1. 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 95-97%; however, due to their high power, typically in the MW range, 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. During normal operation, the heat sources of VFDs mainly include isolation transformers, reactors, power units, and control systems. Among these, the heat dissipation of power devices (main circuit electronic switches), the heat dissipation design of power units, and the heat dissipation and ventilation design of power cabinets are the most important. 2. Heat Dissipation Design of Power Devices For IGBT or IGCT modules, the PN junction temperature should not exceed 125℃, and the package temperature should be 85℃. Studies have shown that if the temperature fluctuation of a component exceeds ±20℃, its failure rate will increase by 8 times. The heat dissipation design of power devices is related to the operational safety of the entire equipment. 2.1 Matters to be noted when designing the heat dissipation of power devices (1) Select components and materials with good heat resistance and thermal stability to increase their allowable operating temperature; (2) Reduce the heat generation inside the equipment (device). To this end, more low-power devices, such as low-loss IGBTs, should be selected, and the number of heat-generating components should be reduced as much as possible in the circuit design. At the same time, the switching frequency of the device should be optimized to reduce heat generation; (3) Use appropriate heat dissipation methods and appropriate cooling methods to reduce the ambient temperature and accelerate the heat dissipation speed. Taking the most common unit cascaded high-voltage frequency converter as an example, the heat design of one of the power units is carried out. The power device uses IGBT, and its circuit is shown in Figure 1. 2.2 Estimation of power loss During steady-state operation of the equipment, the total power loss of the rectifier diodes, IGBTs and freewheeling diodes in the power unit is the power dissipation of the heat sink. Therefore, the first step in thermal design is to estimate the total power consumption of the above devices. Figure 1 Power unit circuit diagram (1) The power loss of IGBT generally includes on-state loss, off-state loss, turn-on loss, turn-off loss and drive loss. When estimating, the on-state loss, turn-on loss and turn-off loss are mainly considered; On-state loss of each IGBT: Switching loss of each IGBT: (2) For the freewheeling diode, the on-state loss and turn-off loss are mainly estimated; On-state loss: Turn-off loss: (3) The power loss of the rectifier diode under low frequency conditions is mainly the on-state loss. A simple way to determine its on-state power consumption is to directly find the relationship curve between the on-state power loss and the average on-state current given by the manufacturer. The total power consumption of the above power unit is: p = (pss + psw) × 4 + pd × 6 (5) 2.3 Calculation of junction temperature under steady state The calculation of junction temperature is based on the simplified thermal resistance equivalent circuit shown in Figure 2. The simplified thermal resistance equivalent circuit of the above power unit is shown in Figure 2. Figure 2 Thermal resistance equivalent circuit diagram of IGBT In Figure 2: rθ(jc) is the steady-state thermal resistance from the junction of the device to the reference point of the casing, which is provided by the manufacturer. Generally, the upper limit value is given in the datasheet or the transient thermal resistance curve is given as the steady-state value of t→∞; rθ(ca) is the thermal resistance of the casing directly to the air without passing through the heat sink, which is usually not considered; rθ(cs) is the contact thermal resistance from the casing to the heat sink, which is usually given by the manufacturer in the datasheet; rθ(ca) is the thermal resistance from the reference point of the heat sink to the reference point of the environment, and its value is determined by the type, size and cooling method of the heat sink; ta is the ambient temperature. (1) Static thermal resistance for the thermal resistance of the device under thermal equilibrium conditions: 2) Transient thermal resistance Since power electronic devices operate in a periodic switching state, it is necessary to consider whether the junction temperature fluctuation caused by transient thermal resistance exceeds the maximum junction temperature. Transient thermal resistance reflects the combined effect of thermal resistance and heat capacity of the heat carrier in the heat dissipation path. Transient thermal resistance can be obtained by the following formula: The average and maximum junction temperatures under periodic pulse power consumption loads can usually be calculated by referring to the transient thermal resistance curves given by the manufacturer. Figure 3 shows the transient thermal resistance curve of the IGBT module of EUPC model BSM400GA120DLC: zthjc=f(t). Figure 3 Transient thermal resistance curve of IGBT module (3) Calculation of junction temperature under steady state By analyzing the above method, all power losses of the entire power unit are obtained, and then the junction temperature of the power electronic device or the thermal resistance of the heat sink is calculated according to the following formula. At the same