Implementation of VDE-AR-N 4105 for determination of critical nodes in lv grids

[AnyaHe]

When determining critical nodes for lv grid reinforcement, we follow the VDE guideline VDE-AR-N 4105 to calculate the voltage drop. Two things are not implemented correctly:

1. After determining the mv impedance, it has to be translated to low voltage in order to calculate the voltage drop correctly. This is not happening so far.
2. The impact of power flows caused by generators or loads is only taken into account for nodes lying behind the respective connection point. The nodes lying in front of the grid connection point do not "feel" the power flow induced by the load or generator in the way it is implemented so far. In reality it would induce a voltage drop which has to be taken into account.

[AnyaHe]

ToDos:

• [x] Introduce voltage ratio to translate mv impendance
• [x] When calculating voltage drop use decendants instead of successors
• [x] Overthink function voltage_delta_stub()
• [x] Overthink handling of exceedance of voltage boundaries at station bus bar (additional transformer?, check_tech_constraints.py line 437)
• [ ] When merging with dev pay attention to calculation of trafo impedance and meta frequency (marked as ToDo in code)
• [ ] Check sign conventions and feedin mode