Turbines-pompes de hautes chutes

Pump-turbines operating under high head

Société NEYRPIC - Grenoble

Abstract

Hydraulic energy-storage methods have been gaining worldwide popularity, especially plant operating under very high head (formerly equipped with thernary units), which is showing increasing promise. The growing tendency today is to equip this type of plant with reversible pump-turbines for operation under more than 500 m head, as considered in this report. Present possibilities are as follows : a) Single-stage pump-turbines for up to 600-700 m head, b) Multi-stage pump-turbines with fixed guide-vanes, c) Two-stage pump-turbines with adjustable guide-vanes. Essential problems associated with each above alternative are discussed in this report. 1. Single-stage pump-turbines (Fig. 1) These are an "extrapolation" of medium-head pumpturbines. Associated problems become more difficult with increasing head. 1.1. "Hydraulic" problems First among these is cavitation during pump operation. Relative flow velocity to the bladings is considerable under high head. To ensure acceptable cavitation conditions, therefore, blade leading edges require careful designing by a method based on calculated flow data. Prototype design must be sufficiently accurate to reflect the characteristics determined on the scale model and ensure the absence of local cavitation-generating faults and associated high erosion velocities, i.e. proportional to the fifth power of flow velocity. Choice of specific speed is critical. If this speed is very low, optimal turbine and pump efficiencies will be affected by high runner disc losses. Too high a specific speed will cause excessive flow velocity in the machine and, for cooling reasons, may result in a prototype machine speed at which the motor-generator cannot operate economically. Pressure, guide-vane torque and runner thrust fluctuations at transient conditions after disconnection of the machine during turbine or pump operation have a more pronounced effect under high head. 1.2. Mechanical design requirements Guide-vane stem diameter and length requirements increase with head. Figure 8 compares the proportions of different guide-vane types for specific speeds of 28 and 50. This is a further essential factor in pump-turbine design. Pump-turbines operating under high head require long, comparatively inaccessible inter-blade channels. In addition to complicating manufacture of very accurate blading geometries, this may set a definite bottom limit for the lower specific speed range under a given head. 2. Multi-stage pump-turbines with fixed guide-vanes The units at Electricité de France's Saint-Hélène power station in the La Coche scheme are a typical example of this design. Leading particulars of the scheme : Peak installed output : 320 MW Average head : 916 m Leading particulars of the power units : Individual output : 79,5 MW under 930 m (turbine operation) Individual discharge : 8,50 m³/s under 890 m (pump operation) Speed : 600 rpm Number of stages : 5 Max. runner diameter 1.915 m Spherical valve diameter : 1 m The cross-section in Figure 9 shows how the runner is fitted to the shaft, return channel positions and the fixed guide-vane assembly layout. Figure 10 shows runners during assembly to the shaft. The machine in Fig. 11 resembles a multi-stage centrifugal pump, except for the following distinct features : a) It can operate as a turbine at a fixed output of 79,5 MW (no adjustable components) b) As a pump, it can start up at reduced power with the runner "churning". c) Peak pump output is obtainable between close limits. d) Turbine start-ups are controlled by spherical valves, or more exactly, by a system designed to dissipate the jet energy in the pipe. This starting-up system is an original feature of the La Coche power units (Fig. 12). Runner starting-up flow rate during turbine operation is controlled by the spherical valve seal, the final design of which was established by scale model tests aimed at obtaining a stable annular jet dissipating its energy inside the pipe without causing vibration. The effectiveness of this method has been fully confirmed by industrial operating experience. Adjustable guide-vanes, however, are necessary for power-governed operation under very high head. A two-stage pump-turbine design with adjustable guide-vanes now at the design stage is discussed in the next section of the report. 3. Two-stage design with adjustable guide-vanes This design offers the following advantages : a) 15 % more output than multi-stage pumps with fixed guidevanes b) Power modulation c) Easy pump start-ups (with guide-vanes c1osed) Fig. 12 shows a design of this type for a machine to produce 300 MW under 900 m head. Conclusion Two-stage pump-turbines with adjustable guide-vanes appear to be an original solution for the problem of equipment operating under very high head. The upper head limit for this type of design will probably be 1000 m. A very high head is obviously an attractive proposition for energy accumulation applications. Some countries have even envisaged producing such heads by artificial means, e.g. using abandoned collieries. Design problems for this type of equipment become increasingly complicated at greater head. This of course reflects on the manufacturer, who must have correspondingly substantial facilities available to be able to produce suitable equipment.