Pump-turbine Unit Research Articles

Starting and Braking of a Large Variable Speed Hydrogenerating Unit Subjected to Converter and Sensor Faults

Doubly fed induction machine (DFIM) is increasingly being preferred in large rated variable speed pumped storage power plants (PSPP), because it provides better energy efficiency due to optimization of pump/turbine efficiency. Increased efficiency results from the flexibility of operating speed at different hydraulic heads. In doubly fed configuration, partial rated power converters feed the rotor circuit. Parallel-connected back-to-back voltage-source converter on rotor side is responsible for starting, braking (dynamic/regenerative), and speed (real power) control of the unit. This paper presents smooth starting and regenerative braking, and assesses performance under power converter and sensor faults of a 250-MW DFIM pump turbine unit to be commissioned in Tehri PSPP, India. Survivability of the unit during faults is analyzed based on current, speed, and dc-link voltage. Further, this paper investigates fault-tolerant operation of 250-MW DFIM unit at open-circuit fault in converters. A 2.2-kW DFIM is tested in the laboratory to corroborate the simulation.

Steady and transient performance of a pump-turbine on an open-loop test rig: pump mode

In order to study steady and transient characteristics of the pump turbine, a model pumped storage system was established in State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University. Different from typical closed-loop test rigs, this open-loop system has a constant upstream and downstream water level, two parallel pump-turbines, surge tank, power grids, etc., to achieve similar operating modes as in prototype power station. This paper aims to investigate the pump performance of the pumpturbine unit, including steady state and transient state. The structure of the present paper is organized as follows: 1) Steady pump performance of a model pump-turbine under three guide vane openings (GVO) was obtained by point-by-point measurements with the the adjustment of spherical valve; Meanwhile, the source of steady hysteresis as well as hump instabilities was identified by pressure analysis. It was found that the pressure instabilities in draft tube caused the drop in the energy performance curve as well as the hysteresis. 2) By dynamically adjusting the opening of the spherical valve, performance curves from nominal flow discharge to zero discharge were obtained under a constant GVO. The results showed that the performance curves of the torque and the efficiency could be measured in a quasi-steady way with sufficient switch time of the spherical valve, despite a clear dynamic loop of the energy performance curves. 3) The performance curves of a model pump-turbine were extended from high flow discharge region to nominal flow discharge operating point through a transient experiment in which the head varied.

Modeling and Sizing of an Undersea Energy Storage System

This paper presents modeling and sizing of an undersea energy storage system (USS). The USS, which is placed at the seabed, consists of a concrete sphere, a reversible pump-turbine unit, a permanent magnet synchronous machine, and a steel pipe through which water flows into/out of the sphere from/to the deep ocean. A novel mathematical model is derived to describe the governing equations of the USS operation. It is assumed that the USS operates in parallel with a direct-drive wave energy converter (DDWEC) and a novel sizing algorithm is developed to obtain the required USS capacity/size for regulating the DDWEC output power fluctuations during a designated period of time. In order to determine the DDWEC output power profile, wave spectrums are required. Real significant wave height and dominant wave period data, which are measured and collected by a buoy station in the Gulf of Maine, are used to generate wave profiles based on the modified two-parameter Pierson-Moskowitz spectrum. Eventually, Monte Carlo simulations with a thousand generated wave profiles are carried out to determine the optimal size of the USS for all possible scenarios. Furthermore, through promising results obtained from hardware-in-the-loop studies, the effective role of the USS in regulating the wave energy converter output power fluctuations is demonstrated.

Investigation of Pumped Storage Hydropower Power-Off Transient Process Using 3D Numerical Simulation Based on SP-VOF Hybrid Model

