Abstract: To achieve energy conservation and reduce production costs, the sintering plant of a steel group company carried out frequency conversion energy-saving retrofitting on the main sintering fan of its 180m² sintering production line. The equipment has a rated power of 5600kW and is equipped with a HARSVERT-VA10/410 sensorless vector control high-voltage frequency converter of equivalent power. To ensure a good operating environment for the high-voltage frequency converter and avoid protection shutdown due to overheating, Leadway also equipped the high-voltage frequency converter with an independent air-water cooling system to solve the equipment heat dissipation problem. The air-water cooling system failed to achieve the goal of stabilizing the ambient temperature effectively. Through on-site analysis, improvements were made to the cooling system. Keywords: Sintering fan, 5600kW high-voltage frequency converter, equipment heat dissipation, air-water cooling system. I. Existing Problems To achieve energy conservation and reduce production costs, the sintering plant of a steel group company carried out frequency conversion energy-saving retrofitting on the main sintering fan of its 180m² sintering production line. The equipment has a rated power of 5600kW and is equipped with a HARSVERT-VA10/410 sensorless vector control high-voltage frequency converter of equivalent power. To ensure a good operating environment for the high-voltage frequency converter and avoid protection shutdown due to overheating, Leadway also equipped the high-voltage frequency converter with an independent air-water cooling system to solve the equipment's heat dissipation problem. After the equipment was installed and put into operation, when the high-voltage frequency converter was at 80% load and using open duct cooling, the equipment operating temperature could be maintained below 76℃ for the transformer and 33℃ for the power cabinet when the outdoor ambient temperature was less than 28℃. However, when using the closed-loop air-water cooling system, the temperature of the high-voltage frequency converter transformer cabinet exceeded 113℃ and the power cabinet temperature reached 38℃. Based on these observations, the air-water cooling system failed to achieve the goal of stabilizing the ambient temperature. Therefore, an on-site investigation and system cause analysis were conducted on the cooling system. The structural principle diagram of the air-water cooling system is shown in Figure 1. II. Cooling System Operating Condition Analysis 1. Equipment Selection Analysis The rated power of the high-voltage frequency converter in this project is 5600kW, with an efficiency of 96%. 4% of the loss is mainly dissipated into the environment as heat. To ensure safe operation, the equipment adopts an advanced, mature, stable, and reliable air-water cooling system. This system features high cooling power, high unit heat exchange efficiency, sealed room, low dust entry, low operating costs, and low maintenance. First, the power selection and allocation of the cooling devices were verified. The maximum heat dissipation power of the high-voltage frequency converter is 5600 kW × 4% = 224kW. Based on the local climate temperature and operating conditions, the design margin of the cooling devices is 1.13. That is, the heat exchange power of the cooling devices should not be less than 253.1kW, and the actual designed installed cooling power is 255kW. The power cabinet is equipped with three 45kW cooling devices, and the transformer cabinet is equipped with two 60kW cooling devices. The cooling system is designed with a total cooling air volume of 100,000 m³/h, including three 20,000 m³/h booster fans used in conjunction with the power cabinet. The power cabinet itself has an effective exhaust volume of 34,400 m³/h from eight 4,300 m³/h fans. The actual cooling system configuration exceeds the ventilation requirements of the power cabinet, meeting operational requirements. The transformer cabinet itself has an effective exhaust volume of 21,500 m³/h from five 4,300 m³/h fans. The actual cooling system configuration exceeds the ventilation requirements of the transformer cabinet, meeting operational requirements. From the above data, it can be seen that the design of the booster fan section of the cooling system fully considers the system's effectiveness and safety. Even if a single device fails in the cabinet top or among the booster fans, the system can still maintain sufficient ventilation efficiency to ensure system stability. Therefore, the selection and proportion of cooling system equipment are normal and there are no problems. 2. Air Path System Analysis To analyze the reasons for the cooling effect problem, actual measurements and data analysis were first conducted on the air path circulation section on-site. This verifies whether the actual air volume, air pressure and other operating indicators of the fan meet the requirements. The wind speed at the entrance of the power cabinet and transformer cabinet, the emergency air duct exhaust port and the indoor exhaust port of the cooling device were measured at multiple points using the wind pressure and wind speed measuring device. The circulating air volume of open ventilation cooling and closed circulation cooling was compared and analyzed by the measured data. (1) When only the top fan of the high voltage inverter cabinet is turned on, the wind speed at the power cabinet door and transformer cabinet door, emergency exhaust port and cooling device exhaust port were measured at multiple points. The average data is as follows: The data shows that when the top fan of