Two French pumped storage hydropower plants of a similar vintage and sharing some design features – but with specific hydraulic design requirements – are being upgraded, one to variable speed. Work so far suggests that such a conversion is an effective approach.
|In a world first, Le Cheylas pumped storage power plant, near Grenoble in France, is being converted from fixed to variable speed operation.|
By Sylvain Antheaume, Geoffrey Darona, Jean-Bernard Houdeline, Yves Labrecque, and Patrick Laurier
Today, variable output renewable power generation, such as wind and solar, represents an ever larger share of the world’s power output. But it represents a major challenge for grid stability. The ideal complimentary solution for intermittent renewable energies is pumped storage plants (PSP) that can support rapidly changing grid requirements.
Electricité de France (EDF) operates a fleet of about 100 GW of electricity generation in its home country and more than 135 GW worldwide. Of this, 5 GW are PSPs using synchronous generators. The large PSPs were commissioned in France between 1975 and 1990 in response to booming nuclear development. A key enabler was reversible Francis pump turbine machines, which allowed large capacities per unit (>200 MW). Alstom (formerly Neyrpic) was amongst the early developers of this technology and supplied the power plants of Revin and Le Cheylas, which are operated by EDF.
After more than 30 years of operation, heavy maintenance is required though, offering the opportunity to improve performance in relation with new operating requirements.
Pumped storage power plant retrofit and refurbishment opportunities
For a power plant operator, the first need can be simply to replace worn parts and to refurbish the major components to retrieve the original level of performance at the time of the first commissioning.
However, for the customer, this can also be the time to modify the existing equipment to adapt to new market requirements and to benefit from the latest technology developments to optimize the investment (refurbishment cost/profit ratio).
The most common and sought after requirements include increasing generating output, pumping discharge and efficiency, as well as improving operational reliability, and reducing operational instabilities and widening the operating range.
To meet these requirements, a detailed hydraulic, mechanical and electrical investigation is needed, which can lead to a:
— Change of the unit speed or implementation of the variable speed technology,
— New runner,
— Modification of the distributor,
— Replacement of guide vanes,
— Reshaping of the stay vanes and/or stay ring,
— Change of part of the draft tube cone.
This kind of analysis must be conducted very accurately to fulfill the customer’s needs, especially the hydraulic investigation, which has a direct impact on the performance of the pump-turbine units to be refurbished.
Revin refurbishment plan
The Revin PSP is located in the North East of France and has been operated by EDF since its commissioning in 1976. Its only purpose is energy storage because there is no catchment area to feed the upper reservoir.
Featuring four units of 200 MW each working under gross heads from 220 to 246 meters, the main objectives of the Revin retrofit project are to secure reliable operation for another 30 years, as well as increase the turbine and pump efficiencies and improve the pump-turbine behavior within the entire operating range to optimize the use of the available hydraulic energy.
The Revin project was prepared by EDF in three phases: pre-feasibility, detailed feasibility studies and tendering.
The pre-feasibility phase was conducted in October 2008 and covered the following three possible scenarios:
No. 1: Replace or repair worn parts back to original condition, replace wickets gates of Unit 3 (showing early signs of cracks in the upper trunions), refurbish the guide bearings and thrust bearing, rehabilitate the distributor’s inferior rotating servomotors (IRS), and improve of the shaft seal design.
No. 2: Same as No. 1, plus redesign the distributor kinematics with the removal of the IRS.
No. 3: Same as No. 2, plus design of a new runner and wicket gates to increase performance. With this scenario, the weighted average efficiency increase was estimated at about 3% in turbine mode and 3% in pump mode.
For each of the three scenarios, an extensive technical and economic study was conducted, taking into account the expected scope of supply, timing of and estimated duration of unit down-time.
Eventually, EDF decided to start detailed feasibility studies for scenarios No. 1 and No. 3. Based upon these studies, EDF selected scenario No. 3.
Four hydro equipment manufacturers (Alstom, Andritz Hydro, Rainpower and Voith Hydro) pre-qualified at the conclusion of a European tendering process and the tender was issued in July 2010. Functional guarantees, including the expected weighted averaged efficiencies in turbine and pump mode, were determined during this phase.
After the bids were opened and following analysis of the technical and commercial offers, EDF awarded Alstom the contract in July 2011.
General presentation of the contract
Alstom proposed a new hydraulic design for the runner and guide vanes in order to improve the efficiency. A new head cover, bottom ring and distributor mechanism were proposed to improve reliability and to reduce the site works period. This included the removal of the inferior rotating servomotors that were installed during the commissioning in order to set a torque of closing tendency on the entire stroke. It also included the improvement of the shaft seal design and the replacement or repair of the other worn or defective parts.
The contract included mechanical and hydraulic studies, model tests, dismantling and reassembly of the pump-turbines, supply and transport of the new or renovated parts, piping and special tools. On-site performance measurement tests on the unit are also to be performed.
