def write_ltc_filter_aux_file(file_name) -> str:
    """Helper to save an aux file which creates a branch filter to get
    on load tap changing (LTC) transformers.

    :param file_name: Path to save the .aux file.
    :returns: Name of filter ("ltc_filter").
    """
    # String defining the filter.
    s = \
        """
DATA (Filter, [ObjectType,FilterName,FilterLogic,Number,FilterPre,Enabled,DataMaintainerAssign])
{
"Branch" "ltc_filter" "AND" 1 "NO " "YES" ""
   <SUBDATA Condition>
     LineXFType = "LTC"
   </SUBDATA>
}"""
    with open(file_name, 'w') as f:
        f.write(s)

    return "ltc_filter"