25.6.3Transformer polarity

An important characteristic to identify for transformers in power systems – both power transformers and instrument transformers – is polarity. At first it may seem inappropriate to speak of “polarity” when we know we are dealing with alternating voltages and currents, but what is really meant by this word is phasing. When multiple power transformers are interconnected in order to share load, or to form a three-phase transformer array from three single-phase transformer units, it is critical that the phase relationships between the transformer windings be known and clearly marked. Also, we need to know the phase relationship between the primary and secondary windings (coils) of an instrument transformer in order to properly connect it to a receiving instrument such as a protective relay. For some instruments such as simple indicating meters, polarity (phasing) is unimportant. For other instruments comparing the phase relationships of two or more received signals from instrument transformers, proper polarity (phasing) is critical.

Polarity markings for any transformer may be symbolized several di erent ways:

Transformer polarity symbols

The marks should be interpreted in terms of voltage polarity, not current. To illustrate using a “test circuit28” feeding a momentary pulse of DC to a transformer from a small battery:

Pushbutton momentarily pressed

Note how the secondary winding of the transformer develops the same polarity of voltage drop as is impressed across the primary winding by the DC pulse: for both the primary and secondary windings, the sides with the dots share the same positive potential.

28The battery-and-switch test circuit shown here is not just hypothetical, but may actually be used to test the polarity of an unmarked transformer. Simply connect a DC voltmeter to the secondary winding while pressing and releasing the pushbutton switch: the voltmeter’s polarity indicated while the button is pressed will indicate the relative phasing of the two windings. Note that the voltmeter’s polarity will reverse when the pushbutton switch is released and the magnetic field collapses in the transformer coil, so be sure to pay attention to the voltmeter’s indication only during the time of switch closure! This is an application where an analog voltmeter may actually be superior to a digital voltmeter, since the instantaneous movement of a mechanical needle (pointer) is easier to visually interpret than the sign of a digital number display.

Transformers with multiple secondary windings act the same, with each secondary winding’s polarity mark having the same polarity as every other winding:

To emphasize this important point again: transformer polarity dots always refer to voltage, never current. The polarity of voltage across a transformer winding will always match the polarity of every other winding on that same transformer in relation to the dots. The direction of current through a transformer winding, however, depends on whether the winding in question is functioning as a source or a load. This is why currents are seen to be in opposite directions (into the dot, out of the dot) from primary to secondary in all the previous examples shown while the voltage polarities all match the dots. A transformer’s primary winding functions as a load (conventional-flow current drawn flowing into the positive terminal) while its secondary winding functions as a source (conventional-flow current flowing out of the positive terminal).

Transformer polarity is very important in the electric power industry, and so terms have been coined for di erent polarity orientations of transformer windings. If polarity dots for primary and secondary windings lie on the same physical side of the transformer it means the primary and secondary windings are wrapped the same direction around the core, and this is called a subtractive transformer. If polarity dots lie on opposite sides of the transformer it means the primary and secondary windings are wrapped in opposite directions, and this is called an additive transformer. The terms “additive” and “subtractive” have more meaning when we view the e ects of each configuration in a grounded AC power system. The following examples show how voltages may either add or subtract depending on the phase relationships of primary and secondary transformer windings:

Subtractive transformer

Transformers operating at high voltages are typically designed with subtractive winding30 orientations, simply to minimize the dielectric stress placing on winding insulation from inter-winding voltages. Instrument transformers (PTs and CTs) by convention are always subtractive.

30The IEEE standard C57.12.00-2010 (“IEEE Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers”) states that single-phase transformers having power ratings of 200 kVA and below and high-voltage winding ratings of 8.66 kV and below must have additive polarity, and that all other types of power transformers must have subtractive polarity.