Load Rejection Research Articles

Dynamic Controllability Study of Extractive and Pressure‐Swing Distillations for Tetrahydrofuran/Water Separation

AbstractControllability features of the extractive distillation process and the pressure‐swing distillation process were studied with dynamic tools and compared in the case study of separation of a mixture of 50 mol % tetrahydrofuran and 50 mol % water. Process simulations with Aspen Plus and Aspen Plus Dynamics were carried out to obtain 99.9 mol % product purity. Control structures for the two distillation techniques were determined. Load rejection studies were completed for ±10 % feed throughput and ±10 % feed composition disturbances, for both processes. The comparison of the dynamic responses, integral absolute errors, and integral square errors shows that the pressure‐swing distillation process has better controllability features than extractive distillation.

A many-objective optimization strategy for unified optimal operation of pumped storage hydropower station under multiple load rejection conditions

Pumped storage hydropower stations are essential for the efficient integration of renewable energy, while frequent conditions conversion make them prone to fall into terrible transient processes, posing a stiff challenge for sustaining stable operation. Especially when simultaneous or successive load rejection happens, the hydrodynamic pressure can be tremendous because of hydraulic interference and outrageous flow changes. Researchers have actively pursued for optimal control under extreme conditions, while their concern is solely on a specific condition. The circumstance may happen that operation strategy admirable for certain condition is generally inappropriate or even highly hazardous for another. Following this idea, a many-objective optimization strategy for unified optimal operation is proposed, which intends to pinpoint the optimal strategy of turbine mode suitable for multiple load rejection conditions with marvelous transient process performance. In the first place, the model foundation of pumped storage hydropower stations characterized by one-tunnel-two-units is primarily established, which incorporates logarithmic curve projection for precise emulation on pump-turbines. Meanwhile, a novel constrained many-objective optimization strategy is proposed as optimization basis, which possesses distinguished exploration and exploitation attributes taking the merit of reference direction and ε-constraint. The effect analysis of operation scheme is conducted for a preliminary understanding of stability characteristic and dynamic quality. The calculation example applied to actual case shows that, compared with other state-of-art counterparts, the proposed many-objective optimization algorithm effectively harmony the contradictory targets under multiple load rejections, and satisfactory performance can be obtained with existence of adequate safety margin disregarding the variation of operation parameters. These results demonstrated the effectiveness and applicability of the proposed strategy for the insurance of stable operation.

Fluctuation in the Water Level of the Air Hole of the Gate Shaft in the Pumped Storage Power Station

In some pumped storage stations, water spray from the air hole occurs during load rejection. In order to avoid this phenomenon, it is necessary to study the change of the air hole water level during the transient process of the power station. A transient simulation of a pumped storage power station was carried out to study the variation in the maximum water level of the air hole and gate shaft. The transition process of a power station with or without a surge tank was analyzed, and the changes in the water level of the gate shaft and air hole were compared based on different gate shaft areas, and the influence of the gate shaft location on the changes in the water level of the gate shaft and air hole was determined. The following results were obtained: when the power station has a surge tank, the maximum water levels of the gate shaft and air hole are basically consistent; when the station has no surge tank, the maximum water level difference between the gate shaft and air hole gradually increases as the gate shaft area increases; the greater the distance between the gate shaft and inlet, the larger the maximum water level difference between the air hole and gate shaft; overflow measures for the air hole are required if the maximum water level of the air hole exceeds the design value.

Analysis of the Integrated Effect of Temporary Overvoltages, PV Transformer Connection and Overcurrent Protection in Hybrid PV-Wind Energy System

The integration of solar and wind energy in hybrid PV/Wind energy systems (HPWES) have allowed it to be more reliable renewable energy source. However, the overvoltage protection for HPWES is scarcely covered in literature. To fill this gap, an investigative analysis of the effect of temporary overvoltages (TOVs) in HPWES is presented in this paper. Sources of TOV considered were ground faults and load rejection events. The HPWES was simulated and tested for TOVs under different fault conditions. The results showed that the TOV response was dependent on three more elements; overcurrent protection relays (OCRs), surge protective devices (SPDs) and the type of the PV transformer connection. The analysis showed that an interactive effect between these three elements after the occurrence of TOVs. This interactive effect has affected the tripping time of the OCRs which in turn affected the TOV duration and the energy absorbed by SPDs. It was concluded that the impact of the type of transformer connection and OCR tripping settings upon SPD could be critical that in some cases could lead to SPD failure. Conclusions are presented to the designers of HPWES to consider them during the design phase to overcome possible failures of SPDs.

