For 3ø-transformer specification, see C57.105-1978
…Application of Transformer Connections in Three-Phase Distribution Systems Excerpted..
- §6.2 …The effect of load unbalance is most easily explained in terms of one single-phase load… With delta-connected secondaries, all three windings are involved. The winding connected to the two load phases supplies 2/3 of the total load, and the other two windings are each loaded to 1/3 the total load as shown in Fig 11B. (If the connection is Δ·Δ, this statement is true only if each phase has the same transformer impedance. See 8.2.)
§8.2.2 …When the three legs of a Δ·Δ bank have different impedances (either ohmic or referred to a common kVA base), the currents within the Δ are not balanced even through the external currents are balanced. The asymmetry of the impedances may be explained or calculated in terms of an EMF which gives rise to a circulating current superimposed on the load current…
Is there impedance dissymmetry in an open·delta set? Aside from loading—voltage imbalance and individual-transformer temperature rise may be critical present and near-term considerations. Do not forget C84.1 annex-D impact.
[This message has been edited by Bjarney (edited 04-07-2005).]
