Featured Application The results and ideas in this paper have a high practical significance, thus can easily be applied by grid operators and any other interested parties. Firstly, the discovered principles of voltage sags propagation can be used for the planning and implementation of a PQ monitoring system, as has successfully been carried out in Lithuania. Secondly, the created BRELL-based test scheme can be used and expanded in further researches. Thirdly, the proposed methodology, involving both reliability indexes and probabilistic analysis of the primary causes of faults in the PQ field, can be used by grid operators, while the method created for the estimation of functions (distributions) similarity can be used universally. Fourthly, in a wider context, the findings will be useful for grid planning, the framework of national energy strategies and legal documents, also as a PQ database for machine learning in anticipation of AI algorithms.Abstract Nowadays, voltage sag continues to remain a critical PQ issue in the industry. Since it is not possible to install a voltage analyzer on every node, optimal monitoring locations must be determined. However, during the PQ monitoring campaign of the Lithuanian DSO grid, the execution of this task was inhibited by the lack of knowledge and literature about the fundamentals of voltage sags propagation. Therefore, the first part of this paper investigates the propagation paths of voltage sags by using a voltage sag matrix method on the created test grid, which for the first time is BRELL-based. This paper is the first to not only investigate voltage sags propagation paths (both downward and upward) in detail, but also to investigate them complexly, encompassing all four types of short-circuits, both voltages (phase-to-phase and phase-to-ground), and interconnections with other PQ events (interruption, transient, unbalance). The result has established a generalization of voltage sags propagation mechanisms by inductive reasoning and laid the foundation for the further development of PQ theory. The second part of this paper focuses on the Lithuanian DSO grid and is one of the first to investigate voltage sags/interruptions through the prism of both relationship with reliability indexes (SAIFI and SAIDI) and primary causes. For this purpose, we created our own scientific methodology, applying known probabilistic and statistical methods along with newly proposed approaches, in particular for ACR success evaluation and estimation of functions similarity.
