Turbo Alternator in Thermal Power Plant
Q. What is alternator function?
Q. What is the working principle of alternator?
Q. What is the operation principle of alternator
Q. Which type of alternator is used in a generating station?
- Alternator used in thermal plants are termed as turbo alternators as they are coupled we steam turbines, which run at 3000 rpm, hence such alternators have only two poles for producing 50 Hz frequency of voltage induced.
- Turbo alternator have cylindrical rotors, hence a uniform air gap exists between rotor and stator. In designing the rotor of a turbo-alternator, care has to be taken to avoid critical speed. The rotor and stator may be air cooled upto 40 MW size. Above this mostly the rotor and stator is hydrogen gas cooled. For turbo alternators above 150 MW rating, hollow stator conductors may be employed. Cooling of such conductors take place depending upon the size of the machines. Turbo alternators are mostly rated at 11 kV, 3-phase star-connected stator windings are mostly used. In India, the frequency of generated voltage being 50 Hz. For large power systems, higher voltages of 22 kV or 33 kV may be employed. The main exciters are compound d.c. generators of 115 V or 230 volts rating capable of supplying required excitation for main generators on full-load and with an overload of about 20% at rated power factor. Minimum of 10 % overload capacity under normal operating conditions is provided for in case of generators.
- Static exciters are also now – a-days in much use for alternator. Largest available size of turbo alternator is 550 MW sets up to 1000 MW are being designed and installed. D.C. excitation from the exciters is supplied to cylindrical rotor having d.c. winding with the help of brushes, resting on the slip rings connected to d.c. winding of rotor and Insulated from each other and the shaft itself.