Analysis and experimental verification of improved two-level converter behaviors under unbalanced AC conditions

Abstract

An improved two-level converter (I2LC) represents a practical design trade-off between the conventional two-level converter (C2LC) and modular multilevel converter (MMC), in which the MMC like AC and DC fault responses and relative simplicity of C2LC power circuit and control are prioritized. Therefore, this paper presents theoretical analysis, which helps to develop broader understandings of the I2LC, and identification of the needed countermeasures to ensure correct operation during balanced and unbalanced grid conditions. The correctness of the findings and observations drawn from the presented analysis has been validated with the aid of scaled-down simulations and experimentations of the I2LC. Unlike the C2LC, it has been revealed that the I2LC can offer two concurrent operational objectives at system level during unbalanced AC grid conditions, such as balanced output currents and ripple-free DC link current. Further investigation conducted on the performance of the I2LC based two-terminal DC link during asymmetrical AC faults corroborates the theoretical analysis and scaled-down simulation and experimental studies.