THREE-PHASE TRANSFORMER BANKS USING SINGLE-PHASE TRANSFORMERS

Principles in using single-phase transformers to create a three-phase banks.

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 of 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 threephase 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 singlephase 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 derate 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, as shown in Figure 9.16. 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 shown in Figure 9.16, then the bank provides a single-phase lighting leg in addition to a three-phase power circuit.