Averaged Dynamic Model of Three-level NPC Grid-following Inverter for Examining Neutral-point Instability

In this paper, the dynamics of neutral-point (NP) voltage oscillations and associated instability phenomena in a three-level (3L) neutral-point clamped inverter (NPCI), operating as a grid-following (GFL) inverter, are analytically modeled. We depart from the traditional space-vector analysis and adopt a time-domain description in synchronous (dq) reference frame for system modeling. In particular, the NP voltage on the dc- side is identified as an additional system state unlike in a traditional 2L-inverter model. We obtain an averaged dynamic model of the current-controlled 3L GFL-inverter in state-space form. While the model is nonlinear and time varying, it comprehensively captures all pertinent dynamics of the GFL-inverter while acknowledging the participation of power- and control- circuit parameters in determining the NP-stability. We present a block-schematic dq-domain equivalent circuit of the average 3L GFL-inverter model delineating the 2L inverter control model contained with it and the additional dynamics arising from switching the NP branch. We utilize this model for two purposes: (a) feedback linearization of the plant model using feed-forward compensation of nonlinear and time-varying terms, which facilitates controller design using classical control synthesis techniques, and (b) improving injected grid-current quality. A full-order switched-model simulation of a three-phase GFL-inverter system in MATLAB®Simulink validates the accuracy of the proposed average dynamic model for NP-stability studies as well as grid current quality improvement.