Design calculation based on thermal simulation analysis of high voltage VFD (Variable-frequency Drive) unit

1 Introduction

Large-scale power electronic equipment, such as high-power high-voltage VFD (Variable-frequency Drive), often requires extremely high reliability. The main form of failure affecting power electronic equipment is thermal failure. According to statistics, more than 50% of electronic thermal failures are mainly caused by the temperature exceeding the rated value. As the temperature increases, the failure rate also increases. Therefore, the thermal design of high-power high-voltage VFD (Variable-frequency Drive) power devices is directly related to the reliability and stability of the equipment.

From the perspective of structural design, heat dissipation technology is the key link to ensure the normal operation of equipment. Since Sunwoele's high-voltage VFD (Variable-frequency Drive) equipment has a large power, generally MW level, it will generate a lot of heat during normal operation. In order to ensure the normal operation of the equipment, it is very necessary to dissipate a large amount of heat, optimize the heat dissipation and ventilation scheme, carry out reasonable design and calculation, and realize the efficient heat dissipation of the equipment to improve the reliability of the equipment.

2 Calculation of heat dissipation

When the high-voltage VFD (Variable-frequency Drive) is working normally, the heat sources are mainly isolation transformers, reactors, power units, control systems, etc. Among them, the heat dissipation of the power devices as the electronic switches of the main circuit, the heat dissipation design of the power unit, and the heat dissipation and ventilation design of the power cabinet are the most important. For igbt or igct power devices, the pn junction must not exceed 125°C, and the package shell is 85°C. Studies have shown that if the temperature fluctuation of components exceeds ±20°C, the failure rate will increase by 8 times.

2.1 Heat dissipation design considerations

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

(2) Reduce the heat generation inside the equipment (device). For this reason, more micro-power consumption devices should be selected, such as low-loss igbts, and the number of heating components should be reduced as much as possible in circuit design, and the switching frequency of devices should be optimized to reduce heat generation;

(3) Adopt appropriate heat dissipation methods and appropriate cooling methods to reduce the ambient temperature and speed up heat dissipation.

2.2 Calculation of exhaust air volume

In the case of the worst ambient temperature, calculate the minimum wind speed when the maximum temperature of the radiator reaches the requirement. The exhaust air volume is determined according to the redundant amplification ratio according to the wind speed. The calculation formula of exhaust air volume is: Qf=Q/(Cp*ρ*△T) where:

Qf: The air volume required by the forced air cooling system.

Q: The total thermal power consumption of the cooled equipment, W.

Cp=1005J/(kg*℃): air specific heat, J/(kg*℃).

ρ=1.11(m3/kg): air density, m2/kg.

△T=10℃: The temperature difference between the inlet and outlet air, in ℃.

The fan model is determined according to the air volume and wind pressure, so that the fan works at the highest efficiency point, which not only increases the life of the fan but also improves the ventilation efficiency of the equipment.

2.3 Air duct design

The series air duct is formed by the radiators of each power module facing each other up and down to form a corresponding air duct up and down. It is characterized by a series of passages formed by multiple power units up and down. The structure is simple, and the air duct is vertical so that the wind resistance is small. However, due to the problem of sequential heating of the air from bottom to top, the upper power unit has a small ambient temperature difference and poor heat dissipation.

In the parallel air duct, the air enters from the front of each power unit, and the corresponding air inlets are arranged in parallel. After being collected in the rear air chamber, it is drawn out by the fans. At the same time, the entire power cabinet generally adopts a redundant method, with multiple fans running in parallel. The overall heat dissipation effect is good, and the reliability of the equipment is improved. However, an air chamber is formed behind the cabinet, which increases the volume of the equipment. At the same time, due to the different distances from the rear end of each power unit to the fan, the air flow of each power unit is inconsistent, which is a difficult point in the design.

According to the characteristics of series air ducts and parallel air ducts, Sunwoele's high-voltage VFD (Variable-frequency Drive) selects the parallel air duct design and forms a unique structural patent technology.

3 Simulation analysis

The simulation software can be used to