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The Sichuan-Yunnan diamond block is a significant region of material flow from the Tibetan Plateau. However, its northwest electrical boundary remains undefined. Our study conducted a magnetotelluric profile from Chazha to Luomai across the geological boundary of the Sichuan-Yunnan block in its northwest sector. Using the nonlinear conjugate gradient three-dimensional inversion method, we obtained the deep electrical structure across the northern part of the block. Our findings reveal distinct segmental characteristics in the electrical structure along the profile. Marked by a high-resistivity zone along the Dedeng-Batang-Riyu fault, two high-conductivity layers exist in the upper-middle crust of the southwestern and central sections of the profile, at depths of 5–20 km and 10–30 km, respectively. Thus, our study suggests that the northwest boundary of the Sichuan-Yunnan diamond block, contrary to previous studies that considered it a transitional boundary, is located at the Dedeng-Batang-Riyu fault. Here, the boundary is a high-resistivity barrier, electrically isolating the Qiangtang block in the southwest from the Sichuan-Yunnan block in the northeast. The two significant high-conductivity zones in the upper-middle crust appear within the block's interior, contrasting with previous assumptions of lower crustal flow. [ABSTRACT FROM AUTHOR]

The new energy system aims to provide reliable, affordable and sustainable energy, and ultimately achieve carbon neutrality. But the unstable renewable energy sources including wind and solar increase the risk to the grid and the power system. The pumped–storage power stations are a good solution to compensate then fluctuating energy and can increase the robustness of the grid. However, the pump–turbine units also suffer from high pressure-pulsations, especially at off-design operating conditions. The flow inside the unit under off-design conditions has very complex hydrodynamic characteristics and generates significant stresses in the prototype pump–turbine units. This work investigates the transient flow characteristics of an extra– high–head pump–turbine prototype with splitter– blades with unsteady CFD simulations. The results show that the pressure-pulsation caused by rotor-stator interaction is the dominant pressure-excitation to the pump– turbine prototype. The excitation frequency is strongly related to the blade number in the rotating frame and the guide-vane number in the non- rotating frame, as well as the rotating frequency of the unit. The flow characteristics at a high pressure-pulsation working condition are extremely complex and unstable. It reduces the efficiencies of the pump–turbine prototype and can generate severe vibrations and stresses on the structural component of the unit. The calculated results of this research can be applied to the structural model to invest in the dynamic characteristics of the pump–turbine unit in the next step.

During transient processes in extra–high–head pumped–storage power plants, the prototype pump–turbine-units must be able to withstand sudden changes in water pressure within the delivery system. Extremely high or low pressures can act on structures in the short or long term and can cause damage, challenging the efficient, safe and healthy operation of prototype pump–turbine-units. So it is very important to adequately evaluate the structural robustness and stability of the extra–high–head pump–turbine prototype in the design period. In this study, the structural characteristics of an extra–high– head prototype pump–turbine-unit at extreme-pressure working- conditions were conducted. First, the full-scale CAD geometries for the prototype pump–turbine-unit were created. Next, the CAD geometries were meshed with tetrahedron elements to obtain the numerical simulation model. Then apply the extreme- pressure loads on the simulation model to evaluate the structural deformation and stress of the pump–turbine-unit prototype. From calculation results, it can be found that the stresses on some structural components are beyond the yield strength, and the high-stress locations have to be redesigned to meet the industrial requirement. Based on the calculation results, technical recommendations are given to optimize the current design. The developed evaluation method in this study can also be applied to evaluate pressured pipes, tanks, and other forms of fluid machinery.

The Francis runner is a critical component of the Francis turbine generator unit, playing a central role in converting water energy into rotating mechanical energy that drives the generator in hydropower stations. In-depth analyses of the flow characteristics of the Francis runner under various operating conditions and avoiding fatigue damage of the Francis runner are crucial to the reliability and efficiency of hydropower operation. In this paper, the flow dynamics of a large Francis turbine runner are analyzed under three representative loading conditions—low partial load, high partial load, and full load—and the flow-induced stress of the runner is analyzed under these loading conditions. It was found that the maximum static and dynamics stresses of the runner at three representative loading conditions are located at the chamfered surface where the blade trailing edge connects to the runner crown. The maximum static stresses of the Francis runner are 284 MPa, 352 MPa, and 381 MPa at low partial load, high partial load, and full load, respectively, and they are above the allowable stress limits, as half of the yield stress of the runner material of 550 MPa. The peak-to-peak values of runner dynamic stress at low partial load, high partial load, and full load are 15 MPa, 25 MPa, and 14.6 MPa, respectively. The high stress invoked by the unsteady flow under various loading conditions in this runner was the cause of the fatigue breakage of the runner blades. The results of this investigation have important reference values for mitigating fatigue damage in similar Francis runners and optimizing unit operation. [ABSTRACT FROM AUTHOR]

