Reference Current Saturation Methods for Modular Multilevel Converters in HVDC Systems

Abstract

This paper addresses the current saturation challenge in modular multilevel cascade converters (MMCCs). The operation of the MMCC requires the control of ac, dc and circulating currents while accounting for limitations introduced by active or passive components such as semiconductor devices, capacitors, or inductors. Traditionally, current saturation methods focus primarily on limiting the ac currents, often neglecting the effect of their internal dynamics on the cluster currents. As a result, not all current limits are respected. This paper presents saturation methods that provide insight into the cross influences of the three current types, enabling simultaneous enforcement of all current limits. The proposed approaches ensure that the actual currents remain within the prescribed limits during all operating conditions. The effectiveness of the saturation method is validated through high-fidelity MATLAB/Simulink simulations of a high-voltage dc (HVDC) system based on MMCCs.