Fix DC junctions in simplify_network

config/config.default.yaml

@@ -1110,6 +1110,7 @@ clustering:
     remove_stubs_across_borders: false
   cluster_network:
     algorithm: kmeans
+    allow_ac_dc_mixing_in_bus_clusters: false
     hac_features:
     - wnd100m
     - influx_direct

config/schema.default.json

@@ -565,6 +565,11 @@
               ],
               "type": "string"
             },
+            "allow_ac_dc_mixing_in_bus_clusters": {
+              "default": false,
+              "description": "Controls whether clustering is allowed to mix AC and DC buses within a bus cluster. If true, mixed clusters are coerced to AC before aggregation. If false, mixed clusters are kept separate.",
+              "type": "boolean"
+            },
             "hac_features": {
               "description": "List of meteorological variables contained in the weather data cutout that should be considered for hierarchical clustering.",
               "items": {
@@ -5961,6 +5966,11 @@
           ],
           "type": "string"
         },
+        "allow_ac_dc_mixing_in_bus_clusters": {
+          "default": false,
+          "description": "Controls whether clustering is allowed to mix AC and DC buses within a bus cluster. If true, mixed clusters are coerced to AC before aggregation. If false, mixed clusters are kept separate.",
+          "type": "boolean"
+        },
         "hac_features": {
           "description": "List of meteorological variables contained in the weather data cutout that should be considered for hierarchical clustering.",
           "items": {
@@ -12033,6 +12043,11 @@
               ],
               "type": "string"
             },
+            "allow_ac_dc_mixing_in_bus_clusters": {
+              "default": false,
+              "description": "Controls whether clustering is allowed to mix AC and DC buses within a bus cluster. If true, mixed clusters are coerced to AC before aggregation. If false, mixed clusters are kept separate.",
+              "type": "boolean"
+            },
             "hac_features": {
               "description": "List of meteorological variables contained in the weather data cutout that should be considered for hierarchical clustering.",
               "items": {

config/test/config.clusters.yaml

@@ -29,7 +29,7 @@ clustering:
   temporal:
     resolution_elec: 24h
   simplify_network:
-    remove_stubs: true
+    remove_stubs: false
     remove_stubs_across_borders: false
 
 solving:

doc/release_notes.rst

@@ -8,6 +8,8 @@ Release Notes
 
 .. Upcoming Release
 .. =================
+* Fix: Prevent over-aggressive HVDC simplification in simplify_network for branched/multi-terminal DC topologies (e.g. UK/Shetland edge cases). Supernode detection now only collapses true chain nodes (degree 2) and preserves DC junctions (degree 3+) so branches are not dropped(https://github.com/PyPSA/pypsa-eur/pull/2147).
+
 * Fix: Activate losses for `H2 pipeline retrofitted` links by default, to ensure consistency with `H2 pipeline` links.
 
 * Fix: Re-introduce capital costs for non-bicharging discharge links in ``add_electricity.py``, e.g. fuel cells.

scripts/cluster_network.py

@@ -415,11 +415,6 @@ def clustering_for_n_clusters(
     bus_strategies.setdefault("substation_lv", lambda x: bool(x.sum()))
     bus_strategies.setdefault("substation_off", lambda x: bool(x.sum()))
 
-    # TODO Quick Fix for osm-prebuilt-version 0.6
-    for way_i in ["way/140248154", "way/975637991"]:
-        if way_i in n.buses.index:
-            n.buses.loc[way_i, "carrier"] = "AC"
-
     clustering = get_clustering_from_busmap(
         n,
         busmap,
@@ -431,6 +426,54 @@ def clustering_for_n_clusters(
     return clustering
 
 
+def apply_carrier_mixing_policy(
+    n: pypsa.Network, busmap: pd.Series, allow_ac_dc_mixing_in_bus_clusters: bool
+) -> pd.Series:
+    """
+    Handle AC/DC buses before clustering.
+
+    If ``allow_ac_dc_mixing_in_bus_clusters`` is True, mixed AC/DC clusters are
+    kept as-is. If it is False, buses in mixed clusters are split by appending
+    the carrier to the cluster label, for example ``cluster::AC`` and
+    ``cluster::DC``.
+
+    Parameters
+    ----------
+    n : pypsa.Network
+        Network providing bus carrier information.
+    busmap : pandas.Series
+        Mapping from bus name to cluster label.
+    allow_ac_dc_mixing_in_bus_clusters : bool
+        Whether mixed AC/DC clusters are allowed.
+
+    Returns
+    -------
+    pandas.Series
+        Busmap, possibly with carrier suffixes added.
+    """
+    busmap = busmap.astype(str)
+    carrier_by_bus = n.buses.carrier.reindex(busmap.index).astype(str)
+
+    mixed_clusters = carrier_by_bus.groupby(busmap).nunique().loc[lambda s: s > 1].index
+
+    if allow_ac_dc_mixing_in_bus_clusters:
+        if len(mixed_clusters):
+            logger.warning(
+                "`allow_ac_dc_mixing_in_bus_clusters` is enabled. Coercing bus carrier to AC in %s mixed clusters.",
+                len(mixed_clusters),
+            )
+            mixed_bus_i = busmap.index[busmap.isin(mixed_clusters)]
+            n.buses.loc[mixed_bus_i, "carrier"] = "AC"
+        return busmap
+
+    if len(mixed_clusters):
+        logger.info(
+            "Splitting %s mixed AC/DC clusters by carrier before aggregation.",
+            len(mixed_clusters),
+        )
+    return busmap.str.cat(carrier_by_bus, sep="::")
+
+
 def cluster_regions(
     busmaps: tuple | list, regions: gpd.GeoDataFrame, with_country: bool = False
 ) -> gpd.GeoDataFrame:
@@ -516,7 +559,7 @@ def busmap_for_admin_regions(
             buses_subset.to_crs(epsg=3857),
             admin_regions.loc[admin_regions["country"] == country].to_crs(epsg=3857),
             how="left",
-        )["admin"]
+        )["admin"].astype(str)
 
