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When it came time to design the turbine-generator units for the new 76.7-MW Ribeiradio facility, the opportunity arose to model them off the 193.5-MW Bemposta II units, saving time and money.
By Miguel Roque, Alexandre Ferreira da Silva, Christine Monette and Dominik Laufer
Significant time and cost savings resulted during the hydraulic development of the turbine unit for the 76.7-MW Ribeiradio facility because the process was based on the Bemposta II unit, which has a similar speed. With the goal to perfectly adapt turbine characteristics for the requirements of Ribeiradio, available hydraulic components were further optimized by means of computational fluid dynamics (CFD) in combination with model testing. Thereby, special focus was given to the part load behavior.
In addition to the hydraulic optimization at part load, the mechanical design of the Ribeiradio turbine also contains several features that support a smooth and stable operation at low load.
The Ribeiradio-Ermida multipurpose hydroelectric scheme is the first large-capacity hydro project developed in the Vouga River basin, located in central Portugal. The site of this development is about 100 km southeast of the city of Porto. Figure 1 indicates the geographical orientation of the Ribeiradio-Ermida reservoirs.
Ribeiradio, the upstream scheme and most important part of the project, was commissioned on May 22, 2015, and includes a 74-m-high, 262-m-long concrete gravity dam that has three gated spillways designed for a total discharge of 2,750 m3/sec. The powerhouse contains a single turbine-generator unit that delivers a maximum capacity of 76.7 MW, and the overall average annual generation is 117 GWh.
Ribeiradio also operates in a grid stabilization scenario, offering Secondary Band Regulation service. The Secondary Band is defined as the margin of power variation, in both directions, in which the secondary regulator may act automatically within a period of time of less than 5 minutes, starting from the current point of operation.
Ermida Reservoir, located downstream, was designed to modulate the flow discharged from the Ribeiradio scheme to minimize flow variations in the final course of the Vouga River, which is required due to the recreational use of the river banks.
Portugal’s public water utility, Instituto Nacional da Agua (INAG), developed preliminary studies for the Ribeiradio-Ermida hydro project during the end of the 20th century, with two main goals: water supply and flood control.
In 2005, the Portuguese government defined the framework for granting rights for the use of hydropower on the Vouga River, and the concession to develop this facility was acquired in 2007 by EVIVA Energy (Martifer Renewables). That same year, Energias de Portugal (EDP) joined EVIVA Energy to develop the Ribeiradio hydroelectric project. On July 5, 2010, EDP acquired complete ownership of the project.
Developing Ribeiradio equipment based on Bemposta II
In 2008, EDP awarded Andritz Hydro a contract to supply electromechanical equipment for Bemposta II hydroelectric plant, then under development on the Douro River. The vertical Francis unit EDP chose for Bemposta II had a rated head of 64 m and was originally targeted at a maximum capacity of 193.5 MW. For Bemposta II, a hydraulic model development was performed and the hydraulic performance was proven during a model acceptance test at the independent laboratory of the EPFL in Switzerland.
Due to the excellent operating behavior of the Bemposta II prototype at full load, EDP desired to extend the continuous turbine operating range up to 205.5 MW. In 2015, Andritz Hydro performed a feasibility study regarding the topic and could afterwards approve the power output extension for unrestricted operation.1
On Dec. 10, 2010, EDP contracted Andritz Hydro GmbH, Ravensburg, to supply the electromechanical equipment for the Ribeiradio powerhouse. The specific speeds of Bemposta II and Ribeiradio are very similar and the operating ranges are overlapping.
It is noticeable in Figure 2, that the head range for Ribeiradio is significantly larger than for Bemposta II, with a ratio between maximum and minimum net head of 1.52 for Ribeiradio, whereas it was only 1.15 for Bemposta II. Further, the weighted efficiency points for Ribeiradio are widely distributed both in head and in discharge. As 20% of the weighted points are located at 70% load and below, a good performance at part load was of particular interest. Consequently, one main target of the hydraulic development was the achievement of flat efficiency characteristics.
To perfectly adapt the turbine to the boundary conditions of Ribeiradio, model development was carried out in the Andritz Hydro laboratory, using the hydraulic model of Bemposta II as basis and optimizing the turbine components.
In the course of the Ribeiradio unit’s hydraulic development, optimized profiles for stay vanes and guide vanes were defined and a new runner developed. The design for the spiral casing of Bemposta II was re-used as is and was scaled fully homologous.
Both draft tubes have a similar hydraulic contour and were adapted to the individual powerhouse design requirements.