The transient characteristic of the power-off process is investigated due to its close relation to hydraulic facilities’ safety in a pumped storage hydropower (PSH). In this paper, power-off transient characteristics of a PSH station in pump mode was studied using a three-dimensional (3D) unsteady numerical method based on a single-phase and volume of fluid (SP-VOF) coupled model. The computational domain covered the entire flow system, including reservoirs, diversion tunnel, surge tank, pump-turbine unit, and tailrace tunnel. The fast changing flow fields and dynamic characteristic parameters, such as unit flow rate, runner rotate speed, pumping lift, and static pressure at measuring points were simulated, and agreed well with experimental results. During the power-off transient process, the PSH station underwent pump mode, braking mode, and turbine mode, with the dynamic characteristics and inner flow configurations changing significantly. Intense pressure fluctuation occurred in the region between the runner and guide vanes, and its frequency and amplitude were closely related to the runner’s rotation speed and pressure gradient, respectively. While the reversed flow rate of the PSH unit reached maximum, some parameters, such as static pressure, torque, and pumping lift would suddenly jump significantly, due to the water hammer effect. The moment these marked jumps occurred was commonly considered as the most dangerous moment during the power-off transient process, due to the blade passages being clogged by vortexes, and chaos pressure distribution on the blade surfaces. The results of this study confirm that 3D SP-VOF hybrid simulation is an effective method to reveal the hydraulic mechanism of the PSH transient process.

Cavitation behavior study in the pump mode of a reversible pump-turbine

Abstract Cavitation is an important issue of reversible pump-turbines especially in the pump mode. It usually causes noise, vibration, material-damage and operation stability on the pump-turbine unit. To diminish the bad influences of cavitation, the cavitation behavior in the pump mode of a pump-turbine is experimentally and numerically investigated. Results show that the best range of inception cavitation number and the best range of critical cavitation number have no intersection. Influenced by the incidence angle on the leading edge, the best inception cavitation range occurs around the impeller design condition. However, the best critical cavitation range is found at partial-load. To find a proper cavitation criterion, the development of cavitation is studied in detail. The relationship among the critical cavitation, the vapor volume and the fluid volume below the vaporization pressure is analyzed. At partial-load, cavitation incepts at a higher cavitation number than under the impeller design condition. During the cavitation number's decreasing, the vapor volume under impeller design condition transcend it at partial-load. Finally, the impeller design condition has a higher critical cavitation number than the partial-load. Considering the existing cavitation before critical cavitation, the inception cavitation standard is strongly recommended for the pump-turbine and other high-energy hydraulic turbomachineries.

Optimized Blade Design of Counter-Rotating-Type Pump-Turbine Unit Operating in Pump and Turbine Modes

In this study, a counter-rotating-type pump-turbine unit was optimized to improve the pump and turbine mode efficiencies simultaneously. Numerical analysis was carried out by solving three-dimensional Reynolds-averaged Navier–Stokes equations using the shear stress turbulence model. The hub and tip blade angles of the rear impeller (in the pump mode) were selected as the design variables by conducting a sensitivity test. An optimization process based on steady flow analysis was conducted using a radial basis neural network surrogate model with Latin hypercube sampling. The pump and turbine mode efficiencies of the unit were selected as the objective functions and they combined into a single specific objective function with the weighting factors. Consequently, the pump and turbine mode efficiencies of the optimum design increased simultaneously at overall range of flow rate, except for low flow rate of turbine mode, compared to the reference design.

The interaction effect of hydraulic transient conditions of two parallel pump-turbine units in a pumped-storage power plant with considering “S-shaped” instability region: Numerical simulation

Abstract In this paper the interaction effect of hydraulic transient flow for two parallel pump-turbine units which are installed on the same penstock, has been numerically investigated. Siah Bishe pumped storage power plant located at the north of Iran, has been considered as the case study. At First, simultaneous emergency shutdown of two units is simulated. Then the state in which the load of unit 1 is rejected while the unit 2 operates normally, has been studied. For this purpose, the governing equations of transient flow in waterways are solved by using the method of characteristics. Since for the same unit speed in “S-shaped” region of pump-turbine characteristic curves, three different values for unit discharge and unit torque are available, the characteristic curves are implemented in the code as the polar form by using modified Suter transformations. Results show that in the case of two units’ emergency shutdown, spiral case pressure rise and draft tube pressure reduction, increase by 6.9% and 54.4% in comparison with the one unit emergency shutdown. In the second case, the maximum pressure in the spiral cases and the minimum pressure in the draft tubes decrease by 1.2% and 6.4% respectively. In the recent case, the speed-control governor of unit 2 improves the transient state of the whole system by the wicket gates adjusting.

