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 semiconductors 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-dependent temperature
behavior 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, then the temperature variations must be reduced with more efficient cooling.
In this example the IGBTs and diodes are integrated into 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 dependence 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 on-shore wind power station.
Low frequencies for on-shore wind power stations:
On-shore wind power stations are mostly equipped with double-fed asynchronous generators with a 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 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 be designed with the help of 3D-CFD.