1. Cooling design of the frequency converter for a wind power station
Dipl.-Ing Karim Segond

2. Agenda Frequency converter and generator IGBT power module
Thermal management
Heat transfer
Materials
Design
Low frequency load of the IGBT
Temperature cycles of the IGBT
1D thermal equivalent circuit: Cauer and Foster models
3D thermal and flow calculation of the IGBT module with FloEFD

3. Frequency converter & generator
Frequency converter & generator Field of application of frequency converter
Photovoltaic
Wind power stations
Electric cars
Locomotives
Gas power plants
Feedwater pump drive trains …

4. Frequency converter & generator Components & function
Rectifier, intermediate circuit, inverter
Integrated rectifier and IGBT power modules
Function
Modulation of amplitudes and frequencies of AC-Voltage:
Continuous speed control of drive train
Synchronisation for supply to the network

5. Frequency converter & generator Wind power application

6. Frequency converter Wind turbine generator
Generator type
Double-fed asynchronous generator with slip-ring rotor
2/ 3 of the installed on-shore plants
Design of the frequency converter
Only for slip power
For about ½ of the total power
Challenges
Minimal load change due to wind variations
Low frequency load of the power modules

7. IGBT power module Manufacturer Semikron Power P=372 kW Size
106x61x30 mm

8. IGBT power module Insulated Gate Bipolar Transistor
Electronics switch ON/OFF
Characteristics
IGBT Chip is made of Silicon
High block voltage rigidity due to i- Layer
Production from losses due to the switching
Limited dynamic characteristics

9. IGBT power module Operating mode
Feed of a gate voltage (GE)
Flood of the loading equipment
Formation of a conductive channel

10. Thermal management Heat transfer Conduction
Thermal conductivity λ [W/mK]
Thermal resistance Rth [K/W]
Convection
Free and forced convection
Heat transfer coefficient α [W/m²K]
Heat radiation
Emissivity coefficient of the surface ɛ [ - ]

11. Thermal management Heat radiation
radiation heat at max.
Chip temperature
P(ɛ) ≈ 140 mW/cm² ≈ 1/1000 * Pv
 negligible

12. Thermal management Materials
Layer
Materials λ
[W/mK]
Chip
Si, SiC
124
Solder
SnAgCU 57
Baseplate Cu
390
Insulator
Ceramic 24
Heatsink Al
235

13. Thermal management Materials: TIM
The thermal interface material is placed between the base plate and the heat sink.
Without TIM λ=0.026 [W/mK] (air) With TIM λ= [W/mK]
Best practise: thermal resistance measurement!, see DynTIM Webinars

14. Thermal management Design
High current density and small sizes require an incessant improvement of the cooling
Cooling mediums
Air, watter or heat pipes
Theses cooling methods can be calculated with FloEFD!
Cooling methods
Avoiding or reducing losses
Good conductivity of the materials
Thermal paths as short as possible
Spreading of the heat
Air cooling with heat sinks
Use of fans

15. Low frequency load of the IGBT
Thermal management
Low frequency load of the IGBT
Thermal challenge
Change of load at low frequencies
Large temperature difference
Strong thermo-mechanical load of the materials
Reduction of the life expectancy
Service required more often

16. Temperature cycles of the IGBT
Thermal management
Temperature cycles of the IGBT
The life expectancy depends on the temperature cycles
Excessive temperature variations lead to material deterioration
Weakest part is the solder
A temperature difference of 25K corresponds to a one milion cycles, which is only one year operation!

17. Temperature cycles of the IGBT
Thermal management
Temperature cycles of the IGBT
The reliability of power electronic components can be characterised by testing them at the limits of their operation
Power Tester 1500A from MicReD is commercially avalaible

18. 1D thermal equivalent circuit Cauer and Foster models
Thermal management
1D thermal equivalent circuit
Cauer and Foster models
Advantages
Acceptable approximation of the temperatures
Can be used in mathematical tools or electric circuit simulator
Fast
Disadvantages
Only one dimensional
For large thermal model, it gets complicated and the precision decreases

19. Thermal management Cauer model
Analogy to the electrical circuit
Each section is represented by a temparature node
Simple model of the thermal-flow path
Coupling of the thermal characteristics

20. Karim Segond www.e-cooling.de
3D thermal and flow simulation of the IGBT power module of a wind power generator with FloEFD
Karim Segond

21. Agenda CAD geometry Boundary conditions materials volume flow
50 Hz operation
Input of the losses
Results of the flow calculation
Results of the thermal calculation
Comparison results vs. tests
Low frequency operation
Summary

22. CAD-Geometry Frequency converter
Diode modul
IGBT power modul
Fans
Heatsink
CAD Geometry with the courtesy from Semikron

23. CAD-Geometry IGBT power module with heatsink
CAD Geometry free of mistakes required, rarely available …

24. CAD-Geometry Components of the IGBT module
TIM

25. Boundary conditions Materials

26. 50 Hz operation / Set-Up

27. Boundary conditions Materials

28. Boundary conditions Pressure drop vs. volum flow
Curves with the courtesy from Semikron

29. 50 Hz operation Stationary calculations
Same fan and volume flow for all four cases
Change of the IGBT and diode losses
Losses are homogeneously spread in the chips of the IGBT and diodes
In the following plots the losses correspond to a an IGBT loss of 57.5 W per chip.

30. 50 Hz operation The mesh

31. 50 Hz operation Free convection in the casing
Very low velocities ; v = 0.01…0.02 m/s

32. 50 Hz operation Streamlines
Flow velocities on the heat sink are the highest
Very low pressure losses  high flow efficiency

33. 50 Hz operation Chip Temperature
T = 60…100°C
Temperature differences due to the pre-heating

34. 50 Hz operation Heat sink
Temperature distribution ; T = 50…90°C

35. 50 Hz operation Heat sink

36. 50 Hz operation Streamlines colored with the temperatures
Heating of the air
Part of the air is not used for cooling

37. Pv_IGBT [W] Pv_Diode T_Mess [°C] T_Sim 23 17 46.5 49.69 57.5 40 75
50 Hz operation Comparison calculation vs. tests
Pv_IGBT
[W]
Pv_Diode
T_Mess
[°C]
T_Sim 23 17
46.5
49.69
57.5 40 75
83.75
43.5 34
65.4
72.28
40.5
35.5 67
73.03
Reasons for differences
Thermal characteristics of the TIM material
Not precise temperature measurement
Inlet volume flow fixed (instead of fan curve dependant)

38. Low frequency operation
FloEFD transient option switched on
Input of the losses:
For f = 10 Hz and f = 0.1Hz
Input of the losses for 10 Hz

39. Low frequency operation Surface temperatures (10 Hz and 0.1 Hz)
Video frequency is real frequency

40. Low frequency operation Air Temperature in casing for 10 Hz
Video frequency is real frequency

41. Low frequency operation IGBT temperatures for 0.1Hz
Calculation is converged

42. Summary Time and cost reduction thanks to FloEFD
FloEFD suited for stastionary as well as transient calculations
Optimisation of the design
Good thermal input about TIM required
Short period of training for new users
CAD- Geometry is required for the simulation