Enhanced Dynamic Performance and Control of an MMC-HVDC Transmission System

This paper presents the modeling, control, and simulation of a two-terminal modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission system rated at $\mathbf{1 0 0 0}$ MVA with a DC link connected to AC networks. The studied system is developed in MATLAB/Simulink using an aggregated MMC representation in order to preserve the dominant converter dynamics while maintaining acceptable computational efficiency. A hierarchical control structure is implemented, in which the outer control loops regulate the DC-link voltage as well as the active and reactive power, while the inner control loops regulate the converter currents in the synchronous dq reference frame. In addition, dedicated control actions are included for submodule capacitor-voltage balancing and circulating-current suppression in order to improve converter stability and reduce internal losses. The HVDC link is modeled using a lumped Tsection representation, allowing the main DC-side dynamics to be captured under both steady-state and transient operating conditions. The overall system performance is evaluated through several simulation scenarios, including converter energization, DC-voltage build-up, active- and reactive-power reference tracking, and a DC fault applied on the transmission line. The simulation results show that the proposed control structure ensures effective DC-voltage regulation around 640 kV, accurate tracking of the active- and reactive-power references, satisfactory capacitor-voltage balancing, and stable converter operation under normal and disturbed conditions. The obtained waveforms also confirm the effectiveness of the protection sequence during DC fault conditions and the good dynamic performance of the studied MMC-HVDC system.