Post on 03-Jun-2018
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GENERATORS
MAINTENANCE
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Typical Generator Maintenance
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Generators
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System controls
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REASONS WHY GENERATORS FAIL
FOREIGN OBJECT DAMAGE
Problem: Objects can come from external sources or
failure of internal components, they can pick up energy from the spinning rotor and do extensive damage
Prevention: Inspect on a regular basis all internal parts
that are prone to failure or can be dislodged.
Inspection tests can be a combination of visual inspection along with ultrasonic or magnetic particle
tests on rotating components
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STATOR WINDING VIBRATION
Problem: Primarily a design related problem that
affects large (>300Mw) generators which have
insufficient end winding bracing to limit the
movement of end turns. Prevention: Proper bracing of the end winding is
required to limit motion caused by steady state and
transient electromagnetic forces.
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ROTOR WINDING DISTORTION
Problem: Rotor winding distortion caused by poor end
turn blocking support design or by foreshortening of
the rotor coils. Foreshortening is caused by thermal forces which compress rotor coils.
Prevention: Proper design of rotor coils and bracing to
support the coils under axial load is essential.
Rotors should be tested for turn to turn shorts at operating speed.
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STATOR WINDING VIBRATION
Problem: Primarily a design related problem that
affects large (>300Mw) generators which have
insufficient end winding bracing to limit the movement of end turns.
Prevention: Proper bracing of the end winding is
required to limit motion caused by steady state and
transient electromagnetic forces.
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OVERHEATING
Problem: Overheating of the rotor or stator can lead to
insulation failure, shorting of turns and ground
faults. Overheating can result from blocked
ventilation passages caused by shifting insulation components or slot wedges.
Prevention: Inspect on a regular basis to ensure all rotor
wedges are "locked" in place preventing migration
and thus blocking of cooling passages.
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CONTAMINATION
Problem: For air cooled machines, dirt and dust cause
tracking which can lead to electrical ground faults.
Prevention: Inspect air filterson a regular basis , the filters must be checked and cleaned regularly.
Polarization index (PI) tests give a good indication of
overall cleanliness of the rotor winding.
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ROTOR VIBRATION
Problem: There are many causes, turn-to-turn shorts,
rotor coil foreshortening, electrical grounds,
mechanical imbalances, overheating, etc.
Prevention: Comprehensive vibration measuring iseffective
combined with a regular maintenance program.
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STATOR WEDGE LOOSENESS
Problem: When stator wedges become loose, coils can
vibrate causing insulation wear leading to ground
faults or turn-to-turn shorts. Prevention: Inspect on a regular basis tightness of
wedge blocks.
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STATOR CORE DAMAGE
Problem: Stator core looseness can occur over time as
pre-tensioned through bolts relax. A loose core
results in insulation wear to coils and laminations
resulting in hot spots and core-to-coil failures. Prevention: Inspect bolt tightness on a regular basis.
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GENERATOR MAINTENANCE The following tests which should be carried
out annuallyin addition to vendor recommended
maintenance.
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A) INITIAL GENERATOR TEST Megger rotor winding
Megger exciter armature winding
Megger exciter field winding
Complete polarization index (pi) on mainstator
Check bearing insulation
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B) INSPECTION ON STATIC EXCITER Remove exciter end cover
Examine condition of diode carrier
Examine exciter armature/stator for contamination
Examine exciter armature/stator for winding wear Check pmg magnets for contamination
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C) ELECTRICAL
CONTROL/PROTECTION PANEL
Visually inspect external surfaces of panel
Complete insulation resistance checks of panelwiring
Check function of all relays
Check all fuses
Check all lamps
Check operation of all switches
Check operation of panel heaters Run generator; recalibrate avr
Secondary inject all protective relays
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E) ROTOR EARTH FAULT
PROTECTOR
Carry out static checks to confirm operation of
detector
Carry out functional check to confirm operation of
detector
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F) SLIP-RINGS AND BRUSH-GEAR (IF
FITTED)
Check all brushes for grade and length
Check condition of brush holders/mountings
Check that spring tensions are correct
Check conditions of slip-rings Check mechanical run-out of slip-rings
Check cooler and leakage alarms