New protection algorithms for HVDC grids

233 p.Los sistemas HVDC representan una alternativa prometedora para futuras expansiones del sistema eléctrico gracias a las ventajas que presentan en comparación con el transporte convencional en corriente alterna. Además, el interés por desarrollar redes HVDC multiterminales ha crecido en los últimos años, sin embargo, su implementación se ha visto ralentizada debido a la complejidad que presenta la protección ante faltas en estos sistemas. El objetivo principal de esta tesis es proponer un nuevo algoritmo de protección contra faltas, apropiado para dichas redes y capaz de superar las limitaciones presentes en algoritmos existentes. El algoritmo propuesto es un algoritmo de tensión de inductancia basado en el cálculo del ratio entre las medidas de tensión tomadas a ambos lados de la inductancia limitadora y la derivada de dicho ratio. Es capaz de detectar faltas rápidamente y de discriminar de manera selectiva entre faltas dentro y fuera de la zona de protección. También se propone una metodología para la selección del valor umbral necesario para la operación de algoritmos locales. A continuación, sedesarrolla un esquema de protección completo que se compone de protecciones de línea primaria y de respaldo, protección de barra y protección ante fallo del interruptor. Esta última protección es, así mismo, un nuevo algoritmo propuesto en la tesis, que presenta una operación más rápida que algoritmos convencionales de detección de fallo en el interruptor. Finalmente, la operación del esquema de protección propuesto es validada y analizada a través de simulaciones en un modelo de red de cuatro terminales con diferentes escenarios de falta, comparándolo con algoritmos existentes

Management and Protection of High-Voltage Direct Current Systems Based on Modular Multilevel Converters

The electrical grid is undergoing large changes due to the massive integration of renewable energy systems and the electrification of transport and heating sectors. These new resources are typically non-dispatchable and dependent on external factors (e.g., weather, user patterns). These two aspects make the generation and demand less predictable, facilitating a larger power variability. As a consequence, rejecting disturbances and respecting power quality constraints gets more challenging, as small power imbalances can create large frequency deviations with faster transients. In order to deal with these challenges, the energy system needs an upgraded infrastructure and improved control system. In this regard, high-voltage direct current (HVdc) systems can increase the controllability of the power system, facilitating the integration of large renewable energy systems. This thesis contributes to the advancement of the state of the art in HVdc systems, addressing the modeling, control and protection of HVdc systems, adopting modular multilevel converter (MMC) technology, with focus in providing services to ac systems. HVdc system control and protection studies need for an accurate HVdc terminal modeling in largely different time frames. Thus, as a first step, this thesis presents a guideline for the necessary level of deepness of the power electronics modeling with respect to the power system problem under study. Starting from a proper modeling for power system studies, this thesis proposes an HVdc frequency regulation approach, which adapts the power consumption of voltage-dependent loads by means of controlled reactive power injections, that control the voltage in the grid. This solution enables a fast and accurate load power control, able to minimize the frequency swing in asynchronous or embedded HVdc applications. One key challenge of HVdc systems is a proper protection system and particularly dc circuit breaker (CB) design, which necessitates fault current analysis for a large number of grid scenarios and parameters. This thesis applies the knowledge developed in the modeling and control of HVdc systems, to develop a fast and accurate fault current estimation method for MMC-based HVdc system. This method, including the HVdc control, achieved to accurately estimate the fault current peak value and slope with very small computational effort compared to the conventional approach using EMT-simulations. This work is concluded introducing a new protection methodology, that involves the fault blocking capability of MMCs with mixed submodule (SM) structure, without the need for an additional CB. The main focus is the adaption of the MMC topology with reduced number of bipolar SM to achieve similar fault clearing performance as with dc CB and tolerable SM over-voltage