IGBT-Umrichtersysteme für Windkraftanlagen

Modern wind energy plants are a re-discovery of a technology with a long tradition. Today´s plants are dominated primarily by their high portion of power electronics. Only complex inverter systems render it economically possible to process the electric current produced by large-scale plants in such a way that it can be compatibly fed into the grid. The problems and load conditions of the frequency inverter of a wind energy plant are presented using the concept of a doubly-fed induction machine as generator. A detailed analysis of the load conditions shows that they differ considerably from those of traction drives. Therefore, the objective of this work is the development of a special tool for the dimensioning and life time prediction of the IGBTfrequency inverters. As a first step a power loss model for the determination of the actual loss in the inverter is developed using information of data sheets and of measurements. The thermal model of the IGBTs is derived from the basic physics. For the determination of the heat flow, the heat sink used is divided into macro cells. The heat removal of a heat sink strongly depends on its structure and – above all – on its flowmechanical properties. Therefore, the possibility to determine the operating point of the combination heat sink and fan, solely on the basis of the geometric data of the heat sink, is especially important. Starting with the basic physics the model is developed, simulated and verified by comparison with measurements. The combination of the individual models can be simulated as a network model with established circuit simulation tools. In order to verify the results of the simulation of the total system, a long time-test bench for cyclic loading of the IGBTs under real inverter operation is developed and built. Its results are compared with simulation. The failures observed during the long time cyclic tests are analysed. By this analysis failures could be clearly determined as bond-wire-lift-off, which were caused by cyclic loading. Furthermore, new failure mechanisms, which reduce life time, have been detected and analysed, e. g. in the area of the thermal interconnection between the IGBT-module and the heatsink. Finally, the work describes a simplified model derived from the experiences made. It is implemented on a microcontroller in order to calculate the chip temperature in real-time and thereby to enable a continuous life-time prediction during operation of the plant.