grids without slack nodes

[FlorianShepherd]

Pypower allows grids withouth slack nodes and changes the first PV-bus to a slack node if none is available. I think it would be better if a warning / error is raised instead and the toolbox should get a function to change a PV node to a slack node.

See bustypes.py: `

pick a new reference bus if for some reason there is none (may have been

# shut down)
if len(ref) == 0:
    # bugfix Pypower: must be an numpy array!
    ref = zeros(1, dtype=int)
    ref[0] = pv[0]      # use the first PV bus
    pv = pv[1:]      # take it off PV list

`

[lthurner]

With the new connectivity check, we can now detect islands. I would suggest the following behaviour:

• if an island has neither gen nor ext_grid, set it out of service (now already done by connectivity check)
• if one generator but no ext_grid is in one island, treat the generator as a slack
• if there is at least one ext_grid in an island, proceed as usual
• if no ext_grid and multiple generators are in one island, throw an error

[FlorianShepherd]

Sounds valid.

[FlorianShepherd]

Fixed. We agreed on the following:

• If an island is present (=connected buses without a slack/ext_grid), no matter if it contains gens, the isolated buses will be set "out of service"
• If no slack / ext_grid is available in the whole net, an error is raised

pandapower.diagnostic can be used to identify unsupplied buses with gens. These can be manually converted to ext_grid if desired. Otherwise the behaviour of the power flow algorithm would be implicit.