With power or distribution transformers, polarity is important only if the need arises to parallel transformers to gain additional capacity or to hook up three single-phase transformers to make a three-phase bank. The way the connections are made affects angular displacement, phase rotation, and direction of rotation of connected motors.
Polarity is also important when hooking up current transformers for relay protection and metering. Transformer polarity depends on which direction coils are wound around the core (clockwise or counterclockwise) and how the leads are brought out.
Transformers are sometimes marked at their terminals with polarity marks. Often, polarity marks are shown as white paint dots (for plus) or plus-minus marks on the transformer and symbols on the nameplate. These marks show the connections where the input and output voltages (and currents) have the same instantaneous polarity.
More often, transformer polarity is shown simply by the American National Standards Institute (ANSI) designations of the winding leads as H1, H2 and X1, X2. By ANSI standards, if you face the low-voltage side of a single-phase transformer (the side marked X1, X2), the H1 connection will always be on your far left. See the single-phase diagrams in figure 1.
If the terminal marked X1 is also on your left, it is subtractive polarity. If the X1 terminal is on your right, it is additive polarity. Additive polarity is common for small distribution transformers.
Large transformers, such as GSUs at Reclamation powerplants, are generally subtractive polarity. It is also helpful to think of polarity marks in terms of current direction. At any instant when the current direction is into a polarity marked terminal of the primary winding, the current direction is out of the terminal with the same polarity mark in the secondary winding. It is the same as if there were a continuous circuit across the two windings.
Polarity is a convenient way of stating how leads are brought out. If you want to test for polarity, connect the transformer as shown in figure 14.
A transformer is said to have additive polarity if, when adjacent high and low-voltage terminals are connected and a voltmeter placed across the other high- and low-voltage terminals, the voltmeter reads the sum (additive) of the high- and low-voltage windings. It is subtractive polarity if the voltmeter reads the difference (subtractive) between the voltages of the two windings. If this test is conducted, use the lowest AC voltage available to reduce potential hazards.
An adjustable ac voltage source, such as a variac, is recommended to keep the test voltage low.
SOURCE: Transformers: Basics, Maintenance and Diagnostics