Turbine Start-up Research Articles

Experimental investigation of a FSFC variable speed pump-turbine prototype – Part2: runner fatigue reduction

Abstract The demonstration of flexibility technologies to enhance ancillary services of Hydropower Plants (HPPs) is showcased by the XFLEX HYDRO H2020 European Project. The potential of Full Size Frequency Converters (FSFC) is numerically and experimentally assessed by a case study on the Z’Mutt HPP. The investigated unit is a 5 MW variable speed reversible Francis pump-turbine fed by a FSFC that provides full speed control from −100% to +100%, allowing for fast start-up and mitigation of damage caused to different components. In this context, several turbine start-up sequences with controlled guide vane opening and rotational speed are developed, implemented, and tested on prototype scale. The definition of the different sequences, their impact on penstock fatigue and further FSFC capabilities like fast active power control in pump mode and fast power reversion are presented in part 1 of this contribution. Part 2 deals with fatigue damage investigations of the runner with respect to variable speed operation in generating mode using coupled numerical and experimental techniques. Strain signals from the runner blades are extrapolated to the critical areas to assess the relative fatigue damage induced by each turbine start-up. The scaled normal stress approach is applied, which is a critical plane method, adapted for multiaxial stress states. The results reveal that runner damage is mitigated by more than two orders performing a variable speed start-up compared to a classical one with fixed speed technology. Simultaneously, the delay for active power provision is significantly shortened. Thus, the study approves that fast and frequent turbine start-ups are realizable without affecting the runner’s service life thanks to the FSFC.

Analysis of mode-switching of a contra-rotating pump-turbine based on load gradient limiting shutdown and startup sequences

Abstract The contra-rotating pump-turbine (CRPT) is investigated as a runner configuration enabling more efficient low-head pumped hydro storage (PHS). To enhance the flexibility of low-head PHS with a CRPT, it is important to examine mode-switching. This study examines two mode-switching sequences for the CRPT, one turbine-to-pump and one pump-to-turbine. The mode-switching sequences are made as a combination of shutdown and startup sequences of each mode. The goal is to avoid pump and turbine brake modes while maintaining smooth load gradients. The turbine shutdown of the turbine-to-pump mode-switching involves a multi-stage valve closure to reduce the rotational speed of the runners, followed by the pump startup based on a previously optimised sequence. The pump shutdown of the pump-to-turbine mode-switching involves a gradual valve closure and rapid speed down of the runners at the end of the sequence. This is followed by the turbine startup, where a valve opens to generate sufficient flow over the runners before increasing the rotational speed of the runners simultaneously. Both mode-switching sequences require approximately 5 s each, and the largest loads are experienced during the pump mode parts of the sequences. The analysis is carried out using transient computational fluid dynamics with the OpenFOAM open-source software.

CFD Simulation of the turbine start-up of a pump-turbine with investigation of the usage of a porous domain for modelling very small guide vane angles

Abstract With increasing number of transient operations, such as start and stop procedures in pump-turbines, CFD simulations of these sequences have become more important to understand the occurring flow phenomena and evaluate their impact on the pump-turbines lifetime. In the present work, the numerical setup for transient CFD simulations is discussed and a CFD simulation of a turbine start-up using a 29 million node mesh and the SBES turbulence model is carried out. The results demonstrate reasonable accuracy when comparing the overall trends with the available measurement data. However, significant underestimation of torque fluctuations as well as draft tube pressure fluctuations during the initial phase of the start-up is observed. In a subsequent study, the modelling of very small guide vane angles by a porous domain is investigated. An analytical function for the loss coefficient of the porous domain is applied up to different threshold values of the guide vane angle. A guide vane angle of 1° is identified as the best option regarding accuracy and computational effort.

Transfer and measurement of power plant transients on a high-performance closed-loop test rig

Abstract As hydro power plants become more and more flexible in their operation, the associated transient processes and the resulting loads on the hydraulic machinery are becoming increasingly important. In addition to numerical analysis, measuring of the transient processes using model test rigs plays an important role. The aim is to determine the effects of the transient operating modes in model tests and to transfer the knowledge gained to the prototype. Therefore, the real transient processes of a hydro power plant and the load on the hydraulic machine must first be reproduced on the test rig. The transient turbine start-up of a hydro power plant is analysed in more detail. The first step is to model the entire hydro power plant and calculate the corresponding start-up process. The boundary conditions applied to the hydraulic machine are transferred to the model machine on the test rig using similarity laws; a time scale adjustment is made using the Strouhal number. The time dependent boundary conditions at the model machine are set by varying the rotational speed of a service pump arranged in the test rig and the transient start-up process is measured.

