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There are several types of transformer connections, which determine how the primary and secondary windings are interconnected. Depending on the application, a transformer's primary and secondary winding will feature four different connections. These connections are:

• Star-Star (Y-Y)

• Delta-Delta (D-D)

• Star-Delta (Y-N)

• Delta-Star (D-Y)

1. Delta (Δ) Connection: In a delta connection, the three winding ends of each phase are connected in a closed loop, forming a triangular configuration. This arrangement doesn't require a neutral point. Delta-connected transformers can continue to supply balanced three-phase power even if one of the windings fails open. 

2. Wye (Y) Connection: A wye connection connects one end of each of the three windings to a common point, called the neutral point or star point. This forms a Y-shaped configuration. The wye connection provides a neutral conductor, which is often used for grounding and connecting single-phase loads. It also allows for easier implementation of unbalanced loads and provides better voltage regulation on the secondary side compared to delta.

3. Delta-Wye (Δ-Y) Connection: This is a combination where the primary winding is connected in delta and the secondary winding is connected in wye. It's commonly used in power distribution to step down high-voltage transmission levels to lower voltages suitable for consumer use. This configuration allows for a neutral point on the secondary side while maintaining the advantages of a delta primary for fault tolerance.

4. Wye-Delta (Y-Δ) Connection: In this configuration, the primary winding is connected in wye and the secondary winding in delta. This setup is less common than Δ-Y but can be useful for starting induction motors with high starting torque requirements. By using a delta-connected secondary during startup, the starting current is reduced, easing the stress on the power supply.

5. Scott-T Connection: This is a specialized connection used primarily for interconnecting two-phase systems with a three-phase system. It uses a special transformer configuration with two windings per phase on the primary side and two independent windings on the secondary side, allowing for phase transformation.

6. Open-Delta Connection: This is an arrangement used as a temporary measure when one transformer of a delta bank fails or needs maintenance. It involves using only two transformers to form an incomplete delta connection, which can still provide some level of service but with reduced capacity and no redundancy.

Each connection type has its advantages and disadvantages, and the choice depends on factors such as the required voltage levels, load characteristics, system grounding requirements, and fault current considerations.