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Frequency converter is an electric energy control device which transforms power frequency power into another frequency by using the on-off function of power semiconductor devices. With the rapid development of modern power electronics and microelectronics technology, the high voltage and high power frequency conversion speed control device has been mature. The high voltage problem which has been difficult to solve has been solved. In recent years, it has been solved by the series of devices or unit series.

In large power electronic equipment, with the increase of temperature, the failure rate also increases. Therefore, the thermal design of high power high voltage converter power device is directly related to the reliability and stability of the equipment. High power high voltage converter often requires high reliability, and the main form of failure of power electronic equipment is thermal failure. According to statistics, more than 50% of the electronic thermal failure is mainly caused by temperature exceeding the rated value. From the aspect of structure design, heat dissipation technology is the key link to ensure the normal operation of equipment. High voltage inverter equipment has high power, generally MW level, which will generate a lot of heat during normal operation. In order to ensure the normal operation of the equipment, it is necessary to transmit a large amount of heat, optimize the heat dissipation and ventilation scheme, design and calculate reasonably, and realize the high-efficiency heat dissipation of the equipment, which is very necessary to improve the reliability of the equipment.


Heat dissipation calculation:


When the high voltage converter is working normally, the heat source is mainly isolated transformer, reactor, power unit, control system, etc. the heat dissipation of power device, the design of power unit and the design of heat dissipation and ventilation of power cabinet are the most important. For IGBT or IGCT power devices, PN junction of IGBT or IGCT should not exceed 125 ℃, and the package shell should be 85 ℃. Some studies show that the temperature fluctuation of components exceeds ± 20 ℃, and the loss efficiency of components increases by 8 times.


Precautions for heat dissipation design:


(1) Select components and materials with good heat resistance and thermal stability to improve the allowable working temperature;


(2) Reduce the heat generated inside the equipment (device). Therefore, we should choose more micro power devices, such as IGBT with low consumption loss, and minimize the number of heating components in circuit design, and optimize the switching frequency of the devices to reduce the heat generation;


(3) Adopt proper cooling method and proper cooling method to reduce the environment temperature and speed up the heat dissipation.


Exhaust air calculation:


Under the worst environment temperature, calculate the minimum wind speed when the maximum temperature of radiator reaches the demand. The exhaust air volume is determined according to the redundant magnification according to the wind speed. The formula of exhaust air volume is qf=q/ (CP) ρ △T)


Where:


QF: the air volume required for forced air cooling system.


Q: Total thermal power consumption of the cooled equipment.


CP = 1005j/ (kg ℃): specific heat of air, j/ (kg ℃).


ρ= 1.11 (m3/kg): air density.


△ t = 10 ℃: temperature difference of air at inlet and outlet.


According to the air volume and air pressure, the fan model is determined, which makes the fan work at the highest efficiency point, which increases the fan life and improves the ventilation efficiency of the equipment.


Air duct design:


The series air duct is formed by the radiator of each power module, which is characterized by the series path formed by several power units, with simple structure and small wind resistance caused by vertical air duct; However, the air heating from the bottom to the top causes the small temperature difference and poor heat dissipation effect of the power unit.


In parallel air duct, the air inlet from the front of each power unit is arranged in parallel with the corresponding air inlet. After being summarized in the rear air bin, the fan shall be drawn out. At the same time, the whole power cabinet generally adopts redundant method, and multiple fans are running in parallel, and the overall heat dissipation effect is good, and the reliability of the equipment is improved. However, the air chamber should be formed behind the cabinet, which increases the volume of equipment. At the same time, due to the different distance between the rear end of each power unit and the fan, the air flow of each power unit is inconsistent, which is the difficulty of design.


According to the characteristics of series air duct and parallel air duct, the high voltage inverter chooses the design of parallel air duct, and forms a unique patent technology of structure.


Simulation analysis:


The simulation software can be used to analyze the heat dissipation, temperature field and the internal fluid motion state of the system efficiently, accurately and simply in different structures and levels. According to the simulation results, the heat dissipation structure is evaluated and modified, and then simulated again until the results meet the requirements. In this way, we have a good control of thermal failure, which greatly improves the reliability and stability of the equipment.


Frequency converter is a kind of equipment which can make the motor run at variable speed and achieve energy saving effect. In general, the motor with rated voltage between 3KV and 10kV is called high voltage motor. Therefore, the inverter developed for motor running in 3KV to 10kV High voltage environment is generally called high voltage converter. Compared with low-voltage inverter, high voltage inverter is suitable for high-power wind power and pump frequency conversion and speed regulation, and can achieve remarkable energy saving effect.


