Transformer Connection Sections

Transformer Connection Designations

The transformer connection designation indicates the winding connection method and the phase relationship between the line voltages of the primary and secondary windings. It consists of two parts: letters and a number. The letters on the left denote the connection configurations of the high-voltage and low-voltage windings, while the number on the right is an integer from 0 to 11.

This number represents the phase shift of the low-voltage winding's line voltage relative to that of the high-voltage winding. Multiplying the number by 30° yields the phase angle by which the secondary voltage lags the primary voltage. This phase relationship is commonly illustrated using the "clock method," where the primary line voltage vector is represented as the minute hand fixed at the 12 o'clock position, and the corresponding secondary line voltage vector acts as the hour hand, pointing to the hour indicated by the number in the designation.

Representation Method

In transformer connection designations:

• "Yn" denotes a star (Y) connection on the primary side with a neutral conductor (n).

• "d" indicates a delta (Δ) connection on the secondary side.

• The number "11" means that the secondary line voltage UAB lags the primary line voltage UAB by 330° (or leads by 30°).

Uppercase letters represent the connection type of the primary (high-voltage) winding, while lowercase letters represent the secondary (low-voltage) winding. "Y" or "y" stands for a star (wye) connection, and "D" or "d" for a delta (triangle) connection. The number, based on the clock method, indicates the phase displacement between the primary and secondary line voltages. The primary line voltage vector is taken as the minute hand fixed at 12 o'clock, and the secondary line voltage vector as the hour hand, pointing to the corresponding hour.

For example, in "Yn, d11," the "11" signifies that when the primary line voltage vector points to 12 o'clock, the secondary line voltage vector points to 11 o'clock—indicating a 330° lag (or 30° lead) of the secondary UAB relative to the primary UAB.

Basic Connection Types

There are four fundamental transformer connection configurations: "Y, y," "D, y," "Y, d," and "D, d." In star (Y) connections, two variants exist: with or without a neutral. The absence of a neutral is not specially marked, whereas the presence of a neutral is denoted by adding "n" after "Y."

Clock Method

In the clock representation, the high-voltage winding’s line voltage vector is treated as the long hand (minute hand), always fixed at the 12 o'clock position. The low-voltage winding’s line voltage vector is treated as the short hand (hour hand), pointing to the hour that corresponds to the phase displacement.

Application of Standard Designations

• Yyn0: Used in three-phase power transformers within three-phase four-wire distribution systems, supplying combined power and lighting loads.

• Yd11: Applied in three-phase power transformers for low-voltage systems above 0.4 kV.

• YNd11: Employed in high-voltage systems above 110 kV where the neutral point of the primary winding must be grounded.

• YNy0: Used in systems where the primary winding requires grounding.

• Yy0: Utilized in three-phase power transformers dedicated to three-phase power loads.