PhD Defence by Arash July

Offshore energy hubs can integrate large amounts of offshore wind power and transmit electricity to multiple onshore grids through multi-terminal MMC-HVDC networks. However, interactions among power converters, their controllers, and network dynamics can cause instability, including sub-synchronous oscillations.

This thesis develops detailed small-signal models to reveal the mechanisms behind these instabilities and explain how disturbances propagate through the DC link between offshore and onshore converters. It identifies new oscillatory interactions and proposes control strategies to damp the oscillations and reduce dynamic coupling. The proposed strategies are validated through frequency-domain analysis, time-domain simulations, and real-time simulation experiments.

The thesis also introduces a scalable, passivity-based stability-assessment framework using power transfer functions. This framework allows stability to be assessed locally at the individual converter level without requiring a model of the entire interconnected system.

Principal Supervisor: Professor Mehdi Savaghebi, DTU Engineering Technology
Co-Supervisor: Professor Nicolaos A. Cutululis, DTU Wind and Energy Systems

Chairperson at Defence: Associate Professor Kristina Vaarst Andersen, DTU Engineering Technology

Examiners:
Associate Professor Kaushik Das, DTU
Professor Ramon Blasco-Gimenez, Universitat Politècnica de València
Associate Professor Sanjay Chaudhary, Aalborg University