A semi-implicit slip algorithm for mesh deformation in complex geometries, implemented in OpenFOAM

Many engineering applications of computational fluid dynamics (CFD) comprise extensive movement of objects that necessitate complex dynamic mesh treatments. In particular, the mesh motion process frequently requires a proper slipping of mesh points on highly curved surfaces. The currently available implementation of explicit slip boundary conditions in OpenFOAM fails to allow large deformations of the mesh without severely degrading the mesh quality and inverting some of the cells. Thus, a robust semi-implicit slip algorithm, based on the Laplacian smoothing methodology, is developed in the present work to tackle this issue. The algorithm is in fact performed in two steps, one explicit and one implicit. The OpenFOAM implementation of the algorithm includes different mesh motion solvers and boundary conditions, based on the displacement or velocity of points. The method is first verified using simple, yet relevant, test cases, and it is shown that the developed algorithm significantly outperforms some of the well-known proprietary CFD codes. Then, it is applied to a complex practical CFD case study. An engineering application that requires the features of the developed mesh motion algorithm is the transient operation of Kaplan turbines. These double-regulated machines simultaneously adjust the guide vane and runner blade angles while changing the operating condition. CFD simulations of such transient operations are highly complex, as they involve mesh deformation of the guide vane passage and simultaneous mesh deformation and rigid-body rotation of the runner blade passage. The mesh deformation requires points to slip on the curved hub and shroud surfaces while preserving the cell quality in tiny blade clearances. Therefore, the feasibility of the developed algorithm is evaluated for a load rejection sequence of a Kaplan turbine model. Program summaryProgram Title: Semi-implicit slip mesh motionCPC Library link to program files:https://doi.org/10.17632/wztc26vh7b.1Developer's repository link:https://github.com/salehisaeed/semiImplicitSlipLicensing provisions: GPLv3Programming language: C++Nature of problem: CFD simulations of numerous engineering fluid flows, such as transient operation of hydraulic turbines, involve an immensely complicated mesh motion process consisting of simultaneous mesh deformation and mesh slipping on highly curved surfaces. The available standard mesh motion methodology in OpenFOAM lacks some features to simulate this elaborate mesh motion. The introduced program addresses this problem by developing a new dynamic mesh algorithm.Solution method: The program implements a robust semi-implicit algorithm for slipping the mesh points on curved surfaces. The algorithm includes two steps, namely, an explicit step based on the general slip condition and an implicit step based on the Dirichlet condition. It employs the Laplacian smoothing equations to spread the mesh deformation into the domain. Additionally, a solid-body rotation may be added on top of the deformed mesh, which could be useful for modeling the runner region in transient operation of Kaplan turbines which contains simultaneous mesh deformation and solid-body rotation of the mesh.

Finite Volume Method for Modeling the Load-Rejection Process of a Hydropower Plant with an Air Cushion Surge Chamber

