An Improved Design for a Heat Sink of a Power Conversion System Adapted to the High Altitude and Cold Environment of the Plateau

Currently, there are many studies on power conversion system (PCS) in the industry, but there are few studies on high-altitude and plateau application scenarios. This paper takes the extreme environment of plateau and high altitude as the research background, uses PLECS software to establish a thermoelectric simulation of a three-phase LCL grid-connected inverter, and calculates the total heat loss of the power devices IGBT and diodes in the PCS and the power loss of the grid-connected filter. Combining the overall heat generation of IGBTs, diodes and filters and considering the characteristics of high-altitude extreme environments, two liquid-cooled radiators with different structures were designed to measure the thermal resistance and pressure loss of the two radiators (reflecting Flow resistance) and heat dissipation efficiency are quantitatively calculated. The results show that the heat dissipation performance of the liquid cooling radiator designed by the latter is better than that of the former, verifying that by improving the channel structure and size of the liquid cooling radiator, the thermal resistance and flow resistance of the radiator itself can be reduced, and its heat dissipation performance can be improved.


Introduction
In recent years, with the rapid development of new energy and high-power power electronic devices, the market demand for energy storage converters (PCS) has gradually increased, and the global energy storage converter market has become fiercely competitive.When many companies design high-power energy storage converters, they first consider safety, stability and efficiency, followed by economy.There are many factors that affect safety, stability, and efficiency, among which the design of the heat dissipation system of the energy storage converter (PCS) has become a key issue.The quality of the cooling system design directly affects whether the converter can work safely and stably in the long term [1].The reason is that the heat source in the converter is mainly the power switching device, and these power switching devices are relatively sensitive to temperature rise.If the heat generated by the heat source cannot be dissipated in time, excessive temperature rise will reduce the performance of the power device.Performance and lifespan.In addition, the alternating thermal stress generated between the various layers of materials in the IGBT module will cause the solder layer to fatigue and fall off.Literature [2] analyzes the thermal resistance change rules of the IGBT module under different shedding degrees of the solder layer, establishes an improved thermal network model of the IGBT module that considers thermal resistance changes, and combines the calculation results of the improved thermal network model with the three-dimensional finite element temperature field simulation distribution results A comparison was made to verify the feasibility and effectiveness of the improved thermal network model.Literature [3] analyzes the device aging mechanism through IGBT module aging experiments, and studies the impact of IGBT module junction temperature on the electrical performance, reliability and life of the module, laying the foundation for converter status monitoring and long-term stable and reliable operation.In practical applications, most IGBTs use N-channel IGBTs that are combined with PNP transistors and N-channel MOSFETs.The maximum operating junction temperature of this IGBT module in the switching state is generally 150 degrees.Once the temperature exceeds the junction temperature of the power device, Temperature may cause the power device to permanently fail, ultimately reducing the reliability and life of the energy storage converter.
What is more noteworthy is that the application scenarios of energy storage converters are gradually increasing, but there are few studies on application scenarios in extreme environments such as plateau and high altitude.Because the air density and air pressure in high altitude plateau areas are affected by altitude, their air density and air pressure will decrease, the temperature will also decrease.Obviously, the air-cooled radiator can no longer meet the heat dissipation needs of the converter [4].Facing this extreme environment of high plateau and high altitude, there is an urgent need to design a liquid-cooled PCS radiator that can meet the needs of high-altitude and cold areas.
Facing the extreme environments of plateau, cold and high altitude, this article designs two different structures of liquid cooling radiators, and analyzes the key factors affecting the heat dissipation performance of liquid cooling radiators.The latter is achieved by improving the channel structure of the liquid cooling radiator and optimize the liquid cooling system to reduce flow resistance to achieve higher heat dissipation.The results show that by improving and optimizing the flow channel structure of the former liquid cooling radiator, its heat dissipation efficiency is significantly improved, verifying the feasibility and significance of the improvement method.

