Optimal Droop Control Strategy for Coordinated Voltage Regulation and Power Sharing in Hybrid AC-MTDC Systems

TL;DR

The paper tackles economic and dynamic stability challenges in hybrid AC-MTDC systems with high renewable penetration. It proposes an OPF-based coordinated droop control that embeds frequency regulation loops inside MMCs to jointly regulate DC voltage and AC frequency. The method solves a two-stage optimization to minimize generation costs while keeping DC voltages near their rated values, and validates the approach on a modified Nordic 32 system with a four-terminal MTDC grid using Julia-based OPF and EMTP time-domain simulations. Results show consistent cost savings compared to active power or adaptive droop controls and improved voltage and frequency resilience under disturbances, highlighting practical potential for future HVDC-connected grids.

Abstract

With the growing integration of modular multilevel converters (MMCs) in Multi-Terminal Direct Current (MTDC) transmission systems, there is an increasing need for control strategies that ensure both economic efficiency and robust dynamic performance. This paper presents an enhanced Optimal Power Flow (OPF) framework for hybrid AC-MTDC systems, integrating a novel droop control strategy that coordinates DC voltage and AC frequency regulation. By embedding frequency control loops into the MMCs, the method enables system-wide coordination, enhancing power sharing and improving system resilience under disturbances. The proposed strategy dynamically adjusts converter operating points to minimize generation costs and DC voltage deviations, thus balancing economic objectives with system stability. A modified Nordic test system integrated with a four-terminal MTDC grid is used to validate the approach. Optimization is performed using Julia, while the system's dynamic performance is evaluated through electromagnetic transient simulations with the EMTP software. Case studies across multiple scenarios demonstrate that the proposed method consistently achieves lower generation costs than active power control and adaptive droop control strategy while maintaining stable control characteristics. The results highlight the method's capability to deliver cost-effective operation without compromising performance, offering a promising solution for the coordinated control of future hybrid AC-DC transmission networks.

Figures (12)

• Figure 1: Configuration of a 4-terminal AC-MTDC system.
• Figure 2: Overview of MMC design.
• Figure 3: MMC operation diagram.
• Figure 4: Voltage-frequency control in the outer control of loop of the d-q control system.
• Figure 5: Flowchart of the proposed strategy.