Developing the Baixo Sabor Pumped Storage Cascade in Portugal

The Baixo Sabor scheme was originally envisioned as a single pumped storage facility but later reconfigured to feature two plants in a cascade. Learn about challenges overcome and lessons learned from this development work.

By Alexandre Ferreira da Silva, Alexandre Maia Magalhaes, Benjamin Benz and Heike Zoeller

The Baixo Sabor hydroelectric scheme on the Sabor River in Portugal was built to generate electricity and create a strategic reserve of water in the region. Although it was originally planned to be a single facility, Energias de Portugal ultimately decided the best option was to divide the available head between two dams and powerhouses.

This article discusses why this arrangement was selected, how the two facilities function together, and the hydraulic development of both pump-turbines for the unique site characteristics. Challenges faced and overcome during this development include wide head variation, a long dry season in the region, and the selection of a shut-off device.

Background

The Sabor and Douro rivers meet upstream of four run-of-river plants on the Douro River – Valeira, Regua, Carrapatelo and Crestuma-Lever. The Baixo Sabor scheme, which began commercial operation in early 2016, improves operational management of the Portuguese Douro watershed and, in particular, of these four plants, which depend heavily on the flow released by the upstream Spanish hydropower plants.

Together the two reservoirs in the Baixo Sabor scheme can store about 642 cubic hectometers of water. In dry years, the water reserve at Baixo Sabor ensures EDP, through these four downstream plants, can provide capacity of 150 MW every weekday for two months and 700 MW during peak hours.

Unique site conditions and operating ranges at the Baixo Sabor plant result in a wide operating range for the pump-turbines.
Unique site conditions and operating ranges at the Baixo Sabor plant result in a wide operating range for the pump-turbines.

Equally important is the Baixo Sabor scheme’s role in controlling the Sabor River peak flood and, thus, the peak flood in the Douro River downstream of the confluence with the Sabor River.

The Baixo Sabor hydroelectric scheme is a combination of pumped water and natural stream flow to store and release energy. During the feasibility studies, two solutions were evaluated from the point of view of the total benefits versus costs – one scheme with only one dam and a scheme with two dams. To avoid geological difficulties and a long waterway that would lead to massive excavation works and significant head loss, it was concluded that the best solution for the Baixo Sabor hydroelectric scheme was the inclusion of two hydropower plants in a pumped storage cascade.

The main companies that did work on this project were:

  • EDP – Gestao da Producao de Energia, S.A., engineering and project management;
  • Odebrecht Portugal (former Bento Pedroso Construcoes e Lena Engenharia Construcoes), civil construction work;
  • Andritz Hydro GmbH Ravensburg (Germany), Andritz Hydro GmbH (Austria) and Ensulmeci (Portugal), equipment supply. The consortium Andritz Hydro and local partner Ensulmeci received the contract for the delivery, installation and commissioning of the complete electromechanical equipment in 2009. In 2012, Andritz assumed also responsibility for the contracted scope of supply end services of Ensulmeci;
  • EIP – Eletricidade Industrial Portuguesa, S.A., power transmission line;
  • Consulgal, Consultores de Engenharia e Gestao S.A., civil construction works, equipment erection and environmental supervision; and
  • Tabique Engenharia, Lda., health and safety supervision.

Project overview

The two hydropower plants of the Baixo Sabor scheme are located 12.6 km and 3 km upstream of the confluence of the Sabor and Douro rivers. With a total installed capacity of 189 MW at Baixo Sabor (upstream) and Feiticeiro (downstream), average generation is 460 GWh per year, which provides an equivalent reduction of CO2 emissions of 1,037 kt per year.

The reservoir behind the upstream dam, a 123 m high and 505 m long concrete arch dam, stores most of the water, with a gross volume of 1,095 hm3. The net volume (active storage) and the normal operating storage are 630 hm3 and 172 hm3, respectively. This site has a large water level variation between the full reservoir level (234 m above sea level) and the minimum drawdown level (205.5 masl). The minimum continuous normal operating level is at elevation 227.4 masl.

