There can be advantages to using
single-phase transformers to make a three phase bank instead of
building a three-phase unit. For instance, it may be impossible or
impractical to fabricate or ship a three-phase transformer with an
extremely large MVA capacity.
A bank of three single-phase
transformers may then be the solution, although the total size,
weight, and cost of three single-phase units will probably exceed the
size, weight, and cost of one three phase unit.
An additional advantage of the bank
arrangement is that a failure of one single-phase unit will usually
be less costly to repair than a failure a larger three-phase unit.
Furthermore, one spare single-phase transformer is usually all that
is required to assure sufficient reliability for the entire bank.
With a three-phase transformer, an
additional spare three-phase transformer would be required, so the
total cost of the installation plus a spare transformer is twice the
cost of the installation alone. The total cost of a bank of single
phase transformers plus a spare is only 133% the cost of the bank
alone.
Therefore, the total cost of a bank of
single-phase transformers plus a spare is probably less than the cost
of a three-phase transformer plus a spare.
Either Y or Δ connections are possible
with single-phase transformers connected in banks. It is extremely
important that the single-phase transformers are carefully matched
when they are banked together, especially when the Δ connection is
used.
Using mismatched transformers in the Δ
connection will result in excessive circulating currents that will
severely de-rate the bank or cause overheating.
One interesting configuration for a
three-phase bank is the open Y-Δ connection used extensively in
rural distribution systems. The open Y-Δ connection uses two
single-phase transformers. An open Y-Δ connection requires only two
phases plus the neutral on the primary side of the bank in order to
develop a three-phase voltage at the secondary.
This is an obvious cost saving (in
addition to the avoided cost of a third transformer) when the
installation is far away from a three-phase primary circuit. If one
of the transformers is center-tapped as then the bank provides a
single-phase lighting leg in addition to a three-phase power circuit.
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