"Additive polarity" and "subtractive polarity" are terms applied to the arrangement of insulators on single-phase, distribution-type transformers. The low-voltage bushings are on the side of the tank and the high-voltage ones are usually on the top (but sometimes on the opposite side of the tank). The relative polarities of the voltages on the two windings is very important when paralleling transformers or connecting several units in a three-phase configuration.
When one low-voltage and one high-voltage bushing (both to the left-hand side when facing the low-voltage bushings) are interconnected and one winding energized by a test voltage, the voltage appearing between the other pair of bushings (one LV, one HV) will be either the numerical sum of the two coil voltages (additive polarity) or the difference (subtractive polarity) as shown in diagram above. The test voltage is not usually at rated voltage. It can be any convenient level.
