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The vector group indicates the phase shift between the primary and secondary windings of the transformer, which affects the system's symmetry, fault currents, and protection schemes. Here are the key criteria considered when selecting the vector group for a power transformer:

1. System Configuration: The transformer's vector group must match the electrical system it will be integrated into. This includes considerations for three-phase systems, interconnected networks, and the presence of parallel transformers or phase shifters.

2. Voltage Levels and Phasing: Different voltage levels in the network may require specific vector groups to maintain the correct phase relationships. This is particularly important when connecting transformers across different voltage levels to ensure smooth power flow and minimize circulating currents.

3. Phase Shift Requirements: In some cases, a specific phase shift between primary and secondary voltages is desired to improve power factor, control reactive power flow, or balance loads in the system. Transformers with appropriate vector groups can provide this functionality.

4. Parallel Operation: When transformers are operated in parallel, their vector groups must be identical to avoid circulating currents and ensure that they share load symmetrically. This is vital for efficient and safe operation.

5. Earthing and Neutral Systems: The neutral earthing arrangement (solidly earthed, impedance earthed, or unearthed) can influence the choice of vector group, especially regarding the handling of zero-sequence currents and earthing transformer requirements.

6. Protection Coordination: The transformer's vector group should support the effective operation of protective relays and schemes. Incorrect vector groups can lead to miscoordination, resulting in unnecessary tripping or failure to detect faults.

7. Harmonics and Resonance: Certain vector groups may be more susceptible to harmonic amplification or resonance conditions within the system. Selection should consider the impact on power quality and potential mitigation measures.

8. Standardization and Interchangeability: Adhering to standard vector groups (e.g., Dyn11, Yyn0, Ynd1) promotes interchangeability, simplifies maintenance, and ensures compatibility with existing equipment.

9. Future Expansion and Flexibility: Anticipating future changes in the electrical network, such as additional transformers or system modifications, may influence the selection to allow for easier integration and adaptation.
   

In summary, the selection of a power transformer's vector group involves a comprehensive analysis of the system's technical requirements, operational needs, and compliance with standards to ensure reliable, efficient, and safe power transmission.