Merge pull request #2217 from retoflow/fix/unbalanced_powerflow_trafo…

…_results

fix key error in unbalanced powerflow if trafo index is not consecutive or not starting at zero

pandapower/results_branch.py

@@ -341,7 +341,7 @@ def _get_trafo_results_3ph(net, ppc0, ppc1, ppc2, I012_f, V012_f, I012_t, V012_t
     if len(gap_trafo_index > 0):
         for i_trafo in gap_trafo_index:
             Iabc_sum = [0, 0, 0]
-            lv_bus = net.trafo.lv_bus[i_trafo]
+            lv_bus = net.trafo.lv_bus.iat[i_trafo]
             V_bus_abc = np.array([[net.res_bus_3ph['vm_a_pu'][lv_bus] * net.bus['vn_kv'][lv_bus]],
                                   [net.res_bus_3ph['vm_b_pu'][lv_bus] * net.bus['vn_kv'][lv_bus]],
                                   [net.res_bus_3ph['vm_c_pu'][lv_bus] * net.bus['vn_kv'][lv_bus]]])