Les groupes bulbes De Röstin en Avignon :L'essor d'une technique

Bulb units : from Röstin to Avignon The upsurge of a technique

Direction de l'Equipement, Electricité de France.

Résumé

The story of the bulb unit began with the original designs for which the firm Escher-Wyss took out patent rights in 1932/33 (Fig. 1). The first machines of this type to be commissioned were two 168 MW sets at Röstin power station in Pomerania in 1936 (Fig. 2). They were also the first axial-flow sets ever built. However, Arno Fischer lost interest in the Escher-Wyss design soon after taking over its patent rights, and turned to the Harza design featuring a peripheral generator (patent rights taken out in 1919 and 1924-Fig. 3). Seventy-three Harza units giving a total output of 100,000 kW were installed in Bavaria between 1937 and 1943 (Fig. 4). French designers began to consider axial units for their Rance tidal power scheme and low-head applications in general shortly after the Second World War. The first notable step forward was made by M. Guimbal in 1949, when he recommended the use of oil as a generator sealing and cooling fluid as a means of reducing generator dimensions. Neyrpic produced and commissioned two 800 MW sets of this type for the French Railway Castet ,Plant in 1954 (Fig. 6). Electricité de France favoured small asynchronous (and occasionally synchronous) sets in the 50-100 kW range for heads from 1.50 m up to 4 m (Figs 7 and 8). In addition, standard 800 kW units of the 'Maulde' type were developed for heads in the neighborhood of 13 m (Fig. 9). With the advent of more efficient insulating materials, the 'oil technique', to which the bulb design owed its increasing popularity, gave way to the 'air technique', which is the only suitable cooling method for large-capacity bulbs. Forty 'Maulde' sets were commissioned between 1961 and 1970 and are still in service. Tidal power soon became one of Electricité de France's main development targets. Since a 20 MW bulb design produced by Neyrpic for the Rance scheme in 1943 (Fig. 5) had turned out to be too cumbersome, Electricité de France invited leading European manufacturers in 1951 to submit design proposals for axial flow units. Ali the designs submitted in 1952 were of the 'downstream-bulb' type with three bearings, since no solution had been found to the problem of designing a bearing capable of taking the loads produced by an overhung runner. In 1958, two experimental 5 MW downstream-bulb sets were installed at Cambeyrac, and two 14 MW sets at Argentat; ail four sets featured air-cooled asynchronous generators (Fig. 10). Further design work and model tests finally produced the answer to the bearing problem, and in 1957, the upstream unit at Wadrineau installed in a well__the first successful set with a large overhung runner (3.05 m dia.) ever produced__confirmed its promoters' faith in this design and marked the break-through of the 'upstream-bulb' layout for both large and small units. The advantages of the 'upstream-bulb' design include smaller length and reduced weight, hence lower cost, higher efficiency and greater ease of dismantling. However, this type of unit appears to be limited to an output range of up to 0,4 MW/r.p.m. In 1959, i.e. before implementation of the Rance project, two large experimental sets were commissioned, one at Beaumont-Monteux and the other at Saint-Malo (Fig. 1l). Considerable progress then followed, and by the time the Rance project came into being, unit outputs had been raised from 9 to 10 MW, runner diameters were down to 5.35 m (from 5.80 m) and units were running at 93.75 r.p.m. instead of 88.2 r.p.m. Similarly, generators were operating under an absolute air pressure of 2 kg/cm2, which allowed the bulb diameter to be reduced from 5 m to 4.38 m. The final outcome of this comprehensive design programme were the large bulb sets commissioned on the Rhone, Rhine and Garonne (see table), i.e. fifty-eight units producing a total installed output of 1,500 MW, i.e. 8 TWh annually. The advantages of the bulb over the vertical-shaft Kaplan design are summed up in a table referring to the Compagnie Nationale du Rhône's plant at Pierre-Bénite, which shows a cost saving of $ 1 million for each set. Other tables compare the dimensions and electrical features of the two designs. Until recently, ail bulb units had their guide-vanes and runner blades controllable but a further step towards simpler design and lower cost has been made with guide-vanes and runner blades fixed. The Sauveterre plant will be' the first to be equipped with this type of unit (2 X 30 MW), which is expected to result in a 30% saving on weight over the 'double-control' design. For the bulbe units the shut-off valve is located downstream from the runner i.e. as in vertical Kaplan layout, which allows the bulb set to be operated as an orifice passing 70% of the rated discharge al sub-synchronous speed. This does away with the need for a conventional bypass system whilst resulting in a cost saving the Compagnie Nationale du Rhône has estimated to amount to 10 million Francs for its Vallabrègues plant. It was the Rance tidal project which gave French designers the impetus for their significant break-through in bulb unit design. Interest in bulb unit technology has been steadily gaining ground in a number of countries, and thus, bulb units can now be claimed to have entered the convention al field barely ten years after they first appeared.