This paper investigates the stability of a modular multi-level converter (MMC)-based HVDC system connected to a weak ac grid. Impedance analysis is used to study the interactions between the weak ac grid and the converter. For this, the impedance of the MMC is first derived considering various outer- and inner-loop controllers. The Nyquist-stability criterion is then used to analyze the stability of the MMC connected to the weak ac grid for various case studies. The impact of circulating current controller on the stability of the MMC connected to the weak ac grid is also validated for two different types of outerloop controllers. Finally, time-domain simulations are carried out using MATLAB/Simscape Power System toolbox to validate the analytical results

The aim of this paper is to investigate the risk for subsynchronous control interaction (SSCI) in Type 4 based wind farms connected to series-compensated transmission lines. For this purpose, the frequency-dependent input admittance of the turbine is derived and the power-dissipation properties of the turbine are used to identify those control parameters and operating conditions that mainly impact the behavior of the wind turbine in the subsynchronous frequency range. It is shown that the Type 4 wind turbine presents a non-dissipative behavior in the subsynchronous frequency range, which is mainly dependent on the speed of response of the synchronization loop, the reactive power controller and the active-power output. For a radial connection, a high series-compensation level also negatively impacts the risk for SSCI. The generalized Nyquist criterion is used to assess the stability of the system and the theoretical findings are verified using detailed time-domain simulations.

Grid-forming (GFM) converters are a promising solution to enable large scale integration of renewable energy sources into the power system. However, due to the intrinsic voltage-source behaviour of GFM converters, current limitation during large grid disturbances is challenging. This paper presents a novel limitation strategy that preserves the GFM properties of the converter and at the same time effectively limits the converter current to the desired value. Through the limitation of the converter's internal voltage, stable operation even during faults and in case of large frequency disturbances in the grid is achieved. Experimental results show the effectiveness of the proposed current limitation strategy in case of various grid disturbances.

This paper deals with the derivation of the ac-side input admittance of a grid-connected modular multilevel converter (MMC) under both open- and closed-loop ac-side current control strategies. The contribution of the internal dynamics of the MMC on the input admittance depends on the type of ac-side controllers implemented. For this reason, the input admittance is first derived with the MMC operating in open loop on the ac side. The results are then modified to include various ac-side controllers in order to analyze the impact of the internal dynamics of the MMC on the total input admittance. Based on the findings, recommendations are given on modeling the input admittance of the MMC. Finally, the derived analytical models are verified using a detailed switching model of the MMC in the frequency domain.