Experimental investigation of a FSFC variable speed pump-turbine prototype - Part1: penstock fatigue reduction and fast active power regulation

Abstract The Z’Mutt pumping station is located in Canton Valais-Wallis, in Switzerland and is part of the Grande Dixence hydroelectric scheme. The unit 5 of this station has recently been upgraded with a 5 MW variable speed reversible Francis pump-turbine equipped with Full Size Frequency Converter, FSFC, allowing the unit speed to be adjusted from −100% to +100%. This paper presents the methodology based on 1D numerical simulations including CFD/FEM runner damage hill chart to optimize the turbine start-up sequence to minimize both runner and penstock fatigue by taking advantage of the FSFC variable speed technology. FSFC allows for precise start-up trajectories in the n11-Q11 frame, to avoid runner high damage operating points, while reducing penstock pressure variations as compared to a classical fixed speed technology. This methodology resulted in the definition of 6 different FSFC start-up sequences, as well as the classical fixed speed start-up, which have been successfully implemented and tested on site. Site tests of fast transition from turbine to pump and vice versa are also showcased. Finally, fast active power regulation tests performed in pump mode are presented to show the ability of FSFC variable speed technology to provide ancillary grid service in pump mode.

Numerical assessment of transient flow and energy dissipation in a Pelton turbine during startup

The Pelton turbine, known for its high application water head, wide efficient operating range, and rapid start-stop capability, is ideal for addressing intermittent and stochastic load issues. This study numerically analyzes the transient two-phase flow and energy dissipation during the startup of a Pelton turbine. Dynamic mesh technology controlled nozzle opening changes, and momentum balance equations managed runner rotation. Findings showed that the runner speed initially increased rapidly and then more slowly, and flow rate matched the nozzle opening variations. Runner torque first rose linearly, then decreased, with the fastest decline during nozzle closing. Hydraulic efficiency peaked early in nozzle reduction but then dropped sharply. Strong vortices formed due to upstream inflow and downstream backflow impact in the distributor pipe. The jet needle and guide vane improved flow in the converging section of nozzle, but flow began to diffuse with increased stroke. Initially, the jet spread fully on the bucket surface, but later only affected the bucket tips. Pressure fluctuations in the water supply mechanism were primarily due to jet needle motion, with higher amplitude during movement and lower when stationary. These fluctuations propagated upstream, weakening over distance. Reynolds stress work and turbulent kinetic energy generation, respectively, dominated energy transmission and energy dissipation, with their maximum contribution exceeding 96% and 70%. High-energy clusters corresponded to jet impact positions, highlighting jet-bucket interference as crucial for energy transport. This study established a performance evaluation method for Pelton turbine startups, supporting further investigation into characteristic parameters, flow evolution, and energy dissipation patterns.

Variations of flow patterns and pressure pulsations in variable speed pump-turbine during start-up process

Abstract In recent years, variable speed pumped-storage (VSPS) technology has become a hotspot of research and application due to its excellent power grid regulation capabilities. Since the speed of a VSPS unit is changeable, when switching operating conditions, more stable operating trajectories can be chosen to reduce vibration and damage. The simulations of transient processes of VSPS units normally use 1D method, which cannot judge the quality of operating conditions because of no flow pattern and pressure pulsation information. Using 3D CFD simulation based on OpenFOAM, we analyse the flow patterns and pressure pulsations when a VSPS unit with a full size frequency converter (FSFC) is in the turbine start-up process. The typical operating points in the variable speed transient trajectories obtained from 1D simulation are studied. It is found that in the initial stage of start-up, high-speed circulation flow arises in the vaneless space, leading to flow separation and large pressure pulsation. The results can provide a reference for the operation of VSPS power stations, so as to avoid operating conditions with adverse flow patterns and pressure pulsations.