실험계획법(2k 요인시험)을 활용한 날개각의 변화가 엇회전 펌프수차의 효율에 미치는 영향에 관한 연구

This paper reports the efficiency of a counter-rotating pump-turbine with various blade angles. Three-dimensional steady Reynolds-averaged Navier-Stokes equations with the shear stress transport turbulence model were discretized by the finite volume method and solved on hexahedral grids to analyze the flow in the pump-turbine unit. The numerical results for the pump and turbine modes were verified by a comparison with their experimental data. The blade angles at both the hub and shroud of the counter-rotating impeller/runner were employed as geometric parameters to investigate their effects on the efficiency of the counter-rotating pump-turbine. The main effects on the efficiency of the geometric parameters were analyzed from their interaction through the 2k factorial design method. These results showed remarkable increases in both the pump and turbine efficiencies compared to the reference model.

Assessment of power step performances of variable speed pump-turbine unit by means of hydro-electrical system simulation

The paper explores the improvement in ancillary services that variable speed technologies can provide for the case of an existing pumped storage power plant of 2x210 MVA which conversion from fixed speed to variable speed is investigated with a focus on the power step performances of the units. First two motor-generator variable speed technologies are introduced, namely the Doubly Fed Induction Machine (DFIM) and the Full Scale Frequency Converter (FSFC). Then a detailed numerical simulation model of the investigated power plant used to simulate power steps response and comprising the waterways, the pump-turbine unit, the motor-generator, the grid connection and the control systems is presented. Hydroelectric system time domain simulations are performed in order to determine the shortest response time achievable, taking into account the constraints from the maximum penstock pressure and from the rotational speed limits. It is shown that the maximum instantaneous power step response up and down depends on the hydro-mechanical characteristics of the pump-turbine unit and of the motor-generator speed limits. As a results, for the investigated test case, the FSFC solution offer the best power step response performances.

On the Implementation of Variable Speed in Pump-Turbine Units Providing Primary and Secondary Load-Frequency Control in Generating Mode

This paper analyses different control strategies for the speed control loop of a variable-speed pump-turbine unit equipped with a doubly fed induction generator, operating in generating mode in an isolated power system with high penetration of intermittent renewable energy. The control strategies are evaluated and compared to each other in terms of the amount of water discharged through the pump-turbine and of the wicket gates fatigue while providing primary and secondary load-frequency control. The influence of the penstock length and the initial operating point on the performance of each control strategy is studied in detail. For these purposes, several simulations have been performed with a suitable dynamic model of the pumped-storage hydropower plant and the power system. The results of the paper indicate that a proper control strategy would consist in updating the reference speed according to the power generation schedule and keeping it constant within each scheduling period (typically 1 h).

3D Two-way coupled TEHD analysis on the lubricating characteristics of thrust bearings in pump-turbine units by combining CFD and FEA

The thermal elastic hydro dynamic (TEHD) lubrication analysis for the thrust bearing is usually conducted by combining Reynolds equation with finite element analysis (FEA). But it is still a problem to conduct the computation by combining computational fluid dynamics (CFD) and FEA which can simulate the TEHD more accurately. In this paper, by using both direct and separate coupled solutions together, steady TEHD lubrication considering the viscosity-temperature effect for a bidirectional thrust bearing in a pump-turbine unit is simulated combining a 3D CFD model for the oil film with a 3D FEA model for the pad and mirror plate. Cyclic symmetry condition is used in the oil film flow as more reasonable boundary conditions which avoids the oil temperature assumption at the leading and trailing edge. Deformations of the pad and mirror plate are predicted and discussed as well as the distributions of oil film thickness, pressure, temperature. The predicted temperature shows good agreement with measurements, while the pressure shows a reasonable distribution comparing with previous studies. Further analysis of the three-coupled-field reveals the reason of the high pressure and high temperature generated in the film. Finally, the influence of rotational speed of the mirror plate on the lubrication characteristics is illustrated which shows the thrust load should be balanced against the oil film temperature and pressure in optimized designs. This research proposes a thrust bearing computation method by combining CFD and FEA which can do the TEHD analysis more accurately.

