Semi‐hot redundant control method for modular multilevel converter‐based high‐voltage direct current systems under submodules fault

Abstract

A modular multilevel converter‐based high voltage direct current (MMC‐HVDC) system involves a large number of cascading submodules (SMs). The failure of SMs may critically degrade the system performance. Therefore, several redundant SMs can be incorporated and reasonably controlled to ensure the system reliable and continuous operation even if certain SMs fail. This paper proposes a semi‐hot redundant method for MMC‐HVDC systems. In the proposed strategy, when the system operates normally, the redundant SMs are controlled in a lower voltage range. These SMs do not require frequent charging and discharging, which decreases the switching loss. When ordinary SMs fail, the faulty SMs are bypassed and the corresponding number of redundant SMs can be promptly recharged to the rated voltage to replace the faulty SMs. The semi‐hot reserved SMs can shorten the transient process of faulty SMs replacement, and in this process, the absence of faulty SMs does not considerably influence the system. Thus, the original control and modulation strategies of the system do not need to be modified when several SMs fail. The performance of the proposed method is compared with those of the existing methods, and its validity is demonstrated through a small‐scale experimental prototype.

MVDC MMC Redundancy Design Based on Availability and Cost Considering Submodule Degradation

“…Maintaining the MMC's high reliability is crucial, and research has been conducted on maintenance, inspection [5], [6], [17], and redundancy plans to enhance the reliability. Several strategies have been proposed, including suggesting control strategies that incorporate redundant modules to enhance the reliability of the MMC arm unit [13], presenting approaches to calculate system reliability considering economic factors and lifespan with redundancy [14], and determining the causes of component lifespan reduction to calculate reliability, along with comparing the methodologies depending on redundancy [15]. Ensuring redundancy in the MMC is considered the most effective method [18].…”