A strategy for Le Cheylas: The first fixed to variable speed upgrade
Le Cheylas is located closed to Grenoble in the French Alps and has been operated by EDF since its commissioning in 1979.
The plant has two purposes: average annual electricity generation of 550 GWh using the Arc River flow conveyed from Longefan to Le Flumet Reservoir by a 20 km tunnel, and electricity storage on a daily cycle by pumping at night from Le Cheylas Reservoir into Le Flumet.
There is only one powerhouse, equipped with two identical 254 MW reversible units working under gross heads from 245 to 261 meters, each located in a shaft. A water conveyance system common to the two units connects the powerhouse to the two reservoirs.
Like most PSPs, Le Cheylas is equipped with two synchronous rotating speed units. This technology imposes a fixed power input per unit in pumping mode: the pump is either stopped or operating at full capacity for the whole range of heads. Because of this, other types of flexible generation plants must be kept online when the pumps are in operation.
In a country like Austria, where hydro generation represents a large proportion of the energy mix, flexibility during off-peaks hours is obtained by running simultaneously hydro units in generation while others are in pumping mode. This kind of operation mode may limit greenhouse gas emissions at the cost of using part of the energy generated. It literally results in using part of the intermittent generation to heat up the water in the upper reservoir. The fixed operating speed also limits the ability of the hydraulic unit, when operating in turbine mode, to keep a high efficiency over a large head and flow range.
Recently introduced in Europe, variable speed technology for PSPs brings additional flexibility. One key benefit of variable speed units with respect to conventional units is their capability to regulate their power both in pumping and generation modes, while conventional units can only regulate in generation mode.
The flexibility in pumping mode is then obtained without wasting energy, as the hydraulic unit efficiency is almost constant over the operating range.
Although several new variable speed PSPs were recently ordered in Europe or are under construction (Linthal 2015: 1000 MW, Nant de Drance: 942 MW, Venda Nova: 800 MW), upgrading conventional PSPs to variable speed is much more complex than developing new plants because the existing pump-turbines and civil structures must be accommodated. Additionally, variable speed motor generators are bigger than conventional generators and require the installation of large power electronics equipment close to the units in generally very constrained cavern areas.
The refurbishment of Le Cheylas is also being developed through the eStorage project, funded by the European Commission. Its members include transmission system operator Elia, EDF, Imperial College, engineering consultancy DNV GL and Alstom.
Unit 1 was dismantled for maintenance in 2010. This allowed verification of all major unit components (spherical valve, pump-turbine, and motor-generator). Their condition was rather good at the time and several operations were performed then. The unit was put back in operation in 2011 and will continue to operate at synchronous speed.
Unit 2 was last inspected in 2010 and was found to also be in good condition. However, making a different selection between maintaining existing parts and installing new parts allowed unit downtime to be shortened and to gain efficiency. For instance, a new cover has been ordered instead of the existing one. Finally, it was also decided to convert this unit to variable speed technology, as a “demonstrator” within the framework of the eStorage project.
|The refurbishment of Le Cheylas aims to demonstrate the cost-effectiveness of upgrading pumped storage plants to variable speed to meet changing operational requirements.|
Hydraulic technical issues
Revin and Le Cheylas are Neyrpic designs from the 1970s. The specific speed (Nq) of the existing turbines is Nq>44 rpm for both projects. Therefore, the two projects share many hydraulic features, including a double curvature spiral case, radial stay vanes, and the same draft tube design. However, the existing Revin design has a nine bladed runner with 16 guide vanes, whereas the Le Cheylas design has a seven blade runner with 20 guide vanes.
For obvious reasons of cost and planning, only non-embedded parts, the runner and guide vanes, were modified. These components have the highest flow velocities and bring the best efficiency/cost trade-off. The existing draft tube, stay ring and spiral casing for both projects were kept.
In the 1970s, the hydraulic design of pump-turbines was strongly influenced by the Francis non-reversible turbine design. This can be seen in the profiles of the existing stay vanes at Revin, which look very much like Francis turbine stay vanes, i.e, radial profiles. While this is suitable for a flow in turbine mode, this is not appropriate in pump mode. With conventional new guide vane designs, some separation flow and even backflow can be observed, increasing the head losses in pump mode.
To avoid this kind of issue, the stay vane profiles must be changed. This issue, which was identified at the tender stage, was solved with a new hydraulic design. Optimal loading on the guide vane profiles and an adequate adaptation of the incoming flow on the existing stay vane profiles in pump mode was achieved.
In addition, the relative azimuthal position of the guide vanes and stay vanes was not optimal in terms of head losses on the existing design. This is the reason why the new guide vanes’ angular position was defined. The hydraulic profile and pitch circle diameter of the guide vanes were also changed. EDF’s decision to change the covers made this possible.