The runner blades for both projects are typical X-blade designs. This design has substantial advantages for controlling the flow field at the outlet of the runner. Particularly, the variation of the draft tube feeding from part load to full load is not as pronounced as for conventional runners. The operating point with no swirl behind the runner is shifted towards smaller discharge. The positive effects of this feature are flat efficiency characteristics and better part-load rope behavior in the draft tube, which have both been stringent requirements for Ribeiradio.
Generally, high specific speed runners with low Thoma number Sigma tend to show a rougher operation behavior at part load (the Thoma number Sigma = NPSH/H is a dimensionless factor which is used for assessment of cavitation risk). Therefore, part-load optimization poses a challenge for the design stage. The use of the X-blade philosophy in combination with the application of draft tube fins is an innovative way to maintain pressure pulsations below critical values.
For high specific-speed Francis runners, such as at Ribeiradio, further focus is placed on the runner leading-edge zone, where velocity triangles of inlet flow differ strongly over net head and discharge. Managing the leading-edge cavitation at maximum head without an excessive shift of the hill chart peak location toward too-large net heads is a sophisticated hydraulic design task.
The following design features help in this context, as they provide a mass throttle effect at the runner inlet: bigger inlet blade angles from mid to band, negative lean angles of the leading edge in circumferential direction and stronger twisting of the blade surface near the leading edge in the X-blade direction. The mass flow change from peak toward full load at the outer sections is not that distinct and the flow angle in front of the leading edge varies only within a small band. Therefore, the variation of the incidence angles from full to part load and from maximum to minimum head is minimized.
The area cavitation on the runner blades is handled by adjusting the openings and the lengths of the profiles at the critical sections. Despite the small Thoma numbers of Bemposta and Ribeiradio, both runner designs are very resistant to cavitation.
During the hydraulic development of Ribeiradio, efficiency characteristics in part load were significantly improved. Therefore, the weighted efficiency for Ribeiradio could be maximized.
Besides performance, other important hydraulic quantities (e.g., cavitation, runaway, guide vane torques, pressure pulsations and hydraulic axial thrust) were measured during model testing and the related guarantees were verified.
The hydraulic performance was also validated with a site performance test. Besides performance, various hydraulic guarantees of importance such as guide vane leakage, power oscillations and pressure pulsations were also validated during site testing.
Draft tube fins
Draft tube fins are a well-known and effective measure to improve part load behavior by avoiding problems caused by the part-load vortex.2 Depending on the draft tube profile, fins with different proportions have proven effective. Four fins are set at 45 degrees against the draft tube symmetry plane.
The main effect of draft tube fins on the part-load pulsation is to reduce the amplitude of the pressure source and therefore also the synchronous pulsation. Accordingly, it is the method of choice for suppressing cavitation-related draft tube resonance as well as power swings.
Air admission or air injection is often beneficial by smoothing out the disruptive high-frequency components of noise and vibration. In addition, aeration sometimes removes flow instability by manipulating the hydraulic transmission behavior - in particular lowering the draft tube natural frequency.
The turbine axis is, because of its radial pressure gradient due to swirling flow, the most suitable location for draft tube aeration by means of atmospheric air. In the case of Ribeiradio, air is admitted through the hollow turbine shaft.
Part load operation challenge and trends in numerical methods
Since 2002, Andritz has performed strain gauge measurements on various sizes and types of Francis runners, contributing significantly to understanding of behavior over the entire range of operation, from standstill to full power.
At high power, the main component of the dynamic stress seen by the runner is a well- defined sinusoid at the guide vane passing frequency, called the rotor-stator interaction phenomenon (RSI). Ten years ago, the challenge was to explain and predict RSI stresses that can be responsible for rapid runner cracking at normal operation. Several papers have been written on this subject3,4 and RSI stress prediction is now sufficiently accurate to ensure reliable runner designs.
Currently, the challenge for numerical methods lies in the prediction of low load dynamic stress and transient stress predictions. Stress predictions in those conditions were far from priority when hydropower plants were primarily operated at their best efficiency point. However, with the requirement of flow control and grid stabilization scenarios, Francis runners are more likely to operate at part and low load for an extended period of time and/or to be subjected to numerous start-ups.
Figure 3 presents the waterfall normalized amplitude spectrum of a reference Francis runner blade strain gauge measurement.5 From this diagram, several phenomena with specific frequencies in the rotating reference system can be identified, such as the once-per-revolution hydraulic imbalance and its harmonics present at all power levels, the part load rope at 0.7 to 0.8 times the rotation speed from 56% to 72% of the maximum power, and the RSI phenomenon at the guide vane passing frequency. Low-load dynamic stresses are characterized as stochastic because they include wide band frequency content and various amplitudes. The real fatigue hurdle of some Francis turbine runners is this high-amplitude stochastic stress at low load with wide frequency content.