The pipe systems of hydropower plants are complex and feature special pipe types and various devices. When the Method of Characteristics (MOC) is used, interpolation or wave velocity adjustment is required, which may introduce calculation errors. The second-order Finite Volume Method (FVM) was presented to simulate water hammer and the load-rejection process of a hydropower plant with an air cushion surge chamber, which has rarely been considered before. First, the governing equations were discretized by FVM and the flux was calculated by a Riemann solver. A MINMOD slope limiter was introduced to avoid false oscillation caused by data reconstruction. The virtual boundary strategy was proposed to simply and effectively handle the complicated boundary problems between the pipe and the various devices, and to unify the internal pipeline and boundary calculations. FVM results were compared with MOC results, exact solutions, and measured values, and the sensitivity analysis was conducted. When the Courant number was equal to 1, the results of FVM and MOC were consistent with the exact solution. When the Courant number was less than 1, compared with MOC, the second-order FVM results were more accurate with less numerical dissipation. As the Courant number gradually decreased, the second-order FVM simulations were more stable. For the given numerical accuracy, second-order FVM had higher computational efficiency. The simulations of load rejection showed that compared with the MOC results, the second-order FVM calculations were closer to the measured values. For hydropower plants with complex pipe systems, wave velocity or the Courant number should be adjusted during MOC calculation, resulting in calculation error, and the error value is related to the parameters of the air cushion surge chamber (initial water depth, air cushion height, etc.). The second-order FVM can more accurately, stably, and efficiently simulate the load-rejection process of hydropower plants compared with MOC.

Two Decentralized Dynamic State Estimation Schemes for Multimachine Power Systems with Transmission Losses

This article addresses the problem of estimating the state of a multimachine power system (MPS). To work with power networks with lossy transmission lines, a variation of the classical third-order MPS is proposed by considering generators’ electrical power injected into the grid as a state variable. Based on a linear decentralized estimation model (tailored for a specific purpose), the state variables of generators (load angle, relative speed, and electrical power) together with terminal voltage magnitudes are estimated in a decentralized fashion through two new robustly convergent linear Luenberger estimators, one based on load angle measurement and the other based on relative speed measurement. The new MPS estimation model includes a set of robustly quick observable states, one per machine, which allows capturing the interaction with other generators, transmission line losses, unknown disturbances, and model errors. The result is a design superior to other related estimation techniques such as the extended Kalman filter (EKF) or Sliding Modes Perturbation Observer (SMPO) in terms of (i) a conventional-like simple pole placement-based tuning, (ii) low online computational load and disturbance rejection capability, and (iii) small gain-based convergence assessment. The performance of the proposed state estimation scheme is illustrated in a 3-machine power system under different operational conditions.

A Study of the Velocity Field During Mitigation of Vortex Breakdown in Model Francis Turbine at High Load

Abstract Off-design operation leads to the development of flow instabilities like vortex breakdown phenomenon which manifests as an enlarged vortex core in the draft tube at high load operating conditions. These flow instabilities are known to potentially have detrimental effects on turbine performance necessitating investigations on their formative and mitigation mechanisms. This work clarifies the evolving velocity maps characterizing vortex breakdown seen in a model Francis turbine draft tube during the transition from high load to best efficiency point. Velocity measurements have been performed inside a draft tube cone using a 2D particle image velocimetry system. Results show a wake-like velocity profile characterizing the vortex core in the draft tube cone at high load condition. The vortex core is a centrally located flow feature embodying a quasi-stagnant flow with recirculation regions. Surrounding the core, an axial outflow is seen with shear layers arising at the interface of core and outflow due to a substantial velocity gradient. Mitigation of this vortex core through a load rejection operation was further investigated. It is seen that as the flowrate approaches the best efficiency point, the shear layers between the outflow and central stagnation region break. The breakup leads to an axially dominated and streamlined flow. This is enabled by the reduction of the swirl until no central flow separation at the stagnation point occurs. The flow at the best efficiency point is thus devoid of the vortex core due to the absence of flow stagnation, the primary instability causing the core development.

Stress Characteristic Analysis of Pump-Turbine Head Cover Bolts during Load Rejection Based on Measurement and Simulation

