Jghrist,
The non-broken line currents are 120-degree apart, because both sides of the line are earthed by neutral circuit: one side by the primary of the YY transformer, and on another side, even if there is a Delta winding, the neutral circuit is provided by a high-Z star load or (usually) the...
Dear Appuni,
When one phase in 66kv line is opened, an unballancy occures in your system currents. In this situation you have only 2 phase currents (in your 66kv line) that make a 120 degrees angle. This causes a big neutral current (that is equal to the sum of current vectors) in the neutral...