Analysis of PCS Switch Time Sequence
In order to adapt to the high-altitude, cold and weather-resistant environment of Xizang, the design of high-power converters should pay more attention to stability and high efficiency.The inverter design in this article selects the NPC three-level topology.Compared with the traditional two-level inverter, this inverter topology type has smaller voltage stress on the switching tube, less switching loss, and also has a multi-level circuit.The advantage is that the output waveform is closer to a sine wave and the highorder harmonic components are low, which is beneficial to the design of the filter [5].In order to avoid increasing the complexity of control, the inverter topology with higher level number is not adopted in this paper.
For high-power power electronic converters with high integration, air-cooled heat dissipation cannot guarantee complete sealing in plateau environments.Wind sand and salt spray can easily cause equipment aging.The thin air caused by low air pressure will reduce the heat dissipation efficiency of air cooling.Therefore, Liquid cooling radiator is selected for heat dissipation of the converter.
In order to study a liquid-cooled radiator with high sealing, high weather resistance, long life, low cost, and high reliability, it is necessary to calculate and analyze the heat effect of power electronic converter, so that the heat dissipation capacity of the radiator can be matched.Figure 1 is the main circuit diagram of the NPC three-level inverter used in this article.During the analysis process, the loss of the weak current control circuit is ignored, and the loss of the main power loop is mainly analyzed.The losses include the loss of active components and passive components.Component losses [6].The losses of active components, i.e.IGBTs, generally include on-state losses, off-state losses and switching losses.Turn-off leakage current loss is generally very small and can be ignored, while switching loss and conduction loss are the main parts of IGBT losses.Passive components include resistors, capacitors, and inductors in the circuit, among which inductors generate the most heat.is the conduction and reverse recovery loss of the diode.The leakage current loss when the IGBT is turned off is very small and can be ignored here.
The IGBT losses studied in this article are analyzed below.The PCS studied in this article works in grid-connected mode, with a total rated power of 100kW, a single-phase rated current of 150A, and a lithium iron phosphate battery with a nominal voltage of 800V on the DC side.The operating condition is 80% SOC and its voltage is 860V, the single capacitor voltage of the split capacitor is 430V, the current outer loop is used as the control target, the SPWM modulation method is adopted, and the switching frequency is 15kHz.In this paper, the IGBT switching timing is analyzed by taking a threephase bridge arm as an example (as shown in figure 2) in rated operating mode, assuming control has reached steady state.The conduction of the IGBT switches T1, T2, T3, and T4 of a single bridge arm is represented by 1, and the turn-off is represented by 0. The output high level is +1.The output low level is -1, and the output level 0 is 0. Hex is its hexadecimal expression, and the true values of all switch states can be listed as shown in table 1.
In each power frequency cycle at rated power, calculate the losses of 4 IGBTs.The voltage and current waveforms of IGBTs are shown in figure 3. From the symmetry of SPWM, we only need to calculate the loss of a single tube in one power frequency cycle to calculate the loss of a bridge arm.
under SPWM modulation.The envelope of the duty cycle of a single IGBT in each power frequency cycle is a sine wave in half the cycle, and remains off in the other half cycle.The voltage and current of the four IGBTs on a single bridge arm are shown in figure 4. At a switching frequency of 15kHz, calculations and experiments show that there are a total of 150 turn-on processes and 150 turn-off processes.Taking into account the tail current of the IGBT and the turn-on and turn-off time, the loss expressions for a single turn-on and turn-off can be calculated.as follows: The detailed waveforms of the tailing current and voltage are shown figure 4. According to the law of SPWM modulation, in a power frequency, the first half of the power frequency cycle of the IGBT turn-on process includes from 1100 to 0110, 0110 to 1100, and the second half of the power frequency cycle includes 0011 to 0110, which includes from 1100 In the process of reaching the intermediate transition state, the reverse recovery loss of the diode generally occurs in the intermediate transition state.The reverse recovery process of the diode and the turn-on and turn-off time of the IGBT require detailed IGBT data query.According to the requirements of the liquid-cooled PCS designed in this article, Infineon's F3L150R07W2E 3_B11 was selected.This is a single-tube 650V, 150A three-level module., integrated with 2 clamping diodes.

Power Loss Analysis and Simulation
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Establish an NPC three-level thermoelectric coupling simulation topology in the power electronics simulation software PLECS (as shown in figure 5).The gray part in figure 5 is used to simulate heat generation and dissipation for the IGBTs and clamping diodes in the three bridge arms.The radiator and the outside world (temperature 25°C) dissipate heat through three-level thermal resistance and heat capacity, respectively simulating the heat transfer process from IGBT to aluminum substrate, aluminum substrate and then to the radiator.The waveform in figure 6 is the power loss of the IGBT in the three three-phase bridge arms.The waveform in figure 7 is the power loss of the six clamping diodes.Figure 8 is the total power loss added together.The heat loss is shown in watts as follows.Figure 6, 7, and 8 above all show the instantaneous heating power after the converter reaches steady state, and the heat dissipation power of the radiator is an equilibrium value.Therefore, it is necessary to calculate the power frequency of the IGBT and clamping diode in the three-level circuit at a power frequency.The overall average power loss during the cycle.The period average calculation formula is as follows.
Where, y(t) is the output signal.u( )  is the continuous input signal.T is the average time specified.
n is the number of current cycles.

