Analysis and Mitigation of Neutral Line Current Ripple in a Dual 3L-NPC Converter System for Bipolar DC Distribution

Bipolar dc distribution systems based on a three-level neutral point clamped (3L-NPC) converter typically require a voltage balancer (VB) to achieve full bipolar dc voltage balancing. Recent studies have shown that certain grid transformer configurations allow the full control of dc-side pole power flows by multitasking the converter-side windings to carry the required dc balancing current through a dedicated neutral line (NL). NL-based NPC converter systems offer a simpler and more cost-effective alternative to VB-based systems, but suffer from significant NL current ripple caused by the pulsewidth modulation (PWM) switching of the converter. This article addresses the issue on two fronts. First, an in-depth analysis of the NL current ripple is conducted in a dual NPC converter system. Analytical expressions are derived to characterize the harmonic distribution under both phase disposition (PD) and alternate phase opposition disposition (APOD) PWM strategies. Second, based on the analysis, an improved PD-PWM strategy incorporating triple degrees-of-freedom (PD-TDoF) is proposed for dual NPC converters, effectively reducing current ripple and significantly enhancing NL current performance compared to conventional PD-PWM. The theoretical analysis and effectiveness of the proposed PD-TDoF strategy are validated through laboratory experiments.