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Before putting into operation, ultra-high voltage transformers need to undergo strict handover tests to ensure their safe and reliable operation. The grading capacity of the grading ring on the transformer will affect the accuracy of the handover test. When there are problems with high altitude environments or grading ring structures, their grading ability will change. It is particularly important to study and design grading rings that can have good grading ability in different situations. This article first uses COMSOL software to build a three-dimensional model of the withstand voltage test circuit of a 500 kV transformer. Secondly, the influence of changes in the pipe diameter and ring spacing of the equalizing ring on the equalizing capacity of the equalizing ring is studied. Finally, the variation of equalizing capacity of the equalizing ring in a rough state is studied. It is found that as the diameter of the equalizing pipe and the ring diameter increase or the distance between the rings decreases, the equalizing ability of the equalizing ring will be enhanced, with the greatest impact brought by the change in pipe diameter. When the grading ring has a rough surface, its grading ability will significantly decrease. Among them, the grading ability decreases most significantly when adhering to conical particles, and decreases least when concave, indicating that the smoother the surface of the grading ring, the better the grading ability. This study can provide some assistance for the design of grading rings for high altitude ultra-high voltage transformers.

Fault ride-through is a prerequisite for ensuring continuous operation of a variable-speed pumped storage unit with a full-size converter (FSC-VSPU) and providing support for the renewable energy and power grid. This paper proposes low-voltage ride through (LVRT) and high-voltage ride through (HVRT) strategies for FSC-VSPU to address this issue. Firstly, the structure of FSC-VSPU and its control strategy under power generation and pumping conditions are described. Subsequently, the fault characteristics of the FSC-VSPU under different operating conditions are analyzed. More stringent fault ride-through technical requirements than those for wind turbines are proposed. On this basis, the fault ride-through strategies of combining fast power drop, rotor energy storage control, power anti-regulation control, dynamic reactive current control, low power protection, and DC crowbar circuit are proposed. Simulation case studies conducted in PSCAD/EMTDC verify the correctness of the theoretical analysis and the effectiveness of the LVRT and HVRT strategies in this paper. [ABSTRACT FROM AUTHOR]

The variable speed pumped storage unit with a full-size converter (FSC-VSPSU) can provide fast and flexible regulation capacity for the power grid, assisting the rapid development of the new energy-dominated power systems, and its application is gradually becoming widespread. The excitation system of FSC-VSPSU is crucial for maintaining the motor-port voltage and further supports the stable operation of the entire unit. However, there is currently no research on excitation system modeling, and their essential operating characteristics have not been discussed in detail, which needs further study. With this aim, this paper firstly establishes a detailed transfer function model for FSC-VSPSU, which is then used to study the influence rules of FSC-VSPSU parameters on the operating characteristics. In addition, based on the summarized operating characteristics, the excitation system can be optimized, and its complexity is greatly reduced while ensuring the original performance of the unit. Finally, the results of the simulation can verify the correctness of the established model and the explored operating characteristics. [ABSTRACT FROM AUTHOR]

Background: Traditional risk evaluation models have been applied to guide public health and clinical practice in various studies. However, the application of existing methods to data sets with missing and censored data, as is often the case in electronic health records, requires additional considerations. We aimed to develop and validate a predictive model that exhibits high performance with data sets that contain missing and censored data., Methods and Results: This is a retrospective cohort study of coronary heart disease at Weihai Municipal Hospital on unique patients aged 18 to 96 years between 2013 and 2021. A total of 169 692 participants formed our study population, of which 10 895 participants were diagnosed with coronary heart disease. Models were built for the risk of coronary heart disease based on demographic, laboratory, and medical history variables. All complete samples were assigned to the training set (n=110 325), whereas the remaining samples were assigned to the validation set (n=59 367). The area under the receiver operating characteristic curve value was 0.800 (95% CI, 0.794-0.805), and the C statistic was 0.796 (95% CI, 0.791-0.801) in the derivation cohort, and the corresponding values were 0.837 (95% CI, 0.821-0.853) and 0.838 (95% CI, 0.822-0.854) in the validation cohort. The calibration curve demonstrated its good calibration ability, and decision curve analysis showed its clinical usefulness., Conclusions: Our proposed risk prediction model has demonstrated significant effectiveness in handling the complexities of electronic health record data, which often involve extensive missing data and censoring. This approach may offer potential assistance in the use of electronic health records to enhance patient outcomes.