Abstract : A water-making agent was screened and modified for evaluation. The modified agent replaced the original imported water-making agent in the deionization tank. Three years of operation results showed that the modified cooling water production system operated well, and the water production effect was superior to the technical indicators of the imported agent. The time required for the conductivity of the cooling medium (deionized water) to decrease from 0.601 μS/cm to 0.200 μS/cm was shortened from 60 min to 20-30 min, and the system operating cycle was extended from 72 h to 120 h. Keywords : Medium-voltage frequency converter; Cooling water production system; Localization; Natural gas; Deionized water; Conductivity In 2001, after the fertilizer plant of Lanzhou Petrochemical Company of China National Petroleum Corporation changed its feedstock from residual oil to natural gas, in order to meet production and power demands, the company purchased a SIMOVERT-MV type medium-voltage frequency converter (3575 kW) from Siemens, Germany, to control the compressor motor. This frequency converter is a key piece of equipment ensuring the stable operation of the ammonia synthesis plant, and it is currently the only water-cooled frequency converter in China (other similar equipment uses gas-cooled circulation). The cooling water system used for cooling the circuit boards is the core unit of this equipment. This system uses deionized water with a conductivity of 0.2-0.6 μS/cm to cool the circuit boards. During equipment operation, once the conductivity of the deionized water exceeds 0.6 μS/cm, the circulation bypass valve of the water system automatically opens to forcibly purify the deionized water. When the conductivity of the deionized water drops to 0.2 μS/cm, the bypass valve automatically closes, and the system enters normal cooling circulation. Before the technical upgrade, the deionized water was forcibly purified once every 72 hours, taking 1 hour each time. Since the cooling medium (deionized water) is in direct contact with the 4400 kV operating voltage inside the equipment, its conductivity directly determines the safety of the equipment. If the conductivity of the deionized water in the system exceeds 1.5 μS/cm (the safety limit), it will cause the inverter control system to trip, resulting in a complete shutdown of the ammonia synthesis unit. More seriously, when the conductivity of the deionized water exceeds 1.5 μS/cm, the insulation layer of the water circulation cooling system will be damaged, leading to short circuits, equipment breakdowns, and even thyristor explosions. [b][align=center]For more details, please click: Localization of Medium-Voltage Inverter Cooling Water System Technology[/align][/b]