In this paper, analyses of Francis turbine failures for powerful Pumped Hydraulic Energy Storage (PHES) are conducted. The structure is part of PHES Chaira, Bulgaria (HA4—Hydro-Aggregate 4). The aim of the study is to assess the structure-to-concrete embedding to determine the possible causes of damage and destruction of the HA4 Francis spiral casing units. The embedding methods that have been applied in practice for decades are discussed and compared to those used for HA4. A virtual prototype is built based on the finite-element method to clarify the influence of workloads under the generator mode. The stages of the simulation include structural analysis of the spiral casing and concrete under load in generator mode, as well as structural analysis of the spiral casing under loads in generator mode without concrete. Both simulations are of major importance. Since the failure of the surface between the turbine, the spiral casing, and the concrete is observed, the effect of the growing contact gap (no contact) is analyzed. The stresses, strains, and displacements of the turbine units are simulated, followed by an analysis for reliability. The conclusions reveal the possible reasons for cracks and destruction in the main elements of the structure.

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.

In the paper analyzes of Francis turbine failures for a powerful Pumped Hydraulic Energy Storage (PHES) are conducted. The structure is part of the PHES Chaira, Bulgaria (HA4 - Hydro-Aggregate 4). The aim is assessment of the structure-to-concrete embedment for possible cause of damages and destructions of the HA4 Francis spiral casing units. The embedment methods applied in the practice for decades are discussed and compared to that used for HA4. An innovative virtual prototype is built based on the finite element method to clarify the influence of workloads under generator mode. The stages of the simulation include:- complete structural analysis of the spiral case and the concrete under loads in generator mode;- complete structural analysis of the spiral case under loads in generator mode but without the concrete.Both procedures of simulations (with and without the concrete) are of major importance. Since a failure of the surface between the turbine, the spiral case and the concrete is observed the effect of the increasing contact gap (no contact) is analyzed. The stresses, strains and displacements of the turbine units are simulated followed by important analysis for reliability. The conclusions reveal the possible reasons for cracks and destruction in the main elements of the structure.

In the present paper, the subject of investigation is the reliability assessment of the single-stage reversible Hydropower Unit No. 3 (HU3) in the Bulgarian Pumped Hydro-Electric Storage (PHES) plant “Chaira”, which processes the waters of the “Belmeken” dam and “Chaira” dam. Preceding the destruction of HU4 and its virtual simulation, an analysis and its conclusions for rehabilitation and safety provided the information required for the reliability assessment of HU3. Detailed analysis of the consequences of the prolonged use of HU3 was carried out. The Supervisory Control and Data Acquisition (SCADA) system records were studied. Fault Tree Analysis (FTA) was applied to determine the component relationships and subsystem failures that can lead to an undesired primary event. A Failure Modes and Effect Analysis methodology was proposed for the large-scale hydraulic units and PHES. Based on the data of the virtual simulation and the investigations of the HU4 and its damages, as well as on the failures in the stay vanes of HU3, it is recommended to organize the monitoring of crucial elements of the structure and of water ingress into the drainage holes, which will allow for detecting failures in a timely manner.

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 because of 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 total destructions of the stay vanes. Based on the historical data and the operating modes, as well as of the identification of the critical elements an objective assessment of the risks of damage of the stay vanes and of entire unit is proposed. A virtual prototype is built for 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 dominated reason for the stay vane complete destruction. The results of the research are used to propose measures for possible rehabilitation and/or replacement of the unit.

In the present paper subject of the investigations is the reliability assessment of the single-stage reversible Hydropower Unit No. 3 (HU3) in the Bulgarian Pumped Hy-dro-Electric Storage (PHES) plant “Chaira”, which processes the waters of the “Belmeken” dam and “Chaira” dam. Preceding destruction of HU4 and its virtual simulation, analysis and the conclusions for the rehabilitation and safety provided the information for the possible processes in HU3. Detailed analysis of the consequences of prolonged use of HU3 was carried out. The Supervisory Control and Data Acquisition (SCADA) system records were studied. Fault Tree Analysis (FTA) is applied to determine the component relationships and subsystem failures that can lead to an undesired primary event. The functional structure of the system was depicted as a causal chain of failure effects. The probability of system failure was estimated based on the failure probabilities of the primary events. The effects of static loads, dynamic loads and low-cycle loads were investigated. Based on the experience and the investigations of the HU4 and its damages, as well as of the failures in the stay vanes of HU3 it is recommended to organize monitoring for water ingress into the drainage holes, which will allow detecting failures in a timely manner.