code: def check_voltage(grid, mode):
      
  
      crit_nodes = {}
  
      if mode == 'MV':
          # load max. voltage difference for load and feedin case
          mv_max_v_level_lc_diff_normal = float(cfg_ding0.get('mv_routing_tech_constraints',
                                                              'mv_max_v_level_lc_diff_normal'))
          mv_max_v_level_fc_diff_normal = float(cfg_ding0.get('mv_routing_tech_constraints',
                                                              'mv_max_v_level_fc_diff_normal'))
  
          # check nodes' voltages
          voltage_station = grid._station.voltage_res
          for node in grid.graph_nodes_sorted():
              try:
                  # compare node's voltage with max. allowed voltage difference for load and feedin case
                  if (abs(voltage_station[0] - node.voltage_res[0]) > mv_max_v_level_lc_diff_normal) or\
                     (abs(voltage_station[1] - node.voltage_res[1]) > mv_max_v_level_fc_diff_normal):
  
                      crit_nodes[node] = {'node': node,
                                          'v_diff': max([abs(v2-v1) for v1, v2 in zip(node.voltage_res, voltage_station)])}
              except:
                  pass
  
      elif mode == 'LV':
          raise NotImplementedError
  
      if crit_nodes:
          logger.info('==> {} nodes have voltage issues.'.format(len(crit_nodes)))
  
      return [_['node'] for _ in sorted(crit_nodes.values(), key=lambda _: _['v_diff'], reverse=True)]
docs: Checks for voltage stability issues at all nodes for MV or LV grid
  
      Parameters
      ----------
      grid : GridDing0
          Grid identifier.
      mode : str
          Kind of grid ('MV' or 'LV').
  
      Returns
      -------
      :any:`list` of :any:`GridDing0`
          List of critical nodes, sorted descending by voltage difference.
  
      Notes
      -----
          The examination is done in two steps, according to [#]_ :
          
          1. It is checked #TODO: what?
          
          2. #TODO: what's next?
          
      References
      ----------
      .. [#] dena VNS