It is not uncommon for pump-turbine units in pumped storage power plants to experience load rejections due to the sudden disconnection of the generator from the power grid. Load rejection can suddenly increase the rotating speed of the pump-turbine and cause strong pressure fluctuations in the flow passage of the pump-turbine unit. During load rejection, the strong pressure fluctuations caused by the water hammer effect can cause strong structural vibrations, high stresses and even damage to the turbine runner, head cover, stay ring, bottom ring, head cover bolts and bottom ring bolts. In order to study, in detail, the flow-induced stress characteristics of the prototype pump-turbine unit, and the pressure variations during load rejection in a high-head pumped storage power plant were measured first. Then the measured data were used to set up computational fluid dynamics (CFD) simulations in the entire flow passage of the prototype pump-turbine and to calibrate the simulation results. The calculated pressure distributions in the flow passage during load rejection were exported and mapped on the finite element model of the stationary structures of the pump-turbine unit so that the flow-induced stresses on the head cover, stay ring, bottom ring, head cover bolts and bottom ring bolts can be calculated. The results of the analysis show that the maximum stresses in the head cover bolts and bottom ring bolts are located on the rounded corner of the bolt near the stay ring and that the stresses in the bolts vary with time during load rejection. The maximum stresses of the head cover bolts are higher than the maximum stresses of the bottom ring bolts, and the maximum stresses of the bolts are above two-thirds of the yield strength of the bolt material. It is recommended to use larger nominal diameter bolts to avoid damage to the connecting bolts of the pump-turbine unit.

Evolution mechanism of water column separation in pump turbine: Model experiment and occurrence criterion

Whether the superposition of pressure pulsation and water hammer pressure caused by the fast closing of guide vanes causes water column separation in draft tube inlets is a hot topic in engineering and academic fields. It is also not known whether the bridging of water column separation endangers the operation of the units. This study uses a model pumped storage power station platform with two units to perform water column separation. Data on the pressure pulsation at different measurement points, discharges, and speeds are collected by an acquisition system, whereas the development process of water column separation is characterized by a high-speed camera. The measured pressure is decomposed into the water hammer pressure, which migrates with the unit operation point and pressure pulsation. The test results reveal that the pressure in the inlet section of the draft tube below the vaporization pressure that lasts for several seconds is a sufficient and necessary condition for water column separation to occur. The water hammer pressure has a significant influence on the water column separation, while low-frequency pressure pulsation has less influence. Water column separation is divided into four stages: starting, improvement, separation, and bridging. The main frequency of the pressure pulsation in the water column separation process is the frequency of the water hammer caused by load rejection and has a value of 0.47f/fn. Cavity bridging generates a frequency of 0.30f/fn. The flow-speed curve remains in the braking condition of turbine for 1.25 s. The separation and part of the bridging stage occur under the braking condition of turbine.

Robust Reference Tracking and Load Rejection on Non-Linear System Using Controllers

DC-DC converters form a class of highly nonlinear systems and are widely used in most practical applications. Positive Output Elementary Luo Converter (POELC) is a fourth-order converter used to regulate dc voltages in low power applications with varying input voltage and load conditions. POELC is employed to regulate dc voltages for sensitive load applications like SMPS, powering hard disks etc. The control of higher-order converters is always a challenging task due to the highly nonlinear nature of these converters. Hence the development of an optimal controller for POELC is proposed in this paper. The proposed control methods are investigated for transient and steady-state performances considering practical operating conditions. The controllers are designed, simulated and the performance of POELC with various controllers is evaluated in the MATLAB/Simulink environment. The PI controller exhibits better performance in the linear region whereas, in the nonlinear region, due to time variance and switching nature. To minimize the sensitivity to external disturbances and to improve the stability margins, an LQR is designed to regulate the output of POELC. The result shows that a well-designed LQR feedback can stabilize the system and enhances its performance in terms of Integral Absolute Error (IAE), Integral Square Error (ISE). Certain case studies are discussed concerning practical applications.

Load Rejection Overvoltage of Distribution-Connected IBRs

Inverter-based resources are proliferating in distribution systems across the globe, along with battery energy storage systems. This in part is attributed to a descending trend observed in their costs. These inverter-based resources, however, create new problems for electrical utilities planners and engineers. One such issue is how to measure, test, and manage load rejection overvoltage. This phenomenon takes place upon sudden islanding of a power system area such that it becomes supported by grid-following inverter-based resources only. This letter presents background, practical methods to test the behavior, as well as two case studies of utility-scale generation and energy storage connected to a distribution feeder.

