Phase-leg power equalization method of modular multilevel converter with APOD strategy under unbalanced fault condition

Abstract

An active power oscillation damping (APOD) control is one of the representative unbalanced fault compensa-tion strategies. Recently, various studies have been performed on the effect of the APOD with the internal power dynamic analysis of a Modular Multilevel Converter (MMC); however, most of the papers have considered only the active power component for analyzing the internal dynamics of the MMC. During the APOD operation, while reactive power is supplied to the grid for the voltage compensation, phase-leg power imbalance can be aggravated. To enhance stability and reliability, the phase-leg power imbalance phenomenon should be sufficiently considered at the development stage of the controller. This research discovers that the phase-leg power imbalance rate could be determined by the magnitude and phase of the negative sequence component of the grid voltage and the amount of reactive power. In addition, the unbalanced power component can be accurately calculated only based on the grid voltage information and the reactive power reference. Also, this paper proposes a phase-leg power equalization method (PPEM) using unbalanced power component estimation. Furthermore, an improved control structure is also proposed to achieve more accurate phase -leg power equalization using the internal leg-energy dynamics analysis. The effectiveness of the proposed methods is verified through high-fidelity PSCAD/EMTDC time-domain simulations using Point-to-Point (PTP) MMC-HVDC system and Hardware-in-the-Loop Simulation (HILS) with the MMC-MVDC system.