feat: add logic to map_generator_to_bus_by_sub function

prereise/gather/griddata/hifld/data_process/generators.py

@@ -73,10 +73,15 @@ def map_generator_to_sub_by_location(generator, substation_groupby):
 def map_generator_to_bus_by_sub(generator, bus_groupby):
     """Determine a likely bus for a generator to be connected to, based on the bus with
     the lowest voltage within the generator's specified substation.
+
     :param pandas.Series generator: one generating unit from data frame.
     :param pandas.GroupBy bus_groupby: data frame of buses, grouped by substation ID.
+    :return: (*int/pd.NA*) -- bus ID if the generator has a substation ID, else pd.NA.
     """
-    pass
+    if pd.isna(generator.sub_id):
+        return pd.NA
+    else:
+        return bus_groupby.get_group(generator.sub_id)["baseKV"].idxmin()
 
 
 def build_plant(bus, substations):