Flow-Induced Dynamic Behavior of Head-Cover Bolts in a Prototype Pump-Turbine during Load Rejection

In order to ensure stable grid operatiFon and improve power quality, active or passive load rejection of pumped storage power stations (PSPS) inevitably occurs from time to time. The rapid closing of the guide vanes will cause drastic changes in pressure pulsations in the flow channel of the pump-turbine (PT) unit. The high-level pressure pulsations during load rejection transfer to the entire flow passage of the PT unit and generate strong vibrations on the head-cover and the connecting bolts. In this study, the 1D/3D joint simulation of the pipeline in a pumped storage power station and the turbine flow channels including the flow domains of the runner, crown chamber, band chamber, upper and lower labyrinths and pressure balance tubes is carried out first. Then, by applying the calculated pressure loads on the head-cover, stay vanes and bottom ring of the PT unit, the flow-induced dynamic behavior of the structures including the head-cover bolts is analyzed in detail. The results demonstrate that pressure loads on head-cover bolts change dramatically during the load rejection process. The flow-induced deformation of the inner head-cover during the load rejection is larger than that of other structures, and the flow-induced displacement and stress of different head-cover bolts are not uniform. The achieved conclusions in this study can be a useful reference for the design and operation of head-cover bolts for other PT units and high-head Francis turbine units.

Cuproptosis-Related lncRNA Gene Signature Establishes a Prognostic Model of Gastric Adenocarcinoma and Evaluate the Effect of Antineoplastic Drugs.

One of the most frequent malignancies of the digestive system is stomach adenocarcinoma (STAD). Recent research has demonstrated how cuproptosis (copper-dependent cell death) differs from other cell death mechanisms that were previously understood. Cuproptosis regulation in tumor cells could be a brand-new treatment strategy. Our goal was to create a cuproptosis-related lncRNA signature. Additionally, in order to evaluate the possible immunotherapeutic advantages and drug sensitivity, we attempted to study the association between these lncRNAs and the tumor immune microenvironment of STAD tumors. The TCGA database was accessed to download the RNA sequencing data, genetic mutations, and clinical profiles for TCGA STAD. To locate lncRNAs related to cuproptosis and build risk-prognosis models, three techniques were used: co-expression network analysis, Cox-regression techniques, and LASSO techniques. Additionally, an integrated methodology was used to validate the models' predictive capabilities. Then, using GO and KEGG analysis, we discovered the variations in biological functions between each group. The link between the risk score and various medications for STAD treatment was estimated using the tumor mutational load (TMB) and tumor immune dysfunction and rejection (TIDE) scores. We gathered 22 genes linked to cuproptosis based on the prior literature. Six lncRNAs related to cuproptosis were used to create a prognostic marker (AC016394.2, AC023511.1, AC147067.2, AL590705.3, HAGLR, and LINC01094). After that, the patients were split into high-risk and low-risk groups. A statistically significant difference in overall survival between the two groups was visible in the survival curves. The risk score was demonstrated to be an independent factor affecting the prognosis by both univariate and multivariate Cox regression analysis. Different risk scores were substantially related to the various immunological states of STAD patients, as further evidenced by immune cell infiltration and ssGSEA analysis. The two groups had differing burdens of tumor mutations. In addition, immunotherapy was more effective for STAD patients in the high-risk group than in the low-risk group, and risk scores for STAD were substantially connected with medication sensitivity. We discovered a marker for six cuproptosis-associated lncRNAs linked to STAD as prognostic predictors, which may be useful biomarkers for risk stratification, evaluation of possible immunotherapy, and assessment of treatment sensitivity for STAD.

