add lines as s_nom_min not s_nom

[pz-max]

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 envs/environment.yaml.

Describe the Bug

Lines are currently added as s_nom. If lines are oversized in a network and can reduce for the optimal case s_nom_opt. 1) deconstructing lines makes no sense (yet) and 2) this can lead to negative objective values.

pypsa-earth/scripts/base_network.py

Lines 353 to 359 in e011735

	def _set_lines_s_nom_from_linetypes(n):
	# Info: n.line_types is a lineregister from pypsa/pandapowers
	n.lines["s_nom"] = (
	np.sqrt(3)
	* n.lines["type"].map(n.line_types.i_nom)
	* n.lines.eval("v_nom * num_parallel")
	)

We should probably add lines as s_nom_min

[pz-max]

@GbotemiB discovered the bug and @davide-f found out about the potential reason

[davide-f]

As a revision, in the mentioned code, it is ok to define it as s_nom, but it is better to specify s_nom_min in prepare_network.
Actually, we have a section of code that should be doing that but it seems not to be working for some reason.

The code is in the set_transmission_limit function here:

pypsa-earth/scripts/prepare_network.py

Line 188 in e4edb17

def set_transmission_limit(n, ll_type, factor, costs, Nyears=1):