Which connection configuration allows a control transformer to output higher voltages?

In a series connection configuration for transformers, multiple transformers are connected end-to-end, allowing the output voltage to be the sum of the individual transformer outputs. This means that if each transformer outputs a certain voltage, when they are connected in series, the total voltage available at the output will be greater than that of any single transformer.

This method is particularly useful when higher voltages are required for specific applications, as it effectively increases the voltage level while keeping the current within manageable limits.

The other connection options do not facilitate increasing the output voltage in the same way. For example, a single-phase connection typically supports the rated output of the transformer without increasing it. A parallel connection would maintain the same voltage but allow for higher current capacity, and transformer stacking is generally a method of physical arrangement rather than a direct electrical connection configuration for increasing output voltage.

Thus, selecting a series connection is the correct choice when the aim is to achieve higher output voltages from transformers.