An aluminum factory has experienced several load shedding as result of faults in 500 kV overhead lines of the electric grid, external to the plant. The load shedding from two of the four potlines of aluminum production, are consequence of the actuation of the protection system against voltages sags installed on the thyristors rectifiers, which under certain conditions considered dangerous, causes the trip of the rectifiers to avoid possible damage to the thyristors due to commutation failure, shedding in this way 360 MW of the total load of the aluminum factory. In order to assess the sensitivity of the protection system to voltage sags caused by faults in the external network, a comprehensive study of the system was performed through digital simulations of failures in several overhead lines, considering different types of faults, locations, scenarios, etc. The parameters and topology of the network are available in detail in the PSS/E program power flow files, but since it is necessary to determine the waveform of the voltage at the plant bus bars, the studies must be performed with a program able to calculate electromagnetic transients such as the ATP. A program was developed to make the conversion of the PSS/E power flow data to ATP format. This paper presents a description of the program, together with the results and conclusions of the study.Instituto de Investigaciones Tecnológicas para Redes y Equipos Eléctricos (IITREE)