In pump mode, for the Revin optimization, the head loss reduction in the distributor was well identified by CFD.
The calculations were done at the discharge design point for each case with the optimal corresponding guide vane opening. The improvement of the flow behavior in pump mode due to the new guide vane design is important for the pump performance level. However, it is also interesting to use it as a means to indirectly increase turbine efficiency by making design choices oriented towards turbine performance.
A new runner was designed in line with the new guide vanes and the most promising manufactured at model scale for validation.
The results presented below are expected prototype values from the model acceptance tests results using IEC 60193 standard scale-up formulas.
Pump mode at Revin
One of the many hydraulic objectives of the retrofit was to increase the flow discharge in pumping mode. This target was achieved with an increase of about 4.5% in discharge for the head range.
Regarding the hydraulic efficiency in pump mode, the weighted average gain is about 2.6% with a best improvement of about 3.0% for the highest head level.
Regarding the turbine mode, the weighted average efficiency gain is about 2.8%. In comparison with the existing design, the largest efficiency improvement is remarkable for the maximum output with a significant increase under the highest head of 240 m (water column).
It is also important to note that the new design is able to deliver about 10% of additional output even though this was not in the scope of the Revin retrofit.
In order to take advantage of power regulation in pumping mode, the hydraulic design must allow increased water flow variation. As the original design was not foreseen to operate in such conditions, the hydraulic design had to be re-engineered.
The key design target was to allow a power variation in pump mode of 80 MW under the full head range (i.e. constant power variation close to 30% of the unit’s nominal power, leading to a quite extended speed variation of +/- 7%).
The pump is the key part of a variable speed upgrade. When compared to a “classical” pump-turbine refurbishment, two new hydraulic challenges generally constrain the design.
Generally, the lower pump power limit is set by an instability area where it is not possible to operate (called the hump zone) or by the cavitation limit on the suction side of the runner blades. The high power limit is defined by the cavitation phenomena on the pressure side of the runner blades. On the Le Cheylas project, these are the cavitation limits that define the operating area, taking into account the existing submergence. Compared to the existing design, the free cavitation zone was extended thanks to an adequate optimization of the runner hydraulic design, allowing the discharge range of the unit to increase by a factor of two.
Since the complete motor/generator is to be changed, the rated revolving speed of the unit can be also changed. Nevertheless, it was decided to keep the existing rated revolving speed as the appropriate solution to maximize the pump power variation.
As with Revin, model tests for Le Cheylas project have been performed for several runners and guide vane designs in order to confirm the CFD investigation results. This experimental process was completed with the model acceptance tests in the presence of EDF’s representatives to validate the final new design.
Pump mode at Le Cheylas
Regarding the hydraulic efficiency in pump mode in comparison with the existing design, the largest improvement is about 1.5%.
Thanks to an adequate optimization of the hydraulic design applied to the variable speed possibility, the expected input power variation is at least 80 MW whatever the net head operating range.
Regarding turbine mode, the reduction of revolving speed of the unit to the minimum value (n/n = -7%) shifts the initial operating area of Le Cheylas towards to the best efficiency line. This advantage given by the speed adjustment possibility combined with the efficiency improvement from the new hydraulic design allows a significant increase in the weighted average efficiency level.
Model tests on the turbine and simulations on the generator side have been performed in order to evaluate the expected efficiency level of the new equipment. Compared to the former unit, the expected hydraulic weighted average efficiency in turbine mode increases by 5.2% and by 0.8% in pump mode.
Converting PSPs to variable speed
Retrofitting existing pump-turbine units can bring a significant performance improvement to a power plant with time and money expenditures much lower than those required to build a new PSP.
The hydraulic and mechanical challenges depend on the type of retrofit and are complex because many design parameters are already imposed by the existing water passage surrounding the runner. For a variable speed upgrade, constraints coming from the civil structure or hydraulic circuit are the most difficult and costly to overcome.
EDF and Alstom leveraged their respective experience in PSP design and operation to successfully conduct two different PSP projects up to the model test and mechanical design stages.
For the Revin project, based on model test results, pumping time has been decreased by 4.5% while average pump and turbine cycle efficiency has been increased by almost 3%.
The Le Cheylas example demonstrates that converting existing synchronous units into variable speed can be a very expedient and cost-effective solution to increase power and the regulation capabilities for plant operators, facilitating the integration of intermittent renewable energy generation into the electrical grid.
Dr. Sylvain Antheaume and Geoffrey Darona are hydraulic engineers with the Alstom Global Hydro Technology Centre. Dr. Jean-Bernard Houdeline is a pumped storage plant hydraulic expert with Alstom. Yves Labrecque is head of the technical office of Alstom Hydro’s office in Sorel-Tracy, Quebec, Canada. Patrick Laurier is hydromachinery expert in the Hydro Engineering Centre of Electricite de France.