We wish to model the voltages developed across a switch used to ground a transmission line. The line is open circuit at the far end and is parallel to an in-service energised line. Hence the line in question is electrostatically charged by the the adjacent line.
With the switch closed, we want to determine the voltage transient across the switch when it is suddenly opened. How can this be done?
We can model the steady state conditions using Tralin but believe that we need to simulate the opening of the switch to determine the likely voltages appearing across the switch.
The scenario you described would not lead to any transient across the switch. Since the switch is initially closed, there is no charge accumulated on the de-energized line. When the switch is thrown open, the de-energized line will quickly charge due to capacitive coupling to the adjacent line.
When the switch is closed after being open for a while, however, there will be a transient generated across the switch. Article  shows how to find the transient once the mutual coupling between the lines and the capacitance to ground of the de-energized line are known. In Figure 2(a) of the article, C1, C2 and C3 are the mutual capacitances between the the three phases of the energized line and the de-energized line, and C is the capacitance to ground of the de-energized line. You can use TRALIN to compute all those quantities. Formulas 1-5 then give the resulting transient.
: J. Ma and F.P. Dawalibi, “Capacitive discharge of metallic structures close to high voltage power lines”, 2002 3rd International Symposium on Electromagnetic Compatibility, Beijing, China, May 21-24 2002
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