fix to avoid negative objective value (#961)

* fix to avoid negative objective value

* revert change

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* added rebase voltage for buses

* added release note

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

doc/release_notes.rst

@@ -48,6 +48,8 @@ E.g. if a new rule becomes available describe how to use it `snakemake -j1 run_t
 
 * Create files where the code outputs the value of the objective function. `PR #1033 <https://github.com/pypsa-meets-earth/pypsa-earth/pull/1033>`__
 
+* Minor bug-fixing for s_nom_min. `PR #961 <https://github.com/pypsa-meets-earth/pypsa-earth/pull/961>`__
+
 PyPSA-Earth 0.3.0
 =================
 

scripts/simplify_network.py

@@ -126,8 +126,9 @@ def simplify_network_to_base_voltage(n, linetype, base_voltage):
     are moved from their starting bus to their ending bus. The
     corresponding starting buses are removed as well.
     """
-    logger.info(f"Mapping all network lines onto a single {int(base_voltage)}kV layer")
 
+    logger.info(f"Mapping all network lines onto a single {int(base_voltage)}kV layer")
+    n.buses["v_nom"] = base_voltage
     n.lines["type"] = linetype
     n.lines["v_nom"] = base_voltage
     n.lines["i_nom"] = n.line_types.i_nom[linetype]
@@ -1100,6 +1101,7 @@ def merge_isolated_nodes(n, threshold, aggregation_strategies=dict()):
         "substation_lv",
         "substation_off",
     }.intersection(n.buses.columns)
+
     n.buses = n.buses.drop(buses_c, axis=1)
 
     update_p_nom_max(n)
@@ -1111,6 +1113,7 @@ def merge_isolated_nodes(n, threshold, aggregation_strategies=dict()):
     s_threshold_fetch_isolated = cluster_config.get("s_threshold_fetch_isolated", False)
 
     n = drop_isolated_nodes(n, threshold=p_threshold_drop_isolated)
+
     if p_threshold_merge_isolated:
         n, merged_nodes_map = merge_isolated_nodes(
             n,