Fix DC junctions in simplify_network

config/config.default.yaml

@@ -1105,6 +1105,7 @@ clustering:
     remove_stubs_across_borders: false
   cluster_network:
     algorithm: kmeans
+    allow_ac_dc_mix: true
     hac_features:
     - wnd100m
     - influx_direct

config/schema.default.json

@@ -565,6 +565,11 @@
               ],
               "type": "string"
             },
+            "allow_ac_dc_mix": {
+              "default": true,
+              "description": "Controls whether clustering is allowed to mix AC and DC buses within a 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": {
@@ -5949,6 +5954,11 @@
           ],
           "type": "string"
         },
+        "allow_ac_dc_mix": {
+          "default": true,
+          "description": "Controls whether clustering is allowed to mix AC and DC buses within a 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": {
@@ -12002,6 +12012,11 @@
               ],
               "type": "string"
             },
+            "allow_ac_dc_mix": {
+              "default": true,
+              "description": "Controls whether clustering is allowed to mix AC and DC buses within a 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": {

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).
+
 * The lockfile update workflow now excludes packages published within the last 7 days to reduce the risk of pulling in broken or yanked releases (https://github.com/PyPSA/pypsa-eur/pull/2130).
 
 * The industry reference year and the ammonia production data have been updated to 2023 (https://github.com/PyPSA/pypsa-eur/pull/2103)

scripts/cluster_network.py

@@ -431,6 +431,33 @@ def clustering_for_n_clusters(
     return clustering
 
 
+def apply_carrier_mixing_policy(
+    n: pypsa.Network, busmap: pd.Series, allow_ac_dc_mix: bool
+) -> pd.Series:
+    """Apply carrier mixing policy to a busmap before clustering."""
+    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_mix:
+        if len(mixed_clusters):
+            logger.warning(
+                "`allow_ac_dc_mix` 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 +543,7 @@ 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 +613,11 @@ def update_bus_coordinates(
     admin_regions["y"] = admin_regions["poi"].y
 
     busmap_df = pd.DataFrame(busmap)
+    busmap_df["admin"] = busmap_df["busmap"].astype(str).str.split("::", n=1).str[0]
     busmap_df = pd.merge(
         busmap_df,
         admin_regions[["x", "y"]],
-        left_on="busmap",
+        left_on="admin",
         right_index=True,
         how="left",
     )
@@ -603,7 +631,7 @@ 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 +714,9 @@ def update_bus_coordinates(
                 features=features,
             )
 
+        allow_ac_dc_mix = params.cluster_network.get("allow_ac_dc_mix", False)
+        busmap = apply_carrier_mixing_policy(n, busmap, allow_ac_dc_mix)
+
         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_mix: bool = Field(
+        True,
+        description="Controls whether clustering is allowed to mix AC and DC buses within a 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)
             )