Numerical studies of sCO2 Brayton cycle

This work describes the one-dimensional, thermo-hydraulic model of the sCO2 cycle Sofia developed to investigate optimal control methods and the behaviour of the cycle during operation. This dynamic model includes all devices such as turbomachinery, heat exchangers, valves, and piping including heat loss, in line with concept of the 1 MWe sCO2 cycle, to be realised in the site of a fossil power plant in the Czech Republic.The model assembly and calculations were conducted using the commercial Modelica-based library ClaRa + using the simulation environment Dymola and in combination with another Modelica-based library, UserInteraction; the real-time simulations, with some parameter changes during the calculation, are made and described in this paper.Nominal parameters were achieved during the steady-state simulation, except for the lower mass flow of sCO2. Transient simulation of power turbine start-up from standby state and results are also presented in this paper. The nominal state is achieved with the semi-automatic procedure in approx. 3 h. The simulation results allow more detailed analyses of control methods and a better understanding of real device control and behaviour during start-up, shutdown, or other transients.Careful manipulation of turbine valves in coordination with the pressuriser operation was identified as crucial for optimal control of the system. Also, the initial amount of CO2 in the pressuriser affects its behaviour during transients.

Aerodynamic characterization of a H-Darrieus wind turbine with a Drag-Disturbed Flow device installation

This study examines the impact of the Drag-Disturbed Flow device on the startup and operation of HDWT (H-Darrieus wind turbine). The device plays a crucial role in enhancing the startup performance of the HDWT and improving the overall wind energy utilization. This work presents a study where two-dimensional computational fluid dynamics (CFD) evaluations are performed on various D-DF (Drag-Disturbed) devices. The HDWT utilizes the NACA0018 airfoil type, while the D-DF device employs triangular, oval, and NACA0018 airfoil structures. The text explains the installation process and operational principles of the D-DF device. It also verifies the device's meshing and accuracy. Furthermore, this study conducts 2-D simulations to analyze the impact of various D-DF devices on the moment and wind energy consumption coefficients of the HDWT. Additionally, the process by which D-DF devices enhance the performance of the HDWT is explained by an analysis of the vortex cloud diagram. The results indicate that the HDWTs equipped with D-DF devices experience significant performance improvements during the wind turbine startup phase. Specifically, the Drag blade effectively enhances the moment coefficient of the HDWTs at low tip speed ratios by an average of approximately 41.74 %. Additionally, once the HDWTs are started and operating at high tip speed ratios, all D-DF devices, except for the DF-Tri, can be utilized within the λ = 1.2–2 range to enhance the torque coefficient of the HDWT by an average of about 7.53 %. This study aims to improve the operating performance of the current HDWT, lower the wind turbine cut-in wind speed, and enhance the wind energy consumption of the turbine.

Patterns of Growth of an Internal Annular Crack Under the Influence of Thermal Stresses During Turbine Startup

Patterns of Growth of an Internal Annular Crack Under the Influence of Thermal Stresses During Turbine Startup

Research on the Numerical Method of Pollutant Emission during Gas Turbine Start-up

It was of great research value and significance to predict the pollutant emission from the gas turbine start-up process by numerical method, which could improve the operation and regulation of gas turbines. In the paper, the computational fluid dynamics(CFD)-chemical reaction network (CRN) method was used to study the pollutant emission of gas turbines. According to the control process of the start-up control module, the start-up process had been divided into 12 states, and the CFD numerical analysis was carried out. Based on the data of Damkohler number (Da), temperature and velocity obtained by CFD, the chemical reaction network model of each state point was established. The research showed that the design parameter value of the state point was the basis of compiling the calculation process and source code; The basic data and partition basis for the chemical reaction network could been provided by the results of CFD analysis, which was an important guarantee for accurate prediction of NOx; The method of CFD-CRN could accurately predict NOx emission of gas engine during start-up, and provided reasonable and reliable research means and mathematical methods.

ДОСЛІДЖЕННЯ ЕФЕКТИВНОСТІ ТЕПЛООБМІННОГО ПРОЦЕСУ В ОХОЛОДЖУВАЧІ ТУРБІННОГО МАСТИЛА МО-2 В ЗАЛЕЖНОСТІ ВІД СХЕМИ РОЗТАШУВАННЯ ТРУБОК У ТРУБНИХ ҐРАТАХ

