2- Generator Maintenance

8/12/2019 2- Generator Maintenance

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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

2- Generator Maintenance

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