Fitzgerald's Electric Machinery book explains the following on chapter 2.7:


The delta-delta connection has the advantage that one transformer can be removed for repair or maintenance while the remaining two continue to function as a three-phase bank with the rating reduced to 58 percent of that of the original bank; this is known as the open-delta, or V, connection. The Y - Y connection is seldom used because of difficulties with exciting-current phenomena. (footnote 5)

(footnote 5): Because there is no neutral connection to carry harmonics of the exciting current, harmonic voltages are produced which significantly distort the transformer voltages

I don't understand how there is "no neutral connection" in the Y-Y system, as it is the only system where both primary and secondary do have the neutral connection. What am I missing here?

  • \$\begingroup\$ You don't need a neutral for a balanced load and balanced supply but, a Y-Y transformer has serious problems in producing a balanced output on light loads. That's why you don't see this configuration used in transformers. \$\endgroup\$
    – Andy aka
    Dec 21, 2023 at 18:32
  • \$\begingroup\$ @Andyaka Thank you, so the book's explanation is incorrect? Also, can you explain or direct me to a source that explains (mathematically) why the Y-Y transformer would struggle providing balanced output on light loads? \$\endgroup\$
    – aulven
    Dec 21, 2023 at 19:23
  • \$\begingroup\$ I don't know of a book that explains this but also, I can't say that the quote above is incorrect; for me, any primary coil impedance imbalances will produce significant output voltage imbalances on light loads due to the star formation and, that is why a Y primary is never used without significant reasons. \$\endgroup\$
    – Andy aka
    Dec 21, 2023 at 19:42


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