Download - Operation and Maintenance of AC Generator
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Operation and
Maintenance of
AC Generator
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Maintenance
To supply stabilized power, generatormust always be set at the condition of
fully enabling the exhibition of its
performance. In case of handling and
maintaining the generator, its
performance must be fully understood
and the unexpected fault must be
avoided by taking proper means ofmaintenance.
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Maintenance
The particular points that need to bepaid consistent attention are the
following: - Isnt there any abnormality of the
respective parts of generator itself?
Isnt there any harmful matter in the
surrounding atmosphere
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When abundant dust is in presence in the windpassage, such dust will stick to coil and the dust
will lead to the cause of short-circuit failure andgrounding failure. When the generator is startedafter a long time, since there may be chances oflowering the insulation resistance of coil, performthe drying thoroughly.
please pay due attention by comparing thetemperature rise and vibration through the feelingobtained by touching your hand on the generator,and by sensing abnormal noise or odor.
As for the confirmation of abnormal odor, pleaseconfirm it by opening the covers once in a while.
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As for drying there are methods likeresorting to space heater and the method
of permitting the flow of current directlythrough the coils.
For either of the above two methods,please avoid over-heating locally as well
as raising the temperature above 80C.Though inspection in details of the
respective parts is carried out at the timeof trimming the generator, during dailyinspection, please inspect the machineby your sense as whether anyabnormality compared to previouscondition has arisen or not.
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1.1 Periodical inspection
To avoid failure in advance, it is
indispensable to watch and inspect the
generator at all times.
By preparing checklist, please perform
the inspection and trimming
periodically. Hereunder, points that
need to be paid attention especially dateof inspection and methods of inspection
are given.
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Respective tightening bolts
and connecting location
Inspection and maintenance within the extent
considered possible must be performed once a year
without taking out the rotor. In particular,
inspection of stator winding and re tightening ofrespective bolts and nuts must be carried out.
Tightening of set bolts must be done once after half
a month from the date of commencing running and
furthermore inspection and re tightening of these
bolts must be done at the rate over 3 times withinone year thereafter.
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BearingPrior to starting, always confirm the flow of lubricating
oil by observing flow sight. After the starting ifbearing is overheated considerably within a shorttime or if abnormality of noise or odor is detected,there is the need of dismantling and inspection.
The causes of overheating of bearing are given
hereunder. Lubricating oil does not flow Use of filthy oil Deficient erection and coupling
Defect of oil supply pipe is in presence and shortageof lubricating oil arises. Generation of rust or presence of scar on the journal
part during the time generator is laid of. Deficient magnetic center.
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1.2 Major requirement of
inspection and maintenanceat respective time is tabulated
in the following table: -
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Item
Item
Place of
inspection
Daily
inspection
(during
running)
Monthly
inspection
(during
stop)
Annual
inspection
1.Static part
1.Noise
2.Temperatu
re
3.Vibration
1.Insulation
Resistance.
2.Air gap
3.Cleaning
1.Maintenance of
winding.
2.Maintenance of
core
Parts.
3.Inspection oflead wires.
4.Inspection of
bolt and
Nuts.
2.Rotating
part1.Noise
1.Inspectio
n
resistance.
1.Loosening of
spigot fitting part
and balanceweight.
2.Contact with
static part.
3.Inspection of
lead wires.
4.Retightening ofbolts and nuts.
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3.Bearingpart
1.Noise
2.Tempera
ture
3.Vibratio
nLubricatin
g
condition.(
oil leak, oilquantity)
1.Oil leak2.Cleanin
g
1.Examinatio
n of
Impairment
of journal.
2. Wear of
journal
3. Magnetic
center
4.
Others
1.Odour
2.Abnorm
al noise
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1.2(1). WINDING AND
INSULATION RESISTANCE
1.2(a) Measurement of InsulationResistance
Insulation resistance is the resistance against
leakage current that flows in the interior and on thesurface of insulation when certain value of D.C.voltage is imposed. In case the generator is installedat a place where high humidity exists or when thegenerator is kept idle over a long period of time,
generally the insulation resistance drops due todeposit of moisture on the insulation surface.Therefore insulation resistance should be measuredprior to running of the generator.
