def extend_distribution_substation_overvoltage(network, critical_stations):
    """
    Reinforce MV/LV substations due to voltage issues.

    A parallel standard transformer is installed.

    Parameters
    ----------
    network : :class:`~.grid.network.Network`
    critical_stations : :obj:`dict`
        Dictionary with :class:`~.grid.grids.LVGrid` as key and a
        :pandas:`pandas.DataFrame<dataframe>` with its critical station and
        maximum voltage deviation as value.
        Index of the dataframe is the :class:`~.grid.components.LVStation`
        with over-voltage issues. Columns are 'v_mag_pu' containing the
        maximum voltage deviation as float and 'time_index' containing the
        corresponding time step the over-voltage occured in as
        :pandas:`pandas.Timestamp<timestamp>`.

    Returns
    -------
    Dictionary with lists of added transformers.

    """

    # get parameters for standard transformer
    try:
        standard_transformer = network.equipment_data['lv_trafos'].loc[
            network.config['grid_expansion_standard_equipment'][
                'mv_lv_transformer']]
    except KeyError:
        print('Standard MV/LV transformer is not in equipment list.')

    transformers_changes = {'added': {}}
    for grid in critical_stations.keys():

        # get any transformer to get attributes for new transformer from
        station_transformer = grid.station.transformers[0]

        new_transformer = Transformer(
            id='LVStation_{}_transformer_{}'.format(
                str(grid.station.id), str(len(grid.station.transformers) + 1)),
            geom=station_transformer.geom,
            mv_grid=station_transformer.mv_grid,
            grid=station_transformer.grid,
            voltage_op=station_transformer.voltage_op,
            type=copy.deepcopy(standard_transformer))

        # add standard transformer to station and return value
        grid.station.add_transformer(new_transformer)
        transformers_changes['added'][grid.station] = [new_transformer]

    if transformers_changes['added']:
        logger.debug("==> {} LV station(s) has/have been reinforced ".format(
            str(len(transformers_changes['added']))) +
                    "due to overloading issues.")

    return transformers_changes