Effective cooling of the turbine oil is important for the reliability and optimal operation of the turbogenerator. The oil is used for lubrication and cooling of the support and radial bearings of the turbine, bearings of the generator and reducer, gears and splined rollers. It is also used as a hydraulic fluid in the hydraulic turbine start-up system. For reliable and safe operation of the system, it is necessary to maintain the correct temperature of the lubricant. Research is aimed at studying the influence of the layout of the heat exchange tubes in the tube grids on the cooling efficiency of the TP-22 turbine oil. With the help of the SolidWorks software complex, a geometric model of the MO-2 turbine oil cooler was created with three schemes for the placement of heat exchange tubes in tube grids: corridor, radial and checkerboard. Adhering to the parameters of the technical characteristics of this cooler, the nominal parameters of temperatures and consumption of coolants (cooling water and turbine lubricant TP-22) were chosen. The result of the calculations were the temperature fields of the coolants and the trajectories of the movement of lubricant particles in the intertube space of the cooler. When creating a geometric model and further research, the main technical characteristics of the shell-and-tube horizontal cooler MO2 were adopted, namely: the temperature of the turbine oil at the inlet is 45°С, the nominal flow rate of the oil is 3.1 m3/h, the temperature of the water at the inlet is 20°С, the nominal flow rate water - 15 m3/h. The conducted computer modeling made it possible to investigate the influence of the layout of the heat exchange tubes in the tube grids on the cooling efficiency of the turbine lubricant in the MO-2 shell-and-tube cooler. It was shown that the most effective, among the studied schemes, is the checkerboard arrangement of heat exchange tubes in tube grids. This scheme makes it possible to reduce the consumption of cooling water, relative to the nominal one, which will allow the use of water pumps of lower power.

A Concept for Safe and Less Expensive Acceleration of a Marine Steam Turbine Start-up

A Concept for Safe and Less Expensive Acceleration of a Marine Steam Turbine Start-up

Numerical Investigation of Flow and Structural Characteristics of a Large High-Head Prototype Pump–Turbine during Turbine Start-Up

Transient processes that occur in pumped storage power plants can cause high-pressure conditions, which in turn can result in vibrations in the pump–turbine structure and even damage to structural components. It is therefore crucial to research the transient process of the large pump–turbine units and the flow-induced vibrations of the structural components. The three-dimensional flow field and structural field models of a high-head prototype pump–turbine were constructed to study its flow characteristics and structural characteristics under the turbine start-up. Calculations and analyses were performed on the pressure variation and the flow-induced stress concentrations of the pump–turbine during start-up in turbine mode. The simulated pressure distributions during the turbine start-up were mapped onto the finite element calculation model of the structures of the pump–turbine to calculate the flow-induced stress concentrations. This study provides a reference to improve the design and operation of high-head prototype pump–turbines based on the findings of the flow and structural characteristics.

Start-up process optimization of pumped-storage unit with Tilt–Integral–Derivative controller

Focused on the hydraulic–mechanical coupled transient behaviors of the pumped-storage unit (PSU), this paper discusses the theoretical implementation and the parameter optimization of tilt–integral–derivative (TID) controller in turbine start-up processes. A novel learning backtracking search algorithm (LBSA) is adapted to optimize the control parameters, and a comprehensive index combining the relative deviation of the rotational speed and the relative deviation of the volute’s water pressure is designed as the objective function. A nonlinear mathematical model which can reflect hydraulic and mechanical dynamic characteristics of the pumped-storage governing system (PSGS) is established for the simulation of the transient period. Simulation experiments of two typical operating conditions are carried out under no-load condition. The simulation results show that the TID controller attaches significance to suppressing the amplitude of water hammer pressure and damping the hydraulic fluctuations. It will also restrain the trend of state trajectories oscillating back and forth in the unstable “S” shaped regions.

Monitoring the Gas Turbine Start-Up Phase on a Platform Using a Hierarchical Model Based on Multi-Layer Perceptron Networks

Abstract Very often, the operation of diagnostic systems is related to the evaluation of process functionality, where the diagnostics is carried out using reference models prepared on the basis of the process description in the nominal state. The main goal of the work is to develop a hierarchical gas turbine reference model for the estimation of start-up parameters based on multi-layer perceptron neural networks. A functional decomposition of the gas turbine start-up process was proposed, enabling a modular analysis of selected parameters of the process. Real data sets obtained from observations of the turbo-generator set located on a North Sea platform were used.

