Circulating current suppressing strategy for mmc hvdc based on nonideal proportional resonant controllers under unbalanced grid conditions
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This document presents a new control strategy for modular multilevel converter high voltage direct current (mmc-hvdc) systems under unbalanced grid conditions. It introduces an innovative inner loop current control method using non-ideal proportional resonant controllers and derives an analytical expression for circulating currents that highlights their asymmetry in unbalanced conditions. The proposed circulating current suppression strategy, verified through simulation, enhances system adaptability to grid frequency fluctuations.
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Circulating current suppressing strategy for mmc hvdc based on nonideal proportional resonant controllers under unbalanced grid conditions
- 1. CIRCULATING CURRENT SUPPRESSING STRATEGY FOR MMC-HVDC BASED ON NONIDEAL PROPORTIONAL RESONANT CONTROLLERS UNDER UNBALANCED GRID CONDITIONS ABSTRACT Modular multilevel converter (MMC) is considered as a promising topology for voltage- source converter (VSC) high voltage, direct current (HVDC) applications. This paper presents a new control strategy for MMC-HVDC under unbalanced grid conditions. First, a new inner loop current control strategy based on non ideal proportional resonant (PR) controllers in stationary αβ frame is designed, which is more concise compared to the existing dual sequence current control scheme. Second, an analytical expression for circulating current is obtained which shows that the circulating currents will be asymmetric under unbalanced grid conditions and can be decomposed into positive-, negative-, and zero-sequence component. In order to suppress all these components, a new circulating current suppressing strategy is analyzed and designed also based on non ideal PR controllers. Application of non ideal PR controllers makes the control system well adapt to the fluctuation of grid frequency. The effectiveness of the proposed control strategy is verified through a simulation case of a 251-level MMC-HVDC transmission system using real-time digital simulator.