Neutral point clamped converter DC-link center point current analysis in the dq0 reference frame

This work proposes a novel analytical method for the analysis of the local average value of the neutral point current in an NPC converter operating with sinusoidal ac port currents. This method is based on the static modeling of the converter performed in a rotating frame coordinate system (dq0) and considering that the modulation strategies can be implemented with a carrier based approach. The importance of such an analysis lies on the fact that low frequency periodic oscillations in the neutral point current increase the dc-link capacitors currents and voltage ripple. The increased ripple lead to more stringent demands for the dc-link capacitors, increasing their losses, cost, volume and weight.