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I have a transformer for troubleshooting. It is a 3 phase DELTA-WYE Step down (11KV/400V) transformer, the low side voltages (secondary) are

Line to line voltages:

RED-YELLOW : 400 V YELLOW - BLUE : 400V BLUE- RED : 400V

Line to Neutral Voltage:

RED-Neutral : 400 V YELLOW - Neutral : 400 V BLUE- Neutral : 05 V

What are the possible causes that might cause the issue? So far I have found that the transformer rating has not been impacted. I could not figure out if the transformer has grounded neutral or not.

What should be the list of causes?

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To figure this out, you must need to identify the type of grounding is adopeted on the 2o side of the transformer. Usually the center of WYE is directly grounding and so is the neutral. Said that, you should measure 230 V between any phase to neutral. By now, it seems like the transformer has an isolation problem on its windings (my guess).

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