Definition of New Critical Distances for Transformers Re-energization during a Black Start

Abstract: This study aims to find new critical distances for transformer re-energization during network restoration after a blackout. Energizing a power transformer, in a grid that is lightly loaded due to the restoration process, involves a risk of temporary harmonic overvoltage. The overvoltages can, if they exceed a certain limit for a certain time, be harmful for the components of the grid. The rule which already exists at RTE states that there is need of a simulation study if the distance between the transformer to be energized and the closest load exceeds a certain length, depending on the transformer type. However, the critical distances already used are not satisfying, because in most cases the overvoltages are far from being dangerous, even for longer distances. This fact is confirmed in the study of a real restoration configuration in the thesis. Moreover, the analysis of the studies that have led to the critical distances shows that some of their modelling assumptions are questionable. The auxiliaries of the nuclear unit, in particular, have to be modelled to obtain correct results. Taking into account these remarks, new critical distances have then been determined. For this determination the models are chosen to be as conservative as possible. The new critical distances now depend on the type of nuclear unit and on the rating of the transformers to be energized.