Thermal characterisation and reliability analysis of power electronic devices in wind and solar energy systems

Batunlu, Canras (2016) Thermal characterisation and reliability analysis of power electronic devices in wind and solar energy systems. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Power electronic converters (PECs) are used for conditioning the flow of energy between renewable energy applications and grid or stand-alone connected loads. Insulated gate bipolar transistors (IGBTs) are critical components used as switching devices in PECs. IGBTs are multi-layered devices made of different coefficient of thermal expansion (CTE) based materials. In wind and solar energy applications, IGBT’s reliability is highly influenced by the operating conditions such as variable wind speed and solar irradiance. Power losses occur in switching transient of high current/voltage which causes temperature fluctuations among the layers of the IGBTs. This is the main stress mechanism which accelerates deterioration and eventual failures among IGBT layers due to the dissimilar CTEs. Therefore, proper thermal monitoring is essential for accurate estimation of PECs reliability and end lifetime. Several thermal models have been proposed in literature, which are not capable of representing accurate temperature profiles among multichip IGBTs. These models are mostly derived from offline modelling approaches which cannot take operating conditions and control mechanisms of PECs into account and unable to represent actual heat path among each chip. This research offers an accurate and powerful electro thermal and reliability monitoring tool for such devices. Three-dimensional finite element (FE) IGBT models are implemented using COMSOL, by considering complex heat interactions among each layer. Based on the obtained thermal characteristics, electro thermal and thermo mechanical models were developed in SIMULINK to determine the thermal behaviour of each layer and provide total lifetime consumption analysis. The developed models were verified by real-time (RT) experiments using dSPACE environment. New materials, such as silicon carbide (SiC) devices, were found to exhibit approximately 20°C less thermal profile compared to conventional silicon IGBTs. For PECs used within wind energy systems, PECs driving algorithms were derived within the proposed models and by adjusting switching frequency PECs cycling temperatures were reduced by 12°C which led to a significant reduction in thermal stress; approximately 27 MPa. Total life consumption for the proposed method was calculated as 3.26x10-5 which is approximately 1x10-5 less compared to the other both methods. Effects of maximum power tracking algorithms, used in photovoltaic solar systems, on thermal stress were also explored. The converter’s thermal cycling was found approximately 3 °C higher with the IC algorithm. The steady state temperature was 52.7°C for the IC while it was 42.6 °C for P&O. In conclusion, IC algorithm offers more accurate tracking accuracy; however, this is on the expense of harsher thermal stress which has led to approximately 1.4 times of life consumption compared to P&O under specific operating conditions.