I. Introduction High-voltage frequency converters (VFDs) require extremely high reliability in industrial production fields such as power, chemical, coal mining, and metallurgy. Several factors affect the reliability of VFDs, among which heat dissipation and ventilation are crucial aspects of the design process. Currently, VFDs come in various types, including high-low-high, direct series connection, neutral-point clamped multilevel, and cascaded unit types. Generally, the efficiency of these types of VFDs can reach 96-98%; however, due to their high power, they still generate a significant amount of heat during normal operation. To ensure the normal operation of the equipment and dissipate this heat, optimizing heat dissipation and ventilation schemes, and conducting reasonable design and calculations to achieve efficient heat dissipation are essential for improving equipment reliability. High-voltage VFDs have a large power output, with 4% of their power loss primarily dissipated as heat in the operating environment. If the operating temperature of the VFD room is not addressed promptly and effectively, it will directly jeopardize the safe operation of the VFD itself; ultimately, excessive temperature will cause the VFD's overheat protection to trip. To ensure a good operating environment for the frequency converter, measures must be taken to control the temperature of both the frequency converter and the operating environment. II. Cooling Methods Based on accumulated experience in frequency converter engineering applications, complete frequency converter cooling system solutions are provided for different application environments. Commonly used cooling methods include: (1) Open duct cooling; (2) Air-conditioning closed cooling; (3) Air-water cooling closed cooling; (4) Water cooling of the equipment body; (5) Combination of the above methods. 1. Open Duct Cooling 1.1 Cooling Process Cool air enters the frequency converter through the ventilation inlet filter in the frequency converter room. After cooling the unit, the hot air is discharged from the frequency converter's duct outlet. 1.2 Installation Method Open duct cooling installation is relatively simple. Just open two ventilation inlets on the wall of the inverter room, install filters, and then extend the air outlet duct outwards from the top shroud of the inverter cabinet, as shown in Figure 1 below: 1.3 System Features (1) Simple construction, high maintenance; (2) Low cost; (3) Operational stability depends on the local environment 2. Air Conditioning Closed Cooling 2.1 Capacity Selection Principle Select the air conditioner capacity according to the heat output of the inverter and the usable area of ​​the control room. 2.2 Installation Method When installing air conditioning in the inverter room, the space of the inverter control room should be as small as possible and well-sealed to avoid the heating effect caused by high outdoor temperatures in summer. The air conditioner can be installed on both sides of the inverter according to the actual site conditions. The specific equipment layout is shown in Figure 2 below. 2.3 System Features (1) Rapid and efficient cooling (2) Child lock function to prevent misoperation (3) Wide-angle air supply, uniform and comfortable room temperature (4) Anti-cold air design, comfortable air supply (5) Independent dehumidification (6) Low temperature and low voltage start-up (7) Outdoor unit withstands high temperature operation (8) Indoor closed cooling (9) Cooling power has a margin 3. Air-water closed cooling 3.1 Basic principle The air-water cooling system is a high-efficiency, environmentally friendly and energy-saving cooling system, and its application technology is in a leading position in China. Its appearance and principle are shown in Figure 3 below: The hot air from the inverter is connected to the radiator with fixed water cooling pipes through the air duct. The radiator is filled with cold water with a temperature lower than 33℃. After the hot air passes through the heat sink, it transfers heat to the cold water and becomes cold air blown out from the heat sink. The heat is carried away by the circulating cooling water, ensuring that the ambient temperature in the inverter control room does not exceed 40℃. The air cooler must be installed in a closed environment. In order to improve the cooling effect, the space for the equipment should be as small as possible. The water flowing into the air cooler is industrial circulating water. To protect the equipment, the circulating water must have a neutral pH and be free of impurities that could corrode or damage copper and iron. The inlet water pressure is generally 0.2–0.3 MPa, and the inlet water temperature is ≤33℃. Air cooler maintenance is simple and easy, typically performed every six months, including flushing the cooling pipes. 