     return buses["busmap"]
 
@@ -586,10 +629,25 @@ def update_bus_coordinates(
     admin_regions["y"] = admin_regions["poi"].y
 
     busmap_df = pd.DataFrame(busmap)
+
+    # Determine admin for each bus via spatial join of bus coordinates
+    # to the administrative polygons
+    buses_gdf = gpd.GeoDataFrame(
+        n.buses[["x", "y"]].copy(),
+        geometry=gpd.points_from_xy(n.buses["x"], n.buses["y"]),
+        crs=geo_crs,
+    )
+
+    # Find nearest admin region for each bus
+    admin_geo = admin_regions.copy()
+    admin_geo["admin_id"] = admin_geo.index
+    joined = gpd.sjoin_nearest(buses_gdf, admin_geo, how="left")
+    busmap_df["admin"] = joined["admin_id"].astype(str).reindex(busmap_df.index)
+
     busmap_df = pd.merge(
         busmap_df,
         admin_regions[["x", "y"]],
-        left_on="busmap",
+        left_on="admin",
         right_index=True,
         how="left",
     )
@@ -603,7 +661,7 @@ def update_bus_coordinates(
     if "snakemake" not in globals():
         from scripts._helpers import mock_snakemake
 
-        snakemake = mock_snakemake("cluster_network", clusters=60)
+        snakemake = mock_snakemake("cluster_network", clusters=50)
     configure_logging(snakemake)
     set_scenario_config(snakemake)
 
@@ -686,6 +744,14 @@ def update_bus_coordinates(
                 features=features,
             )
 
+        allow_ac_dc_mixing_in_bus_clusters = params.cluster_network[
+            "allow_ac_dc_mixing_in_bus_clusters"
+        ]
+
+        busmap = apply_carrier_mixing_policy(
+            n, busmap, allow_ac_dc_mixing_in_bus_clusters
+        )
+
         clustering = clustering_for_n_clusters(
             n,
             busmap,

scripts/lib/validation/config/clustering.py

@@ -65,6 +65,10 @@ class _ClusterNetworkConfig(BaseModel):
         "kmeans",
         description="Clustering algorithm to use.",
     )
+    allow_ac_dc_mixing_in_bus_clusters: bool = Field(
+        False,
+        description="Controls whether clustering is allowed to mix AC and DC buses within a bus cluster. If true, mixed clusters are coerced to AC before aggregation. If false, mixed clusters are kept separate.",
+    )
     hac_features: list[str] = Field(
         default_factory=lambda: ["wnd100m", "influx_direct"],
         description="List of meteorological variables contained in the weather data cutout that should be considered for hierarchical clustering.",

scripts/simplify_network.py

@@ -135,13 +135,14 @@ def split_links(nodes, added_supernodes):
 
         seen = set()
 
-        # Supernodes are endpoints of links, identified by having lass then two neighbours or being an AC Bus
-        # An example for the latter is if two different links are connected to the same AC bus.
+        # Supernodes are buses that are not simple chain nodes within the component.
+        # A chain node has degree 2 inside the component; endpoints (degree 1),
+        # junctions (degree >=3), and AC buses are kept as supernodes.
         supernodes = {
             m
             for m in nodes
             if (
-                (len(G.adj[m]) < 2 or (set(G.adj[m]) - nodes))
+                (len(set(G.adj[m]) & nodes) != 2)
                 or (n.buses.loc[m, "carrier"] == "AC")
                 or (m in added_supernodes)
             )
@@ -241,9 +242,6 @@ def split_links(nodes, added_supernodes):
 
     _remove_clustered_buses_and_branches(n, busmap)
 
-    # Change carrier type of all added super_nodes to "AC"
-    n.buses.loc[added_supernodes, "carrier"] = "AC"
-
     return n, busmap