Neutral point control in multi level converters applying novel modulation schemes

This paper investigates the behavior of the neutral point (NP) voltage and current for generalized multilevel converters introduced in recent years (ANPC: active neutral point clamped converter, SMC: stacked multicell converter). The unified approach introduced in this paper shows that all considered topologies have the same NP current properties when applying standard modulation schemes. A simple graphical representation is used to explain how the NP currents of the different phases combine in a multiphase converter. New modulation schemes are proposed and their impact in the NP current is demonstrated. The current that can be injected when needed is maximized. A new NP control scheme without using the common mode (CM) voltage is introduced. Experiments confirm that the NP voltage can be controlled by the dynamic application of the different modulation types, even if there is a static CM applied to the output voltage. A 5-level SMC converter is used for the experimental verification of the NP current characteristics and the new NP control strategy.