Pandapower converter: fix 3-Ph transformer loading Calculation

src/power_grid_model_io/converters/pandapower_converter.py

@@ -2017,12 +2017,19 @@ def _pp_trafos_output_3ph(self):  # pylint: disable=too-many-statements
         # Only derating factor used here. Sn is already being multiplied by parallel
         loading_multiplier = pp_input_transformers["df"] * 1e2
         if self.trafo_loading == "current":
-            ui_from = pgm_output_transformers["i_from"] * pgm_input_transformers["u1"]
-            ui_to = pgm_output_transformers["i_to"] * pgm_input_transformers["u2"]
-            loading_a_percent = np.maximum(ui_from[:, 0], ui_to[:, 0]) / pgm_input_transformers["sn"]
-            loading_b_percent = np.maximum(ui_from[:, 1], ui_to[:, 1]) / pgm_input_transformers["sn"]
-            loading_c_percent = np.maximum(ui_from[:, 2], ui_to[:, 2]) / pgm_input_transformers["sn"]
-            loading = np.maximum(np.sum(ui_from, axis=1), np.sum(ui_to, axis=1)) / pgm_input_transformers["sn"]
+            # since "i_from" and "i_to" are (n, 3) arrays while "u1" and "u2" are (n,) arrays, ValueError is generated
+            # during broadcast
+            ui_from = pgm_output_transformers["i_from"] * pgm_input_transformers["u1"][:, None]
+            ui_to = pgm_output_transformers["i_to"] * pgm_input_transformers["u2"][:, None]
+            # for phase wise loading, sn_ph = sn / 3, v_n = v / sqrt(3), so (i * u / sqrt(3)) / (sn / 3)
+            # ==> sqrt(3) * i * u / sn
+            loading_a_percent = np.sqrt(3) * np.maximum(ui_from[:, 0], ui_to[:, 0]) / pgm_input_transformers["sn"]
+            loading_b_percent = np.sqrt(3) * np.maximum(ui_from[:, 1], ui_to[:, 1]) / pgm_input_transformers["sn"]
+            loading_c_percent = np.sqrt(3) * np.maximum(ui_from[:, 2], ui_to[:, 2]) / pgm_input_transformers["sn"]
+            # for total loading, phase wise powers are be calculated using v_n instead of v.
+            loading = (
+                np.maximum(np.sum(ui_from, axis=1), np.sum(ui_to, axis=1)) / np.sqrt(3)
+            ) / pgm_input_transformers["sn"]
         elif self.trafo_loading == "power":
             loading_a_percent = (
                 np.maximum(