Investigation of MMC-HVDC operating region by circulating current control under grid imbalances

The maximum-allowable voltage/energy ripple and peak arm current define the operating limits of a modular multilevel converter (MMC). Furthermore, under unbalanced ac-grid conditions, the control targets of a power electronics converter with regards to balanced active power, reactive power or grid currents affect the power capabilities and operating region of the MMC. The objective of this paper is to investigate and quantify the operating region of an MMC under unbalanced ac-grid conditions and the impact of circulating current control and system control targets on the limits of the MMC operating region. It is demonstrated that higher order harmonic injection in the circulating current through a reference calculation based on instantaneous information extends the operating region of the MMC under grid imbalances, reducing capacitor voltage ripples compared to a direct elimination of the second order harmonic regardless of the converter control target. The efficacy of both circulating current control methods is validated through real-time digital simulations of an 800 MVA benchmark MMC-based high-voltage dc (MMC-HVDC) transmission system.