In recent years, the country's railway industry has accelerated development and achieved gratifying results, especially in the field of high-speed trains. In order to further reach the level of rail transit technology in developed countries, there are still many technologies that need further research.

Due to the unparalleled advantages of traditional internal combustion engine or steam engine traction, electric traction has advantages such as high power, fast speed, high energy utilization efficiency, and low environmental pollution. Nowadays, the traction systems of diesel locomotives, electric locomotives, and high-speed trains all use electric transmission. At present, there are two types of traction transmission methods for electric drive: (1) AC-DC transmission method. In this transmission method, the main function of the traction inverter is to rectifier the AC power through a four quadrant rectifier to output DC power. By adjusting the DC voltage, each DC motor is controlled, which belongs to voltage type speed regulation. (2) In the AC-DC-AC transmission mode, the main function of the traction converter is to rectifier the AC power through a four quadrant rectifier and output DC power. The DC power is then output by the inverter into AC power with adjustable voltage and frequency to control each AC motor, which belongs to variable frequency speed regulation. The characteristics of DC motors are: commutation problem with current, large volume, heavy weight, short service life, consumption of more non-ferrous metals, complex equipment, high cost, and difficult maintenance and repair during operation. The speed improvement space of trains using DC motors for speed regulation is limited. The characteristics of AC motors are: small size, small weight, long service life, simple structure, reliable operation, relatively low cost, and simple maintenance and repair. Due to the use of AC motor traction, trains have fast speed regulation and can achieve higher speeds. Therefore, high-speed trains often use AC-DC-AC speed regulation for traction transmission.

 

 

     

 

                                          

                                                 

         

 

                                                                                                                                                                                   

 

 

 

 

 

 

In AC-DC-AC speed regulation, the main electrical component used in traction inverters is IGBT. IGBT is a high-frequency on/off power component that consumes electrical energy during operation. It converts electrical energy into thermal energy. Usually, the current flowing through the IGBT is relatively large, and the switching frequency of the IGBT is also high, resulting in significant energy loss of the device. If the generated heat cannot be dissipated in time, the junction temperature inside the IGBT will exceed the maximum value of 125 ℃, and the IGBT may be damaged. Statistics show that for every 2 ℃ increase in temperature of electronic components, reliability decreases by 10%; The lifespan at a temperature rise of 50 ℃ is only 1/6 of that at a temperature rise of 25 ℃. Therefore, only by quickly and timely dissipating the generated heat can the normal operation of IGBT be maintained.

There are several types of IGBT heat dissipation: air cooling, heat pipe, and liquid cooling. This study mainly investigates the cooling form of liquid (water) without phase change.

IGBT water cooling principle: Water cooling is a closed liquid circulation device that circulates the water ethylene glycol mixture in the cooling system pipeline through the installation substrate of power electronic components. The heat generated by the power module is carried away by the circulating water, and then the water pump pumps this mixture into the air water heat exchanger, dissipating the heat to the surrounding air environment, achieving the cooling effect.

The cooling system structure diagram is shown in Figure 1 below:

 

Figure 1 Structure diagram of IGBT water-cooling system in high-speed train traction inverter

1. Heat sink, 2. Fan, 3. Electronic components, 4. Cooling and heat dissipation substrate (containing medium and micro channels)

 

 

　　The flow of medium inside the cooling and heat dissipation substrate is shown in the following figure.

 

objective

The timely and efficient dissipation of heat generated by IGBT depends on the size of the wall temperature on the contact surface between IGBT and the cold plate, and the smaller the wall temperature, the better, that is, the lower the wall temperature of IGBT, the better.

As we know from Newton's cooling formula,

(1)

Among them,QHeat flow rate for IGBT

hIs the surface heat transfer coefficient

sThe surface area of the contact side between the IGBT and the cooling and heat dissipation substrate

t_wWall temperature on the contact side between IGBT and cooling and heat dissipation substrate

For the temperature of the cooling liquid

Heat flow rateQThe decrease in can causet_wThe decrease in IGBT power, but with constant IGBT power, causest_wThe space for decline is very limited.

surfacesThe increase in product can causet_wThe decrease in surface area is due to limitations in actual object weight, volume space, and the requirements of high-speed trains themselves, resulting in a decrease in surface areasThe increase is very limited, making itt_wThe space for descent is greatly constrained.

The temperature of the cooling liquidThe decrease in can causet_wThe decrease in temperature, but the temperature of the cooling liquid​​​​​​​The decrease in temperature is influenced by regional, climatic, and external temperature changes; Affected by the heat exchange between the heat exchanger and the fan, as well as the heat exchange between the air and the heat; Causing significant fluctuations in the heat dissipation effect of the radiator, which is not conducive to effective reductiont_w。

The surface heat transfer coefficienthImproving can causet_wIt can effectively reduce the decrease without being restricted by other conditionst_w。

So, when surface heat transfer occurshWhen the coefficient is at its maximum,，t_wMinimum, which means the surface wall temperature of IGBT is minimized, making the operation of IGBT safer and more reliable!

Therefore, how to obtain the maximum surface heat transfer coefficient of the cooling and heat dissipation substratehBecoming the key to the problem is also the purpose of this study.

 

Experimental principle:

In order to obtain the surface heat transfer coefficient h, it is necessary to measure the surface area s on the contact side between the heating block (IGBT uses heating blocks instead) and the cooling and heat dissipation substrate, measured in meters ²; Measure the temperature of the heating block; Measure the inlet temperature tw and outlet temperature tfout of the cooling liquid, using

，Obtain the temperature of the coolant, in K; The heating of the heating block is achieved by using a DC stabilized power supply. The current I and voltage U in the circuit are measured using an ammeter and a voltmeter, respectively. Using the formulas P=UI and Q=P, the heating power Q is obtained, in units of W. Finally, apply Newton's cooling formula:

(2) Calculate the surface heat transfer coefficient h.
In order to obtain the maximum surface heat transfer coefficient, we use micro channels for the flow channel of the coolant. The internal structure of the micro channels is shown in Figure 3:

 

Figure 3

 

The experimental system is shown in Figure 4:

Figure 4: Experimental System Diagram

The system mainly includes one constant temperature water tank, one electric water pump, one water purification device and water supply system, one ethylene glycol replenishment system, two DC power systems (including two volt meters, two ammeters, two sliding resistors, two light bulbs, and several wires), two heating blocks, one cooling and heat dissipation substrate (containing media and micro channels), two data acquisition boards, and one computer.