and the full-load capacity of the open-delta, or
V-connected, system is
or
The ratio of the load that can be carried by two
transformers connected in open delta to the load that can
of the original load.
be carried by three transformers in closed delta is
The rating of each transformer in open delta
necessary to supply the original 150-kW load is
of the closed-delta rating.
and two transformers require a total rating of
For example, a 150-kW, three-phase balanced load
2 x 86.6= 173.2 kW, compared with 150 kW for three
operating at unity power factor is supplied at 250 volts.
transformers in closed delta. The required increase in
The rating of each of three transformers in closed delta
transformer capacity is
is
when two transformers are used in open delta to supply
and the phase current is
the same load as three 50-kW transformers in closed
delta.
Three single-phase transformers with both primary
and secondary windings wye connected are shown in
The line current is
figure 3-36. Only 57.7 percent of the line voltage
If one transformer is removed from the bank, the
remaining two transformers would be overloaded.
is impressed across each winding, but full-line current
flows in each transformer winding.
Three single-phase transformers delta connected to
To prevent overload on the remaining two transformers,
the primary circuit and wye connected to the secondary
the line current must be reduced from 346 amperes to
circuit are shown in figure 3-38. This connection
200 amperes, and the total load reduced to
Figure 3-38.--Wye-wye transformer connections.
3-32