The general heat dissipation method of frequency converter


According to the structure analysis of the current frequency converter, the heat dissipation can be divided into three types: natural heat dissipation, convection heat dissipation, liquid cooling heat dissipation and external environment heat dissipation.


（1） Natural heat dissipation


For small capacity frequency converter, natural heat dissipation mode is generally used, and its use environment shall be well ventilated, and there is no easy to attach dust and floating objects. The driving objects of this kind of frequency converter are mostly air conditioning, CNC machine tools, etc., with small power and good environment.


Another type of converter using natural heat dissipation method does not have to be small, that is explosion-proof converter. For such a small capacity converter, you can choose a general type of radiator, which requires the heat dissipation area to be as large as possible within the allowable range, and the spacing between fins is smaller, so as to increase the heat radiation area as much as possible. For explosion-proof converter with large capacity, it is recommended to use heat pipe radiator if natural heat dissipation is used. Heat pipe radiator is a new type of radiator in recent years. It is a product of combination of heat pipe technology and radiator technology. It has high heat dissipation efficiency, which can make the capacity of explosion-proof converter larger, up to hundreds of kVA. Compared with ordinary radiator, the difference is that the volume is relatively large and the cost is high. Compared with water cooling, this cooling method has advantages: water cooling devices, water-cooled radiators and essential water circulation system are used for water cooling, and the cost is higher than that of using heat pipe radiator. The industry reflects the good performance of heat pipe radiator, and it is worth popularizing.


Another way of natural heat dissipation is "through wall" natural heat dissipation, which can reduce 80% of the heat at most. Its feature is that the main body of the frequency converter is completely isolated from the radiator through the electric control box, which greatly improves the cooling effect of the converter components. The biggest advantage of this heat dissipation method is to clean the radiator regularly and ensure the protection level of the electric control box is higher. As a common cotton spinning enterprise, because of the excessive cotton floc, it is often easy to block the ventilation channel of the frequency converter, which leads to the overheat fault of the frequency converter. It can be solved well by using the wall type natural heat dissipation.


（2） Convective heat dissipation


Convection heat dissipation is a common way of heat dissipation. With the development of semiconductor devices, the radiator of semiconductor devices has also been developed rapidly, which tends to be standardized, serial and universal; The new products are developed to the direction of low thermal resistance, multi-function, small volume and light weight, which is suitable for automatic production and installation. Several major radiator manufacturers in the world, with up to thousands of series, have been tested, and provide the curve of power and heat resistance of radiator, which provides convenience for users to select accurately. At the same time, the development of the cooling fan is also quite fast, showing the characteristics of small volume, long life, low noise, low power consumption, large air volume and high protection. For example, the common low power converter cooling fan is only 25mm × 25mm × 10mm； The long-life fan of Sanyo in Japan can reach 200000h, and the protection level can reach ipx5; There are also large axial flow fans with large air volume in Germany, with a discharge air volume of 5700m3 / h. These factors provide a very broad choice space for designers.


Convection heat dissipation is because the device (fan and radiator) used is easy to choose, and the cost is not too high. The capacity of the converter can be achieved from dozens to hundreds of KVA, even higher (parallel unit method) is widely used.


(1) Cooling of fan installed in frequency converter


The internal fan is generally used for small capacity general frequency converter. By installing the inverter correctly, the heat dissipation capacity of the internal fan of the converter can be maximized. The internal fan can take away the heat inside the converter. The final heat dissipation is carried out through the iron plate of the box where the frequency converter is located. The cooling method of fan installed in the converter is only suitable for control box with separate frequency converter and control box with less control elements. If there are several inverters in the control box of frequency converter, or other electrical components with large heat dissipation, the effect of heat dissipation is not very obvious.


(2) Cooling of fan installed outside converter


By adding several fans with the function of air exchange and convection in the control box where the inverter is installed, the cooling effect of the converter can be greatly improved and the temperature of the working environment of the converter can be reduced. The capacity of fan can be calculated by the heat dissipation of frequency converter. Here is a general choice:


According to the experience, we calculate the heat generated by every 1kW power consumption, the exhaust air volume of the fan is 360m/h, while the power consumption of the converter is 4-5% of its capacity. Here, we can calculate the relationship between the fan and its capacity according to 5%


For example: if the power of the converter is 90 kW, then:


Exhaust air volume of fan (m3/h) = capacity of frequency converter × 5% × 360m/h/kW=1620m/h


Then, the fan that meets our requirements is obtained by selecting the fan model of different manufacturers through the exhaust air volume of the fan. Generally speaking, fan cooling is the main means of frequency converter heat dissipation at present, especially in the case of large control cabinet and the electrical components owned in the control cabinet working and heating simultaneously. It is suitable for highly integrated centralized control cabinet and control box. In recent years, due to the continuous progress of science and technology, the cooling fans are not as large as those in the previous years, and there are many small and powerful fans. Cost performance is also better than other cooling methods.