3.2 Construction and Installation A single system consists of one frequency converter and two air coolers; a single unit failure will not significantly impact the system. The specific equipment layout is shown in Figures 4 and 5. The air cooler can be installed on the front or back of the frequency converter, depending on the actual site conditions. If the on-site water supply is insufficient, an independent cooling water system can be provided to adapt to the site conditions. This solution adds an independent mechanical cooling tower, water treatment device, and booster pump station to the existing cooling system to meet the independent cooling requirements of the on-site equipment frequency conversion retrofit project. It also reserves a certain amount of cooling water for future equipment additions to the on-site industrial cooling water system. 3.3 Safety Performance Evaluation The equipment is installed outside the wall of the high-voltage frequency conversion distribution room. It is directly connected to the exhaust port on the top of the frequency converter cabinet through the air duct, which improves the equipment operating efficiency of the cooler and can directly cool the hot air discharged from the frequency converter. At the same time, it avoids the serious accident that could occur if the cooling water pipeline is laid out in the high-voltage room and leaks water, endangering the operation safety of the high-voltage equipment. In the design of the air-cooling system, in order to prevent the condensation of cold air on the outlet side of the air cooler from being discharged into the room, the air outlet, wind speed and other indicators of the air cooler are designed and calculated to ensure safe and stable operation under good pressure. In addition, in order to prevent water from leaking into the room after the air cooler leaks, a water spray plate is installed on the outlet side of the air cooler; when there is leakage or water accumulation, it can be directly discharged to the outside. The complete cooling system solution effectively reduces the failure rate of the auxiliary system and the degree of impact on the operation safety of the main equipment. 3.4 System Features (1) The equipment is simple and quick to install. (2) The equipment has a long service life, low failure rate and reliable performance. (3) The operating cost of the equipment is 1/3 to 1/4 times that of an air conditioner with the same heat exchange power. (4) The indoor airtight cooling system is clean and hygienic; the frequency converter requires low maintenance. (5) No filter cleaning is required. (6) Ventilation cooling can be used in case of an accident without affecting equipment safety. (7) The cooling power has a margin . 4. Water cooling of the equipment body From the perspective of heat dissipation, water cooling is ideal. However, the water circulation system has high process requirements, complex installation, and a large amount of maintenance work. Moreover, once water leaks, it will bring safety hazards. Therefore, water cooling should not be used in situations where air cooling can solve the problem. 5. Comparison of Cooling System Indicators and Operating Costs The comparison of indicators and operating costs for various cooling methods is shown in the table below: Cooling Method | Open Duct | Air Conditioning | Closed-Loop | Air-Water Cooling | Body Water Cooling Equipment | Low | High | High | Very High | Cooling Effect | Average | Good | Good | Good | Applicable Range | ≤1600kW | ≤1000kW | ≤6000kW | >6000kW | Applicable Region | 0-35℃ | No Requirements | With Cooling Water | No Requirements | Applicable Environment | Clean | Dust | High | Dust | No Requirements | Temperature Control | Local Temperature | 22-35℃ | 20-35℃ | 20-30℃ | Maintenance | High | Low | Small | Small | Small | Operating Cost | Low | Very High | Low | High | Operating Cost (RMB/kW) | 0.0291 | 0.178 | 0.0458 | 0.089 III. Conclusion Based on factors such as regional differences (north and south), temperature, humidity, and equipment safety, adhering to the service concept of "the most suitable is the best," we provide perfect high-voltage variable frequency cooling system solutions according to the actual needs of users, striving for optimal system performance, safety, stability, and high economy. In high-power and ultra-high-power high-voltage frequency converters above 1600kW, the air-water cooling system still possesses strong adaptability and technical advantages. The 2×3400kW induced draft fan frequency converter project at a power plant in Shanxi Province fully demonstrated that the completely sealed air-water cooling method resulted in excellent operational performance and high safety, showcasing the advantages of this cooling method in high-power and ultra-high-power product applications. It significantly improved the safety and stability of the product, while saving substantial secondary energy consumption and maintenance costs, reflecting the project's excellent overall economic benefits. In conclusion, developing and selecting new, efficient heat dissipation technologies for cooling high-voltage frequency converters is an important measure to improve equipment reliability.