Because pump operation is exposed to higher heads than turbine operation, the best efficiency point of the unit in turbine mode is shifted toward higher heads.
Because pump operation is exposed to higher heads than turbine operation, the best efficiency point of the unit in turbine mode is shifted toward higher heads.

The upstream dam has a gated surface free fall spillway with a design discharge capacity of 5,000 m3/s and a downstream plunge pool. An intake feeds water to two pressure tunnels of 5.7 m diameter, 242 m and 331 m long. The powerhouse is equipped with two reversible Francis pump-turbine units of 76.5 MW each, installed in two separate shafts with a height of 79 m and 11.5 m diameter.

The downstream facility includes a 45 m high and 315 m long concrete gravity dam provided with a gated surface spillway, also designed for 5,000 m3/s flow and a roller bucket-stilling basin. It creates a reservoir with a gross volume and net volume (active storage) of 30 hm3 and 12 hm3, respectively.

The waterway includes a water intake followed by two separate pressure tunnels of 4.2 m diameter, 110 m and 121 m long. The two tunnels cross midway to the powerhouse. The powerhouse has two shafts of 22 m diameter, each containing one 18 MW reversible Francis pump-turbine unit.

Challenges for Baixo Sabor plant

The pump-turbines of Baixo Sabor are designed to work about eight and five hours per day in turbine and pump mode, respectively. In addition, the site experiences a wide head variation of Hmax/Hmin = 1.163, which can increase to Hmax/Hmin = 1.24 due to plant usage in the market. Operation during dry years can lead to an extraordinary head variation of 1.54 (see Figure 1 on page 7).

To enable this extraordinary operating range for fixed speed pump-turbines, stability limits in both operating modes must be shifted and the cavitation limits need to be respected for the overall head range.

In addition, improving the performance level in both pump and turbine mode is always a development objective. The hydraulic development of a pump-turbine is often a compromise between pump and turbine mode. The pump operation is exposed to higher heads than the turbine operation due to the hydraulic losses of the waterway system. This leads to a runner diameter that is bigger than required for turbine operation exclusively. Thus, the best efficiency point in turbine mode is shifted toward higher heads (see Figure 2).

To improve turbine performance and allow operation at lower heads, the best efficiency point has to be shifted closer to the operating range. This can be realized by improving the stability behavior of the pump at maximum head, by shifting the pump characteristic towards larger energy coefficients and by increasing the pressure loading of the runner blades. All measures independently result in a smaller runner diameter.1

Cavitation free pump operation over the whole head range – 67.5 m to 104 m – without a variable speed drive or without a negative effect on performance of the pump-turbine is not possible. But the plant seldom operates below 89.4 m, expected to be only about every 10 years for a short period. Additionally, maximum discharge at the river site is limited to 88 m³/s per unit. To improve cavitation behavior at very low heads and to respect this maximum discharge requirement, it was decided to reduce the guide vane openings compared to the openings of the optimal pump envelope from the point of the required discharge limitation (see dashed lines, Figure 3). The efficiency reduction that accompanies this opening reduction does not have a decisive economic impact on the power plant due to the limited operation time. The other aspects mentioned before outweigh the loss in efficiency for the head range mentioned.

Reducing the guide vane openings compared to the openings of the optimal pump envelope from the point of the required discharge limitation was designed to improve cavitation behavior of the units at very low heads.
Reducing the guide vane openings compared to the openings of the optimal pump envelope from the point of the required discharge limitation was designed to improve cavitation behavior of the units at very low heads.

