Marche en déchargeur des turbines-hélices à pales orientables (kaplan et bulbes) application aux usines du Rhône

Relief operation of variable-pitch Kaplan and bulb turbines The case of power plants on the Rhône

Directeur délégué à la Compagnie Nationale du Rhône.

Résumé

In low-head power schemes with a long diversion channel, a most economical way of avoiding unduly high storage levels is to provide relief arrangements at the power house enabling at least half the rated plant discharge to be bypassed through the diversion canal. Arrangements of this type are necessary because of the moderate size of the flows liable to ,be released at the river barrages while the bypassed river section remains below a certain level. Quick-acting relief facilities able to pass roughly half the rated total power plant flow are also required at the power house as a means of attenuating waves caused by power sets cutting out, which are a particularly bad navigation hazard. Analysing the causes of discharge variations in power plants, it is seen that troublesome cutting-out of power units (i.e. involving more than half the units) due to internal power house faults can be almost completely eliminated, and that insidious power variations due to units cutting out a long way away can he transformed into local effects. This only leaves faults originating from the network to deal with; though these are seldom troublesome, one mays on the average expect one such fault each year, whose effects it is most important to attenuate. This makes some form of relief arrangement absolutely essential, which may be dimensioned solely for operation during the infrequent short emergencies when it will be required. The relief arrangements may either be a separate structure from the power house, with quick-opening gates__i.e. a gated bypass__or alternatively, the power sets themselves can be set for no-load relief operation passing a high flow. It is shown that a correctly dimensioned separate structure is expensive and that it is more economical to do without it if the power units can be made to do the same job cheaper. The developments which led to use of the power units for relief operation are reviewed, showing the various stages of research and development, which the manufacturers concerned describe in detail in a separate paper. The effect of such outside factors as the increasingly widespread use of individual downstream gates and the advent of bulb units on the design and development work is demonstrated. It is shown how this work finally led to three different policies for power plants on the Rhône, one being a layout without agate designed for a relief capacity of around 100 cu.m./sec. per unit under 10 m head, another featuring a slow-operating downstream gate finally giving very calm operation with a 200 cu.m./sec. flow, after an initial period at a lower flow, and the third, with a quick-acting downstream gate enabling an immediate transition (i.e. with one same time discharge curve) to be archieved from the flow under head to a very high relief flow (200 cu.m./sec. at Pierre-Bénite and at least :i.50 cu.m./sec. at Vallabrègues, both of which have this type of system operational). The Compagnie Nationale du Rhône have thus been able to do away with the bottom relief outlet at Vallabrègues, which has saved at least 10 million Francs compared to the conventional singlebypass layout. Future new power schemes will also be so equipped.