Hydro mechanics

Water from the Grande Dixence reservoir is conveyed to the new underground power house at Bieudron via a long head race tunnel, isolated upstream and downstream by butterfly valves. This is followed by a fully excavated penstock ending in a manifold for three vertical Pelton units. Three spherical valves isolate each unit from the penstock.

The valves

The two butterfly valves, 3.30m in diameter, allow the tunnel and penstock to be dewatered for inspection when necessary. In an accidental pipe burst with discharge of up to 150m3/s (twice the rated discharge), these valves are closed from the power station, or automatically by local detection of water overspeed. The lattice type disc is cast in one piece with trunnions in GS20Mn5 steel. The body, in the same material, is in two pieces with partition bolts tensioned by hydraulic jacks.

The spherical guard valves on the turbine cut off water flow to the turbines in case of loss of control of the nozzle needle valves. Their rated diameter is 1400mm, and the design pressure is 2071m of water column (mWC) — maximum static head plus waterhammer. The rated discharge is 25m3/s per valve, or accidental discharge up to 100m3/s. The valve rotor and its trunnions are cast in a single piece, and without longitudinal or transverse flanges to avoid deformation under pressure. Finally, the valve rotor must be removable for overhauling.

The valve rotor is operated by a straight-stroke hydraulic jack on each side of the body. Oil pressure opens the valves and water pressure from the penstock closes them. The valves are rigidly fastened to the upstream manifold and the anchoring system allows them to move axially in response to pressure changes.

The valves were built by Sulzer Hydro, under licence from Mr Weibel. The valve body is a cylinder with its axis perpendicular to the piping, including the assembly flanges to the upstream and downstream piping and large lateral openings allowing dismantling of the rotor and equipped with bearing blocks. The cast alloy steel construction is sturdy: under test pressures of 3107mWC its largest deformation was as little as 1mm.

The Pelton turbines

In the old Grande Dixence scheme, water from Lac des Dix dropped in two stages, with power stations at Fionnay and Nendaz (878m and 1008m head). The Cleuson-Dixence scheme uses the head — 1734-1883mWC — in a single race. The head loss is 100mWC for the design discharge of 75m3/s, bound by the diameter of the waterways (4.80m for the tunnel and 3.20m for the penstock). The power output is therefore 1100-1200MW, depending on the level in the lake.

The most powerful multijet Pelton built so far is at Sellrain Silz, which has six jets and a head of only 1200mWC. For Cleuson-Dixence two alternatives were considered:

•Three units with five jets of 420MW.

•Four units with four jets of 315 MW

A study by the Cleuson-Dixence Group (GCD), Sulzer Hydro and Hydro Vevey considered:

•Flow in the buckets, evacuation of water and preservation of the following jet.

•The machine’s hydraulic and mechanical design.

•Machine operating characteristics.

•Mechanical behaviour and service life of the runners.

•The dynamic characteristics of the shaft line.

•Model tests for bucket flow analysis.

Water drop impact and erosion need particular attention, considering the very high head of this scheme. When the jet first touches the bucket, the speed composition leads directly to shocks caused by droplet impacts at the rear or in the interior of the bucket tip cutaway. The specific energy of these impacts is directly proportional to the absolute value of the head. Furthermore, in vertical multi-jet Pelton turbines, another source of erosion may appear in the form of the direct or indirect effect on the following jet of the water evacuated from a bucket. The impact of water drops causes degradation of the cylindrical surface of the jet, leading to water droplet impacts on both sides of the bucket edge.

Many multijet turbines present interference between the outflowing water and the jets without causing erosion, because the impact only leads to elastic deformation of the material. But the erosion develops very fast — as the third power of the head — when the head is increased. Experience at Sellrain Silz shows that the erosion is reduced by a factor of about ten when protective shields are installed for the nozzles and the jets.

Erosion predictions for the five-jet solution were slightly worse than those for the four-jet solution. The difference is not decisive, as the result would only be a small difference in maintenance. Erosion is also influenced by other factors: the working sector of each bucket; the shape and number of the buckets; and the angle between the jets.

Visualisation trials were held to analyse bucket inflow and outflow under stroboscopic light, and the impact of the outflowing water on the next nozzle was measured with strain gauges. More detailed analyses was carried out on the major elements — distributor, nozzles and runners.

Both solutions were feasible and neither was significantly better. Taking into account the savings connected with the erection of three units instead of four, which was estimated at SFr60M, and considering ongoing differences in the cost of maintenance, it was decided to use three units.