（3） Liquid cooling


Water cooling is a common way in industrial liquid cooling. For the frequency converter, this device is rarely used for heat dissipation, because of its high cost and large volume when used in small capacity inverter. Moreover, due to the capacity of the general inverter is from several KVA to nearly 100 KVA, the capacity is not very large, so it is difficult to make the cost-effective acceptable to users, This is only used in special occasions (if explosion-proof is required) and frequency converters with a very large capacity.


Water cooled inverter has been used in high power and limited space such as ships and locomotives for nearly ten years in Europe. Compared with the traditional air-cooled converter, the water-cooled converter solves the problem of heat dissipation more effectively, which makes the volume of high-power converter greatly reduced and the performance more stable. The reduction of volume means saving the installation space of equipment, which effectively solves the volume requirements of frequency converter in many special occasions. For example, the 400KW water-cooled inverter of Vacon company of Finland has a volume of only one fifth of the same grade air-cooled inverter.


The results show that the heat dissipation of radiator can be increased by 10-15% under natural cooling condition, 20-30% in forced air cooling condition, and 500-800v after electrophoretic painting. Therefore, when selecting radiator and making processing process, the above-mentioned process treatment will greatly improve the heat dissipation capacity of the radiator, and also enhance the insulation, reduce the adverse effects caused by improper installation of the creepage distance and insufficient electrical clearance.


The heat dissipation effect is closely related to the installation process. The contact area between power module and radiator should be increased as much as possible to reduce the thermal resistance and improve the heat transfer effect. The thermal resistance can be reduced by 25-30% by coating a thin layer of thermal conductive silicone grease between the power device and radiator. If insulation or pad is needed between power device and radiator to facilitate installation, it is recommended to use low thermal resistance materials: Mica, polyester film or copper block, aluminum block. The position of the device on the radiator shall be arranged reasonably. When installing single piece, the device shall be located at the center of the radiator base surface, and the multiple parts shall be evenly distributed during installation. The torque shall be consistent when tightening the device. After installation, it is not suitable to machine the device and radiator, otherwise stress will be generated and thermal resistance will be increased. The single rib radiator is suitable for natural air cooling outside the equipment, which is conducive to the ventilation of power devices and can reduce the temperature inside the machine. When the natural air is cold, the section of the radiator should be parallel to the horizontal plane; When forced air cooling, the flow direction of air flow shall be parallel to the rib direction of radiator


（4） External environment heat dissipation


With the progress of science and technology, air cooling system is gradually applied in steel, cement, petrochemical and other fields. Air cooling system is specially used for high power inverter heat dissipation equipment. Air cooling directly exchanges the hot air of the converter through the air duct through the air-water cooling device, and the heat lost by the converter is directly taken away by the cooling water; The cool air cooled is discharged back to the room. The air-water cooling device can ensure that the temperature of the converter room is controlled below 40 ℃ after the hot air passes through the heat sink, which can meet the requirements of the inverter for environmental operation, thus ensuring the good operation environment in the inverter room. Air cooling system has the characteristics of low energy consumption, high cooling capacity and large air volume.


No matter which heat dissipation method is adopted, the power consumption of the converter shall be determined according to the capacity of the converter, the appropriate fan and the appropriate radiator shall be selected to achieve good cost performance, and the environmental factors used by the frequency converter shall be fully taken into account. In view of the environment is relatively bad (high temperature, high humidity, coal mine, oil field, offshore platform), corresponding measures must be taken to ensure the normal and reliable operation of the frequency converter. From the inverter itself, the adverse factors should be avoided as much as possible, such as sealing treatment can be carried out for the influence of dust and sand. Only the radiator air duct contacts the external air, thus avoiding the influence on the inverter interior; For salt fog, humidity and so on, the insulation spraying treatment can be carried out for each part of the inverter; Frequency converter for field operation shall be protected to prevent rain, sun, fog and dust; For high temperature and high humidity environment, air conditioning and other equipment can be added to cool and dehumidification, and a good environment for the converter can be ensured to ensure reliable operation of the frequency converter. It is shown that the heat dissipation of the converter is handled well not only by the designer, but also by the user in strict accordance with the instructions. There is enough ventilation space and suitable environment, And it is necessary to maintain regularly, especially in coal and other dust industries, to regularly dust the environment and dust the air duct of the converter. In this way, the heat dissipation system of the frequency converter can play a normal function, and the temperature rise of the frequency converter can be within the allowable value, so that the frequency converter can be operated reliably, and bring greater economic and social benefits to the enterprise.