The aim of this doctoral thesis was to investigate control strategies in Voltage Source Converter - High Voltage Direct Current (VSC-HVDC) multi-terminal systems (VSC-MTDC, for short) to improve angle stability in hybrid HVAC/HVDC grids (under large and small disturbances). The work was motivated by the growing interest of TSOs in various parts of the world on this technology. Electrical systems tend to be operated in increasingly stressed conditions (AC lines closer to their stability limits, transmission of large amount of power over long distances and less inertia, among others), which make angle stability a key limiting factor. Meanwhile, future electrical systems are expected to have an ever-increasing number of electronic power converters, such as renewable generators interfaced by power converters, FACTS devices and point-to-point and multi-terminal HVDC systems. Furthermore, hybrid HVAC/VSC-HVDC transmission systems are being considered to facilitate the integration of a large amount of renewable generation and to interconnect different countries in different parts of the world. Due to the fast control of the active and reactive power injections of the VSCs stations, VSC-HVDC multi-terminal systems, when installed, seem to be an attractive alternative to help improving angle stability. Previous work has shown that, indeed, VSC-MTDC systems can improve angle stability of electrical power systems significantly using suitable control strategies. All those strategies had in common the use of global-but-difficult-to-implement measurements, namely, the speed of all the generators of the system and the speed of the centre of inertia (COI). This thesis investigates control strategies based on “global-but-practical" measurements for (a) transient stability improvement (large disturbances) and for (b) power-oscillation damping (small disturbances). First of all, control strategies for P and the Q injections of the VSCs using the weighted-average frequency (WAF) of the converter stations of the VSC-MTDC system have been proposed to improve transient stability. The results presented in this thesis have shown that large and small-signal angle stability can be improved significantly using the proposed control strategies and their implementation is easier than previous approaches that require a Wide Are Measurement System (WAMS) to obtain the measurements of the speeds of all the generators of the system. Due to the promising results obtained for transient stability, supplementary controllers for power-oscillation damping (POD), using the same signals, were also investigated, showing that the WAF could also be useful for this purpose. A coordinated-design algorithm based on eigenvalue sensitivities was used to design the POD controllers of the VSC-MTDC system. The following conclusions have been obtained from the results of this thesis: • The proposed strategy P-WAF (for P injections of the VSCs) improves transient stability significantly. It uses the weighted-average frequency of the AC terminals of the VSC-MTDC system. • The proposed strategy Q-WAF (for Q injections of the VSCs) also improves transient stability significantly. It uses the weighted-average frequency of the AC terminals of the VSC-MTDC system. • The proposed local strategy Q-LWAF (for Q injections of the VSCs) improves transient stability significantly. It behaves very much like strategy Q-WAF, but it requires local measurements, only. • The proposed POD controllers (for P and/or Q injections of the VSCs) damp inter-area oscillations successfully. The coordinated-design algorithm allows the POD controllers to damp a required set of modes, obtaining the required damping ratios.
Descriptors: VSC-HVDC multi-terminal, control, angle stability, transient stability, power oscillation damping
Universidad Pontificia Comillas. Madrid (España)
05 March 2018