Démarrage en pompe des groupes réversibles -Différents procédés utilisables. Avantages et inconvénients.

Chef de Division, Service Matériel, R.E.H. Alpes-Nord, E.D.F., Chambéry Rapporteur du Groupe de travail Schémas de démarrage des groupes réversibles

Résumé

Compared with ternary sets comprising two distinct conventional machines, i.e. a pump and a turbine, reversible pump-turbine units permit considerable reduction in the capital cost of pumped storage schemes provided power demands for pumpwise and turbinewise operation are similar and provided also that time of transition between modes is not a decisive factor. On the other hand, reversible sets rotating either way pose several problems, a major one being the best way to effect pumpwise start-up, more especially when unit power is high. A number of possible procedures are examined and, in each case, advantages, disadvantages and application constraints are stated. Start-up procedures available may be classified as in Figure 1 (table). From the turbine standpoint, start-up may be carried out with the runner either wet or dry. Wet runner start-up is clearly the simpler of the two, although high starting torque is required to overcome the drag of the water. Dry start-up has the disadvantage of requiring expensive compressed-air equipment for dewatering the runner. When unit power is large, the majority of the start-up procedures call for a dry runner. The curves of Figure 2 show the general shape of the variation of pump-turbine drag torque with speed of rotation for wet and dry runners. These curves also point up the advantages of supplying oil at the thrust bearing. Choice between wet and dry modes depends largely on torque available and torque required. As for the choice of the best start-up procedure for a reversible unit, this is by no means a simple matter since it depends on a large number of criteria, the most important being as follows : (i) Characteristics of the unit to be started up, i.e. power, inertia, driving couple, drag couple, reactance, time constants, etc. (ii) Characteristics of the network : short-circuit power, acceptable voltage drop, line and transformer design, etc. (iii) Times allowed for changing from pumping to generation and vice versa. (iv) Number of units required for turbine and pump operation. (v) Local conditions on site e.g. ease of installation of a special unit or possibility of connecting up with a unit in a neighbouring plant suitable for back-to-back start-up. When technically feasible, direct synchronous start-up is the simplest, most economical procedure. However, the present trend towards increased reversible unit power seems likely to create problems. These will be concerned not only with the design and construction of the machines but also with network operation. Such problems may lead to preferential use of other start-up procedures. Start-up under reduced voltage may sometimes have advantages. This is an economical method (especially when reactance coils are used for voltage reduction) which is well suited to fast machines with low start-up reactance. However, use of this method should be confined to machines with low drag torques. Asynchronous start-up motors on the same shaft are suited to most cases. However, this is an expensive solution especially if the plant is made up of several sets. None of the other methods are valid except in rather special cases. However, it is worth noting that progress with the static converter may make this technique competitive with others in cases where schemes use large numbers of sets. No method is perfect and for a given problem several solutions are possible.