Liquid Cooling Radiator Design
The above article uses PLECS software to build a simulation model of IGBT and diode heat loss, and solves the total power loss of the heating power component IGBT and diode, with an average value of 837W within a power frequency cycle.Two liquid-cooling radiators with different sizes and channels are designed for the total heat generation of IGBT and diode, which are marked as liquid-cooling radiator A and B respectively.The liquid-cooling radiator A flow channel is designed as three series zigzag flow channels (as shown in figure 9). Figure 10 shows the combined model of liquid cooled radiator A and IGBT.The liquid-cooling radiator B flow channel is designed as an integrated serpentine flow channel (as shown in figure 12). Figure 11 shows the appearance of liquid cooled radiator B. Figure 13 shows the combination of liquid cooled radiator B and IGBT.The three series back-shaped flow channels are external flow channels, and the aluminum plate covering them is fastened with screws and fitted to the IGBT module to achieve heat dissipation.The integrated serpentine flow channel is an internal flow channel, that is, a long serpentine flow channel is dug deep inside the aluminum plate.The flow channel runs through the entire radiator and can evenly dissipate heat to the entire IGBT module.

Thermal Resistance Calculation
Thermal resistance represents the resistance encountered by heat conduction along the heat flow path, and is a comprehensive parameter that reflects the ability to prevent heat transfer [7].Before calculating and analyzing the thermal resistance of the radiator, the radiator is simplified and it is considered that there are two forms of convection heat transfer thermal resistance and table heat conduction thermal resistance.Define conv R and cond R as the radiator convection thermal resistance and tabletop thermal conductivity thermal resistance respectively, A is the countertop area ( d AL =), then we have: The Reynolds numbers adopted in this article are all less than 2300, so the flow and temperature in the channel can be regarded as fully developed laminar flow, and the flow pressure loss coefficient and Nusselt number have the following relationship.Where, h is the heat transfer coefficient.(10) Where, 1  is the channel aspect ratio.d is the width of the radiator.C is a constant. is the heat sink duty cycle.A is the bottom area of the channel.

2
A the area of the side wall of the channel.

3
A is the area of the upper wall of the channel.N is the number of channels.
Through the analysis of equation ( 1), it can be seen that when the size and material of the radiator are selected, the value of cond R is constant, and the thermal resistance of the radiator will be determined by conv R .The smaller conv R is, the greater the proportion of the average temperature rise of the fluid to the average temperature rise of the radiator table, and the higher the efficiency of the radiator.The corresponding smaller the total temperature rise, the better the heat dissipation effect.Now quantitatively calculate the thermal resistance of liquid cooling radiators A and B, and compare the thermal resistances of the two to verify that the heat dissipation efficiency of radiator B is better than that of radiator A. Use SOLIDWORKS software to accurately measure the model parameters, and consult the fluid and solid thermal conductivity manual tables to obtain the specific parameters of radiators A and B, as shown in table 2. Through the parameters in table 2 and combining equations ( 8), ( 4), ( 6), (7), and (3), the thermal resistance of liquid cooling radiator A is approximately 0.235 /W  , The thermal resistance of liquid cooling radiator B is about 0.109 /W  , It can be seen that the thermal resistance of radiator A is twice as much as that of radiator B. Under the same heat loss, the heat transfer amount of A accounts for about 46.4% of that of B, which means that the total temperature rise of A is larger.Therefore, the heat dissipation efficiency of liquid cooling radiator B is better.

Flow Resistance Analysis
The geometry and size of the flow channel will directly affect the flow state and velocity distribution of the fluid.Flow channels of different geometric shapes will produce different resistances to the fluid, which is called flow resistance.For example, excessive narrowing and curved shapes in the flow channel will lead to increased flow resistance [8].The increase in flow resistance will be accompanied by an increase in flow velocity.When the flow velocity increases to a certain level, not only cannot the heat dissipation performance be continuously improved, but it will also cause circulating water pump noise and consume large amounts of energy.In addition, the viscosity of the fluid will also affect the flow resistance.Fluids with greater viscosity will produce greater internal friction resistance when flowing in the flow channel, which will lead to an increase in flow resistance.Therefore, the fluid must be fully considered during the flow channel design process.viscosity.Generally speaking, the fluid flow rate will also have a certain impact on the flow resistance.The greater the fluid flow rate in the flow channel, the greater the flow resistance.It is not that the greater the fluid flow rate, the better.Therefore, the fluid flow rate needs to be controlled within a reasonable range during the flow channel design process.
According to the heat transfer theory, the channel flow Reynolds number e R , pressure loss p  and heat dissipation efficiency are respectively.
Where, m V is the average flow velocity of the channel.V is the volume flow rate of the coolant.Using the PLECS software, the IGBT module heat loss model was established and the grid-connected filter was added.The simulation time was set to one cycle, and the total power loss of the IGBT, diode and filter was calculated to be 2074W.By consulting the liquid parameter manual table, the specific heat capacity of 40% ethylene glycol aqueous solution at -20°C is 3.334 kJ / kg K , the density is 1071.98 3 kg / m , and the viscosity is 15.75 mPa s .After testing, it is found that the temperature difference between the inlet and outlet of the circulating coolant is about 3.59°C in the steady state.The fluid flow rate is calculated by formula (14) to be 9.7 L / min .
Since the flow resistance is proportional to the dynamic pressure loss, the flow resistance can be reflected laterally by calculating the dynamic pressure loss.According to equation (15), it can be seen that the channel pressure drop loss is related to the pressure loss coefficient and fluid flow rate, in which the fluid density, channel hydraulic diameter and radiator length are fixed values, and the pressure loss coefficient is related to the Reynolds number as shown in equation (5).According to the specific parameter table of the radiator above combined with equation ( 5), equation (13), and equation (15), the results related to the radiator can be solved, as shown in table 3. It can be seen from the calculation results in table 3 that the pressure loss of radiator A is greater than that of radiator B, that is, the flow resistance of radiator A is greater than that of radiator B. Combined with the thermal resistance calculation results above, the thermal resistance of radiator A is the heat resistance of radiator B. Block twice as much.The conclusion shows that the heat dissipation performance of radiator A is not as good as that of radiator B. The calculation results of the heat dissipation efficiency of the two in the above table also verify the correctness of the conclusion.

