Turbine Generator

an electric generator driven by a steam or gas turbine. A turbine generator, which is sometimes called a turbogenerator, is usually a synchronous generator that is directly connected to the turbine of a steam power plant.

Since the turbines at steam power plants that use fossil fuels operate best and most economically at high rotational speeds, turbine generators driven directly by a turbine shaft must also operate at high rotational speeds. The rotational speed n of a turbine generator is given by the equation f = p × n, where f is the AC frequency and p is the number of pole pairs in the turbine generator. In the USSR, the commercial AC frequency is f = 50 hertz; therefore, the highest rotational speed of a turbine generator for which p = 1 is 50 sec^–1.

A turbine generator is an electrical machine of the horizontal type. Its excitation winding is embedded in a rotor with non-salient poles; a three-phase armature winding is located in a stator. The rotor experiences the highest mechanical stress and is therefore manufactured as a solid high-quality steel forging. Because of strength considerations, the linear velocity v of any point of the rotor must not exceed 170–190 m/sec. This requirement limits the diameter of the rotor to D = v/πn = 1.2–1.3 m for n = 50 sec^-1. The relatively small rotor diameter results in a relatively large rotor length. The rotor length, however, is limited by the permissible bending of the turbine shaft and does not exceed 7.5–8.5 m. The surface of the rotor contains longitudinal milled slots, in which the coils of the excitation winding are embedded. The coils are held in place by wedges, which enclose the slots, and by large bindings made of nonmagnetic steel, which cover the ends of the winding. The excitation winding is energized by an electrical machine exciter.

The stator of a turbine generator consists of a shell and a core with slots for the winding. The core is made from several lacquered stacks of electrical-steel sheets 0.35–0.5 mm thick. The individual stacks are separated by vents that are 5–10 mm wide. The winding is held in the slots by wedges, and the ends of the winding are fastened to slip rings at the ends of the stator. The core is embedded in a welded steel shell whose ends are enclosed by shields.

The turbine generators of nuclear power plants have certain distinctive features associated with the relatively low parameters of the steam produced in a nuclear reactor. The steam parameters make it economically expedient to use turbines with a rotational speed of 25 sec^–1. Such a speed requires the use of two pole pairs in the turbine-generator rotor and makes it possible to construct rotors with a larger diameter, that is, up to 1.8 m. The size of the rotor forging, however, is limited by the technological feasibility of fabrication, since the maximum weight of the forging is 140–180 tons.

Turbine generators with an output power of up to 30 megawatts (MW) have a closed air-cooling system. For a power output above 30 MW, hydrogen with a gauge pressure of about 5 kilo-newtons per square meter (kN/m²) is used as the coolant instead of air. The use of hydrogen as the coolant enhances the removal of heat from the cooled surfaces, since the heat capacity of hydrogen is several times that of air, and consequently increases the output power of a turbine generator of a given size. The coolant is circulated by fans on a shaft of the turbine generator, and the heat is released from the surfaces of insulated conductors and from the steel cores. The heated coolant enters a special cooler; for hydrogen cooling, the cooler is built into the turbine generator and the whole cooling system is carefully pressurized. To intensify the cooling at turbine-generator output powers in excess of 150 MW, the hydrogen pressure in the system is increased to 300–500 kN/m². For output powers above 300 MW hydrogen or distilled water is used for internal cooling of the conductors of the winding. If hydrogen cooling is employed in this case, the conductors of the winding have lateral vents; if water cooling is used, the conductors are hollow. In large turbine generators a combined type of cooling is employed. For example, the stator and rotor windings are water cooled, and the stator core is hydrogen cooled.

An increase in the power output of a turbine generator results in a decrease in the amount of materials used per kilowatt (kW) of power produced and, ultimately, a reduction of the manufacturing costs per kW of power produced. Thus, the amount of material used per kW of power produced is 2.75 kg for a 30-MW turbine generator, but only 1.53 kg for a 200-MW turbine generator, 0.64 kg for a 500-MW unit, 0.58 kg for a 800-MW unit, and 0.457 kg for a 1,200-MW turbine generator. Table 1 shows the increase in the power output of turbine generators manufactured in the USSR.

The efficiency of the Soviet turbine generators is 98–99 percent, and the terminal voltage is as high as several tens of kilo-volts.