6000

FIGURE 5 Typical cavitation model test (1 ft3/s = 0.0283 m3/s) (Voith Siemens Hydro)

Pumping Mode If we attempted to start the unit as a pump, from rest and with the spiral case and draft tube filled, the power and inrush current would be excessive. Accordingly, the standard type of compressed air system is provided to depress the tailwater elevation to below the bottom of the impeller, with the wicket gates closed. The load to be overcome at starting then consists of the load of the rotating masses, which must be accelerated to synchronous speed, and the load due to windage. Owing to inevitable leakage past the wicket gates, this windage is substantially greater than that due to dry air. The main penstock valves must also be closed during starting, or else the leakage and "wet" windage would be still further increased at the much higher head.

For the larger units generally employed, a separate starting motor, of the induction type with wound rotor, is mounted directly above the main generator motor. It has not been found feasible, in the motor space available, to design an amortisseur winding capable of sustaining the heat from the inrush current resulting from across-the-line starting of the main generator motor, even at reduced voltage. For the smaller units, however, this may be accomplished. In rare instances, where a main unit is always available, this spare may be electrically coupled to the pumping unit and the two started from rest in back-to-back synchronism. The separate starting motor is usually sized to bring the main unit up to synchronous speed in about 10 minutes, as shown by Figure 11.

When the unit has attained full speed, it is synchronized to the line, the compressed air is cut off, the tailwater rises to fill the draft tube, the wicket gates and main penstock valve are opened gradually to prevent shock, and pumping to the upper reservoir begins.

The maximum transient load on the generator motor thrust bearing occurs just as pumping begins. Prior to the advent of pumped storage, thrust bearings were designed to carry the weight of the rotating parts plus the hydraulic thrust at the steady-state condition. Now a greatly increased thrust of short duration must also be accommodated. Because of the short duration of this transient excess load, it may usually be carried safely by the bearing as designed for the steady-state requirement, depending on the detailed

FIGURE 6 Turbine performance (1 ft3/s 0.0283 m3/s; 1 ft = 0.3048 m; 1 hp 0.746 kW) (Voith Siemens Hydro)
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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