A generalised natural balance model and balance booster filter design for three level neutral point clamped converters

Three level Neutral Point Clamped (NPC) inverters exhibit a natural balancing property that can self-regulate the intermediate Neutral Point (NP) voltage to half the overall DC link voltage. However, the underlying mechanisms of this process are not well understood, its performance under various load conditions has not been properly explored, and the requirements and benefits of (RLC) balance booster filters to enhance the natural balancing response are not clear. This paper presents a generalised model of the NPC natural balancing process that allows these issues to be explored for various load structures and balance booster configurations (i.e. isolated wye, wye with a grounded star point, etc.). The model accurately predicts the dominant balancing time constant, identifies the harmonics that drive the balancing process, and can quantify the steady state balance booster losses for a designed target natural balance response. Extensive simulation and experimental tests have been performed to validate the theoretical model.