The pump turbines of Baixo Sabor are equipped with a ring gate as a shut-off device. The forces acting on the ring gate were determined during the model test for the layout of the servomotors. The forces were measured directly with load cells and indirectly by measuring and integrating the wall pressure at several locations on the gate perimeter. Additionally, a computational fluid dynamics study of the ring gate was performed to support the model test results by simulating various gate openings and to compare the calculated wall pressures with the measurements on the model. As the results of the CFD simulation correlate quite well with the test results, this approach can be used to calculate the forces for additional openings, which were not tested on the model.2

Instability in turbine mode

Commissioning of the Baixo Sabor units was affected by a long dry season in the Douro region, significantly delaying filling of the upper and lower reservoirs. The water levels and operating head were below the exceptional design limits. At these very low heads, a large guide vane opening is required to reach speed-no-load conditions. During start-up, speed and pressure oscillations occurred due to instability, caused by the interaction of the hydraulic system with the turbine characteristic, which led to an instantaneous shut-down of the units.

Pump-turbine characteristics tend to have a positive slope related to dimensionless coefficients at the intersection from turbine operation to reverse pump operation (region of speed-no-load) due to the compromised design of pump and turbine (see Figure 4 on page 12).3 This so-called “instability” is more distinct towards bigger guide vane openings.

At the lower end of the exceptional operating range, the Baixo Sabor pump-turbine has the tendency to be unstable, which was confirmed during commissioning. This unstable behavior can be avoided by excluding a certain number of guide vanes from the synchronous regulating mechanism. These guide vanes are pre-opened during turbine start-up to obtain the required discharge for speed-no-load operation, with a smaller opening of the synchronous guide vanes and ensuring stable conditions.4 The units of Baixo Sabor were designed with the possibility to have additional asynchronous displacement of some of the guide vanes.

Mechanical design

The asynchronous guide vanes follow that movement of the synchronous operating ring of the other guide vanes. The additional opening of the asynchronous guide vanes is made possible through two additional levers. One is affixed to the guide vane stem and movable by using two additional servomotors.

The “instability” in pump turbines resulting from the compromised design of a pump and turbine is more distinct toward bigger guide vane openings.
The “instability” in pump turbines resulting from the compromised design of a pump and turbine is more distinct toward bigger guide vane openings.

One specific feature of the Baixo Sabor mechanical design is the cylindrical ring gate as a shut-off device. This is a proven technology for Francis turbines, but it is one of the first applications for pump-turbines. The ring gate has a diameter of 5,513 mm and is located between the stay vanes and guide vanes.

The ring gate drive mechanism for upward and downward movement is equipped with six double-acting hydraulic servomotors mounted on the upper head cover flange. To avoid inclination of the ring gate cylinder, the servomotors are mechanically synchronized by connecting their operating spindles using chains.

Modified turbine domain and start-up sequence under low heads

The economic benefit of the Baixo Sabor pump-turbines is to provide the possibility of a wide power regulation band to the grid. During commissioning, the turbine characteristic shifted slightly toward higher discharge. This is beneficial for full-load operation, but the units face stability and vibration issues at part load. Because the extent of possible power regulation is most profitable, it was decided to modify the turbine domain by cutting the temporary operating range at the lower end and adding operation possibility at high flows above the continuous limitation of 85 m³/s (see Figure 1).

Unit commissioning had to be executed at the lower end of the exceptional head range due to unfavorable hydrological conditions. During commissioning, it was found that two asynchronous guide vanes were not sufficient to compensate for the no-load instability and the number had to be increased to four. For additional stabilization, the start-up sequence “Turbine start with static frequency converter (SFC)” was added. Thereby, the unit is started in water with the SFC and, after reaching a defined speed, the guide vanes are opened. Through the control of SFC, the speed variations caused by the no-load instability of the unit at very low heads can be compensated. With those additional measures, a stable synchronization of the units is achievable.

Generally, the start-up and speed-no-load conditions are the most damaging operating conditions for hydraulic runners.5,6 The loads on the runner are decreased through a start with closed distributor and by the subsequent slower opening of the guide vanes. The stabilization of the speed through the SFC has the same relieving effect.

Feiticeiro plant issues

Similar to Baixo Sabor, the Feiticeiro units are exposed to a large head variation. The relation between maximum head to minimum head is Hmax/Hmin = 1.38. A temporary turbine operating range is foreseen at low loads but not for an extended head range (see Figure 5).

