Analysis of the Average Neutral-Point Current Limits of the Neutral-Point-Clamped Converter Under Three-Level Modulation

The neutral-point-clamped (NPC) converter employed in applications like bipolar dc-bus, back-to-back converters, power compensator, etc., requires a nonzero average neutral-point (NP) current injection under certain operating conditions. In order to better implement the average NP current injection, this paper studies the maximum and minimum average NP current that can be injected with an NPC converter. For this study, a three-level modulation technique is employed in which one of the three phases is selected to switch between the positive dc-rail and the negative dc-rail while the other two phases switch in two consecutive voltage levels. The maximum and minimum average NP current injection limits are analyzed under different loading conditions. The presented study also evaluates the effects of modulation index and operating power factor on the limits.