1 Introduction
Large power electronic equipment, such as high-power high-voltage frequency converters, often requires extremely high reliability. The main form of failure affecting power electronic equipment is thermal failure. Statistics show that over 50% of electronic thermal failures are caused by temperatures exceeding rated values. As temperature increases, the failure rate also increases. Therefore, the thermal design of power devices in high-power high-voltage frequency converters directly affects the reliability and stability of the equipment.
From a structural design perspective, heat dissipation technology is a crucial aspect of ensuring the normal operation of equipment. Because Sanhuan Company's high-voltage frequency converters have high power, typically in the MW range, they generate a significant amount of heat during normal operation. To ensure the equipment's proper functioning and dissipate this heat effectively, optimizing heat dissipation and ventilation schemes, and conducting reasonable design and calculations to achieve efficient heat dissipation are essential for improving equipment reliability.
2. Heat dissipation calculation
During normal operation, the heat sources of a high-voltage frequency converter mainly include the isolation transformer, reactor, power unit, and control system. Among these, the heat dissipation of the power devices (which act as the main circuit electronic switches), the heat dissipation design of the power units, and the heat dissipation and ventilation design of the power cabinet are the most critical. For IGBT or IGCT power devices, their PN junction temperature must not exceed 125℃, and the package temperature must be 85℃. Studies have shown that component temperature fluctuations exceeding ±20℃ can increase the failure rate by eight times.
2.1 Heat dissipation design considerations
(1) Select components and materials with good heat resistance and thermal stability to improve their allowable operating temperature;
(2) Reduce the heat generated inside the equipment (device). To this end, more low-power devices should be selected, such as low-loss IGBTs, and the number of heat-generating components should be minimized in the circuit design. At the same time, the switching frequency of the devices should be optimized to reduce heat generation.
(3) Use appropriate heat dissipation methods and cooling methods to reduce the ambient temperature and accelerate the heat dissipation rate.
2.2 Calculation of exhaust volume
Calculate the minimum fan speed required for the radiator to reach its maximum temperature under the worst ambient temperature conditions. Determine the exhaust volume based on the fan speed using a redundancy factor. The formula for calculating the exhaust volume is: Qf = Q/(Cp•ρ•△T)
In the formula:
Qf: The air volume required by the forced air cooling system.
Q: Total heat dissipation of the cooled equipment, in 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, in ℃.
Determining the fan model based on air volume and air pressure ensures that the fan operates at its most efficient point, which increases both the fan's lifespan and the equipment's ventilation efficiency.
2.3 Air duct design
The series airflow duct consists of heat sinks of each power module positioned vertically opposite each other, forming corresponding airflow channels. Its characteristics include multiple power units connected in series, a simple structure, and vertical airflow ducts that result in low air resistance. However, due to the sequential heating of air from bottom to top, the upper power units experience a smaller temperature difference, leading to poor heat dissipation. Its structure is shown in Figure 1.
Figure 1. Power cabinet series air duct structure diagram
In the parallel air duct system, air enters from the front of each power unit, with corresponding air inlets arranged in parallel. The air is collected in the rear air chamber and then extracted by a fan. The entire power cabinet typically employs redundancy, with multiple fans operating in parallel, resulting in good overall heat dissipation and improved equipment reliability. However, forming an air chamber at the rear of the cabinet increases the equipment's size. Furthermore, the varying distances from the rear of each power unit to the fan cause inconsistent airflow rates for each unit, posing a design challenge. Its structure is shown in Figure 2.
Figure 2. Parallel air duct structure diagram of power cabinet
Based on the characteristics of series and parallel air ducts, Sanhuan Company's high-voltage frequency converters have adopted a parallel air duct design, which has resulted in a unique structural patent technology.
3. Simulation Analysis
Simulation software can be used to perform efficient, accurate, and convenient quantitative analysis of system heat dissipation, temperature field, and internal fluid motion state at various structures and levels.
Figure 3 Temperature distribution of the power unit heat sink substrate (2 IGBTs)
Based on the simulation results, the heat dissipation structure was evaluated and modified, and then simulated again until the required results were obtained. In this way, we achieved good control over thermal failure, thereby greatly improving the reliability and stability of the equipment.
