Abstract: The water-making agent in the cooling system of the medium-pressure inverter of the natural gas compressor was screened out, and the water-making agent in the decoupler tank was replaced. After more than two years of operation following the system upgrade, the results show that all control indicators have met or exceeded the requirements of Siemens Germany for water production in this system, realizing the localization of this technology. Keywords: medium-pressure inverter; cooling system; localization. After the raw material of Lanzhou Petrochemical Company Fertilizer Plant was changed from residual oil to natural gas in 201, its supporting natural gas compressor invested more than 20,000 yuan to purchase a compressor motor (3575kw) from Siemens Germany to control the medium-pressure inverter. This medium-pressure inverter is the only water-circulating cooling inverter in China (other similar equipment uses gas cooling systems for water production), and the cooling water production system is one of the core components of this equipment. Its main function is to cool the circuit board using deionized water with a conductivity in the range of 0.2–0.6 μS/cm. During equipment operation, once the deionized water conductivity exceeds 0.6 μS/cm, the water purification system's circulation bypass valve automatically opens, forcing the deionized water into the deionization tank for purification. When the deionized water conductivity drops to 0.2 μS/cm, the bypass valve automatically closes, and the system enters normal cooling circulation. Before the technology update, the deionized water was forcibly purified once every 72 hours, with each purification taking 1 hour. Since the cooling medium, the deionized water, directly contacts the 44 kV operating voltage inside the equipment, its conductivity directly determines the safety of the equipment's operation. If the conductivity of the deionized water in the system exceeds 1.5 μS/cm, exceeding 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 exceeds 1.5 μS/cm (the time required for 1.0 to rise to 1.5 or higher is very short), the insulation of the water circulation system will be damaged, leading to short circuits, thyristor explosions, and other malfunctions. During normal cooling circulation, changes in ambient temperature, humidity, and the presence of inhalable suspended matter, copper, iron, silica, etc., will cause the conductivity of the deionized water to gradually increase. The system will force the filter media to circulate to bring the conductivity back to a suitable range, thus maintaining the normal operation of the inverter. Therefore, the deionized water purification system in the equipment is crucial for the proper functioning of the entire medium-voltage inverter. Since the frequency converter was put into operation in 2001 until May 2018, it had been running at full load for more than three years. The water-purifying packing in the desiccant tank of the water cooling system had completely aged, the desiccant effect had significantly decreased, the forced circulation time had become increasingly longer, reaching up to 4 hours in severe cases, and alarms were frequently triggered when the conductivity exceeded 1.0 μS/cm, affecting the normal and stable operation of the medium-voltage frequency converter and endangering equipment safety. The desiccant tank of the water purification system is a component supplied by Siemens AG of Germany, and there is no relevant technology available for reference regarding the internal water-purifying packing. Obtaining technology from Siemens would cost 100,000 yuan per maintenance and replacement, and 300,000 yuan per desiccant tank, requiring replacement annually. The high maintenance costs led the fertilizer plant's leadership and technical personnel to decide to upgrade the frequency converter's cooling water purification system using domestically produced technology. 1. Experimental Section Ll Experimental Equipment, Instruments and Materials 1 set of simulated cooling system water production device; 1 atomic absorption spectrophotometer; 1 ultraviolet-visible spectrophotometer; 1 solid suspended solids analyzer; 3 conductivity meters; 2 pH meters; 1 constant temperature shaker; 1 ion chromatograph; 2 metering pumps; 10 kg water production agent; 10 kg modifier LZ Update Technology Based on relevant literature review, after analyzing and studying the role of the medium-voltage inverter cooling system, a high-efficiency water purification agent was selected in the laboratory and then modified. The modified water production agent was loaded into a 1.0L simulation system to evaluate the water production effect. 1.2.1 Water Production Agent Test 1.2.1.1 Water Production Agent Selection Considering the pure water requirements of the medium-voltage inverter cooling system, eight easily modifiable water production agents were selected as research objects. Under the condition that the water production rate and water production volume are the same in each cycle, the water production performance was evaluated by measuring the conductivity of the produced pure water. The test results are shown in Table 1. As shown in Table 1, the pure water produced by Al water-making agent has the lowest conductivity, making it suitable as a water-making filler in the cooling system. 1.2.1.2 Water-making agent modification Although the selected Al water-making agent has good water-making effect, the water production volume is small, which is difficult to meet the requirements of long-term stable operation of the frequency converter. In order to further improve its water production volume, a certain amount of modifier needs to be added to modify Al to enhance the exchange capacity of its active groups and increase the water production volume. The results are shown in Table 2. As can be seen from the experimental data in Table 2, the pure water produced by the modified Al water-making agent has low conductivity and large water production volume. 1.2.2 Simulated cooling water production system test A cooling system water production device was simulated according to the structural characteristics of the pure water device of the medium-voltage frequency converter (process flow is shown in Figure 1). The trend of pure water conductivity change in one cooling system operation cycle is shown in Figure 2. [align=center] Figure 1 Simulated frequency converter cooling system process flow diagram[/align] As can be seen from the data of one operation cycle of the laboratory simulation device in Figure 2, the conductivity of the cooling water fluctuates little and the operation is stable. The experiment consisted of 10 cycles, each lasting 12 hours. Within one cycle, the system medium conductivity increased from 0.200 μS/cm to 0.601 μS/cm, at which point the water production system started and began producing water. Each cycle required 30 minutes of water production, during which the conductivity decreased to 0.200 μS/cm. [align=center]Figure 2: Trend of pure water conductivity during one cooling system operation cycle[/align] 2. Application The screened and modified water production agent was put into application on May 16, 2004, and by May 16, 2006... The system has been running stably for 2 years, and all technical indicators are better than those provided by Siemens AG of Germany. The time required for the conductivity of pure water in the inverter cooling system to decrease from 0.60 μS/cm to 0.200 μS/cm has been shortened to 20-30 minutes, and the system operation time has been extended from 72 hours to 120 hours. The successful localization of this technology has not only saved foreign exchange for the country, but also contributed to the long-term, safe operation and efficiency of the production equipment of Lanzhou Petrochemical Company. 3 Conclusion (1) After the technical upgrade, the medium-voltage inverter cooling water system has been running normally for 2 years and is in good condition. The water production effect of the upgraded system is better than the technical indicators provided by Siemens AG of Germany; (2) The time required for the conductivity of pure water in the upgraded cooling water system to decrease from 0.60 μS/cm to 0.20 μS/cm has been shortened to 20-30 minutes, and the start-up time interval has been greatly extended; (3) The upgraded system has low operating costs, is easy to maintain, and can ensure the long-term safe operation of the fertilizer plant production process.