Unipolar Double-Star Submodule for Modular Multilevel Converter With DC Fault Blocking Capability

Xing Hu,Jianzhong Zhang,Fujin Deng,Zhe Chen,Yaodong Zhu

doi:10.1109/access.2019.2942137

Xing Hu, Jianzhong Zhang + Show 3 more

Open Access

https://doi.org/10.1109/access.2019.2942137

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Journal: IEEE Access	Publication Date: Jan 1, 2019
Citations: 12	License type: CC BY 4.0

Affiliation: Southeast University, Aalborg University

Abstract

Adopting overhead lines is an effective way to reduce the cost of flexible high voltage direct current (HVDC) systems. However, the dc short-circuit fault is easily occurred and the development of the overhead lines based HVDC system is hampered. This paper proposes a unipolar double-star submodule (UDSSM) with the dc fault blocking capability. Under the dc fault condition, the UDSSM based modular multilevel converter (MMC) could block the fault current by switching all power switching devices off to protect the MMC system. In each proposed submodule, two capacitors work under almost the same condition and could be in series or parallel connection to output different voltage levels. Then two voltage sensors of the two capacitors are redundant each other, which improves the reliability of the MMC system. Additionally, voltage balancing methods for the capacitors under both normal operation and sensor fault conditions of the MMC submodule are developed in this paper. The effectiveness of the proposed submodule and the capacitor voltage balancing strategy are validated by the simulation and experiment results.

Highlights

Modular multilevel converter (MMC) has been widely adopted in high voltage direct current (HVDC) transmission system due to its excellent features, such as high modularity, superior ac output performance, and low voltage stress on power switching devices [1], [2]
The cable instead of overhead line is used in the present flexible HVDC system to reduce the possibility of dc short-circuit fault, which increases the cost of HVDC system greatly [8]
This paper proposes a novel unipolar double-star submodule (UDSSM) for high reliability MMC system with dc fault blocking capability

Summary

INTRODUCTION

Modular multilevel converter (MMC) has been widely adopted in high voltage direct current (HVDC) transmission system due to its excellent features, such as high modularity, superior ac output performance, and low voltage stress on power switching devices [1], [2]. Under this situation, the two capacitors in this submodule are connected in series, which means the same current passes through two capacitors. A sorting process is implemented to regulate the operation states of power switching devices according to the capacitor voltage, the direction of the arm current, and the number of inserted submodules. Both the SCSM and the proposed UDSSM could tolerate the sensor fault, which improves the reliability of the MMC system. The proposed UDSSM has a better dc fault blocking capability than the SCSM

SIMULATIONS

EXPERIMENTS

CONCLUSION