Investigation and Identification of the Causes of the Unprecedented Accident at the “Chaira” Pumped Hydroelectric Energy Storage

The present study deals with an accident analysis of the “Chaira” Bulgaria high-pressure Pumped Hydroelectric Energy Storage (PHES), especially the failures of the Francis large-scale Hydraulic Unit No. 4 (HU4). The causes of overloading and cracking of the stay vanes and their functionality are investigated. Despite many studies on the behavior of the runner and its safe operation, the complete destruction of the system due to the stay vanes failure has not been sufficiently studied and published in the scientific literature. The aim is to assess the reasons that have led to this unprecedented accident of the “Chaira” PHES of the total destruction of the stay vanes. Based on the historical data and the operating modes, as well as on the identification of the critical elements, an objective assessment of the risks of damage of the stay vanes and of the entire unit is proposed. A virtual prototype is built for the simulation of the system behavior in working and emergency conditions. The loads, stresses, and deformations are precisely calculated. The critical elements of the highly-loaded blocks are evaluated. Low-cycle material fatigue is with a high degree of confidence the dominant reason for the stay vanes complete destruction. The results of the research are used to propose measures for the possible rehabilitation and/or replacement of the unit.