Load Rejection Research Articles

Analysis of extremely low water hammer pressures of draft tubes for double units in pumped storage power stations under successive load rejection conditions

With large-scale integration of intermittent energy into power systems, the operating conditions of pumped storage power stations (PSPSs) change frequently, thereby intensifying the risk of load rejections. For multiple units sharing a water conveyance system, when one or more units reject the load, other units also reject the load after a few seconds; this is defined as successive load rejection (SLR). During SLR conditions, pumped storage units produce extreme water hammer pressures, jeopardizing the safe operation of PSPSs. This study reveals the generation mechanism of extremely low draft tube pressure (DTP) during SLR, and clarifies the generating unit and corresponding occurrence moment characteristics of the minimum DTP. The results indicated an extremely low DTP, attributed to the sharp decline in the discharge owing to increased rotational speed in unit's S-shape region and the hydraulic interference owing to the increased discharge of the other unit. When the guide vane closing time (GVCT) was short, the minimum DTP occurred in the subsequent load rejection unit (SLRU) near the first peak of its rotational speed, with the discharge of initial load rejection unit (ILRU) in the rising stage. Conversely, for a large GVCT, the minimum DTP occurred in the ILRU near the second peak of its rotational speed, with SLRU discharge in the rising stage. Finally, engineering measures to improve the extremely low DTP were proposed. Overall, the findings underscore crucial engineering insights for ensuring the safety of PSPSs under extreme conditions.

Using bionic tubercles to control swirling flow instabilities of a hydraulic turbine during the load rejection process

With the development of renewable energy sources, operations of hydroelectric units under off-designed conditions and frequently transition processes have gradually become normalized, which is accompanied by threats from swirling vortex ropes, high amplitude pressure fluctuations, and strong hydraulic vibrations. This study introduces bionic tubercles into the Francis turbine's draft tube and guide vane design to suppress pressure fluctuation and stabilize swirling vortex rope during the load rejection process. Results reveal that the bionic Francis turbine (BFT) reaches its maximum speed of 686.4 rpm at 3.804s, marking a 6.08 % reduction from the prototype Francis turbine's maximum speed during load rejection. Moreover, it significantly diminishes the principal frequency amplitude of low-frequency pressure fluctuations caused by swirling flow instability from the draft tube cone, achieving a reduction by one to two magnitude orders. It is interesting that the scale of the circumferential vortex in the vaneless region and blade vortices within the runner channels is verified to decrease. More importantly, it is attributed that the bionic tubercles effectively disrupt and disintegrate the walled circumferential vortex structures in the draft tube.

A novel multi-excitation transient vibration framework for coupling three- and one-dimensional pumped storage hydropower shafting systems

In vibration models of shafting systems, the hydraulic excitation is difficult to characterize due to the complex and changeable hydraulic factors. Thus, hydropower units are not well understood in terms of their dynamics and stability control under transient processes. A hydraulic–mechanical–electric multi-excitation transient vibration calculation framework is developed for analyzing the relationship between shafting vibration and internal flow regimes. First, the boundary data from penstocks, tailraces, and hydro-turbine are interacted with using one-dimensional and three-dimensional (1D–3D) coupling; Second, user-defined function secondary development is applied to achieve two-stage guide vane closure and the runner's variable speed rotation; Third, based on the computational fluid dynamics results, a multi-excitation vibration model is established to analyze shafting system characteristics. There is less than 1.2% error between the algorithm and the field test in terms of speed peak values. Under braking or reverse pumping modes, various vortice clusters are generated in the blade channel as well as the cascade, blocking the flow passage and leading to the runner's unbalanced force. Three sudden increases in vibration amplitudes of the shafting system have occurred in the radial direction under load rejection, each corresponded to the runner's stall rotations. The change trend in axial vibration amplitudes, however, is closely related to the change in axial hydraulic thrust. Furthermore, in braking and reverse pumping conditions, the axis trajectory is more complex under the action of multiple coupling factors than when only hydraulic factors are considered.