the cabinet is operated alone and the closed circulation cooling method is used, the circulation air duct and the air-water cooling device increase the air path resistance and reduce the effective ventilation volume of the equipment. When the open cooling method is running, the air velocity at the cabinet inlet reaches 1.40 or more to meet the ventilation and cooling requirements of the inverter itself. (2) When only the cooling system booster fan is turned on and closed-loop cooling is used, the wind speed test data of the power cabinet door, transformer cabinet door, and cooling device exhaust port are as follows: The data shows that when closed-loop cooling is used alone, its effective ventilation volume and wind speed can meet the cooling air volume requirements of the cabinet itself. (3) When the high-voltage inverter cabinet top fan and the cooling system booster fan are running at the same time, the wind speed test data of the power cabinet door, transformer cabinet door, and cooling device exhaust port are as follows: The data shows that when the high-voltage inverter cabinet top fan and the closed-loop cooling system fan are all turned on and are in normal operation, the cooling ventilation volume of the system fully meets the operating requirements of the high-voltage inverter. According to Bernoulli's equation, the wind speed-pressure relationship is: WP = 0.5•ro•V² (where WP is wind pressure [kN/m²], ro is air density [kg/m³], and V is wind speed [m/s]); flow rate Q = S×V (where Q is airflow [m³/s], S is area [m²], and V is wind speed [m/s]). The effective ventilation volume and wind pressure of the closed-loop cooling system fully meet the cooling airflow requirements of the cabinet itself. Therefore, even with comparable wind pressure and airflow, the difference in the high-voltage inverter's operating temperature is related to the temperature of the cooling air drawn into the high-voltage inverter. If the temperature of the drawn-in circulating air is too high, the expected cooling effect cannot be achieved. Therefore, the cooling system's heat exchange capacity may not meet the design requirements, failing to effectively remove heat from the high-voltage inverter, resulting in excessively high-temperature circulating air returning to the room. 3. Water Circuit Analysis Following an on-site investigation of the cooling system's water circuit, the comparison between the current on-site technical parameters and design requirements is shown in the table below: The data obtained from the table indicates that the actual operating performance of the air-water cooling system on-site has failed to meet the expected requirements. The air-water cooling system achieves heat exchange by having cooling water flow through the exchange pipes and interact with hot air. The heat exchange capacity of the air-water cooling system primarily relies on flow velocity and water volume to achieve heat exchange efficiency and power. When the flow velocity decreases, the exchange efficiency between the cooling water and hot air decreases; conversely, a decrease in water volume prevents the air-water cooling system from achieving the expected heat exchange power. The project design requires an inlet water pressure of 0.25 MPa, a return water pressure of 0.1 MPa, a water temperature of 33℃, and a total cooling water flow rate of 68 t/h. Based on a seamless pipe resistance coefficient of 0.2, at least φ133 pipes are required. However, the actual pipe cross-sectional area used on-site is only 44.4% of the design requirement, far below the operational needs. Small pipe diameter and low pressure are the fundamental reasons why the air-water cooling device did not reach its rated cooling power, causing heat accumulation in the room and resulting in poorer cooling effect for the high-voltage frequency converter under closed conditions compared to open cooling. III. Improvement Measures To improve the cooling water flow rate of the cooling device, achieve the design operating parameters, and increase the heat exchange power of the existing system, it is necessary to improve the water supply pressure and circulation flow rate of the cooling system. Specific measures include: adding a booster pump at the outlet of the branch pipe from the cooling water pump house main pipe to the high-voltage frequency converter cooling system to increase the water supply pressure, ensuring the inlet pressure of the air-cooled device reaches above 0.25MPa, meeting design requirements. Calculations show the booster pump head needs to be greater than 60m. The pipe diameter from the pump house to the cooling system main pipe will be replaced with a φ133 pipe. The water supply flow rate under the existing pipeline conditions will be improved to meet operational needs. IV. Conclusion Through the modifications, the actual ambient temperature and operating temperature of the cooling system under high-voltage frequency converter operating load rates above 80% have been significantly improved. The highest temperature of the high-voltage frequency converter transformer cabinet is 76℃, and the temperature of the power cabinet is 28℃, which fully meets the operating environment requirements of less than 95℃ for the high-voltage frequency converter transformer cabinet and less than 40℃ for the power cabinet. Practice has proven that the analysis of the cooling system's operating conditions is accurate and the improvement measures are effective. In the application of the cooling system, ensuring that all indicators meet or exceed the operating requirements is a crucial factor in whether the system can achieve the expected cooling effect.