Page 276 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

GENERATOR COMPONENTS, AUXILIARIES, AND EXCITATION

GENERATOR COMPONENTS, AUXILIARIES, AND EXCITATION 13.9

During system faults or during unbalanced electrical loading, negative phase sequence
currents and fluxes occur, leading to induced currents in the surface of the rotor. These cur-
rents will flow in the wedges which act as a “damper winding” similar to the bars in the
rotor of an induction motor. The end rings act as shorting rings in the motor. Arcing and
localized pitting may occur between the end rings and the wedges.

Slip Rings, Brush Gear, and Shaft Grounding

The D leads in the bore are connected through radial copper connectors (which normally
have backup hydrogen seals) and flexible connections to the slip rings (Fig. 13.9). The exci-
tation current is around 5000 A dc for a 660-MW generator. The surface area of the slip rings
must be large to run cool while transferring the current. Figure 13.10 illustrates the brush
gear including brushes and holders of a removable bracket. The holders can be replaced on
power. Constant-pressure springs are used to maintain brush pressure. A brush life should
be at least 6 months. A separate compartment houses the brush gear. A shaft-mounted fan
provides separate ventilation so that brush dust is not spread on other excitation components.
Small amounts of hydrogen may pass through the connection seals and may accumulate in
the brush gear compartments during extended outages. The fan dilutes them safely during
start-up before excitation current is applied. The brush gear can be easily inspected through
windows in the cover. Figure 13.11 illustrates brushless rotor connections.
A large generator produces normally an on-load voltage of 10 to 50 V between its shaft
ends due to magnetic dissymmetry. This voltage drives an axial current through the rotor
body. The current returns through bearings and journals. It causes damage to their surfaces.
Insulation barriers are installed to prevent such current from circulating. The insulation is
installed at all locations where the shaft could contact earthed metal, e.g., bearings, seals,
oil scrapers, oil pipes, and gear-driven pumps.
Some designs have two layers with a “floating” metallic component between them. The
integrity of insulation is confirmed by a simple resistance measurement between the float-
ing component and earth.
If the insulation remains clean and intact, a difference in voltage will exist between the
shaft at the exciter end and ground. This provides another method to confirm the integrity
of the insulation. The shaft voltage is monitored by shaft-riding brush. An alarm is initiated
when the shaft voltage drops below a predetermined value.
It is important to maintain the shaft at the turbine end of the generator at ground level.
A pair of shaft riding brushes ground the shaft through a resistor. Since carbon brushes
develop a high-resistance glaze when operated for extended periods without current flow,
a special circuit introduces a wetting current into and out of the shaft through the brushes.
This circuit also detects loss of contact between the brush and the shaft.

Fans
Fans drive the hydrogen through the stator and the coolers. Two identical fans are mounted
at each end of the shaft. Centrifugal or axial-type fans are used (Fig. 13.12).

Rotor and Alignment Threading

The stator bore is about 25 cm larger than the rotor diameter. The rotor is inserted into the
stator by supporting the inserted end of the rotor on a thick steel skid plate which slides into
the stator, while the outboard end is supported by a crane.

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