def ftp_close(self, connId='default'):
        """
        Closes FTP connection. Returns None.
        Parameters:
        - connId(optional) - connection identifier. By default equals 'default'
        """
        thisConn = self.__getConnection(connId)
        try:
            thisConn.quit()
            self.__removeConnection(connId)
        except Exception as e:
            try:
                thisConn.close()
                self.__removeConnection(connId)
            except ftplib.all_errors as x:
                raise FtpLibraryError(str(x))