Single-Phase Transformers for Three-Phase Operation

Author: Hans Lohninger

Single-phase transformers may be connected in groups in many ways for three-phase transformation. Fig. 17-9 shows a Δ-Δ group and Fig. 17-10 shows a Y-Δ group. In general, groups with Y-Δ connection, but without use of a fourth connection as a neutral, are to be avoided where unbalanced loads must be carried, because an unbalance in load will cause a circulating current of fundamental frequency to flow around the delta and will cause increased heating. If, however, in a Y-Δ bank with ungrounded neutral a primary conductor be opened, as by blowing of a fuse, while the secondary is carrying a three-phase motor load, the bank will no longer attempt to maintain three-phase voltage at its secondary terminals and neither the primary nor the secondary winding will be overloaded. Under these conditions a three-phase motor on the secondary line will still operate single-phase and will attempt to carry its assigned load. The current in one motor line will be twice as large as that in either of the other two lines; and, if the motor is protected by overload relays in only two of its lines, then there is one chance in three that the motor will not be adequately protected against excessive current.

Fig. 17-9. Transformer Connections. 2200/220-Volt, 3-Phase, Δ-Δ

Fig. 17-10. Transformer Connections. 3800/220-110-v, 3-Phase, Y-Δ

Fig. 17-11 shows three single-phase transformers stacked in a common frame for immersion in a single tank for pole mounting.

The A-A connection has many advantages. If the load to be carried may be expected to increase materially in a few years, the transformers may be installed Δ-Δ, but with one unit omitted as in Fig. 17-12. Such a group is called an open delta.

Fig. 17-11. Three "Round-Wound" Transformers Stacked for Immersion in One Tank for Three-Phase Service

Fig. 17-12. Transformer Connections. 2200/220-Volts, Open-Delta, 3-Phase

The open delta eliminates the fixed charges and depreciation on the third transformer until the growing load requires the addition of the third. However, the two transformers so connected have only approximately 58 per cent as much capacity as the complete group of three.1 The reason

1 Principles and Practice of Electrical Engineering, by A. Gray and G. A. Wallace. McGraw-Hill Book Co. for this is as follows: In a group of three the line current to the load is times the single transformer current; whereas, in an open-delta bank each transformer must carry the same current as each of the two lines connected to their "outside" terminals. Therefore, the line current must not exceed the current capacity of one transformer, which is or 0.58, times the three-transformer capacity.

If one of the group of a Δ-Δ bank of transformers fails, it may be removed and the service may be continued with open-delta connections until repairs can be made, although the system will have only 58 per cent of the original capacity.

It is sometimes desirable to increase the distribution voltage of a power system to accommodate a growing load. If the transformers connected to the system are Δ-Δ connected, it is feasible to reconnect them Y-Δ and thus permit raising of the line voltage 73 per cent without replacing transformers. Before deciding upon such a change, however, the adequacy of the insulation in the transformers must be investigated.