On the Redundancy Strategies of Modular Multilevel Converters

Abstract

The modular multilevel converters (MMCs) have become an attractive topology in recent years. The MMC has been employed in several applications as HVDC, energy storage, renewable energy, electrical drives and STATCOMs. One of advantages of MMC based topologies is the inherited fault-tolerant operation associated with the high number of submodules (SMs). This work proposes the study of four redundancy strategies which can be employed in modular multilevel converters: Standard Redundancy (SR); Redundancy Strategy based on Additional Submodules (RAS); Redundancy Strategy based on Additional Submodules Optimized (RASO); and Redundancy Strategy based on Spare Submodules (RSS). These strategies are compared through a case study of a 15 MVA MMC STATCOM. A new approach for the SM capacitance design is proposed, including the effect of the negative sequence current in the converter storage energy variations. The comparisons of the redundancy strategies are accomplished based on dynamic behavior, capacitor voltage balancing, control complexity and power losses. Finally, the advantages and drawbacks of each redundancy strategy are presented.

Existing System

Pulse Width-Modulated Modular Multilevel Converters

Proposed System

This work presented approaches to explore the redundancy capability of MMC. Four strategies are defined and their performances are evaluated through a case study based on a 15 MVA MMC STATCOM. The presented design methodology discusses the effect of the redundancy factor and the utilization factors of semiconductors in the number of SMs. In view of the points aforementioned, this work provides the following contributions: Design of MMC based STATCOM, considering the 4 redundancy strategies for both positive and negative sequence injection; Discussion of the necessary changes in the control strategy when additional SMs are employed, considering phase-shift modulation; Comparison of the dynamic performance of the redundancy strategies when both positive and negative sequences are compensated by a MMC STATCOM; Evaluation of the converter power losses when redundancy strategies are employed.

Architecture

Schematic of the DSCC-MMC STATCOM