time, when calculating the thermal resistance, the fluctuation of power loss and the fluctuation of load should be considered; that is, while considering the average junction temperature, the amplitude of its fluctuation should be considered. Under normal circumstances, the highest junction temperature under given conditions must not exceed its maximum rated value of 150℃, and a 5℃ margin should be considered when calculating the steady-state junction temperature. [b]3. Heat Dissipation and Cooling Design of Power Units[/b] The components in the power unit mainly include rectifier diodes, IGBT (or IGCT) modules, capacitors, fast-acting fuses, bus switch drive circuits, and other protection circuits. Except for the diode rectifier modules and IGBT (IGCT) modules, the other components are installed in the power unit through brackets, etc., and their heat dissipation requirements are met as long as sufficient space and necessary slight air convection are ensured. Therefore, the cooling design of the power unit mainly considers the heat dissipation requirements of the diode rectifier modules and IGBT (IGCT) modules. The temperature rise caused by the power dissipation of power devices needs to be reduced by heat sinks. Heat sinks increase the thermal conductivity and radiation area of ​​power devices, expand heat flow, and buffer the thermal conduction transition process, directly conducting or transferring heat to the cooling medium, such as air, water, or a mixture of water, through thermal conduction. Currently, the commonly used cooling methods in high-voltage frequency converters are forced air cooling, circulating water cooling, and heat pipe radiator cooling. 3.1 Forced Air Cooling The radiator used for forced air cooling is usually a good thermal conductor with many blades. The formula for estimating the thermal resistance (r(sa)) of the radiator is: In formula (9): k is the thermal conductivity of the radiator; d and a are the thickness and area of ​​the radiator, respectively, expressed in cm and cm2; c is a correction factor related to the surface of the radiator and the installation angle. This formula is valid when the air temperature does not exceed 45℃. It is worth noting that the manufacturing process of the radiator will affect its thermal conductivity. For example, cast aluminum alloy, extruded or brazed radiators should be considered separately. At the same time, when selecting a radiator, the following should be considered: the thickness of the radiator root should meet the heat conduction requirements; the number of fins and the corrugations should not generate too much fluid resistance while ensuring the maximum heat dissipation area; the ratio between the height and thickness of the fins should be reasonable. If a large margin for heat dissipation is to be ensured, increasing the length of the radiator is a better choice. 3.2 Circulating Water Cooling for High-Voltage Frequency Converters: Circulating water cooling significantly improves heat dissipation efficiency, resulting in a smaller volume per unit power and a substantial reduction in overall system size. Compared to forced air cooling, the temperature difference between the radiator surface and the fluid is smaller, which can increase power output and reduce chip temperature, extending their lifespan. However, circulating water cooling requires water circulation and treatment equipment, increasing system complexity. When using this method, to prevent rust and freezing caused by pure water, a mixture of water and alcohol is generally used. The mixing ratio affects the thermal resistance of the coolant; a 50% mixing ratio typically increases the thermal resistance by 50%. Under normal circumstances, the water flow rate should be no less than 8 liters per minute. In hot and humid environments, due to the high relative humidity, condensation can occur on water-cooled radiators when the cooling surface temperature is below the dew point, potentially causing insulation damage to components. Therefore, water-cooled high-voltage frequency converters have stricter environmental requirements. Water typically freezes at 0℃. According to standards, the rated temperature difference is 5℃, therefore the operating temperature should not be lower than 5℃; simultaneously, the relative humidity should be ≤90% (at 25℃), and the relative humidity change rate should be ≤5%/h. 3.3 Heat Pipe Radiators Heat pipe radiators are boiling radiators that use water or other heat transfer fluids as the cooling medium, sealed within copper tubes with capillary structures. The heat generated by the power devices is conducted to the fluid through the radiator. After vaporization, the fluid diffuses throughout the copper tube, dissipates heat through the heat sink fins, cools into water, and then flows back to the heat-absorbing surface. Heat pipe radiators have advantages such as strong heat transfer capacity, excellent temperature uniformity, variable heat density, no external equipment required, reliable operation, simple structure, light weight, and maintenance-free operation. They are generally suitable for high-power, discrete component applications. In some special production conditions, such as dusty environments (coal mines, coking plants, some chemical plants), heat pipe radiators can be used because they can achieve complete sealing of the power conversion section. The domestic power electronic converter industry has been using heat pipe radiators for many years. For example, the power rectifier cabinet of the DF4 electric transmission diesel locomotive has been upgraded to use heat pipes instead of the original pure aluminum heat sink; Shanghai Weiteli Welding Equipment Manufacturing Co., Ltd. uses heat pipe heat sinks for cooling IGBTs and diodes in every inverter welding machine with an voltage rating of 400A or higher. However, high-voltage frequency converters using heat pipe cooling have not yet been observed. Considering the above-mentioned cooling methods, heat pipe cooling should be the preferred option. 