Sensitivity analysis of transient flow of two parallel pump-turbines operating at runaway

Transient flow analysis in hydroelectric power plants is one of the most important issues for the prevention of undesirable pressure fluctuations in waterways. In this paper sensitivity analysis of hydraulic transient for two parallel pump-turbine units operating at runaway (unstable S-shaped zone of pump-turbine hill-chart) have been numerically investigated. The characteristics of Siah Bishe power plant located at the north of Iran, has been considered. Firstly, simultaneous operation of two units at runaway is simulated. For this purpose, the governing equations of transient flow are solved along the waterways using the method of characteristics and modified Suter transformations. Then, the sensitivity of pressure rise at the spiral case, water level oscillating at surge tank, rotational speed and flow rate and torque oscillation at unstable zone of hill-chart to moment of inertia, head loss coefficient of connection tunnel of surge tank, connection tunnel diameter and acoustic wave velocity has been performed. The results show that the moment of inertia has considerable effect on the maximum torque at unstable condition of runaway. Also, more moment of inertia can well postpone the peak of pressure and rotational speed. The maximum water level of surge tank and the maximum torque at unstable condition of runaway are strongly affected by head loss coefficient of connection tunnel of surge tank and connection tunnel diameter. By using different values for acoustic wave velocity, output conditions don't show any clear trend.

Analysis of the dynamic response of pump-turbine impellers. Influence of the rotor

This paper deals with the dynamic response of pump-turbine impellers. A pump-turbine impeller is a complex structure attached to a rotor and rotating inside a casing full of water with very small clearances between the rotating and the stationary parts. The dynamic response of this type of structures is very complex and it is very much affected by the connection to the rotor as well as by the added mass and boundary conditions. As a consequence its calculation presents several uncertainties.First, the dynamic response of pump-turbine impellers is introduced. Second an experimental investigation in a real impeller attached to the rotor and inside the machine was carried out. For this investigation, the impeller of an existing pump-turbine unit with an installed power of 110MW and a diameter of 2.87m was studied. For a better analysis of the experimental results a numerical model using FEM was also built-up. Frequencies and mode-shapes were identified numerically and experimentally and the characteristics of the structural response analyzed.To determine the influence of the rotor and supporting structures on the impeller response the results were compared with the ones obtained with the same impeller but suspended (non-connected to the rotor). Experimental and numerical simulation were also used for this case. The changes in the dynamic response due to the rotor connection were determined.Finally the results obtained are compared with the results from other pump-turbine impellers of different designs and general conclusions about the dynamics of this type of structures are given.

Analysis of the pump-turbine S characteristics using the detached eddy simulation method

Current research on pump-turbine units is focused on the unstable operation at off-design conditions, with the characteristic curves in generating mode being S-shaped. Unlike in the traditional water turbines, pump-turbine operation along the S-shaped curve can lead to difficulties during load rejection with unusual increases in the water pressure, which leads to machine vibrations. This paper describes both model tests and numerical simulations. A reduced scale model of a low specific speed pump-turbine was used for the performance tests, with comparisons to computational fluid dynamics(CFD) results. Predictions using the detached eddy simulation(DES) turbulence model, which is a combined Reynolds averaged Naviers-Stokes(RANS) and large eddy simulation(LES) model, are compared with the two-equation turbulence mode results. The external characteristics as well as the internal flow are for various guide vane openings to understand the unsteady flow along the so called S characteristics of a pump-turbine. Comparison of the experimental data with the CFD results for various conditions and times shows that DES model gives better agreement with experimental data than the two-equation turbulence model. For low flow conditions, the centrifugal forces and the large incident angle create large vortices between the guide vanes and the runner inlet in the runner passage, which is the main factor leading to the S-shaped characteristics. The turbulence model used here gives more accurate simulations of the internal flow characteristics of the pump-turbine and a more detailed force analysis which shows the mechanisms controlling of the S characteristics.

Counter-Rotating Type Pump-Turbine Unit Stabilizing Momentarily Fluctuating Power from Renewable Energy Resources

Counter-Rotating Type Pump-Turbine Unit Stabilizing Momentarily Fluctuating Power from Renewable Energy Resources

Resonance investigation of pump-turbine during startup process

The causes of resonance of a certain model pump-turbine unit during startup process were investigated in this article. A three-dimensional full flow path analysis model which contains spiral case, stay vanes, guide vanes, runner, gaps outside the runner crown and band, and draft tube was constructed. The transient hydraulic excitation force of full flow path was analyzed under five conditions near the resonance region. Based on one-way fluid- structure interaction (FSI) analysis model, the dynamic stress characteristics of the pump-turbine runner was investigated. The results of pressure pulsation, vibration mode and dynamic stress obtained from simulation were consistent with the test results. The study indicated that the hydraulic excitation frequency (Zg*fn) Hz due to rotor-stator interference corresponding to the natural frequency of 2ND+4ND runner mode is the main cause of resonance. The relationship among pressure pulsation, vibration mode and dynamic stress was discussed in this paper. The results revealed the underlying causes of the resonance phenomenon.

