Wicket Gate Research Articles

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.

Investigation Of The Influence Of The Turbine Wicket Gates Closure Law Pattern On The Water Hammer Effect During Turbine Off-Design Operation

Hydraulic transients are accelerated by the wicket gate closing in hydroelectric power plants. When the wicket gate is closed, there is a sudden change in velocity due to the closure. Therefore, a study has been carried out here, wherein water hammers on different hydropower components use optimum closure laws: Fast closure laws, slow closure laws, and instant load rejection. Hammer V10i software was used to investigate the phenomenon of pressure transient. The results show that the maximum transient pressure is strongly influenced by a very short closing time and was increased to 41.24% from the slow to the fast closure.
 Furthermore, the results from instant load rejections reveal that the transient pressure will be less than the fast and slow closure. So, the closing law selection can positively influence the entire hydropower plant system. Furthermore, the results show that there was a decrease in pressure near the turbine during the different load rejections, Fast closure, slow closure, and instant load rejection, where 57.7%, 15%, and 0.46%, respectively, and the decrease in turbine rotation speed were as 5.1%, 60%, and 24% respectively. Moreover, results reveal that maximum and minimum flow variation reached -29.75% and 41.2% during the fast closure.
 

Modeling of Optimal Power Generation in Small Hydropower Plants

Abstract Small hydropower plants (SHPs) have gained international attention as a reliable and versatile renewable energy source. Unfortunately, this energy source is not used efficiently. This is because these systems utilize headwaters with lower water capacity called forebays instead of large water reservoirs like dams. Since this hinders the control of electricity generation, the headwater is either wasted before it can be converted into electricity or it is exhausted very quickly and electricity generation stops after a short time. The aim of this study is to prevent the loss of electricity generation through the efficient use of SHP headwater. For this purpose, a mathematical model was created based on two SHPs operating in tandem. The mathematical model obtained was analyzed, and forebay water level simulation graphs for electricity generation at water flowrates of 2000, 4000, 6000, and 8000 kg/h were obtained. With the help of the obtained graphs, critical wicket gate openings for maximum electricity generation at flowrates of 2000, 4000, 6000, and 8000 kg/h were calculated. The results of this study are applicable to all SHPs. These results are also promising in terms of optimizing the use of a renewable energy source.

Modeling and Transient Response Analysis of Doubly-Fed Variable Speed Pumped Storage Unit in Pumping Mode

Doubly-fed variable speed pumped storage (VSPS) unit is an advanced hydropower system. In pumping mode, its input power can continuously be adjusted within a certain range via changing speed. Thus, it overcomes the major defect that the pumped storage unit cannot provide frequency regulation services to the power grid in pumping mode. However, for the pumping mode VSPS, how to establish its model is still a problem. In this article, an electromagnetic-electromechanical transient multi-time scale model of pumping mode VSPS is presented. This model depicts the intrinsic speed-power regulation characteristics and the extrinsic power-frequency coupling characteristics of pumping mode VSPS, which can be used for the power grid frequency regulation analysis. Firstly, the hydraulic subsystem model of VSPS is established within the electromechanical transient time scale. The model is built based on the variable speed characteristics of the reversible pump-turbine (RPT) in pumping mode and considers the hydrodynamics and additional friction effects of the wicket gate in the water conveyance system. Secondly, the electric subsystem model of VSPS is established within the electromagnetic transient time scale. The model mainly describes the operation characteristics of a doubly-fed induction machine (DFIM) and the control properties of the inner and outer loops of the back-to-back (BTB) converter system. Finally, comprehensive simulation and experimental results have fully verified the established model and theoretical analysis.

Battery and Flywheel hybridization of a reversible Pumped-Storage Hydro Power Plant for wear and tear reduction