Conclusion
This article introduces the NPC three-level topology, analyzes its basic working principle and working state transformation in depth, and summarizes the advantages of the three-level topology.In addition, taking a bridge arm as an example, the SPWM modulation method is used to analyze the switching timing of the IGBT within a power frequency cycle.An electrical-thermal simulation of an LCL gridconnected inverter with NPC three-level main circuit was established in PLCES, and the total power loss of all IGBTs, clamping diodes and grid-connected filters in the three bridge arms was calculated.In order to solve the problem of heat loss and heat dissipation caused by the inverter, two radiators with different structures were designed.Considering that the PCS application scenario is in plateau and highaltitude areas, the PCS heat dissipation is designed as a liquid cooling radiator, and the coolant is a 40% ethylene glycol aqueous solution.The mixed solution has low freezing point characteristics, that is, it is not easy to freeze under high altitude and low temperature conditions.
In addition, the models of the two types of liquid cooling radiators designed were simplified, their dimensions were accurately measured, and their channel flow rate, thermal resistance, pressure loss and heat dissipation efficiency were quantitatively calculated based on heat transfer and fluid mechanics theories.Comparing the heat dissipation performance of the two based on the calculation results proves that the heat dissipation efficiency of the radiator designed in the latter is higher than that of the former.It shows that improving the channel of the radiator can improve its heat dissipation performance.
.1088/1742-6596/2731/1/012033 4 SPWM modulation uses double-layer reverse carrier modulation.Within a power frequency period of 0.02s, the driving waveform of the switch tube of a single bridge arm is shown in figure3.

Figure 4 .
Figure 4. Voltage and Current of IGBTs.

Figure 7 .
Figure 7. Power loss of diodes.Figure 8. Total loss of IGBTs and diodes.

Figure 8 .
Figure 7. Power loss of diodes.Figure 8. Total loss of IGBTs and diodes.

Figure 9 .
Figure 9. Liquid cooling radiator A.Figure 10.Liquid cooling radiator A and IGBT combination.

Figure 11 .
Figure 11.Appearance of liquid cooling radiator B.

Figure 12 .
Figure 12.Anatomy of internal channel of liquid cooling radiator B.

Figure 13 .
Figure 13.Liquid cooling radiator B and IGBT combination.

sA
is the total effective convection heat transfer area.fin  is the derivative coefficient of the radiator material.A is the countertop area.b is the thickness of the table surface in contact with the fluid.G is a constant.e R is Reynolds number.Nu is the Nusselt number.f is the EPES-2023 Journal of Physics: Conference Series 2731 (2024) 012033 IOP Publishing doi:10.1088/1742-6596/2731/1/0120338 pressure loss coefficient.f is the fluid thermal conductivity.h D is the hydraulic diameter of the channel.D is the channel height, c b is the channel width.There is the following relationship between the overall dimensions of the radiator and the channel parameters.

cA
is the fluid volume flow rate. is the kinematic viscosity of the fluid.L is the length of the radiator.vQ is the coolant flow rate.P is the total heat loss of the power device.p C is the specific heat capacity of the coolant.T  is the coolant temperature rise. is the fluid density. f
Note: The coolants of liquid cooling radiators A and B are both a mixture of ethylene glycol and water, of which ethylene glycol accounts for 40%.The radiator materials are all aluminum.