Another challenge for the runner development is the high specific speed of the pump-turbines (~280). Most of the pump-turbine projects worldwide are in a speed range between 100 and 180, only a few up to 240. Therefore, a complete new development model was required for this project.

During hydraulic development, an already measured reference model was selected and numerically calculated to adjust the simulation to the measurements. A new hydraulic model was developed to fit the requirements of the project. Design targets were defined to increase pump stability at high heads, flattening the -curve to improve cavitation behavior and extend the vortex-free operating zone and flattening the propeller curves in turbine mode.1 All measures target to a smooth and stable operation over a wide head and load range.

For the Feiticeiro plant, a temporary turbine operating range is foreseen at low loads but not for an extended head range.
For the Feiticeiro plant, a temporary turbine operating range is foreseen at low loads but not for an extended head range.

Finally, after a detailed optimization process, the runner was released for model testing. The results confirmed the tendencies of the CFD optimization and validated the given guarantees. In the end, a new excellent reference model for this high specific speed range was developed.

Mechanical design

The pump-turbines at Feiticeiro have no classical shut-off valve. To reduce leakage through the distributor, specifically during standstill and pump startup, the units are equipped with seals at the lower and upper guide vane faces in the closed position. The seals are integrated in the bottom ring and head cover.

The intake gate is capable of performing emergency closures (i.e., being closed while the machine is running in turbine mode). This requirement is considered during the dimensioning of the servomotors and gate. The non-linear closing behavior of the intake gate is due to the water flow below the cylinder body, which accelerates the closing speed. With 100% load, a closing time of 28 seconds was measured.

Conclusions

Like most hydropower projects, the Baixo Sabor scheme is unique, differentiating itself as a pumped storage cascade by the large head range of the upstream plant with fixed-speed pump-turbines and by the high specific speed of the pump-turbines of the downstream plant.

Work on the Baixo Sabor plant began in mid-2008, with commercial operation in February 2016. Work on Feiticeiro began in mid-2009 with commercial operation in April 2015.

Since commissioning, the units of the Baixo Sabor pumped storage cascade have made their mark on the water and energy management in the upper Douro region.

Notes

1. Kerschberger, P., and A. Gehrer, “Performance Optimization of High Specific Speed Pump-turbines by Means of Numerical Flow Simulation (CFD) and Model Testing,” International Journal of Fluid Machinery and Systems, Volume 3, No. 4, pages 352-359, 2010.

2. Sallaberger, M., C. Gentner, C. Widmer, and U. Henggeler, “Challenges in Design of Pump Turbines,“ Proceedings of HydroVision International 2011, PennWell, Tulsa, Okla., USA, 2011

3. Staubli, T., C. Widmer, T. Tresch, and M. Sallaberger, “Starting Pump-turbines with Unstable Characteristics,” Proceedings of Hydro 2010, 2010.

4. Klemm, D., “Stabilising of the Characteristics of a Pump-turbine in the Range between Turbine Part-load and Reverse Pumping Operation,” Voith Forschung und Konstruktion, Volume 28, 1982.

5. Nennemann, B., J-F., Morissette, C. Monette, and A. Coutu, “Challenges in Dynamic Pressure and Stress Predictions at No-Load Operation in Hydraulic Turbines,“ Proceedings of IAHR, Volume 22, Montreal, Quebec, Canada, 2014.

6. Mende C., et al, “Potential of Start Optimization for Francis Turbines,” Proceedings of IAHR 2013, Lausanne, Switzerland, 2013.

Alexandre Ferreira da Silva is head of the mechanical engineering area and Alexandre Maia Magalhaes is a senior mechanical engineer working in the design of hydroelectric installations with EDP – Gestao de Producao de Energia S.A., a company of EDP – Energias de Portugal, S.A. Group. Benjamin Benz is head of the pump turbine layout center and Heike Zoeller is design engineer with Andritz Hydro.

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