Energy security and environmental challenges are some of the drivers for increasing the electricity generation from non-programmable Renewable Energy Sources (RES), adding pressure to the grid, especially if located in weakly connected (or isolated) islands, like Sardinia. Variable-speed Pumped Storage Hydro Power (PSHP) can offer a high degree of flexibility in providing ancillary services (namely primary and secondary regulations), but due to the hydro-mechanical nature of the equipment, sudden variations in the power output cause wear and tear. Other energy storage devices cannot compete with PSHP in terms of energy and power availability. The aim of this research is to assess the benefits derived from the hybridization of a PSHP with Battery Energy Storage System (BESS) and Flywheel Energy Storage System (FESS), to be installed in the Sardinia island (Italy). A dynamic model of the hybrid plant was made in MATLAB–Simulink® environment. A detailed model of the variable-speed pump-turbine was obtained from experimental data, and a simplified model of a fixed-speed turbine was produced. A detailed FESS model was provided by CIEMAT (Madrid, Spain) and a simplified BESS model was included. A dedicated control strategy to manage the power flows and accounting for State Of Charge (SOC) control, was implemented. A total of 100 combinations of BESS and FESS powers were taken into account, and the control strategy was calibrated for each one of them. The plant was simulated open-loop over a 3600s time period, feeding historical frequency and Automatic Generation Control (AGC) data. The simulations covered three PSHP operation modes: variable/fixed-speed turbine and variable-speed pump, and with/without hybridization. The performances of the hybridization were evaluated with wear and tear indicators for the PSHP (distance travelled by and number of movements of the wicket gate for turbine, fluctuations of the shaft torque for the pump) and capacity loss (life consumption) for the BESS. The results show that all the combinations of BESS and FESS powers result in the reduction of both the travelled distance and number of movements of the guide vanes. The best hybrid combination for the PSHP does not affect the BESS life consumption, which still is always in an acceptable range. A comparison between the non-hybrid variable-speed turbine and the hybrid fixed-speed counterpart shows that the electric powers do not differ substantially, but the hybridization smooths the movement of the guide vanes. The pump torque fluctuations sharply decrease with the hybridization, but more research is needed to validate that the change in the fluctuation index corresponds to a physical phenomenon. Overall, the hybridization improves the plant performances in terms of wear and tear reduction, and the presence of an additional FESS benefits both the BESS and the PSHP. The results also highlight the necessity for more research in variable-speed pumps providing ancillary services, and their impact.

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.

Automated hydraulic design procedure for a Francis turbine spiral casing

The spiral casing of a Francis turbine distributes the water from the penstock to the stay and guide vanes circumferentially and uniformly, which is important for achieving the required flow conditions at the runner entrance. Moreover, the spiral casing is supposed to provide the required inlet velocity in front of the stay vanes and create minimal hydraulic losses. The dimensions and shape of the spiral casing depend on the hydraulic and energy parameters of the turbine. A calculation methodology for a Francis spiral casing hydraulic design is presented in this paper. The methodology developed is based on the main condition to achieve a uniform water flow rate into the stay vanes system and the wicket gate over the entire perimeter. The free vortex flow theory is implemented in this research, where the design is based on the law of constant velocity moment. The parametric definition of the spiral casing makes the geometries generated for certain input combinations suitable for numerical analysis using commercially available Computational Fluid Dynamics (CFD) software. The calculation procedure can be used for any set of energy and geometry turbine parameters, such as water discharge, angle of streamline departing from the spiral casing, and stay ring diameter and height. The automated approach integrating MATLAB and ANSYS Workbench capabilities is presented as a spiral casing design tool. The product is a design solution proposed on basis of the turbine parameters. The spiral casing geometry generation is followed by a CFD analysis. Considering input parameters of different existing Francis turbines, the spiral casing is redesigned accordingly. One of the obtained spiral casings geometry is numerically tested. The results show that the uniform discharge distribution is achieved. The automation of the design procedure allows further optimization based on chosen input parameters.

Modelling of water conduit dynamics using the electromagnetic transient program line model

Almost all electrical power systems literature assumes that the shaft mechanical torque of a water turbine is linearly proportional to the wicket gate position and ignores the dynamics in the shaft torque due to the propagation of the water from the reservoir to the turbine. This propagation is usually modelled very simply using the water start-time equation and ignoring the travelling wave dynamics along possibly long conduits. This simplified modelling may introduce significant errors in the frequencies of oscillation which are in the range of 0.5–2 Hz and often fall within the range of slow oscillations in power system dynamics. This paper introduces the EMTP-H model for water dynamics based on the analogy between the hydraulic equations and the EMTP transmission line equations. The proposed model is validated with a field test and is applied to a simple power system frequency dynamics case.