Review of Research on the Three-Dimensional Transition Process of Large-Scale Low-Lift Pump

Due to the uneven distribution of water resources, there are many water diversion projects around the world, such as the South-to-North Water Diversion Project in China, especially in some plain areas. To transfer water from low to high areas, large low-head pumps have been widely used. The transition process of the pumping station is mainly caused by the sudden change in the flow velocity and pressure of the fluid in the pipeline of the pumping station system caused by the start-up and shutdown processes. The previous research has mainly been based on the one-dimensional characteristic line method. However, due to the characteristics of the low-lift pumping station, the flow passage is short and irregular, and the calculation results often cannot guarantee the accuracy of the calculation. In addition to some faults in the actual operation process, in some pumping stations, accidents or operation-scheduling faults are caused by transient processes, such as a high degree of water hammer, the inability to initiate backward flow, the shutdown load rejection runaway exceeding the standard, and decreased hydraulic efficiency. To avoid transition process failures in the newly designed pumping stations and the modified pumping stations, it is necessary to carry out a research review of the three-dimensional transition process of large low-lift pumps. Especially with the development of computing technology, CFD numerical simulation technology has become the main research method for analyzing the pump transition process. The research on the transition process is mainly based on the combination of numerical simulations and experiments. The reliability of a numerical simulation is verified by an experiment. A numerical simulation can measure some parameters that cannot be measured by an experiment. Dynamic mesh technology has become the main technical means for using CFD numerical simulation to study the three-dimensional transition process, and the secondary development of computing software has become the main trend of future development. This paper analyzes and summarizes the research status of the start–stop transition process of large low-lift pump stations and provides a reference for the protection of the start–stop transition process of pump stations.

Transient analysis of a hydropower plant with a super-long headrace tunnel during load acceptance: Instability mechanism and measurement verification

The current development trend of China's hydropower is moving toward ultra-high heads, complex geological scenarios, ultra-large capacities, and the Tibetan region. The construction of hydropower stations with super-long headrace tunnels is conducive to overcoming the constraints of complex geological conditions and making full use of hydropower resources. However, these stations have unique technical problems that limit their commercial development, including pressure fluctuations during load rejection and the operational stability of the power-frequency regulation. In this study, a potential problem in hydropower stations with super-long headrace tunnels, the instability during load acceptance, is studied by combining prototype experiments with 1D transient numerical simulations. First, according to a test case using a prototype hydropower station, the instability phenomenon occurring during a simultaneous load acceptance of two Francis turbines in a hydropower station with a 20-km headrace tunnel is described. Next, using 1D steady-state and transient simulations, the factors leading to the instability are analyzed in detail. Finally, countermeasures for the safe operation of hydropower stations are proposed based on numerical simulations and supplementary prototype experiments. Hydropower stations with super-long headrace tunnels exhibit a characteristic where the net head decreases sharply with an increase in the flow rate. Therefore, prior to load acceptance, the maximum output achievable by the multiple units under the operating net head has to be carefully determined to avoid over-adjustment of the guide vane opening and detrimental phenomena such as operational instability and extremely low pressure in the draft tube.

On the auto-oscillation effect of Kaplan rotors during load rejection

High vibrations due to auto-oscillation effects can occur at power units with Kaplan type runner during load rejection. These vibrations may be present in the turbine braking mode when the wicket gates are almost closed, and the rotating runner pushes the water towards the draft tube resulting in higher pressure below the runner blades. The upward leakage flow between runner blades and discharge ring is suspected to cause the vibration phenomenon.Within this contribution, a fluid-structure-interaction (FSI) technique is applied to identify the mechanism of self-excitation. The investigation is carried out on a disk that represents the simplified geometry of a Kaplan runner. Computational fluid dynamics (CFD) simulations are used in order to obtain parameters in terms of stiffness, damping and inertia of the fluid to be applied in the following shaft line analysis. Damped modal analyses of the shaft line allow for evaluation of the stability of natural frequencies and corresponding mode shapes of the shaft line. A resulting negative damping indicates the occurrence of auto-oscillation behavior of the shaft line. For the operating condition under investigation, mode shapes with negative damping are obtained with this approach and a dynamic bending moment on the runner due to lateral movement in combination with a tilting at the runner in these mode shapes is identified as source of instability.Based on these findings, an analytical approach for modeling the relevant effect is derived and compared to CFD result as well as results from acoustic finite element modelling. Finally, conclusions for avoidance of self-excited vibrations at Kaplan runners are proposed.