Francis turbines, being widely used in hydropower plants, operate under different loads which significantly affect their hydraulic characteristics and structural dynamics. It is essential to carry out the flow-induced dynamics analysis of the large prototype Francis turbines under different loads in a wide load operation range to optimize the hydraulic performance, ensure structural reliability, and prevent mechanical failure. This work analyzes the flow-induced dynamics of a large Francis turbine prototype with a rated power of 46 MW. Computer-aided design (CAD) models of the Francis turbine unit are first constructed, including the fluid and structural domains. After generating the computational meshes of the flow passages in the Francis turbine unit, Computational fluid dynamics (CFD) calculations are carried out under four typical operating conditions from 25% load to 100% load, and the pressure files obtained from CFD calculations are applied to the finite element model to analyze the flow-induced stresses of the runner. The results show that the pressure inside the Francis turbine runner decreases gradually from the spiral case to the draft tube under 25%, 50%, 75%, and 100% loads, but the local pressure distribution in the crown chamber of the Francis turbine unit varies under different loads. The locations of the maximum stress of the runner under the four different operating conditions vary with the power output. The flow-induced maximum stress of the runner at 25% load is located on the chamfer of the connection between the blade trailing edge and the crown. But from 50% load to 100% load, the maximum stress of the runner appears on the chamfer of the connection between the blade leading edge and the band. From 25% load to full load, the maximum stress of the unit is one-fifth of the yield stress of the runner material, and the runner will not be damaged during normal use. The calculation method with a fully three-dimensional fluid–structure interaction (FSI) method and the conclusions proposed in this study can provide important references for the design and evaluation of other hydraulic turbine units. [ABSTRACT FROM AUTHOR]

Traditional photovoltaic (PV) forecasting methods often overlook the impact of the correlation between different power fluctuations and weather factors on short-term forecasting accuracy. To address this, this paper proposes a PV output forecasting method based on Variational Mode Decomposition (VMD) disturbance feature extraction and the WaveNet model. First, to extract different feature variations of the output and enhance the model's ability to capture PV power fluctuation details, VMD is used to decompose the PV output time series, obtaining IMFs modes representing output disturbances and quasi-clear sky IMF modes. Then, to reveal power changes, especially the underlying patterns of disturbances and their relationship with weather factors, K-means clustering is applied to the IMF modes representing output disturbances, clustering the disturbance IMFs into different power change feature clusters. This is combined with Spearman correlation analysis of weather factors and the construction of an experimental dataset. Finally, to enhance the model's learning ability and improve short-term output forecasting accuracy, the WaveNet model is employed during the forecasting phase. Separate WaveNet models are constructed and trained with the corresponding datasets, and the total PV output forecast is obtained by superimposing the predictions of different IMF modes. Experimental results are compared with traditional methods, demonstrating a significant improvement in forecasting accuracy, with a Mean Absolute Percentage Error (MAPE) error of 6.94%, highlighting the effectiveness of our method and providing strong technical support for the refined management and intelligent forecasting of PV energy. [ABSTRACT FROM AUTHOR]

The large dimensions of the 1000 MW hydroelectric generator sets require high mounting accuracy. Small deviations can lead to asymmetry, which in turn triggers unbalanced magnetic pulls and moments. Therefore, symmetry is a central challenge in the installation and operation of giant hydroelectric generators. In this paper, the effects of radial eccentricity, axial offset, and rotor shaft deflection on the unbalanced magnetic pull and moment are investigated by transient finite element analysis of the asymmetric magnetic field. The results of the time-domain and frequency-domain analyses show that asymmetric operation generates unbalanced magnetic forces and moments. These forces and moments increase linearly with increasing offset or deflection rate. When the eccentricity meets the installation criteria, the unbalanced magnetic pull forces are small and within acceptable limits. This study helps to understand the relationship between asymmetry and unbalanced magnetic pulling forces in large hydroelectric generators, and provides a theoretical basis for standardizing installation deviation control. [ABSTRACT FROM AUTHOR]