Enhanced Optimal Power Flow Based Droop Control in MMC-MTDC Systems

TL;DR

The paper tackles the challenge of coordinating MMC-based MTDC networks under variable renewable generation by embedding an adaptive voltage droop strategy within an OPF framework. It formulates a two-stage optimization that jointly minimizes generation costs and DC-voltage deviations while enforcing AC/DC power-flow constraints and converter loss models, with voltage stability prioritized. A droop-control strategy with variable $k_{droop}$ is implemented in two optimization rounds to achieve near-optimal cost and robust voltage regulation, reinitializing when stability is threatened. Case studies on the Nordic 32 system with a four-terminal MTDC grid demonstrate improved DC voltage regulation and competitive economics, highlighting the approach's potential for reliable, scalable HVDC-AC hybrid systems.

Abstract

Optimizing operational set points for modular multilevel converters (MMCs) in Multi-Terminal Direct Current (MTDC) transmission systems is crucial for ensuring efficient power distribution and control. This paper presents an enhanced Optimal Power Flow (OPF) model for MMC-MTDC systems, integrating a novel adaptive voltage droop control strategy. The strategy aims to minimize generation costs and DC voltage deviations while ensuring the stable operation of the MTDC grid by dynamically adjusting the system operation points. The modified Nordic 32 test system with an embedded 4-terminal DC grid is modeled in Julia and the proposed control strategy is applied to the power model. The results demonstrate the feasibility and effectiveness of the proposed droop control strategy, affirming its potential value in enhancing the performance and reliability of hybrid AC-DC power systems.

Figures (4)

• Figure 1: Overview of investigated configuration of MMC.
• Figure 2: Flowchart of the proposed strategy.
• Figure 3: Nordic test system with a 4-terminal MTDC system 8950069van2015test.
• Figure 4: DC voltage in different scenarios.