Figure 3-36.--Change connections.
winding are reverses the voltages between the three line
three-phase voltage to the load. The line current is equal
to the transformer phase current in the open-delta
wires will become unbalanced, and the loads will not
connection. In the closed-delta connection, the
receive their proper magnitude of load current. In
transformer phase current
addition, the phase angle between the line currents will
be changed, and they will no longer be 120 out of phase
with each other. Therefore, it is important to properly
connect the transformer secondaries to preserve the
symmetry of the line voltages and currents.
Thus, when one transformer is removed from a
delta-connected bank of three transformers, the
Three single-phase transformers with both primary
remaining two transformers will carry a current equal to
and secondary windings delta connected are shown in
figure 3-36. The H1 lead of one phase is always
connected to the H2 lead of an adjacent phase, the X1
lead is connected to the X2 terminal of the
This value amounts to an overload current on each
corresponding adjacent phase, and so on; and the line
transformer of 1.73 times the rated current, or an
connections are made at these junctions. This
overload of 73.2 percent.
arrangement is based on the assumption that the three
Thus, in an open-delta connection, the line current
transformers have the same polarity.
must be reduced so as not to exceed the rated current of
An open-delta connection results when any one of
the individual transformers if they are not to be
the three transformers is removed from the
overloaded. Therefore, the open-delta connection
delta-connected transformer bank without disturbing
results in a reduction in system capacity. The full load
the three-wire, three-phase connections to the remaining
capacity in a delta connection at unity power factor is
two transformers. These transformers will maintain the
correct voltage and phase relations on the secondary to
supply a balanced three-phase load. An open-delta
In an open-delta connection, the line current is limited
connection is shown in figure 3-37.
to the rated phase current of
The three-phase source supplies the primaries of the
two transformers, and the secondaries supply a
Figure 3-37.--Open-delta transformer connection.
3-31