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Measurement of insulation resistance indicates theinsulated condition of the equipment in a verysimple way. However, the value of insulation
resistance measured indicates the condition ofinsulation deterioration. Measurement is essential before running the
generator. Although the measured value indicatesthe absolute value corresponding to the condition ofinsulation at the instant of measurement, acomparative value has more significance. It isessential to measure periodically and to check incomparison the condition of change with referenceto earlier measured values.
Although the insulation resistance increases withtime after the imposition of D.C. voltage the valuethat settles down to a constant should be registered.In case the insulation resistance increases graduallylittle by little this insulation resistance value isindicated by 30 seconds value or 1-minute value.
Si h i l i i h l
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Since the insulation resistance has a closerelationship with the temperature of insulation, whenmeasuring insulation resistance, it is always requiredto measure temperature of insulation at that time andrecord this temperature along with the insulation
resistance. It serves convenient for maintenancepurposes if the relationship between the insulationresistance value and winding temperature at clean anddry condition of winding is available. Generally, theinsulation resistance value drops rapidly together with
the rise of temperature and the relationship betweenthe logarithm of insulation resistance value andtemperature changes as straight line approximately inthe temperature range of usage.
Insulation resistance value differs from the process by
the insulation composition of equipment and thetemperature rise. There are generally three types ofclassification as given below.
1. Initially the insulation resistance drops at once andthen starts rising as the drying effect progresses. This
is most generally seen.
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2. In case the absorbed moisture on the surface issignificant first the insulation resistance rises by thedrying effect and thereafter follows the sameprocess as (1).
3. In the case of drying on site the equipment thathas absorbed moisture excessively and/or large sizemachines, this type of characteristic is frequentlyexperienced. In the case of equipment that hasabsorbed moisture excessively, since the amount ofmoisture absorbed is large, the insulationcharacteristic is to be absorbed before the dryingeffect appears. In the case of large size machines along time is required for the temperature rise ofequipment and the drop portion due to insulationcharacteristic offsets the rise portion due to drying
effect. In both the above cases, a fairly low value forthe measured insulation resistance by meggershould continue without change. About 3 full days(days and nights) may be required to reach therising point
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NOTE: -At the time of measuring insulation
resistance, attention should be paid to the following
points: If rotating type megger is used, rotating it for one-
minute at least should do measurement.
If battery driven megger is used, error increases as
the battery voltage drops.
Prior to measurement, residual electric charge of
winding should be removed. To exclude the effect of connected auxiliary devices,
other circuits should be disconnected at an easy
convenient part and then measurement should be
done.
After the measurement the residual charges should
be discharged.
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1.2 (b) Insulation Resistance (IR) values
required for Running
The minimum permissible insulation resistancevalue changes extensively by the type, rating,dimension and insulation method of generator. Ageneral value cannot be determined. However, thefollowing guideline can be approximately used when
the windings are at normal temperature, forpermitting normal running.
Greater than 1 Meg-Ohm for low voltage statorwinding and the field winding.
Greater than 5 Meg-Ohm for 3.3kV and 6.6kV statorwindings.
Greater than 10 Meg-Ohm for 11kV stator windings.
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Alternatively, the following formula may be used for the
minimum insulation resistance value:
Rated Voltage (V)
__________________________________
Rated Output (kW or KVA + 1000)
Rated Voltage (V)+1/3(rpm)
___________________________________________Rated Output (kW or KVA)+2000+0.5M-ohm
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1.2(c) Prevention of IR Drop It is desirable to run the generator with its winding
condition always set clean and dry. Dust deposited
on the winding surface prevents heat dissipationand becomes the cause of deterioration dependenton its kind. Also, the intrusion of moisture into theinterior forms the cracks on insulation leads to thedrop of insulation resistance. Therefore, daily
attention to cleaning and prevention againstmoisture absorption leads to prevention of IR dropespecially for exciter, which is open type. Althoughgenerators interior presents dry condition duringrunning, there may be moisture absorption duringstoppage of generator at times of high humidity suchas rainy season. To prevent such situations, it isrequired to put-on the space heater immediatelyafter stopping the generator and to raise thetemperature of generator interior higher than theambient temperature.
