Does anyone know how determine how much charge remains on a transmission line once the circuit has been opened but not brought to ground? I have a 7 mile long 69kV (nominal) transmission line that was opened up and never grounded.
It depends on many factors. Are there other lines in the vicinity that can induce voltage on the line in question? No such thing as a dead line unless it is visibly grounded, but what charge might be trapped on it depends on the specifics of that installation.
With another line in the vicinity the trapped charge will be renewed as fast as it can dissipate and the line will remain energized at some voltage. A quick EMTP study would tell you what kind of voltage to expect.
In my last post, I probably misused the term "trapped charge". The trapped charge becomes a DC component once the driving AC source is removed. What will happen with this line is that when the line is disconnected for its source the other line(s) in the vicinity will continue to induce voltage in the subject line. The induced voltages will reach a steady-state level and the line will remain energized at some level.