Comparison of a Z-source inverter and a voltage-source inverter linked with a DC/DC-boost-converter for wind turbines concerning their efficiency and installed semiconductor power

In this paper the power losses of both Z-source- inverter (ZSI) and voltage-source inverter linked with a dc/dc boost inverter (VSI+BC) are compared. Therefore the circuits are simulated in time domain by means of numerical methods whereas the switching and conducting losses are calculated separately and in parallel. A main focus is on mathematical description of the current rms through the IGBTs of the ZSI to derive the conducting losses. It is shown that the losses of the diode in the dc link of the ZSI has a high percentage of the total power losses and cannot be neglected. Besides that the choice of the modulation method has a high influence on the power losses of the ZSI as well. Based on the calculated power losses the efficiency and the installed semiconductor power are investigated and compared for different input voltages. The results show that for an output voltage of 400V the VSI+BC topology outperforms the ZSI in almost all operating points. The reason is that for low input voltages the VSI+BC topology boosts the input voltage to limited 600 V and the VSI works with a modulation index of about 1 very efficiently. However the ZSI has to boost the input voltage to much higher values, since for high voltage gains long shoot- through-states are essential. However, long shoot-through- states are coming along with long zero-states which causes a buck behavior.

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