Classification of Synchronous Generators | Electrical Engineering

The synchronous generators may be classified as: 1. Hydroelectric Generators 2. Turbo Alternators 3. Diesel-Engine Driven Synchronous Generators.

1. Hydroelectric Generators:

The generators employed in hydroelectric power plants are three phase alternating current synchronous generators, called the alternators. Alternator consists essentially of two parts namely armature and field magnet system.

The armature of an alternator is an iron ring, formed of laminations of special magnetic iron or steel alloy (silicon steel) having slots on its inner periphery to accommodate arma­ture conductors and is known as stator. The whole structure is held in a frame which may be of cast iron or welded steel plates.

For minimising eddy current losses due to rotation of field structure in between the stator, the stator core is laminated. The laminations (usually of thickness 0.5 mm or less) are stamped out in complete rings for small machines or in segments (for larger machines) and insulated from each other with paper or varnish.

The stampings also have openings which make axial and radial ventilating ducts to provide efficient cooling. The open slots are more commonly used because the coils can be form wound and insulated prior to being placed in the slots giving least expenditure and more satisfactory winding method. Such slots also facilitates in removal and replacement of defective coils.

The field structure is the largest and heaviest component of alternator (in large ma­chines, it may be 15 m in diameter and 1,000 ton in weight), and is called the rotor. The rotor houses the dc excitation winding and the exciting current is supplied to the rotor through two slip- rings and brushes.

The field windings are connected in series to form the excitation winding which is supplied with dc at 110/220/300 V. Earlier, the excitation was usually supplied by small dc generators (shunt or compound wound type) driven through pilot exciters from the turbo-generator shaft. The recent generators make use of static excitation systems.

The alternators employed in hydroelectric power plants have the following features:

The machines may have horizontal or vertical configuration. The alternators employed in conjunction with impulse turbines are usually of horizontal configuration while those employed with Francis and Kaplan turbines are of vertical configuration. Low speeds (about 50—500 rpm in case of vertical machines and 100—1,000 rpm for the horizontal machines). Protection against runaway speeds is to be provided.

The machines are of large diameter and small lengths. The machines are usually of salient pole type and the number of poles, they consist of, varies from 6—120. These machines should be capable of supplying heavy line charging currents since hydroelectric power plants are usually located at considerable distance from load end. A higher value of short-circuit ratio (around unity) is therefore required. The machines are usually air cooled. Such machines are built in sizes varying from 0.5 MW to over 1,000 MW.

2. Turbo Alternators:

In central power stations, the steam turbine and alternator are directly coupled to avoid transmission losses. Turbo-alternators are high speed machines (3,000 or 1500 rpm) for 50 Hz systems. These machines have horizontal configurations and smooth cylindrical (or non- salient pole) type field structure wound usually for 2 or 4 poles. To reduce the peripheral speed (maximum peripheral speed should not exceed 175 m/s) the diameter of the rotor is kept small and axial length is increased. The ratio of diameter to axial length ranges from 1/3 to 1/2.

Due to high peripheral speed, the rotating part of the turbo-alternator is subjected to high mechanical stresses. As a result the rotor of large turbo-alternator is normally built from solid steel forging. Chromium-nickel-steel or special chrome-nickel-molybdenum steel is used for rotors of turbo-alternators.

The forging has radial slots in which the field copper, usually in the form of strips, is placed. The coils are held in place by steel or bronze wedges and the coil ends are fastened by metal rings. Normally two-thirds of the rotor is slotted for the field winding and one-third is left without slots so as to form the pole faces.

Rectangular slots with tapered teeth are milled out in the rotor so that rectangular conductors can be used for the field windings. Each slot is provided with a ventilation hole at the bottom. To reduce harmful tooth ripples, either stator slots or poles are skewed.

The non-salient field structure used in turbo-alternators has the following special features:

(i) They are of smaller diameter (maximum lm in 2-pole machine) and of very long axial length.

(ii) Robust construction and noiseless operation.

(iii) Less windage (air-resistance) loss.

(iv) Better in dynamic balancing.

(v) High operating speed (3,000 or 1,500 rpm).

(vi) Nearly sinusoidal flux distribution around the periphery, and therefore, gives a better emf waveform than obtainable with salient pole field structure.

(vii) There is no need of providing damper windings (except in special cases to assist in synchronising) because the solid field poles themselves act as efficient dampers.

500 MW units generally use hollow stator conductors. The short-circuit ratio is 0.4 to 0.6.

Turbo-alternators are usually rated at 11 kV with 3-phase star-connection of stator windings. Standard frequency of generation is 50 Hz. Generally turbo-alternators are rated at 0.8 power factor lagging.

Main exciters are dc compound generators of 125 or 250 V 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 the case of turbo-generators. Static exciters are also now in much use.

The small machines (of rating up to 50 MW) are air cooled. However, hydrogen cooling is invariably used for both stator and rotor of medium and large size turbo-generators. There are several advantages of using hydrogen as coolant in place of air in closed circuit venti­lation system such as improved efficiency, increased output rating, increased life, elimination of fire hazard, less noise, smaller size of coolers etc.

The machines are built in size from 10 MW to over 1,500 MW. 500 MW units have been installed at Singrauli, Anpara, Chandarpur, Ramagundem, Trombay, Farakka and Korba.

3. Diesel-Engine Driven Synchronous Generators:

Diesel-engine driven synchronous generators are of small ratings from 100 kVA or less to 1,000 kVA. The speed is generally between 375 to 1,500 rpm, i.e., the number of poles is 16 to 4. Generally 4 to 6 pole alternators are common for diesel driven generators.

The diesel engines are manufactured as horizontal type, and therefore, both the diesel engine and the synchronous gen­erator are mounted horizontally and are connected by a horizontal shaft. Such generators employ salient pole type construction. The torque of the diesel engine is not uniform and this makes the synchronous generator sensitive to torque variations.