Abstract— When selecting components for transformers, it is essential to understand whether the transformer uses delta (Δ) or wye (Y) connections, as these configurations impact the component selection process. In wye (Y) connections, the three windings form a neutral point, and component selection is then based on whether the system is grounded or ungrounded. Additionally, the availability of the neutral affects the transformer designation and component choices. In delta (Δ) connections, no neutral exists, and components are selected based on the voltage between the lines. The kV class for components in delta systems is comparable to that in ungrounded wye connections.
Keywords— Transformer connections, Wye (Y) connection, Delta (Δ) connection
I. GENERAL
To ensure the proper selection of components, it is crucial to understand the type of transformer connections where these components will be applied. Nearly all three-phase transformers (99.9%) fall under either delta (Δ) or wye (Y) connections. These designations are defined by Standard C57.12.00.
Delta and wye connections have distinct characteristics that affect the choice of components.
Figure 1. Examples of Transformer Connections
The next sections contain key considerations to keep in mind when a customer is requesting guidance on selecting the right components for these transformer connections.
II. WYE CONNECTION (Y)
In a Wye (Y) connection, three identical terminals of the three windings are connected together (either three start or three finish) to form the neutral. It is important to note that when the wye connection is grounded, components such as arresters, bushings, tap changers, and fuses are selected based on the phase-to-ground voltage. On the other hand, for an ungrounded wye connection, these components are selected based on the phase-to-phase voltage.
Additionally, when customers specify a wye-connected three-phase transformer, it is important to understand the characteristics of the neutral, which can be classified into the following two categories:
- Neutral Available or Unavailable
- Neutral Available: Designation by Standard C57.12.00. E1Y/E (Ex. 4160Y/2400)
Figure 2. Wye Connection Neutral Available
- b. Neutral Unavailable: Designation by Standard C57.12.00. E1Y (Ex. 4160Y)
Figure 3. Wye Connection Neutral Unavailable
- Neutral Grounded or Ungrounded
- Neutral Grounded: Designation by Standard C57.12.00. E1GrdY/E (Ex. 4160GrdY/2400) Neutral Available or E1GrdY (Ex. 4160GrdY) Neutral Unavailable
Figure 4. Wye Grounded Connection
- Neutral Ungrounded: Designation by Standard C57.12.00. E1Y/E (Ex. 4160Y/2400) Neutral Available or E1Y (Ex. 4160Y) Neutral Unavailable
Figure 5. Wye Ungrounded Connection
The three main differences between these two systems are:
- The number of bushings in the transformer.
- The number of lines in the distribution system.
- The available voltages in the distribution system.
When neutral is available, loads and transformers can be connected between lines (E1) or between lines and neutral at a different voltage (E). For ungrounded systems, the components (tap changers, fuses, arresters, and bushings) are generally of a higher kV class.
III. DELTA CONNECTION (Δ)
Figure 6. Delta Connection
For Delta connections (Δ) the neutral does not exist. There are always three bushings in the transformer and three lines in the distribution system. Loads and transformers are connected only between lines at a unique voltage (E). The kV class of the components is generally the same as that of an ungrounded wye connection.
Download Resource