Impact of modulation strategies on the power losses of the dc-link capacitor of an inverter

Power losses within the powertrain of electric vehicles are crucial for the efficiency as well as the thermal load of its main components. Due to the limited robustness with regard to thermal degradation, the dc-link capacitor of a traction inverter requires good knowledge of its power dissipation. The harmonics occurring in the dc-link current are of vital importance considering the thermal stress on components of the dc-link circuit. Using different modulation strategies, various quantities of the powertrain can be positively influenced and are accompanied by a change of the harmonic characteristics. The effect of the voltage modulation needs to be considered appropriately in a loss model. Therefore, in this paper a new generic approach for the calculation of the current and the related power loss of the dc-link capacitor is presented. The validity of the algorithm was verified by simulation applying two different types of voltage modulation: space vector pulse width modulation (SVPWM) and six-step mode (SSM). The analyses indicate a high accuracy of the loss model with a maximum error of less than 2 %. Using SSM a reduction of the power dissipation of the capacitor of nearly 97 % against the standard modulation strategy SVPWM could be proven. As a result, the outlined methodology may be a basis for real-time computation of the power losses and a thermal protection function of a dc-link capacitor using a broad spectrum of modulation strategies.