This paper presents a practical strategy for utilizing the submodule (SM) redundancy of a modular multilevel converter (MMC) for its fault tolerance. This strategy provides a systematic framework for balancing the tradeoff between two conventional methods for using the active redundancy and, thus, achieves operational flexibility. One of the existing methods improves SM reliability owing to less voltage stress on the SM components by employing all of the SMs to form the ac or dc voltages (voltage-sharing mode). The other avoids transients by keeping the average SM voltage constant at the cost of slightly increased stress on the SM components (fixed-level mode), which, however, can be controlled to provide the grid-adaptive operation by reserving the energy of the SMs not in service. We, thus, develop a new redundancy management scheme by integrating these two methods and exploiting their technical benefits to meet the PQ requirements and MMC control performance. This research provides a theoretical basis and a technical guide to determining the number of SMs, which can further increase the voltage steps as per the MMC and grid conditions. This paper also connects the remaining PQ capability of the MMC at a particular operating point with the SM redundancy concept by defining a potential redundancy, especially useful when the physical redundancy is exhausted. The theoretical findings and efficacy of the proposed strategy are validated through PSCAD/EMTDC time-domain simulations followed by experiments using a nine-level single-phase MMC system.
Bibliographical noteFunding Information:
Manuscript received April 16, 2018; accepted May 29, 2018. Date of publication June 10, 2018; date of current version February 5, 2019. This work was supported in part under the framework of an international cooperation program managed by the National Research Foundation of Korea under Grant 2017K1A4A3013579 and in part by the “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning, granted financial resource from the Ministry of Trade, Industry & Energy, South Korea, under Grant 20174030201540. Recommended for publication by Associate Editor M. T. Bina. (Corresponding author: Kyeon Hur.) J. Kang, H. Kim, and K. Hur are with the School of Electrical and Electronic Engineering, Yonsei University, Seoul 03722, South Korea (e-mail:, firstname.lastname@example.org; email@example.com; firstname.lastname@example.org).
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All Science Journal Classification (ASJC) codes
- Electrical and Electronic Engineering