Numerical investigation of the effect of the closure law of wicket gates on the transient characteristics of pump-turbine in pump mode

Two kinds of two-stage broken line closure laws of the wicket gates, modified A scheme (MAS) and modified B scheme (MBS), are presented to investigate the relationship between the wicket gates closure law and pressure pulsation shutting down pump turbines. The Detached Eddy Simulation (DES) model that is based on the moving mesh technology, is adopted to simulate the process of closure of the wicket gates. The results are analyzed in the time-domain and the time-frequency domain by Continuous Wavelet Transform (CWT). It was discovered that at the vaneless space, there isa intense pulsation at Blade Passing Frequency (BPF) lasting up to 3s in the original scheme (OS) of linear closure law, while the duration is reduced by 16.17% in MAS and has little change in MBS. At the leading edge of the stay vane, the high amplitude pulsation lasting nearly 1s is observed in OS. However, this pulse duration is reduced by 50% in MAS. Interestingly, in MAS, there is also a stabilization period lasting 1s before the wicket gates are fully closed, which is consistent with the pressure characteristics at the outlet of wicket gate. Moreover, the BPF and low frequency are the main components in the time-frequency plots. And the fluctuation energy at BPF and low frequency in the MAS is significantly smaller than the other two schemes. It is important that the torque duration at BPF is significantly reduced by 35% in MAS and slightly reduced in MBS. It is worth noting that the closure law of wicket gates has great effect on the pressure pulsation at BPF. This study is crucial improving the transient characteristics of pump turbines in pump mode by using the two-stage broken line closure law.

Improvement and extension of Cordier diagram for hydraulic turbines

The Cordier diagram for hydraulic turbines has been accurately improved and extended. The first part of this work completes the Cordier diagram by adding the effect of the number of injector nozzles in Pelton turbines. The Cordier lines (or bands) for Francis and Kaplan turbines have been recalculated based on the use of most available empirical relations without artificial smoothing of the boundaries between the different types of hydraulic turbines. Both turbine type and impeller diameter can be more accurately determined. The part of extending the Cordier diagram is based on the application of design equations for hydraulic turbines and the introduction of the turbine shape factor. This shape factor has a clear physical meaning in contrast to the diameter number in the conventional Cordier diagram. For Pelton turbines, it is representative of the ratio between runner diameter and jet diameter; for Francis and Kaplan turbines, it is representative of the ratio between the height of the wicket gate and the diameter of the impeller. The Cordier diagram for the diameter number can be replaced by the new diagram for the turbine shape factors. Exclusively for Francis turbines, the impeller shape factor also combines the blade profile (angle) at the impeller inlet with design parameters. The method presented in this paper can be well applied to create the extended Cordier diagram for the use of shape factors for pumps and compressors.

Numerical Investigation of the Effect of the Closure Law of Wicket Gates on the Transient Characteristics of Pump-Turbine in Pump Mode

Numerical Investigation of the Effect of the Closure Law of Wicket Gates on the Transient Characteristics of Pump-Turbine in Pump Mode

Frequency modelling and dynamic identification of cross-flow water turbines

The cross-flow turbines cover from the point of view hydraulic power the running domain of some well-known turbines such as Pelton, Francis or Kaplan. This type of turbine has a simple construction, long life and low execution cost, which makes it very suitable for on and off grid small to medium hydro power plants. It is quite difficult to establish an exact theoretical dynamic model for this type of turbines, due to the complex flow phenomenon (bi phase flow water and air). In order to obtain the exact dynamic behavior of the hydraulic machine, experimental dynamic identification will be done. In automation, the dynamic properties, represent the fundamental characteristic of the object which must be regulated. When the dynamic properties of the regulated object are obtained experimentally, we analyze the characteristics of the transient regime, which appears because of the application at the system inlet of some stochastic or deterministic signals (sine waves for our case). The hydraulic turbine is modeled as an informational quadrupole having the inlet parameters the movement of the wicket gate and the turbine head and outlet parameters the torque and the speed. In this paper it will be presented the frequency modelling of the cross-flow turbine and the validation of the mathematical model through experimental dynamic identification.

Fluid-structure coupling analysis of inlet ball valve on pumped-storage power station under extreme conditions

While pumped storage power stations (PSPSs) provide clean energy, they are also facing many problems of safe operation. Inlet ball valves bear the brunt of the impact and disturbance from upstream pressure pipeline under extreme conditions on PSPSs. Artificially change the wicket gate and ball valve closing laws can improve the extremums of key indicators. However, the internal flow field in ball valves is complex, and the internal flow field interacts with the external structure. The performance impact studied only by the indicators of the ball valve inlet and outlet is far from enough. In this study, three-dimensional flow field simulation of an inlet ball valve on PSPS was carried out, and the Realizable k-ε turbulence model was selected to simulate the pressure and velocity changes of the flow field inside the ball valve under load rejection conditions. Based on the theory of fluid-structure coupling and CFX dynamic grid, the response characteristics of the ball valve mechanical structure after being subjected to transient flow field were studied, the distribution of related performance parameters of the structure was obtained, and the mechanism that causes stress, strain and total deformation was analyzed. This study found that the ball valve structure is significantly affected by the three-dimensional transient flow of water under extreme conditions of load rejection. The safety of the ball valve can be enhanced by setting expansion joints and strengthening the base.

