connected as a neutral as shown in the second diagram. This setup
produces three different voltages. If the voltage between the center tap (neutral)
and each of the two adjacent phases is 120 V (100%), the voltage across any two
phases is 240 V (200%), and the neutral to "high leg" voltage is ≈ 208 V (173%)