Operation and Maintenance of AC Generator

7/28/2019 Operation and Maintenance of AC Generator

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Operation and

Maintenance of

AC Generator


2/31

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.


3/31

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


4/31

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.


5/31

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.


7/31

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.


8/31

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


12/31

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.


13/31

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


14/31

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


16/31

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.


17/31

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.


18/31

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


19/31

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.


20/31

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.


21/31

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: -


23/31

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


24/31


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.


25/31


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.


26/31


27/31


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


28/31


BEARING LOSS OIL, OIL DRAWN

INTO MACHINE

OIL FOAMS

INSPECT BEARING VENT, USE

DIFFERENT OIL TYPE AFTER

ENQUIRING WITH SUPPLIER. USEANTIFOAM ADDITIVE



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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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


30/31


BEARING LOSES OIL

OIL RETURN OPENING IN OR

UNDER LOWER SEALING

RINGHALF CLOGGED

CLEAN OIL RETURN OPENINGS




CAUSE, OIL BLACKENS


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


31/31


BEARING OVERHEATS, BEARINGTEMP VARRIES OR RISES ABRUPTLY


CAUSE, OIL BLACKENS


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

Documents

Operation and Maintenance of AC Generator