Flow-induced vibration analysis of a prototype pump-turbine runner during turbine start-stop transient process

Recently more start-stops per day of the pump-turbine units are required by the power grid for load regulation. During the turbine start-up and shut-down transient processes, the flow and pressure of the prototype pump-turbine change dramatically at different guide vane opening angles. The extremely unsteady pressure fluctuation in the flow passages can induce large deformation, stress concentration, and strong vibration of the pump-turbine runner. Therefore, it is significantly important to study the unsteady flow characteristics and the corresponding flow-Induced vibration of the runner during turbine start-up and shut-down transient process. In this investigation, a 3D model of the pump-turbine unit including the flow passages of the spiral casing, stay vane, guide vane, runner, and draft tube, crown chamber, band chamber, labyrinth seals, and balance pipes are constructed. The flow characteristics of the unit during the turbine start-up and shut-down process were analysed by coupling the 1D pipeline calculation and 3D turbine flow domain calculation. By mapping the pressure distribution of the fluid domain to the runner structure domain, the flow-induced dynamic behavior of the pump-turbine runner is performed, and the large deformation and stress concentration of the runner are investigated in detail. The flow-induced vibration results achieved are able to provide meaningful suggestions for safe operation and for improving the pump-turbine runner design.

Flow Characteristics Analysis of a High-Head Prototype Pump-Turbine During Turbine Start-Up: Effects of the Clearance

The dramatic changes in the internal flow and the corresponding structural behavior during the turbine start-up transient process of pump-turbines are extremely complex. The clearances in the upper crown chamber and bottom ring chamber affect the results of the flow field and structural field of the pump-turbine runner. Most of the previous studies ignored the effects of the clearance flow field to simplify the numerical simulations. In this study, numerical calculations were performed on the entire flow passage of a high-head prototype pump-turbine during the start-up in turbine mode, and the model with and without the clearance are analysed respectively. The causes of the flow field characteristic difference and external characteristic difference caused by the existence of intermediate clearance in the model are studied in detail. The results show that the clearance flow field has a great influence on the axial forces on the runner, which is mainly due to the higher pressure of the clearance flow field compared with the flow field in the runner; At the same time, because the existence of clearance flow field only has a small effect on the distribution of internal flow field, the hydraulic torque of runner considering clearance effects is basically the same as the one without clearance.

Optimization of turbine start-up sequence of a full size frequency converter variable speed pump-turbine

The stability of the electricity grid will be disrupted by the massive integration of new renewable energies. Hydropower plants have a major role to play in this transformation of the electricity market by increasing their operational flexibility and their ability to provide ancillary services. However, this flexibility may lead to an accelerated degradation of mechanical components. By changing the turbine operating point far from the best efficiency point or by increasing the number of transient manoeuvres such as start and stop sequences, unsteady flow phenomena, cavitation development and additional wear and tear stress the unit’s components and impact its lifetime. The present work aims to provide preliminary insight on the optimization of the start-up sequence of a 5 MW reversible Francis pump-turbine equipped with a Full Size Frequency Converter (FSFC). The goal of the optimization approaches are to determine a start-up sequence which minimizes the runner damage, the penstock fatigue and the water losses. The objective functions are evaluated by 1D hydraulic transient simulations with the SIMSEN software and which it allow to compare the relative mitigation between the conventional fixed-speed start-up and the linear variable-speed start-up equipped with a Full Size Frequency Converter.

Numerical fatigue damage analysis of a variable speed Francis pump-turbine during start-up in generating mode

Variable speed hydropower units offer a large spectrum of grid regulation services and may therefore contribute to the stability of future power supply systems. Full Size Frequency Converters (FSFC) already found real world application in Pumped Storage Hydropower Plants up to a rated power of 100 MW and are even considered scalable up to a few hundred MW. Apart from the extension of the power range and grid regulation capacities, the FSFC technology also provides new control possibilities during transient operations such as start-up in generating mode. Thus, harsh conditions with damaging impact on the hydromechanical components may be avoided by tuning the operating point trajectory in the start-up phase. In this paper, runner fatigue damage during start-up in generating mode of a 5 MW variable speed Francis pump-turbine prototype equipped with a FSFC is numerically analyzed. The fixed speed solution is compared to a variable speed solution following a BEP tracking control strategy. 1D hydraulic transient simulations provide boundary conditions for detailed 3-D CFD/FEA simulations. Full and reduced numerical domains are used and compared. The overall outcome of the present numerical study indicates an important reduction of partial damages using variable speed drives for turbine start-up manoeuvres.