PhD Juan-Carlos GONZALEZ

“Transient stability of high voltage AC-DC electric transmission systems”

The new policy frameworks adopted by national authorities has encouraged the large scale-integration of Renewable Energy Systems (RES) into bulk power systems. The large-scale integration of RES will have consequences on the electricity transmission system as it is conceived today, since the transmission of bulk power over long distances could lead the existing transmission systems to work close to their limits, thus decreasing their dynamic security margins. Therefore more complex transmission systems are needed. Under this scenario, HVDC transmission systems raise as the most attractive solution for the reinforcement and improvement of existing AC networks, not only using point-to-point configurations, but also in a Multi-Terminal configuration (MTDC). The introduction of HVDC transmission systems will eventually result in a hybrid high voltage AC/DC power system, which requires to be analyzed as a unique system in order to understand the interactions between the AC network and the DC grid.

This thesis addresses the transient stability analysis of hybrid AC/DC electric transmission systems. More precisely two questions sought to be investigated: What is the impact of a DC contingency on AC transient stability? How can we take advantage of the DC transmission systems as control inputs in order to enhance AC transient stability?

In the first part of this work, the mathematical models of the hybrid AC/DC grid are described as well as the necessary tools for the analysis of the system taking into account its nonlinear nature. Then, a thorough analysis of transient stability of the power system in the particular case of a DC fault and the execution of the corresponding protection strategies is done. As a complement, stability indicators and tools for sizing future MTDC grids in order to respect the constraints of existing protection strategies are proposed.

The second part of the thesis addresses the control proposals for the modulation of power references of the HVDC transmission systems with the purpose of transient stability enhancement of the surrounding AC system. Firstly, we focus our study in the nonlinear control of point-to-point HVDC links in hybrid corridors. Fast power compensation, injection of damping power and injection of synchronizing power are identified as the mechanisms through which HVDC systems can improve stability margins. Finally, a control strategy for transient stability enhancement via active power injections of an MTDC grid is proposed. The strategy allows to fully use the available headroom of the converters by dealing with power limits in a decentralized way.