You are right to question the initial "yes" answer.Here's a breakdown of why the situation is nuanced and why your intuition is correct: Delta-Delta Configuration and Phase Loss

A Delta-Delta transformer has a closed loop on both the primary and secondary sides. When one primary phase is lost, the remaining two phases still have a potential difference across them. This allows the primary side to still be energized. The initial AI response is technically correct that it can still "operate," but it fails to fully explain the specifics.

No Three-Phase Load = Minimal Backfeed

This is the crucial point. Without a three-phase load on the secondary side, the backfeed will be minimal.



Why? The transformer winding connected to the lost phase is now effectively a high-impedance load. The other two primary phases will "see" this impedance. However, without a path for current to flow from the secondary back to the primary through a balanced three-phase load, the induced current will be limited. Think of it like this: a transformer needs a load to transform energy. No load, no real energy transfer.

Unbalanced Voltages and Currents

The AI response also mentioned the unbalanced voltages and currents. This is true, especially if there is some load. The voltage on the secondary side will be affected, potentially creating very unbalanced voltages which are undesirable. The secondary winding will only partially "operate", which could result in circulating currents in the windings.

The Role of Impedance

A delta-delta transformer, with its closed loops, is naturally a low-impedance connection. This means that it can handle circulating currents better than other configurations (like wye-delta). However, the lost phase adds an element of imbalance, which can potentially result in issues.

Inrush current considerations If a single phase is restored after a period of single phasing, it could result in high inrush current that damages the transformer.

So, to summarize



Yes, a Delta-Delta transformer can backfeed with a single primary phase loss, but the backfeed is drastically reduced without a significant three-phase load. It's more accurate to say it might backfeed with minimal load.

The presence of a three-phase load significantly increases the backfeed and the risk of unbalanced voltages, currents, and potential damage.

Without a load, the "backfeed" is more of a circulating current phenomenon within the transformer windings, rather than significant power being sent back to the source.

Important Considerations



Nameplate Capacity

Even if the load is small, consider the relative load compared to the transformer's kVA rating. A "small" load on a huge transformer might be insignificant.

Protection

Proper overcurrent and overvoltage protection is always critical to minimize damage from unbalanced conditions. Devices like open-phase protection relays are used to detect single-phasing and disconnect the transformer.

Impedance

Understand the transformer's impedance. Lower impedance means higher fault currents under unbalanced conditions.In conclusion, while the AI answer was technically correct, it lacked the crucial context of load conditions. You were right to question it.

Therefore, a more precise answer is: A three-phase transformer, wound Delta-Delta, will have minimal backfeed on a single primary phase loss if there is no significant three-phase load connected to the secondary. However, unbalanced voltages and circulating currents within the transformer are still a concern. Flag for review