Hydroelectric Generator

a generator of electric current driven by a hydraulic turbine.

Usually a hydroelectric generator is a salient-pole synchronous generator whose rotor is connected to the rotor of a hydraulic turbine. The design of a hydroelectric generator is basically determined by the position of its rotor’s axis, as well as by the frequency of rotation and power of the turbine. High-capacity, low-speed generators are usually manufactured with a vertical axis of rotation (with the exception of capsule-type hydroelectric units), whereas high-speed units with a bucket-type hydraulic turbine are made with a horizontal axis of rotation. There are also experimental industrial generators of original design (with phase rotors; counterrotat-ing or flow-through types). Because of the topological and geological characteristics of rivers in the USSR, most highspeed generators are installed with a vertical axis of rotation.

Hydroelectric generators are subdivided by capacity into low-capacity types (up to 50 megawatts [MW]), medium-capacity (50-150 MW), and high-capacity (over 150 MW) and by speed of rotation into low-speed (up to 100 rpm) and high-speed (exceeding 100 rpm). Soviet and foreign hydroelectric generators for conventional use have a range of potential generating voltage of from 8.8 to 18 kV; their power coefficient (cos ø ) ranges from 0.8 to 0.95; the efficiency of high-speed hydroelectric generators ranges from 97.5 percent to 98.8 percent, and of low-speed generators, from 96.3 percent to 97.6 percent.

The first Soviet hydroelectric generators, which had a capacity of 7.25 MW, were constructed in 1925 at the Elek-trosila Plant in Leningrad for the V. I. Lenin Volkhov Hydroelectric Power Plant. In the early 1930’s hydroelectric generators with a capacity of 65 MW were installed at the Dnieper Hydroelectric Power Plant, and in 1939-40, generators were manufactured for the Uglich and Rybiinsk hydroelectric power plants, which at that time were the largest with regard to torque moments, size, and weight. Unique generators were made for the Bratsk (1960) and Kras-noiarsk (1964) hydroelectric power plants, with capacities of 225 and 508 MW, as well as water-cooled capsule-type generators (20 MW) for the Cherepovets Hydroelectric Power Plant; reversible hydroelectric units have been installed at the Kiev Pumped-Storage Electric Power Plant; in 1966 the Uralelektrotiazhmash Plant turned out an economical experimental high-voltage (110 kV) hydroelectric generator with a capacity of 20 MW; and (as of 1971) a 650-MW hydroelectric generator was being planned for installation at the Saian-Shushenskoe Hydroelectric Power Plant.

In the design and installation of hydroelectric generators, special attention is devoted to reinforcing the rotating parts of the hydroelectric unit, as well as to cooling the windings of the rotor and stator. According to the positioning and design of the support bearing or step bearing, a distinction is made between suspended and hooded generators. In the suspended generator the support bearing, which receives the weight of the rotating parts of the hydroelectric unit, as well as the axial pressure of the water on the turbine runner, is positioned above the generator’s rotor, on the upper cross-piece of the unit. In the hooded generator the step bearing is positioned under the generator rotor, on a lower crosspiece or on the turbine casing; the generator shaft rotates in two or three guide bearings. High-capacity, low-speed hydroelectric generators are usually very large; the hooded model is effective in reducing their dimensions and in decreasing their weight. An example of the hooded type is the generator of the Krasnoiarsk Hydroelectric Power Plant. Its speed of rotation is 93.8 rpm, and its rotor is 16 m in diameter and weighs 1,640 tons. The suspended design is preferable for smaller highspeed generators; as compared with the hooded type, it has greater resistance to the mechanical vibrations of the rotor, its supporting step bearing is of smaller diameter, and it is simpler to install. An example is the hydroelectric generator of the Bratsk Hydroelectric Power Plant. Its speed of rotation is 125 rpm, and its rotor is 10 m in diameter and weighs 1,450 tons.

In order to cool such large generators (with a capacity of up to 300 MW), a closed ventilation system is customarily used; it may be indirect, or surface-type, in which air is blown onto the winding from the surface, or forced, in which the air is fed into the current-carrying conductor or between the conductors. It is considerably more efficient to cool the stator windings with distilled water, with forced-air cooling of the rotor windings. The use of forced cooling increases the output of the hydroelectric generators and reduces the expenditure of insulation, copper, and electrical steel.

Hydroelectric generators are usually excited by a DC auxiliary generator mounted on the shaft; large generators also have pilot exciters, which are used to excite these auxiliary generators. In some instances a synchronous generator with rectifiers, which also serves as an auxiliary generator, is used for this purpose.