Possible bug in definition of central bus of the microgrid

[ekatef]

Checklist

• I am using the current main branch or the latest release. Please indicate.
• I am running on an up-to-date pypsa-earth environment. Update via conda env update -f pypsa-earth/envs/environment.yaml.

Describe the Bug

It looks like there may be some mild discrepancy in defining a central bus for each grid.

Currently, such a bus is hardcoded as bus_9:

pypsa-distribution/scripts/build_shapes.py

Lines 76 to 79 in 94db427

	# The bus is the central bus of each microgrid
	microgrid_name = f"bus_9"
	microgrid_shapes.append(microgrid_shape)
	microgrid_names.append(microgrid_name)

This bus_9 is used to fetch both conventional generators and generate time-series of renewable generation. At the same time, bus_9 column is dropped from the demand dataframe:

pypsa-distribution/scripts/build_demand.py

Lines 226 to 227 in 94db427

	# Remove the bus_9 column
	load_df = load_df.drop("bus_9", axis=1)

Which means that load is attached to all the buses, except bus_9:

pypsa-distribution/scripts/add_electricity.py

Line 357 in 94db427

n.madd("Load", demand_df.columns, bus=demand_df.columns, p_set=demand_df)

So, generation and loads are attached to different buses which leads to not meaningful simulation results.

I can miss something, but currently it looks like some fix may be needed here. Would be happy to contribute, but need verify that my understanding is correct. @davide-f could you please give some hints on that?