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1.2(d) Cleaning Method of Winding
Dust accumulated on the insulation reduces theventilation area and the cooling effect. This may
result in over-heating of the generator. Even though
the accumulated dust is not conductive by itself, it
absorbs moisture and becomes conductive thereby
causing short circuit of winding and ground fault.Therefore, it is essential to keep the winding always
in a clean condition. Any of the following methods
may be used for cleaning the windings.
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1.2(e)Wiping with cloth:
This method is especially effective for cleaning the
local parts where the contamination is severe. Dryand clean cloth from which thread pieces do not
come out should be used. Attention should be paid
as not to impair the insulation by rubbing strongly.
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1.3 FAULT DIAGNOSIS
CHART tabulated in thefollowing table: -
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FAULTS POSSIBLE CAUSE REMEDY
EXCITATION CURRENT VARRYING,
INDIVIDUAL WINDING SECTION
OVERHEAT
OVER LOAD REDUCE LOADING
EXCITATION CURRENT VARRYING,
INDIVIDUAL WINDING SECTION
OVERHEAT, HUMM DURING START
UP/RUNNING, RADIAL
OSCILLATIONS
ONE SUPPLY PHASE OPEN
CIRCUITED, ONE SUPPLY
PHASE OPEN CIRCUIT AFTER
SWITCH ON, OPEN CIRCUIT IN
FIELD CIRCUIT
CHECK SWITCHGEAR & SUPPLY
CIRCUIT
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FAULTS POSSIBLE CAUSE REMEDY
EXCITATION CURRENT VARRYING
,HUMM DURING START
UP/RUNNING, RADIAL
OSCILLATIONS
STATOR WINDING
INCORRECTLY CONNECTED
CHECK WINDING
CONNECTIONS
EXCITATION CURRENT VARRYING
,HUMM DURING STARTUP/RUNNING, RADIAL
OSCILLATIONS
INTERTURN SHORT CIRCUIT INSTATOR WINDING
DISCONNECT M/CS, FEEL
SURFACE OF ENDTURNS &CHECK TEMP. RISE.REMOVE
SHORT CIRCUIT.
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FAULTS POSSIBLE CAUSE REMEDY
EXCITATION CURRENT VARYING,
AXIAL OSCILLATIONS
INTERTURN SHORT CIRCUIT IN
FIELD WINDING
CHECK TEMP RISE.
DISCOLOURED VARNISH MAY
BE A HINT.REMOVE SHORT
CIRCUIT. CHECK AIR GAP &
REALIGN AVOID NARROW
GAPS OR SHARP CORNERS
SCREECHING NOISE
ROTOR CORE RUBBING
AGAINST STATOR
CORE.WINDAGE NOISE,
SMALL GAPS OR SHARP
CORNERS IN AIR CIRCUIT
GRINDING NOISEROTATING PARTS RUB AGAINST
STATOR PARTS
REALIGN M/C, CHECK
RUBBING NOISE.
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FAULTS POSSIBLE CAUSE REMEDY
BEARING OVERHEATS
OIL AGED OR
CONTAMINATED, HIGH
VISCOCITY
CLEAN BEARING HOUSING,
RENEW OIL, CHECK VISCOCITY
BEARING OVERHEATS, BEARING
TEMP VARRIES OR RISES ABRUPTLY
WITHOUT EXTERNALLY TRACEABLECAUSE
LOW OIL LEVEL, OIL RING OUT
OF TRUE, ROTATES
IRREGULARLY OR STALLESPECIALLY AT LOW SPEEDS
CHECK OIL LEVEL & TOP UP, IF
NECESSARY, CHECK OIL RINGS
FOR ROUNDNESS, REALIGN &
STRAIGHTEN OR EVEN REPLACE
THEM
OIL BLACKENS PREMATURELY OR
CONTAINS ABRADED MATTER
VISCOCITY OF OIL TOO LOWCHECK VISCOCITY, CHANGE
OIL IF LOW
BEARING LOSS OIL
OIL VISCOCITY TOO HIGH DUE
TO LOW OIL TEMP,
ESPECIALLY DURING START UP
HEAT BEARINGS OR OIL
DURING OR PRIOR TO STARTUP
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FAULTS POSSIBLE CAUSE REMEDY
BEARING LOSS OIL, OIL DRAWN
INTO MACHINE
OIL FOAMS
INSPECT BEARING VENT, USE
DIFFERENT OIL TYPE AFTER
ENQUIRING WITH SUPPLIER. USEANTIFOAM ADDITIVE
BEARING OVERHEATS, BEARING
TEMP VARRIES OR RISES ABRUPTLY
WITHOUT EXTERNALLY TRACEABLE
CAUSE.
