Consider multiple slack nodes in local power flow calculation

[ckittl]

Currently, only one actual transformer connection between two adjacent subgrids is supported. However, #25 provides some steps on the pathway towards multiple connections between adjacent grids: The message protocol of the DBFS algorithm is adapted accordingly. The given changes however still assume, that power is exchanged over one of the existing links, as an appropiate consideration in local power flow is not apparent, yet.

This issue card collects possibly necessary steps towards provinding this feature and might be broken down into further, more precise tasks. A non-comprehensive list of things to consider:

• Do research on different solution strategies
  • Possible options are: slack power distribution factors, ward equivalents, ...
  • The physical power flow commutates over the available links, based on the impedance relations. Here, we need to do some more research to properly reflect this in the DBFS, as we are decoupling the grid, where the dispersal would happen "automatically".
  • Some of the found approaches may be applicable in the highest grid (without further superior grid structure) and not in intermediate levels
• Define a proper test grid, where reference results can be obtained with an industry-standard simulator (for validation purposes)
• Using slack power distribution factors, we most likely need an additional iterative level around local power flow calculation. Thoroughly design this with regard to performance. Check, if this iteration level can be omitted, if there is only one slack node apparent.
• Identify the implementation needs
• If everything is specified, finally rethink if the features should go into simona or better into ie3-institute/powerflow