Abstract
A large high-voltage direct current (HVdc) reactor is required in modular multilevel converter (MMC) projects to realize dc fault current limiting (FCL). This condition may result in adverse effects on MMC control dynamics and oscillation and instability of HVdc systems. In this article, a virtual reactor (VR) control strategy is proposed to reduce a fixed large HVdc reactor. The VR characteristic of MMCs is revealed and analyzed as a controlled reactor, and the inductance of this reactor can be directly controlled by the discharge degree of submodule capacitors. A virtual reactor fault current limiting (VRFCL) control strategy in which VR can effectively limit dc fault current and substitute the fixed HVdc reactor is proposed accordingly. Simulation and experimental results verify the proposed control strategy. The adoption of the proposed strategy can eliminate the undesirable effect caused by an HVdc reactor, and the sub-module (SM) capacitor voltage can be reserved well. In addition, fault clearance time and fault energy dissipation can be reduced. The proposed VRFCL control is beneficial for MMCs to safely ride through dc fault in a continuous operation manner.
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