FORCED LUBRICATION FAILS.
OIL OR COOLING WATER
FAILURE
INSPECT OIL SUPPLY SYSTEM,
REMOVE DISTURBANCES
OIL DRAWN INTO MACHINE
OIL PRESSURE OF FORCED
LUBRICATION SYSTEM TOO
HIGH, MORE THAN 0.5 BAR
REDUCE OIL PRESSURE, INSTALL
PRESSURE REDUCING VALVE
BEARING LOSES OILFORCED LUBRICATED
BEARING FLODDED
ADJUST CORRECT OIL
RATE.CHECK OIL DISCHARGE
FOR OBSTRUCTION. CHECK OIL
LEVEL IN BEARINGS
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FAULTS POSSIBLE CAUSE REMEDY
BEARING OVERHEATS, BEARING
TEMP VARRIES OR RISES ABRUPTLY
WITHOUT EXTERNALLY TRACEABLECAUSE, OIL BLACKENS
PREMATURELY OR CONTAINS
ABRADED MATTER
RESIDUAL OIL PRESSURE OF
BEARINGS WITH JACKING OIL
ARRANGEMENT DROPS
DURING OPERATION WITH
JACKING INOPERATIVE
CHECK OIL PRESSURE PIPING &
NON RETURN VALVE, CHECK
BEARING SHELLS FOR
CANTING. REMEDY
DISTURBANCES
BEARING OVERHEATS
EXCESSIVE AXIAL THRUST,
THRUST FROM COUPLING
EXCESSIVE RADIAL LOAD,
INADEQUATE RADIAL PLAY
INSPECT FOR MAGNETIC
CENTRE. CHECK
ALIGNMENT.REALIGN
BEARINGS
AND/MACHINE.REREAM
BEARING SHELLS.
OIL BLACKENS PREMATURELY OR
CONTAINS ABRADED MATTEREXCESSIVE RADIAL PLAY
CHANGE OIL AFTER
CONSULTATION WITH
SUPPLIER,USE OIL WITH HIGH
VISCOCITY
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FAULTS POSSIBLE CAUSE REMEDY
BEARING LOSES OIL
OIL RETURN OPENING IN OR
UNDER LOWER SEALING
RINGHALF CLOGGED
CLEAN OIL RETURN OPENINGS
BEARING OVERHEATS, BEARING
TEMP VARRIES OR RISES ABRUPTLY
WITHOUT EXTERNALLY TRACEABLE
CAUSE, OIL BLACKENS
PREMATURELY OR CONTAINS
ABRADED MATTER
DAMAGE TO BEARING
LINING. DEFECTIVE BOND
BETWEEN LINING &
SUPPORTING BLOCK OF
BEARING SHELL
INSPECT BOND. REPLACE
LINING.(ENSURE CORRECT
SHAPE OF OIL POCKETS & OIL
GROOVES)
BEARING OVERHEATS
TOO SMALL OIL PACKETS,
TRANSITION TO BEARING
SURFACENOT SMOOTH
ENOUGH
REFINISH OIL POCKETS
BEARING LOSES OIL, OIL DRAWN
INTO MACHINE
SEALING RINGS DEFECTIVE.
GAP BETWEEN SHAFT AND
SEALING RING TOO LARGE.
REFINISH OIL RINGS
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FAULTS POSSIBLE CAUSE REMEDY
BEARING OVERHEATS, BEARINGTEMP VARRIES OR RISES ABRUPTLY
WITHOUT EXTERNALLY TRACEABLE
CAUSE, OIL BLACKENS
PREMATURELY OR CONTAINS
ABRADED MATTER
BEARING CURRENTSINSPECT BEARING INSULATION&
REPLACE IF NECESSARY.
ENCLOSURES VIBRATE
SHOCKS TRANSMITTED FROM
COUPLED MACHINE
RESONANCES, INSUFFICIENT
RIGIDITY
CHECK COUPLED MACHINE.
OBTAIN INSTRUCTIONS FROM
SUPPLIER, REINFORCE
ENCLOSURE