Suggestion to jghrist (Electrical) Apr 20, 2003 marked ///\\\
The "some ground path" is probably through building steel.
///This may be true, however, the 78A were measured between the transformer X2 terminal and switchgear ground bus as posted above. I was interested in this since this connection establishes path for the ground currents to flow. These are also referred to as common mode currents. They should be accounted for in the model by symmetrical components to have proper electrical energy balance and the model accurate. Else, the mathematical model is inaccurate, i.e. somewhat off.\\There can be unbalanced phase current without zero-sequence current.
///Certainly, if modeled so. See above comments.\\ If the three phase currents are unequal but add to zero, then there is no zero-sequence current.
///Agreed. But if the "if" represents the reality.\\ There can be no zero-sequence current if the transformer neutral is isolated. For instance, Ia = 1940.9 <-41.5°, Ib = 1970.3 <-156.6°, Ic = 2100 <80.3°. The currents are not equal (unbalanced), but Ia+Ib+Ic = 0.
///True, based on the "if".\\
There can be no zero-sequence current if the transformer neutral is isolated.
///True, but what remains to be added, there is no any other ground connection, e.g. corner ground from transformer terminal X2 to ground as posted above.\\ It doesn't matter if there is or is not any ground current.
///It just depends on the accuracy of modeling. How to account for the electrical energy balance accurately?\\ If a delta transformer secondary has one corner grounded, a fault to an ungrounded phase would result in ground fault current, but no zero-sequence current.
///True, however, the load is also corner grounded as posted above, creating a ground path with 78A flowing through it. The transformer actually has an open wye connection since the transformer neutral is not grounded and the actual neutral is the ground path carrying three zero sequence currents 3xIo=78A. The symmetrical component modeling needs to be adjusted accordingly.\\