Thermal design of the power electronics at low frequencies

Reliability

IGBTs are exposed to strong thermo-mechanical load variations, which lead to aging, material fatigue abrasion and finally outage. The switch losses and the resulting temperature rises in the IGBT semi-conductors can be considered as constant for frequencies of 50 Hz and more. For lower frequencies the on and off switching is so slow, that it results in a time-dependant temperature characteristics of the chips. The life expectancy of an IGBT type is defined by the number of temperature cycles; this rapidly falls by increased amplitude of the chip temperature variation.
IGBT chips used for the control of the traction motors of metros might experience during their period of use up to 1 to 10 Millions load changes with a temperature variation between 15K and 40K. When neither the chips nor the connections can be upgraded, than the temperature variations must be reduced with a more efficient cooling.

 
Surface temperatures of the IGBTs and diodes for a 0.1 Hz frequency
(video frequency is real frequency), fan on the left

 


Surface temperatures of the IGBTs and diodes for a 1 Hz frequency
(video frequency is real frequency), fan on the left

Calculs & results

In this example the IGBTs and diodes are integrated in a Semikron power-module, which is mounted on a heat sink. The produced losses will be evacuated by forced convection through the cooling fans.

These calculations have been performed with the commercial software FloEFD by switching on the transient option. The time step is one hundredth of the period. The time dependency of the losses is given as input. It is known for such applications that the heat radiation is negligible as the temperatures are too low, therefore it does not need to be simulated. The natural convection is calculated in the casing by switching on the gravitation option of the solver. Thanks to the friendly user interface, the embedment in the CAD tool and a multi-processor solver, the results could be reached quickly.

The calculation domain has been spilt in ½ Million cells for ½ a module. For the calculated worst case with a frequency of 0.1 Hz, the IGBT temperatures vary between and 45 and 60°C. This corresponds to only 1 Milion cycles or one year normal operation of an an on-shore wind power station.

 
Air temperatures in an IGBT-module casing for a  0.1 Hz frequency
(video frequency is real frequency)


Air temperatures in an IGBT-module casing for a  1 Hz frequency
(video frequency is real frequency)

Low frequencies for on-shore wind power stations:
On-shore wind power stations are mostly equipped with double-fed asynchronous generator with slip-ring rotor. The rotor windings are excited with a regulated low frequency current, typically between 0.1 and 10 Hz. This enables the production of a current with the network frequency directly at the generator, independently from the last changes due to changes in the wind speed. The transistors of the frequency converter show strong temperature variations, a more efficient thermal path must therefore be designed with the help of 3D-CFD.

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