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

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