3.4 Other Precautions Regardless of the cooling method used, the installation position of components on the heat sink should be carefully considered. The following points should be noted regarding the layout of devices on the heat sink: (1) The thermal resistance is lowest at the center of the heat sink; (2) When multiple power devices are installed on the same heat sink, the installation position should be determined based on the loss of each device, and the device that generates large losses should be given the largest area; (3) The surface of the heat sink on which the module is installed should be kept within 100 for flatness between screw positions and below 10 for surface roughness. If there are depressions on the surface, it will directly lead to an increase in contact thermal resistance; (4) In order to reduce the contact thermal resistance, a heat dissipation insulating mixture should be evenly applied between the heat sink and the mounting surface of the power component, and a suitable tightening torque should be applied so that the contact thermal resistance between the device shell and the heat sink does not exceed the value required by the datasheet. [b]4. Heat dissipation and ventilation design of the whole machine[/b] The common cooling methods of high voltage frequency converters are forced air cooling of the heat sink, circulating water cooling and heat pipe cooling. Forced air cooling is generally preferred due to its simplicity, absence of condensation issues associated with water cooling, and the complexity of heat pipe radiator design, provided a suitable ventilation structure is determined. When using forced air cooling, the heat dissipation duct needs to be considered during structural design. The design of the heat dissipation duct should be optimized as much as possible while fully considering the heat dissipation requirements of each unit. Common multi-level series-connected high-voltage frequency converters are structurally divided into power cabinets, transformer cabinets, and control cabinets. Power cabinet air duct design typically employs two methods: series air ducts and parallel air ducts. 4.1 Series Air Ducts Series air ducts consist of radiators for each power unit positioned vertically opposite each other, forming corresponding air ducts. Their characteristics include multiple power units forming a series path, a simple structure, and vertical air ducts resulting in low air resistance; however, due to the sequential heating of air from bottom to top, the temperature difference in the upper power units is small, leading to poor heat dissipation. Its structure is shown in Figure 4. Figure 4 Power Cabinet Air Duct Structure Diagram 4.2 Parallel Air Duct In a parallel air duct, air enters from the front of each power unit, with corresponding air inlets arranged in parallel. The air is collected in the rear air chamber and then extracted by a fan. The entire power cabinet generally employs redundancy, with multiple fans operating in parallel, resulting in good overall heat dissipation and improved equipment reliability. However, the air chamber at the rear of the cabinet increases the equipment's size. Furthermore, the varying distances from the rear of each power unit to the fan cause inconsistent airflow rates for each unit, which should be considered during the design phase. 4.3 Selection of Cooling Fan The entire power section uses forced air cooling, requiring a continuous flow of sufficient air at ambient temperature across the radiator surface to achieve thermal equilibrium at a certain temperature. Under stable equilibrium conditions, the convective heat transfer coefficient h of the heat-absorbing medium can be calculated using the formula: p = h × a × Δt, given the system dissipation power p, the effective surface area of ​​the radiator a, and the temperature difference between the radiator surface and the ambient temperature Δt. The United States and Japan stipulate that fan noise should not exceed 65 dB, therefore their specified air velocity is 2-4 m/s. Thus, when considering fan selection, an air velocity of 3-6 m/s should be ensured for the air-cooled heat sink of power semiconductor devices; this generally guarantees that the requirements are met. 5. Conclusion Currently, most high-voltage frequency converters use forced air cooling. However, due to the advantages of water cooling and heat pipe cooling, such as small size, high efficiency, and no pollution, design concepts should be updated and their promotion should be vigorously pursued. In conclusion, developing and selecting new, efficient heat dissipation technologies for cooling high-voltage frequency converters is an important measure to improve equipment reliability and reduce equipment size.