MV grid generation results in multiple subgrids

[nesnoj]

Error:

• In 10 of the 3854 MV grids, e.g. in Berlin 32356, 32391 (eGon-data run 06.04.23) grids are not generated properly
• Log:

  File ".../ding0/ding0/tools/pypsa_io.py", line 1591, in _check_integrity_of_pypsa
    raise ValueError("The graph has isolated nodes or edges")
ValueError: The graph has isolated nodes or edges

Observations:

• Error results from MV grid with multiple graphs (PF cannot be run)
• This happens when connecting MV stubs in fct mv_urban_connect() and 2 stubs (or even more?) share the same root node, see stub_dict["root"]
• When the 1st stub is handled, a cable distributor / branch tee is created from the root node. When connecting the 2nd stub, it is not connected to this very cable distributor - instead, a new cable distributor is created and connected to itself (circle).
• Example: MVGD 32356, 2 stubs with root node (osm_id) 390032673

Next steps

• Quick fix: Delete smaller graph to at least get largest parts of the grid
• Fix: Find out why stubs' root nodes are sometimes redundant