1. Overview

An Open Delta Transformer is a special type of three-phase transformer configuration that uses only two transformers instead of three to provide three-phase power. It’s often used in emergency, backup, or low-power applications when full transformer capacity isn’t needed or one unit has failed.

2. Applications

• Emergency or temporary three-phase supply when one transformer is out of service
• Small industrial loads where full three-phase power is not required
• Cost-effective solution where load is light or intermittent
• Backup supply in rural or remote areas

3. Configuration Details

• Two single-phase transformers are connected in V-shape between three-phase lines.
• The third phase is electrically synthesized, not physically present as a winding.
• It works on the principle of vector addition of phase voltages to simulate the third phase.

4. Electrical Characteristics

• Power Output Capacity:

• Load Handling:

5. Advantages

• Lower initial cost (uses only 2 transformers)
• Emergency continuity if one transformer in a delta bank fails
• Simpler installation for light three-phase loads
• Useful in temporary power setups

6. Limitations

• Limited to 57.7% of full load capacity
• Uneven load distribution between the two transformers
• Not ideal for heavily unbalanced or nonlinear loads
• Reduced efficiency and higher losses under high load

7. Design Considerations

• Transformers should have the same voltage rating and impedance
• Proper phase angle alignment is critical to ensure correct three-phase synthesis
• Impedance matching is important for voltage balance
• Often used with additional voltage regulation or monitoring devices

8. Typical Use Cases

• Backup for closed delta systems
• Small rural substations
• Construction site power supplies
• Small commercial or light industrial facilities
• Diesel generator applications with space/power constraints

9. Standards

Open delta transformers are generally designed according to:

• IEC 60076 – Power Transformers
• IEEE C57.12 series
• IS 2026 – Indian Standard for Power Transformers

10. Summary Table