Range of operation

The operating range of the turbines is a function of the level of the lake, the discharge and the evolution of the net head with regard to head loss.

The individual unit power is 423MW, at maximum head, and electric power at the transformer terminals is 417MWe. The turbines are designed for 25m3/s discharge under l663mWC, minimum net head. This maximum unit discharge is constant under all heads. The scheme has a total power output which varies from around 1100MWe under the minimum head to 1200MWe under the maximum head.

Beyond 72m3/s, the global efficiency of Cleuson-Dixence is comparable to global efficiency at small load of the old Grande Dixence waterways with Fionnay and Nendaz power stations. At larger loads, Bieudron will therefore be operated in parallel with Fionnay and Nendaz.

Additional tests have studied operation with a reduced number of jets, definition of the maximum level in the tail water for safe operation, identification of the impact zones of the deflected jets and finally determination of the runaway speed of the turbines.

The design contract, awarded to GCD, included investigation of the Pelton runners to analyse their mechanical behaviour and deter-mine their service life.

The stress distri-bution in the buckets was analysed using finite element compu-tation. It was also demonstrated that the limit value set by EOS ( = 55MPa) for the pulsating stress due to the jet action at full head was not exceeded.

The centrifugal force of a bucket produces a steady stress = 30MPa at the rated rotational speed.

The study defined the critical cast-flaw dimensions which may become cracks. From a calculation of crack propagation based on mechanics of elastic rupture, the maximum interval between inspections was defined as 400hr of operation at full load. The quality requirement for the finished runner is the absence of any flaw greater than 3mm under the surface, and the absence of any flaws greater than 6mm as far as 50mm deep.

Extrapolating the Woehler curves (LBF tests, Darmstadt) in a humid atmosphere up to 1010 cycles for the 13.4 steel used, and taking into consideration the stresses at the foot of the bucket, the distribution of the load, the residual stresses, etc result in a service life reduced to 50,000hr of full-load operation, with a safety coefficient of four for all damages. Thanks to a favourable design of bucket attachment and to the large ratio between the runner diameter and the bucket width, simple checks are easy to perform. A strict maintenance policy during operation will ensure that this service life can certainly be exceeded. (Cleuson-Dixence is designed for an equivalent of 1000hr of full load operation per year).

Shaft line design

Shaft line design was awarded to ABB, who supplied the generator. The lower bearing and turbine shaft were designed by GCD. The shaft line design covers bearings and coup-lings, and study of the dynamic be-haviour of the shaft line.

The runner torque is trans-mitted to the shaft by a friction coupling with a safety margin of 2.5 in relation to the rated torque and 1.1 in short-circuit condition. Additional mechanical security is guaranteed by a cylindrical sleeve adjusted on the coupling bolt perpendicular to the contact between runner and shaft flange of the turbine, and by radial pins between the flanges of the generator–turbine coupling.

The generator is rated at 465MVA. It is compact, with turbine guide bearing and a combined guide/thrust bearing above the generator.

The first critical bending frequency is between the load rejection speed and the runaway speed. Since this critical frequency is determined by the distance between the generator bearings, and the rotor height is defined by electro-mechanical requirements, using three bearings does not raise this critical frequency above the runaway speed.

Sellrain Silz shows that one must not underestimate the stiffness of the design. Construction of the turbine bearing pads allows radial stiffness to be adjusted, to guarantee sufficient margin between the critical frequency and the rated and runaway speeds. Similarly, the turbine shaft has been designed to separate the first critical torsion frequency by at least 10% from the electrical frequency of the network.

Studies concluded that a runaway unit with normal unbalance and slightly off-centre air gap, could pass critical speed without damage during an event. During extreme runaway, with loss of one bucket, the damage would be limited to the bearings.

Organising the project

The Pelton turbines were ordered in 1993. The runners were cast by Georg Fischer and assembly started in June 1995. Acceptance tests will take place in summer 1998, and the three units will be in service by the end of the year.

Project management had to keep within a very short schedule (five years for civil engineering, three years for installation and commissioning) and tight budget (SFr1000/kW). A detailed organisational concept split the scheme into a set of relatively independent functions, referred to as ‘elementary systems’ and linked to the other systems (thirty for the complete scheme). Each system is documented by a dynamic database which contains all the information necessary for implementation, commissioning and operation.

An extremely strict quality plan was all the more necessary during manufacture, since many sub-contractors had to be controlled between seven European countries.