Operation of the counter-rotating type pump-turbine unit installed in the power stabilizing system

This serial research intends to put a unique power stabilization system with a pumped storage into practical use. The pumped storage is equipped with a counter-rotating type pump-turbine unit whose operating mode can be shifted instantaneously in response to the fluctuation of power from renewable resources. This paper verifies that the system is reasonably effective to stabilize the fluctuating power. It is necessary to quickly increase the rotational speed when the operation is shifted from the turbine to the pumping modes, because the unit cannot pump-up water from a lower reservoir at a slow rotational speed while keeping gross/geodetic head constant. The maximum hydraulic efficiency at the turbine mode is close to the efficiency of the counter-rotating type hydroelectric unit designed exclusively for the turbine mode. The system is also provided for a pilot plant to be operated in the field.

Development of a pump-turbine runner based on multiobjective optimization

As a key component of reversible pump-turbine unit, pump-turbine runner rotates at pump or turbine direction according to the demand of power grid, so higher efficiencies under both operating modes have great importance for energy saving. In the present paper, a multiobjective optimization design strategy, which includes 3D inverse design method, CFD calculations, response surface method (RSM) and multiobjective genetic algorithm (MOGA), is introduced to develop a model pump-turbine runner for middle-high head pumped storage plant. Parameters that controlling blade shape, such as blade loading and blade lean angle at high pressure side are chosen as input parameters, while runner efficiencies under both pump and turbine modes are selected as objective functions. In order to validate the availability of the optimization design system, one runner configuration from Pareto front is manufactured for experimental research. Test results show that the highest unit efficiency is 91.0% under turbine mode and 90.8% under pump mode for the designed runner, of which prototype efficiencies are 93.88% and 93.27% respectively. Viscous CFD calculations for full passage model are also conducted, which aim at finding out the hydraulic improvement from internal flow analyses.

Multiobjective Optimization of a Counterrotating Type Pump-Turbine Unit Operated at Turbine Mode

A multiobjective optimization for improving the turbine output and efficiency of a counterrotating type pump-turbine unit operated at turbine mode was carried out in this work. The blade geometry of both the runners was optimized using a hybrid multiobjective evolutionary algorithm coupled with a surrogate model. Three-dimensional Reynolds-averaged Navier-Stokes equations with the shear stress transport turbulence model were discretized by finite volume approximations and solved on hexahedral grids to analyze the flow in the pump-turbine unit. As major hydrodynamic performance parameters, the turbine output and efficiency were selected as objective functions with two design variables related to the hub profiles of both the runner blades. These objectives were numerically assessed at twelve design points selected by Latin hypercube sampling in the design space. Response surface approximation models for the objectives were constructed based on the objective function values at the design points. A fast nondominated sorting genetic algorithm for the local search coupled with the response surface approximation models was applied to determine the global Pareto-optimal solutions. The trade-off between the two objectives was determined and described with respect to the Pareto-optimal solutions. The results of this work showed that the turbine outputs and efficiencies of optimized pump-turbine units were simultaneously improved in comparison to the reference unit.

Power Stabilization System with Counter-Rotating Type Pump-Turbine Unit for Renewable Energy

Traditional type pumped storage system contributes to adjust the electric power unbalance between day and night, in general. The pump-turbine unit is prepared for the power stabilization system, in this serial research, to provide the constant power with good quality for the grid system, even at the suddenly fluctuating/turbulent output from renewable energies. In the unit, the angular momentum changes through the front impeller/runner must be the same as that through the rear impeller/runner, that is, the axial flow at the outlet should be the same to the axial flow at the inlet. Such flow conditions are advantageous to work at not only the pumping mode but also the turbine mode. This work discusses experimentally the performance of the unit, and verifies that this type unit is very effective to both operating modes.