Numerical Investigation of Symmetrical and Asymmetrical Characteristics of a Preloading Spiral Case and Concrete during Load Rejection

Numerical Investigation of Symmetrical and Asymmetrical Characteristics of a Preloading Spiral Case and Concrete during Load Rejection

Comparison study of two control strategies for the small pressurized water reactor

The control strategy has a significant impact on the control performance of the reactor system. This paper investigates the feedwater flow and steam flow control strategies for small pressurized water reactors. First, the mathematical model of the SPWR Nuclear Steam Supply System is introduced. Second, a dual-constant control strategy was proposed, and the feedwater flow rate and steam pressure control systems under the two control modes were designed. Moreover, a steam bypass control system was designed to avoid excessive steam pressure under the feedwater flow control mode, and the reactor power and pressurize control systems were introduced. Simulation results indicate that the two control modes both meet the SPWR control requirements. However, the feedwater flow control mode has better reactor coolant temperature and steam pressure control performances during the load rejection transient, owing to the timely discharge of heat from the reactor coolant system through the steam bypass system.

Modeling of hydropower plant in islanded mode for different operating conditions

Most countries have access to abundant water resources through rivers and canals. Utilizing this renewable resource, electricity can be generated in an environmentally friendly manner without causing pollution. In rapidly developing countries like India with the abundance of natural resources and diversities, the development of Hydropower is gaining in importance to meet the country’s demand. This work discusses the different operating conditions that may occur in real time of the standalone hydro power generation system. In this work, various operating conditions are considered in terms of faults and disturbances that occur on the load side. These effects of faults and disturbances may be caused in the generating side. It takes into consideration some major events from the load side i.e. small disturbance, load addition, load rejection, large disturbance. In this work, the above-mentioned objectives are achieved by creating a model of a hydro power plant in MATLAB Simulink and keeping its operating environment same, simulate different scenarios related to load side, and study its effect on the generator and generating system. This is achieved by changing the load side for different conditions like introducing a small fault into the system, changing the load on a larger scale, etc The conditions that are introduced are simulated in a period of 10 s time frame. The reaction of the generating side from these conditions is recorded and plotted on parameters that can show the effect directly on the generator.

Optimising orthodox style learning in physical education through a variational training model

Background: The research problem is to improve our understanding of how different training approaches can improve bullet-shot results. Research Objectives: This study aims to investigate the impact of different training models on the learning outcomes of orthodox-style shot put. Methods: The research method employed was an experiment with a one-group pre-test and post-test design to enhance student learning outcomes using a varied training model. The subjects consisted of 32 students from a high school in Palembang. The study was conducted over 30 days, with four weekly meetings. During each meeting, 2-3 training models were presented, resulting in a total of 10 variation training models, namely Passing Techniques in Football Baseball, Techniques that Hold the Repulsion, Hanging the Ball, Resisting Pull, Clap Hand, Reject Target, Throwing Darts, Shoot in Target, and Load Rejection One Shoot. Data analysis used SPSS version 23 to calculate the significant value of the pretest and posttest results. Finding/Results: The study found that using varied training models resulted in significantly better learning outcomes in orthodox-style bullet throwing. Conclusion: The study concluded that varied training models have a considerable positive influence on learning outcomes in the orthodox style of instruction. This suggests that the findings contribute to a deeper understanding of effective training methods in sports education and have practical implications for athletic training programs. We could conduct longitudinal studies to investigate the long-term impact of diverse training on skill retention and entry into competitive athletics.

Flow Characteristics Analysis of Load Rejection Transition Process in Pumped Storage Unit Based on Cavitation Model

Flow Characteristics Analysis of Load Rejection Transition Process in Pumped Storage Unit Based on Cavitation Model

The Influence of Pump-Turbine Specific Speed on Hydraulic Transient Processes

The main porpose of this paper is to perform and present at transient process analysis on the influence of different specific speeds (nq) of the three-model pump-turbines that may implemented at the hydropower pump storage plant (HPSP) Bajina Basta, Serbia. The intention is to analyze the operation regimes along the S-shaped characteristic curve, which creates difficulties in calculations during the load-rejection process. These problems are manifested by an unusual increase in water pressure, followed by an increase in machine vibrations, thus threatening the stability of machine operation. The subject of this research is the four-quadrant (4Q) characteristic curves, presented in the form of Suter curves (the head and momentum parameters (Wh and Wm), in dependence of angle (θ)) for different wicket gate openings, of the three pump-turbine models with different specific speeds (nq = 27, nq = 38 and nq = 50). These 4Q characteristics are used as input data for numerical code developed to calculate the simultaneous load rejection of both pump-turbines. The numerical code is based on the method of characteristics and applied to the test case at HPSP. This is especially relevant when dealing with characteristic S-shaped curves that may lead to serious instability in operation during the transient process. The calculation results show changes in the pressure, speed of rotation and discharge of the machine operation regimes, but the important findings are reflected in further application of the results to determine the parametric flow and speed (Q11 and n11) trajectories of the operating points, where the unstable behavior of a pump-turbine operating in turbine mode is presented and periodically the trajectories also pass through the reverse pump zone.

