Page 268 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

Source: ELECTRICAL EQUIPMENT HANDBOOK

CHAPTER 13
GENERATOR COMPONENTS,

AUXILIARIES, AND

EXCITATION

Figure 13.1 illustrates a sectional view of a large generator. Hydrogen is used to cool most
generators having a rating larger than 50 MW.

THE ROTOR

The rotor is made from a single steel forging. The steel is vacuum-degassed to minimize
the possibility of hydrogen-initiated cracking. Reheating and quenching also harden the
forging. Stress-relieving heat treatment is done following rough machining. Ultrasonic
examination is performed at various stages of the rotor. Figure 13.2 illustrates the winding
slots in the rotor. Figure 13.3 illustrates a rotor cross section and the gas flow.
The generator countertorque increases to 4 to 5 times the full-load torque when a short
circuit occurs at the generator terminals. The rotor and turbine-end coupling must be able
to withstand this peak torque.

Rotor Winding
Each winding turn is assembled separately in half-turns or in more pieces. The joints are at the
centers of the end turns or at the corners. They are brazed together after assembling each turn,
to form a series-connected coil. The coils are made of high-conductivity copper with a small
amount of silver to improve the creep properties. The gas exits through radially aligned slots.
Slot liners of molded glass fiber insulate the coils. These separators of glass fiber are
used between each turn. They insulate against almost 10 V between adjacent turns (Fig.
13.4). The end rings and end disks are separated from the end windings by thick layers of
insulation. Insulation blocks are placed in the spaces between the end windings to ensure
the coils do not distort. The winding slots are cut in diametrically opposite pairs. They are
equally pitched over two-thirds of the rotor periphery, leaving the pole faces without wind-
ing slots. This results in a difference between the stiffness in the two perpendicular axes.
This difference leads to vibration at twice the speed. Equalizing slots are cut in the pole
faces (Fig. 13.5) to prevent this problem from occurring. The slots are wider and shallower
than the winding slots. They are filled with steel blocks to restore the magnetic properties.
The blocks contain holes to allow the ventilating gas to flow.
The average winding temperature should not exceed 115°C. The hydrogen enters the
rotor from both ends under the end windings and emerges radially from the wedges. Figure
13.6 illustrates the fans used to drive the hydrogen through the stator.

13.1
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