BOOSTER TRANSFORMERS CONNECTION & OPERATION BASIC AND TUTORIALS

Ordinary distributing transformers applied as illustrated (Fig. 1) are used when it is necessary to raise, by a fixed percentage, the voltage delivered by a line, as it is when transformer ratios do not give quite the right voltage or when line drop is excessive.

Fig. 1 Boosting transformers.

A booster raises the voltage of any primary circuit in which it may be inserted by the amount of the secondary voltage of the booster (see Fig.1).

EXAMPLE
On a long single-phase 2080-V lighting branch so heavily loaded that the pressure drops more than the amount for which the normal regulation of the feeder will compensate, a 110-V transformer inserted in the line as a booster will raise the pressure of the primary branch on the load side of the booster by 110 V.

This raises the secondary pressure 5.5 percent on all the transformers beyond the booster. With 440-V service supplied by star-connected 230-V transformers, a 10 percent booster in each phase raises the normal pressure of 230/400 V to 253/440 V.

The connections for a simple booster are shown in Fig. 1, I, the line pressure being raised from 2080 to 2184 V, or 5 percent. The connection at II is that for an augmented booster in which the line pressure is raised from 2080 to 2190 V, because the primary of the booster is connected across the line on the far side and the booster is boosted as well as the line.

This gives an increase of 5.5 percent in the line pressure. Figure 1, II, shows a 10 percent simple booster and IV an augmented 11.1 percent booster.

The transformers shown in Fig.1 have a 10:1 or 20:1 ratio, and the percentages shown apply only to transformers of this ratio. If boosters having a ratio of 2080 to 115/230 are used, the percentages are increased by about 10 percent. Figure 1, I, would then become 5.5 percent; II, 6.05 percent; III, 11.1 percent; and IV, 12.2 percent.

DELTA, STAR, AND OPEN DELTA TRANSFORMER CONNECTION COMPARISON

The choice between the methods would be governed largely by the service requirements. When the three transformers are delta-connected, one can be removed without interrupting the performance of the circuit, the two remaining transformers in a manner acting in series to carry the load of the missing transformer.

The desire to obtain immunity from a shutdown due to the disabling of one transformer has led to the extensive use of the delta connection of transformers, especially on the low-potential delivery side. It is to be noted that if one transformer is crippled, the other two will be subjected to greatly increased losses.

Thus, if three delta-connected transformers are equally loaded until each carries 100 A, there will be 173 A in each external circuit wire. If one transformer is now removed and 173 A continues to be supplied to each external circuit wire, each of the remaining transformers must carry 173 A, since it is now in series with an external circuit.

Therefore, each transformer must now show 3 times as much copper loss as when all three transformers were active, or the total copper loss is now increased to a value of 6 relative to its former value of 3. An open-delta installation is made frequently when considerable future increase in load is expected.

The increase can be accommodated by adding the third transformer to the bank at a later date and thus increasing the capacity of the load that can be carried by about 75 percent.

A change from delta to Y in the secondary circuit alters the ratio of the transmission emf to the receiver emf from 1 to 1.73 .

On account of this fact, when the emf of the transmission circuit is so high that the successful insulation of transformer coils becomes of constructive and pecuniary importance, the three-phase line sides of the transformers are connected in “star” and the neutral is grounded.

The windings of most transformers operating on systems of 100,000 V or more are star-connected.


Comparative cost of transformers for different grouping for three-phase service. 
The accompanying table shows the costs of the single-phase transformers, of proper capacities for either a delta or an open-delta grouping, and of a three-phase transformer to serve a 75-kVA installation. The relative costs will be the same for the present date.