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-DCx2014;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 SMsx2019; 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.

Automated summary

In this article, a small-signal model-based analysis is conducted to uncover the cause of dc voltage imbalance in cascaded 3L-NPC converters. Two voltage balancing methods are proposed, one based on PI controllers and the other based on inverse-droop control. Experimental validation demonstrates the effectiveness of both methods in balancing the dc voltages of SMs under changing load conditions and dc bus voltages.