Cooling design of the frequency converter for a wind power station Dipl.-Ing Karim Segond www.e-cooling.de

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

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 …

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

Frequency converter & generator Wind power application

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

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

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

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

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 [ - ]

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

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

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=1 -10 [W/mK] Best practise: thermal resistance measurement!, see DynTIM Webinars

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

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

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!

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

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

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

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 www.e-cooling.de

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

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

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

CAD-Geometry Components of the IGBT module TIM

Boundary conditions Materials

50 Hz operation / Set-Up

Boundary conditions Materials

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

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.

50 Hz operation The mesh

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

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

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

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

50 Hz operation Heat sink

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

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)

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

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

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

Low frequency operation IGBT temperatures for 0.1Hz Calculation is converged

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