Numerical simulation of air injection in part load operating point

The operation hydro turbines in the off-design operating points is often accompanied by pressure pulsations with a large amplitude. One way to deal with such pulsations is to inject the air into hydro turbine flow passage. The aim of this work is to simulate this process. Previously we carried out calculations in frames of homogeneous incompressible three-phase «liquid – vapor – non-condensable gas» mixture model. The computational domain consisted of wicket gate, runner and draft tube. The wicket gate and runner were considered in the periodic stage approach. The results obtained were in good agreement with experimental data at full load operating point. It was shown that the calculations on the previous model failed to accurately simulate influence of air flow rate on the amplitude and frequency of pressure pulsations in part load. Therefore the part load regime is studied carefully in this work: different computational domains are considered, different solvers are used, including ANSYS CFX and CADRUN, developed at the Institute of Computational Technologies. Different models of «liquid – gas» mixture are used in the calculations, compressibility is also taken into account. The flow rate of the injected air ranged from 0.5 to 2 percent of the water flow rate. The influence of the injected air on the flow pattern, including the vortex rope, is investigated. The results obtained are compared with experimental data.

Multi-objective shape optimization of Francis runner using metamodel assisted genetic algorithm

Nowadays evolutionary optimization techniques become an integral part of the design process of hydraulic turbine runners. For optimization the shape of the runner is parameterized by a set of geometrical parameters. The objective functions of each runner geometry, being the efficiency and cavitation performance, are evaluated through CFD analysis of 3D flow-field. As usual, some kind of genetic algorithm is used for searching the Pareto front, due to its inherent possibility of parallel evaluation of different runner variants within one generation. This approach requires extensive CFD computations of hundreds and thousands of runner variants. In order to speed up the optimization process a metamodel (approximate, surrogate model) can be utilized. The metamodel provides an approximation of true dependency of the objective functions on the design parameters. The metamodel is built based on the known objective functions for some initial set of points randomly taken form the design space. Once built, it can be used to replace time consuming CFD computations during the optimization process. In the present paper a metamodel, based on Gaussian processes, is implemented and integrated into a multi-objective genetic algorithm. The obtained algorithm is applied for shape optimization of the Francis turbine runner. The number of the design parameters varied from 4 to 24. The objective functions are multi-point efficiency and cavitation characteristics. True values of the objective functions are evaluated using Reynolds averaged Navier-Stokes computations of the flow field in a reduced turbine domain, including wicket gate, runner and draft tube. These values are used to train the initial metamodel. In order to enhance predictive capabilities of the metamodel, it is retrained periodically during the optimization loop. The results of metamodel-assisted optimization runs are compared to that obtained without metamodel. It is shown that application of metamodel significantly reduces the number of actual CFD computations even for high number of design parameters and thus speeds up the overall optimization process.

Nonlinear Control of Non-Combined Integrated Governor for Kaplan Turbine Generator Set

In this paper, a nonlinear model is established for the Kaplan turbine generator set (KTGS) with dual input governor and generator excitation to solve unsatisfactory regulation performance caused by the deviation from the combination curve during the transient process. According to the model characteristics, the constraint effect of the objective function in multi-objective feedback nonlinear control design is utilized to replace the associative relationship between the wicket gate and the blade. Then, a three-variable integrated nonlinear control strategy (TVINCS) is applied to the wicket gate, blade governor, and generator excitation. Simulation results demonstrate the precise tracking performance of hydraulic and electrical parameters controlled by TVINCS. The controller is designed to guarantee the turbine's output efficiency, and effectively coordinate the wicket gate and blade actions in the dynamic regulation process, providing a superior dynamic adjustment performance for the system under various disturbances.

Optimization and Mechanism of the Wicket Gate Closing Law for High-Head Pumped Storage Power Stations