248.11: Torque Teno virus Load as Functional Marker of Immune Function in Solid Organ Transplantation: A Systematic Review And Meta-Analysis

Background: Balancing immunosuppression to prevent rejection and infection in solid organ transplant recipients (SOTx) remains a challenge. Torque teno virus (TTV), a commensal non-pathogenic virus, has been proposed as functional marker of immunity: higher loads reflect over-immunosuppression, while lower loads reflect under-immunosuppression. Here we present a systematic review and meta-analysis of the current literature and provide evidence on the association between TTV load and infection and rejection in SOTx recipients. Methods: A systematic literature search strategy, deposited in PROSPERO, resulted in 548 records. After screening, 23 original and peer-reviewed articles were included in qualitative assessment, investigating the association between TTV load, and either infection or rejection in SOTx. The Quality in Prognosis Studies (QUIPS) tool was used to assess the risk of bias. Meta-analysis was performed on studies with similar outcomes measures and exposure using a joint modelling strategy. Results: 23 studies were included, most involved retrospective cohorts in which the TTV load was measured within 1-2 years post-transplantation, once or longitudinally. Defined outcomes differed between studies, and included viral (CMV, BK, respiratory), bacterial, and fungal infections. Rejection outcomes included rejection treatment, with or without biopsy confirmation. QUIPS assessment showed varying risks of bias between studies. Twelve out of 17 studies reported an association between high TTV loads and infection (71%), and 13 out of 15 an inverse association between low TTV loads and rejection (87%). Meta-analysis showed an increased risk of infection (OR: 1.16, CI 1.04-1.30; HR: 1.05, CI 0.97-1.14) and decreased risk of rejection (OR: 0.90, CI 0.87-0.94, HR: 0.74, CI 0.71-0.76) per 1 log TTV load increase. Conclusions: Systematic review of the current literature shows an association between TTV load and each outcome in most studies, and this association is supported by our meta-analysis. However, concerns on study quality and the risk of publication bias warrants careful interpretation. Externally validated prediction models using TTV load remain to be built, as these will offer individualized diagnostic or prognostic value to predict infection and rejection in SOTx recipients.

Effects of Hydro Power Plant Releases on Navigation Conditions in the Lower Lock Approach

Waterways are usually designed as multipurpose structures and perform a number of functions in addition to transport. A very common use is the generation of electricity in hydropower plants, water withdrawal for users and flood protection. On canalized rivers, increased attention must be paid to the design of the layout of the individual structures within the barrage. This paper deals with the optimization of barrage layout with respect to the interaction of navigation and hydropower use of the waterway. The navigation conditions below a power plant can be significantly affected by peak operation or by sudden load rejection. These phenomena can generate a negative surge wave with very adverse effects on navigation. In addition, special attention should be paid to the current conditions in the lower lock approach area and the guard wall that separates the lock approach from the weir and the power plant should be optimised. Appropriate design of operational and structural measures can significantly increase the traffic capacity of a waterway. The research was conducted as part of project no. DG18P02OVV004 within a support programme for applied research and experimental development of national and cultural identity, funded by the Ministry of Culture of the Czech Republic.

Variation law of pressure pulsation during variable speed operation of pumped storage units

Variable speed operation technology has become an emerging development direction in hydropower industry. Variable-speed pumped storage plants (VSPSPs) constantly undergo various transient processes, and ensuring the amplitude of pressure pulsation within the control range is critical to the safe operation of variable-speed pumped storage units (VSPSUs). However, its extreme value is usually determined by rule of thumb in design stage, and few researches focus on the relationship between pressure pulsation and variable speed operation of VSPSUs. In this paper, characteristic curves of pressure pulsation are plotted by processing model test results with Gridfit function, taking a real VSPSP as an example. The total pressure during transient processes is predicted by simulation of a numerical model. Load rejection conditions (change initial speed) and large load regulation conditions (change speed command and power command) are simulated, and the influence of operating trajectory on pressure pulsation during variable speed operation is analyzed. The results show that with the increase of initial speed or speed command, the value of pressure pulsation increases; the trajectory passes through more high-amplitude pressure pulsation region and gradually shifts to the S-shaped region, causing the pressure oscillation. This study could provide theoretical reference for the operation of VSPSUs.