Analysis and Mitigation of DC Voltage Imbalance for Medium-Voltage Cascaded Three-Level Neutral-Point-Clamped Converters

The cascaded three-level neutral-point-clamped (3L-NPC) converter and the modular multilevel converter (MMC) are attractive solutions for medium-voltage direct-current (MVdc) applications. Due to their low cost compared to MMCs, cascaded 3L-NPC converters have been adopted in ANGLE-DC—a 30-MVA MVdc link demonstration project in North Wales, U.K. The dc voltage imbalance across submodules (SMs) is a common challenge for both types of MVdc converters. Such imbalance is topology dependent and remains underresearched for cascaded 3L-NPC converters. In this article, a small-signal model-based analysis has been done to reveal that the dc voltage imbalance in cascaded 3L-NPC converters is caused by an unstable system pole. Two voltage balancing methods are presented. The first method is based on PI controllers to precisely regulate SMs’ voltages without influencing output power. However, it relies on communication between a central controller and local controllers within SMs. The second method uses inverse-droop-based control to take over the dc voltage regulation upon loss of communication. Both balancing methods are experimentally validated using a 30-kVA testbed based on the ANGLE-DC project. It has been demonstrated that the dc voltages of SMs can be effectively balanced with both methods during changes of load conditions and dc bus voltages.