The high-head pumped storage power station (PSPS) has complex working conditions and severe transient processes. Under load rejection conditions, the turbine speed and the flow channel pressure will threaten the unit. By adjusting the wicket gate closing law (WGCL), we can effectively alleviate the adverse effects of the transient process. In this study, a full flow channel refined model of a high-head PSPS was constructed, and the influence mechanism of load rejection conditions on key parameters was analyzed by tracking the operating trajectory on the S characteristic curves and simulating the transient flow field inside the pumped turbine by three-dimensional computational fluid dynamics (3D-CFD). Given the contradiction between different parameters in the wicket gate closing process, a third-generation non-dominated genetic algorithm based on reference point selection (NSGA-III) was introduced, and a high-dimensional multi-objective WGCL optimization model was constructed. We compared parameter changes in the two-phase and three-phase WGCL optimized elite solutions and explained the rationality of adopting the three-phase WGCL in high-head PSPSs. By proposing an improved three-phase WGCL, a new scheme—the most suitable WGCL under load rejection—was obtained through precise optimization. The research embodied the excellent effect of the heuristic multi-objective (MO) optimization algorithm based on the Pareto strategy in solving the operation problem of the large fluctuation transient process of hydraulic machinery. It revealed the theoretical basis of engineering adjustment for the safe production and operation of hydropower.

Simulacija i odstranjivanje stick-slip efekta servosistema sprovodnog aparata hidraulične turbine

Friction is a repeatable and undesirable problem in hydraulic systems where always has to be a tendency for its removal. In this paper, the friction model is presented through which the most accurate results are achieved and the way of friction compensation, approached trough technique presented with the mathematical model of a hydraulic cylinder of a hydro turbine wicket gate controlled by a servomechanism. Mathematical modelling of a servo mechanism and hydraulic actuator, and also the simulation of hydraulic cylinder as a part of a hydro turbine wicket gate hydraulic system where the stick-slip phenomenon is present between the system components that are in contact is presented. Applied results in this paper and the theory behind them precisely demonstrate under what circumstances the stick-slip phenomenon appears in such a system. The stick-slip effect is simulated using Simulink and Hopsan software and the analysis of the results are given in this paper. Removal of the stick-slip effect is presented with the design of a cascade control implemented to control the behaviour of the system and remove the appearance of a jerking motion.

Assessment and Performance Evaluation of Water Hammer in Hydroelectric Plants With Hydropneumatic Tank and Pressure Regulating Valve

Abstract The simulation approach of pipeline systems, pressure regulating valve (PRV), and hydropneumatic tank (HPT) in hydropower plants (HPP) is performed. The article used suitable protection devices such as hydropneumatic tank and pressure regulating valves to protect the hydro-electric system against water hammer negative effects. The method of characteristics solves hydraulic transient in the HPP system. This article simulates transient phenomena in a hydro-electric power system using FORTRAN language. The aim was to find out and diminish the water hammer at the entrance of the spiral case as well as the pipeline system. The paper has four major philosophies: simulation with no protection, simulation with hydropneumatic tank, simulation with pressure regulating valve, and the combination of hydropneumatic tank with the pressure regulating valve. The results show that a combination of the pressure regulating valve and the hydropneumatic tank is adequate appliances to lessen the effect of water hammer/transient flow in a hydropower plant system. However, in the transient process of the water diversion system, the gas' kinetic characteristic in the hydropneumatic tank has certain complexity when there is high fluctuation in the surge chamber. The study proved that the wicket gate's stepwise could cut down the impact of the water hammer automatically than applying the linear closure law on the network. The automated pressure regulating valve with a high opening stepwise can decrease the negative impact of water hammer significantly. The results from this research paper show that pipeline diameter has crucial factors that influence hydraulic transient in hydropower plant systems.

Improved Hydroturbine Control and Future Prospects of Variable Speed Hydropower Plant

Hydroturbine control system (HTCS) is a nonlinear, nonminimum phase system comprising of speed/frequency sensors, hydraulic servo motors, wicket gates, gate position feedback mechanism, and proportional integral derivative (PID) controller (governor). Governor controls the guide vane opening (GVO) corresponding to the input (speed/frequency). Nowadays, HTCS becomes more intricate insight of variable speed doubly-fed induction generator for compensating wide variation in water head. Hence, robust control techniques are necessary for the optimal utilization of hydropower potential. In this article, a brief review of HTCS is given and a complimentary sliding-mode controller (CSMC) is designed for GVO serving a 250 MW hydrogenerating unit. The CSMC control law is derived and the stability is verified through the Lyapunov function. Simulation is carried out through the MatLab Simulink environment for a 250 MW HTCS. Then, the performance of CSMC for GVO and its influence on the generator characteristics are analyzed and compared with the traditional PID controller. The simulation results demonstrate the efficacy of the controller during the startup process, subsynchronous, and super synchronous mode of operations. It is inferred that the proposed CSMC has three times faster settling time than that of the traditional PID controller. Experimental validation is carried out with a scaled-down laboratory prototype. Furthermore, future scope and design challenges associated with the hydropower control system are illustrated.