Numerical simulation on the flow characteristics of a pump-turbine in load rejection process

Abstract Pumped storage plays a crucial role in maintaining the stability of the power grid, with the pump-turbine being the key component. The flow characteristics of the pump-turbine during transient processes have a significant impact on the unit’s safety, particularly in load rejection scenarios. In this study, a 1D transient simulation is conducted to analyze the flow characteristics of a pump-turbine during load rejection using the Quasi-static method. The study focuses on the flow patterns and axial hydraulic thrust. The results indicate that the unit operates in turbine, braking, and pumping modes during load rejection, with the streamline becoming chaotic. This complex flow pattern also leads to notable variations in the axial hydraulic thrust, ranging from -92t to 405t. The findings presented in this paper are valuable for ensuring the safety of pump-turbines and can serve as a reference for their optimization and operation.

Research and application of Qingyuan pumped-storage power station multi units control strategy during simultaneous load rejection

Abstract This paper studies the qingyuan pumped-storage power station multi units safety control strategy during load rejection at the same time, through the comparative analysis of several schemes, design the transmission line trip trigger generator tripping logic, optimize the existing protection system control strategy, the formation of the single line power station a few units safety control strategy during load rejection at the same time, the realization of multiple units in the case of lines correctly load rejection, Ensure the safety of unit operation.

Analysis of flow characteristics of a pump turbine under the transient condition of 75% load rejection

Abstract In this study, the flow calculations for a pump turbine under the transient condition of 75% load rejection was conducted. The model incorporated a boundary layer on the surface of the guide vane, with the node coordinates of each element in the boundary layer region being updated. This approach ensured the stability of the boundary layer mesh. Experimental measurements provided parameter curves for inlet pressure, outlet pressure, runner speed, and guide vane opening over time. The pressure fluctuation at each monitoring point was compared with the experimental data, and a good agreement was found. Additionally, the variation curve of the axial hydraulic thrust of the head cover and the pressure distribution nephogram of the head cover at typical moments were analysed. The results of this study could be used as a reference for the safe and stable operation of pumped-storage power stations.

Study on dynamic stress variation of head cover bolt during transient operation of pump turbine

Abstract The head cover bolt is an important connecting part of pump turbine, if it is damaged, it will affect the operation safety of the unit and the power station. During the transient process, such as start-up and load rejection in extreme case, the head cover bolt is subjected to complex load action, and its stress also changes, and extreme cases may occur. The dynamic stress level of the bolt will have a certain influence on its fatigue life, therefore, in order to further study the dynamic stress level and variation law of the bolt, this paper relies on a pumped-storage station, the dynamic stress of the head cover bolt during the start-up and load rejection is measured, and the pressure pulsation between the head cover and the runner and the bladeless zone are collected synchronously. Through the data analysis, the bolt dynamic stress level and variation law and the correlation between stress and pressure fluctuation under the power generation and pumping conditions are summarized.

FSI simulation of a high-head pump-turbine during load rejection process

Abstract High-head pump-turbine suffers large centrifugal and hydraulic forces due to large rotational speed and severe pressure pulsations during transient processes, and the strong vibration may increase the risk of runner failure. Load rejection transient process is crucial for pumped-storage hydropower stations, and the evolution laws of the runner dynamic stress during this process need profound research. In this study, the evolution of the runner dynamic stress of a prototype pump-turbine during a load rejection transient process was simulated using the one-dimensional and three-dimensional coupling (1D-3D) method and the fluid-structure interaction (FSI) method. The results of this dynamic transient process were compared to the results of the static working points. It is shown that the rotational speed dominates the mean value of stress while the fluctuating amplitude of stress is dominated by pressure fluctuations, which are larger in the S-shaped region of characteristics. Compared with the static working point solutions, the transient process solutions can better reflect the stress fluctuations and should be used for safety assessment.

