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

1 Introduction

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

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

2 Heat dissipation calculation

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 power devices used as main circuit electronic switches, the heat dissipation design of power units and power cabinets The heat dissipation and ventilation design are the most important. For igbt or igct power devices, the pn junction must not exceed 125°C, and the packaging shell is 85°C. Studies have shown that if the temperature fluctuation of components exceeds ±20°C, their failure rate will increase by 8 times.

2.1 Thermal design considerations

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

(2) Reduce the heat generated inside the equipment (device). For this reason, more micro-power consumption devices should be used, such as low-loss IGBTs, and the number of heating components should be minimized in circuit design. At the same time, the switching frequency of the devices should be optimized to reduce heat generation;

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

2.2 Calculation of exhaust air volume

Under the worst ambient temperature conditions, calculate the minimum wind speed when the maximum temperature of the radiator reaches the demand. The exhaust air volume is determined based on the wind speed and the redundancy amplification ratio. The calculation formula for exhaust air volume is: Qf=Q/(Cp*ρ*△T) where:

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

Q: Total thermal power consumption of the equipment being cooled, W.

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

ρ=1.11(m3/kg): air density, m2/kg.

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

Determine the fan model based on the air volume and wind pressure, so that the fan works at the highest efficiency point, which increases the life of the fan and improves the ventilation efficiency of the equipment.

2.3 Air duct design

The series air duct is composed of the radiator of each power module facing each other up and down, forming an upper and lower corresponding air duct. Its characteristics are that multiple upper and lower power units form a series connection path. The structure is simple, and the air duct is vertical so that the wind resistance is small; but because the air flows from the bottom There is a problem of sequential heating, resulting in a small ambient temperature difference between the power units above and poor heat dissipation effect.

In the parallel air duct, air enters from the front of each power unit, and the corresponding air inlets are arranged in parallel. It is collected in the air silo at the back and extracted 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 silo must be formed behind the cabinet, which increases the size 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 rate of each power unit is inconsistent, which is a difficulty in the design.

Based on the characteristics of series air ducts and parallel air ducts, Sunwoele Company's high-voltage VFD (Variable-frequency Drive) chose parallel air duct design and formed a unique structural patent technology.

3 Simulation analysis

Using simulation software, you can