Merged voltage control separated by a switch contingency

[annetill]

• Do you want to request a feature or report a bug?

A bug. In case of several voltage controls controlling buses that belong to the same non impedant connected set, they are merged. If we have a switch contingency that modify the non impedant connected set, we are not updating the voltage controls to go back to unmerged voltage controls.
Update: this bug could also happen when we run loadflow from network cache after a switch contingency.

• What is the current behavior?

The spanning trees are computed once in the beginning. They should be recomputed in case of zero impedance branch enabled/disabled. In order to merge and unmerge voltage controls, we have to rethink about how we merge the voltage control when controlled buses belong to the same zero impedance connected set.

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

• What is the expected behavior?

• What is the motivation / use case for changing the behavior?

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

(if a question doesn't apply, you can delete it)

[annetill]

Solved by #718