The broadband frequency content of the low- and part-load stochastic stress requires the use of the so-called rainflow counting method for the evaluation of fatigue damage. This method makes it possible to include all loading phenomena occurring on the turbine runner in the fatigue analysis. Runner fatigue can be calculated using those measured rainflows, an appropriate design fatigue curve in water and the Miner’s rule of cumulated damage. Many published papers discuss this methodology.6,7
For most Francis runners, the part load vortex rope has a weak impact on runner fatigue damage, while the condition giving the highest fatigue damage is the low load condition between speed-no-load and 40% of the maximum power. The description of how the machine is operated, including the number of start-ups and the time at each operating point, is called design reference mission (DRM). With strain gauge measurements, it is possible to define different DRMs and to compare their respective effect on runner fatigue life.
Research engineers in the hydro business have been looking for ways to predict stochastic stresses at low load. Some CFD methodologies are promising,8 but the preparation and calculation time still makes it difficult to use them in the day-to-day runner design processes. Quicker alternatives of computation are being investigated, such as empirical correlation based on strain gauge measurements of similar runners.
This article demonstrates how the hydraulic development of the Francis turbine for Ribeiradio was carried out, using as a basis the existing Bemposta II project, and further optimized in hydraulic components for the particularities of Ribeiradio. Special techniques for CFD design of the runner and some mechanical features in order to verify a smooth and stable part load operation are presented. A turbine with excellent performance characteristics, low vibrations and pressure pulsations was achieved and the hydraulic guarantees could be verified during site testing.
The Ribeiradio hydropower plant is operating with high availability, at low load and with frequent start-stops. It therefore follows the general trend in the electricity market observed during recent years and fulfils the necessity for more flexible power plant operation.
Since the beginning of commercial operation of this facility in 2015, EDP has recorded stable functioning of the turbine over its entire continuous normal operating range and also for the upper and lower momentary abnormal operation. The needs of both the grid operator and the remote-control center for the hydroelectric power plants have been satisfied.
1Ferreira da Silva, A., M. Roque, and D. Laufer, “Bemposta II-Increasing the Maximum Power Output whilst Reducing Wear and Tear,” Proceedings of HydroVision International 2017, PennWell Corporation, Tulsa, Okla., 2017.
2Doerfler. P., M. Sick, and A. Coutu, “Flow-Induced Pulsation and Vibration in Hydroelectric Machinery,” Springer, London, 2013.
3Monette, C., et al., “Hydro-dynamic Damping in Flowing Water,’’ Proceedings of 27th IAHR Symposium on Hydraulic Machinery and Systems, International Association for Hydro-Environment Engineering and Research, Beijing, China, 2014.
4Coutu, A., M.D. Roy, C. Monette, and B. Nennemann, “Experience with Rotor-Stator Interactions in High Head Francis Runner,” Proceedings of 24th IAHR Symposium on Hydraulic Machinery and Systems, International Association for Hydro-Environment Engineering and Research, Beijing, China, 2008.
5Monette, C., et al., ‘’Cost of Enlarged Operating Zone for an Existing Francis Funner,’’ Proceedings of 28th IAHR Symposium on Hydraulic Machinery and Systems, International Association for Hydro-Environment Engineering and Research, Beijing, China, 2016.
6Coutu, A., et al., “Specific Speed Effect on Francis Runner Reliability under Various Operating Conditions,’’ Proceedings of Sixth International Conference on Water Resources and Hydropower Development, Aqua Media International, Wallington, Surrey, UK, 2016.
7Huang, X., et al., “Fatigue Analyses of the Prototype Francis Runners Based on Site Measurements and Simulations,” Proceedings of 27th IAHR Symposium on Hydraulic Machinery and Systems, International Association for Hydro-Environment Engineering and Research, Beijing, China, 2014.
8Nennemann, B., et al., “Challenges in Dynamic Pressure and Stress Predictions at No-Load Operation in Hydraulic Turbines,” Proceedings of 27th IAHR Symposium on Hydraulic Machinery and Systems, International Association for Hydro-Environment Engineering and Research, Beijing, China, 2014.
Miguel Roque, ME, works in mechanical engineering at EDP Gestão da Produção de Energia (EDP-P), an Energias de Portugal subsidiary. Alexandre Ferreira da Silva, ME, is an advisor to EDP-P. Christine Monette, ME, is mechanical analysis team lead of Andritz Hydro-Montreal. Dominik Laufer, ME, is Layout Center Francis lead within the Andritz Hydro Group.