What Are You Looking For?
Transformers typically use two main winding connections: "Delta" and "Wye." In transformer connection groups, "D" denotes a Delta connection, "Yn" denotes a Wye connection with a neutral, and "Y" denotes a standard Wye without a neutral. The "11" indicates that the secondary line voltage Uab lags the primary line voltage UAB by 330 degrees (or leads by 30 degrees).
(1).Y and D Notation
Uppercase letters represent the primary side (or primary winding) connection type, while lowercase letters represent the secondary side (or secondary winding) connection.
"Y" (or "y") indicates a Wye connection, while "D" (or "d") signifies a Delta connection.
Clock notation is used to show the phase difference between the primary and secondary line voltages, where the primary line voltage vector is fixed at the 12 o'clock position, and the secondary line voltage vector points to the appropriate clock hour.
(2).Yn, d11 Notation
"Yn, d11" indicates that when the primary line voltage vector is at 12 o'clock, the secondary line voltage vector points to 11 o'clock. This means the secondary line voltage Uab lags the primary line voltage UAB by 330 degrees (or leads by 30 degrees).
(3).Four Common Connection Groups
By combining two windings, transformers can be configured in four basic groups: "Y, y," "D, y," "Y, d," and "D, d."
Only "Y, y" and "Y, d" are commonly used in China. For the Wye connection, neutral may be included (denoted by "Yn") or omitted. If neutral is included, it is represented by the letter "n" after "Y."
1.Current and Polarity (A, a)
The polarity marking follows a subtractive polarity system. In this system, the high potential point on the same core column in each winding is marked as the same polarity (e.g., "A" and "a" or "•").
With this marking, if current flows into "A" on the primary winding, it flows out of "a" on the secondary winding. Clock notation represents the phase difference between primary and secondary line voltages.
2.Primary and Secondary Coils (Y, y)
In the "Yyn0" configuration, the high-voltage side is in a Wye connection, where magnetizing current is sinusoidal. Due to the transformer’s nonlinear magnetizing curve, the core flux waveform becomes flat-topped, with significant third harmonic content.
3.Zero-Sequence Current
When secondary load is unbalanced, each part carries a zero-sequence current equal to one-third of the neutral line current.
In a "Dyn11" configuration, zero-sequence current can circulate within the primary winding, reducing the magnetic losses. However, in "Yyn0" connections, large transformers are not recommended as zero-sequence flux has no path within the core, leading to additional loss.
4.Single-Phase Short Circuit
In the event of a single-phase short circuit on the low-voltage bus, "Dyn11" transformers have lower zero-sequence impedance, thus providing greater sensitivity to single-phase faults on the low side.
Delta Connection (Y, D)
Delta connections help eliminate third harmonic currents by forming circulating paths within the Delta windings, reducing harmonic transmission to the system. Additionally, the Delta connection can form a circulating path for zero-sequence currents, preventing them from flowing back to the high-voltage side.
High Voltage Side Y Connection: Lowers the cost of insulation by reducing phase voltage.
Low Voltage Side Delta Connection: Reduces conductor cross-sectional area, lowers cost, and mitigates third harmonic currents.
By using either Delta or Wye on one side of the transformer, harmonic currents can be managed effectively, ensuring a sinusoidal waveform without distortions.