Research on Closing Law of Guide Vane in a Francis turbine during the Load Rejection Process

Abstract It has become a general trend for new energy to be connected to the power grid, and the tasks of peak and frequency regulation undertaken by hydropower units are tough. Inevitably, the Francis turbine operates under a transient process, which brings serious pressure fluctuations and instability problems. In the present study, a three-dimensional simulation of Francis turbines with the water pipeline system during the load rejection process is carried out. Results indicated that when a straight line is used to close guide vanes, the Francis turbine generates significant rotational speed, which poses a threat to system safety and stability. Based on this, the sinusoidal closing law proposed in the present study can effectively reduce the maximum rotational speed by 2.6%. After the end of load rejection, the difference in rotational speed between the two closing laws is 5.15%.

Three-dimensional simulations of load rejection processes of a variable-speed pump-turbine under different initial rotational speeds

Abstract The load rejection transient process is one of the most dangerous operating conditions in pumped storage power stations. When the operating point slides through the S-shaped region quickly, the pressure fluctuations increase sharply and the unit vibrates obviously. In variable-speed units, the different initial rotational speeds have significant effects on the operating point trajectories during load rejection processes. In this paper, under the same head and guide vane opening, the load rejection processes with different initial rotational speeds were simulated by using three-dimensional methods, and the variations of rotational speed, discharge and torque were obtained and analyzed. The results show that the changes of initial rotational speeds affect the shapes of velocity triangle at the runner inlet and outlet, and causes the flow separations and vortices in the runner and draft tube, which leads to different efficiency and torque of the pump-turbine under initial steady conditions. In addition, during load rejection processes, the maximum values of macro parameters and pressure fluctuations, and the transitions of flow patterns have obvious differences because of these different initial rotational speeds.

Study on sensitivity evaluation of conveyance system to unit hydraulic transient characteristics in pumped storage plant

Abstract The minimum pressure of draft tube inlet under the condition of successive load rejection is the key factor restricting the unit hydraulic transient characteristics in pumped storage plant, and the layout of conveyance system is an important factor affecting the unit hydraulic transient characteristics. To explore the influence of the conveyance system layout on the unit hydraulic transient characteristics, the author proposed a new method to evaluate the sensitivity of conveyance system layout to the unit hydraulic transient characteristics. Furthermore, the simulation analysis of two case power stations is used to verify the new method.

Study on load rejection test of pumped storage units

Abstract The rise rate of rotating speed, the maximum inlet pressure of the spiral casing and the minimum inlet pressure of the draft tube cone of unit 2 in Jinzhai pumped storage power station were analyzed when 50% and 100% of the rated load were rejected in this paper. The low pass filtering method (cut off frequency is 0.2 times of the frequency) was used to process the pressure data at the inlet of the spiral casing and the draft tube cone to retain the trend term. The peak to peak values (97% confidence) of the pressure pulsation term under different data durations were studied, which provided technical support for rational and accurate calculation of the maximum inlet pressure of the spiral casing and the minimum inlet pressure of the draft tube cone, and provided an example reference for the load rejection test analysis of pumped storage units.

Effect of guide vane closing speed deviation on transition process in pumped storage power plant

Abstract The guide vane closing law is a crucial role in ensuring the hydraulic transients of pumped storage power plant. It is also essential for ensuring the safe operation of pumped storage power plant after long-term operation. Variations in the closing speed between simulated guide vane closing laws and on-site closing laws have been observed. This study first verifies the accuracy of the simulation model through load rejection tests conducted on two units. It then simulates the deviation in guide vane closing speed for these units. The deviation in guide vane closing speed does not impact the regulatory requirements for a single unit. However, variations in the closing speed between simulated guide vane closing laws and on-site closing laws have been observed. This study first verifies the accuracy of the simulation model through load rejection tests conducted on two units. It then simulates the deviation in guide vane closing speed for these units and proposes a reference range. The deviation in guide vane closing speed does not impact the regulatory requirements for a single unit. However, for two units in one pipe, opposite deviations in guide vane closing speed can result in key parameters failing to meet regulatory requirements.

Hydraulic analysis of jet deflection on Pelton turbine deflector based on CFD simulation

Abstract The deflector of the external regulating mechanism of the Pelton turbine can reduce the pressure of the nozzle structure during the emergency load rejection of the unit, and solve the problem that the pressure in the pressure pipe increases too much and the rotate speed of the unit gets too fast. This paper mainly introduces the flow pattern analysis of jet deflection when the Pelton turbine deflector is in working state. The results of calculation and analysis in this paper are based on the Navier-Stokes equations and k-ω SST turbulent model of CFD simulation.