Small-Signal Stability Enhancement With Circulating Reactive Power in Parallel Modular Multilevel Converters

This paper proposes a small-signal stability enhancement method that leverages the admittance shaping effect of the output power in modular multilevel converters (MMCs). The small-signal oscillations, which are caused by the impedance interactions between MMCs and grids, are suppressed by injected reactive power circulating between parallel MMCs. The reactive power references of both MMCs are modified to enable reactive power circulation without changing the control structures or parameters, which is more feasible for practical applications. The total power supplied to the grid remains the same, maintaining the power flow distribution and the grid bus voltage. The amount and direction of the circulating reactive power are constrained by both system stability margin and MMC capacity. The system stability margin is evaluated based on the phase characteristics of the determinant of bus admittance, which reflect the number and positions of system right-half-plane zeros. The effectiveness of the proposed method is validated by real-time simulation results, which are aligned with the theoretical stability analysis. The proposed method is tailor-made to networks with parallel-connected converters, especially at the bus where multiple inverter-based resources are locally integrated.