Where voltage improvement can be obtained by some fixed
amount which will not cause voltages to exist outside permissible limits
during both light and heavy load conditions, taps can be changed on the
distribution transformers on certain portions of the feeder.
For example, assuming an evenly distributed load on a feeder, the
taps on the transformers in the first third of the feeder from the substation
can be changed to lower the secondary voltage a fixed amount; the
taps on the second or center third of the feeder may be left on their normal
setting; and those on the farthest third of the feeder may be changed
to raise the secondary voltage a fixed amount.
The taps on the transformers merely change their ratios of transformation.
If the normal ratio is (say) 20 to 1 to give a secondary voltage of
120 V, tap changes on those nearest the substation would result in a 21
to 1 ratio and a voltage drop of approximately 6 V, which, if subtracted
from a high permissible voltage of 126 V, will still leave a voltage of 120
V; or put another way, the tap change allows the highest permissible
voltage at the substation to be raised 6 V without exceeding the permissible
high-voltage limit at the first consumer. Similarly, on those farthest
from the substation, tap changes can result in a 19 to 1 ratio and a voltage
increase of 6 V, allowing additional voltage drops in the feeder up to
6 V before the permissible low voltage at the last consumer is not met.
