Alt Maintenance

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Alternator Maintenance

Introduction

Generators are the most important as well as the most expensive piece of

equipment in a power system and they are also subject to more possible types

of troubles than any other equipment if not maintained and protected in a

proper way.

The choice of equipment protection ,which is governed by the method of

earthing,type of prime mover,importance of machine etc.,must be carefully

considered.

Before carrying out any maintenance work the generator should be stopped

and isolated electrically from all external loads or supplies.



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Mechanical checks

•It should be checked that machine is received without any damage and is

erected correctly .

•Bearing must be lubricated.

•The rotor should be checked for free running by hand cranking.

•In case of single bearing machine,it should be ensured that split locking

plate is removed before starting the generator.

•The direction of rotation must be correct.

•All the fuse link must be provided.



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Ventilation

It should be checked that all ventilation grids,gland plates and covers are

firmly and correctly fitted in position and the generator inlet and outlet and

ventilation ducts are not obstructed in any way.



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Electrical checks:

1)Insulation check

If the machine is stored for long or suspected to be damped, the insulation

resistance of the winding should be checked and the machine dried out.

While checking insulation resistance of winding it must be ensured that all

connection to these windings including generator control unit and rotating

rectifier assembly are removed.

The insulation resistance of all the winding should be checked using a

500V megger before the generator is put in operation.The insulation

resistance between any winding and earth should not be less than one

megaohm.



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If the insulation resistance is less than one megaohm,the generator should

be cleaned and dried out until the insulation resistance is at least equal to

minimum value.

A method of drying out is to circulate hot air through the machine. It may

be done by by placing heaters inside or near the machine.

Care should be taken that no part of the winding exceeds 90^0 C total

temperature by thermometer.



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Connection checks:

Most of the problems that arise during operation are due to wrong , loose

or snapped connections from switchgear to the machine. Hence it isimportant to check these connections thoroughly.

The connections between GCU and the generator terminals should be

checked as per one of the three connection diagrams,depending upon the

type of AVR input.Check for transients damage or loose connections.



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Running checks and commissioning

Commissioning:•Check for transit damage or loose connections

•Disconnect exciter field leads, using 500V megger,check the exciter field

ground insulation(at least 1Mohm).•Reconnect with correct polarity (see connections in drawing provided).

Caution:Do not megger or flash test the generator winding unless all leads to theGCU have been disconnected.

•Run up the generator on no load to rated speed.The output voltage should build up in a few seconds and stabilize at the rated voltage,which can betrimmed to exact value required .Check that Vtrim covers the requires range

Do Not Start The Generator With Load On.



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•The generator may now be loaded and regulation checked.

•To check underspeed protection ,reduce the speed to 90% of rated speed,

and observe that a small red light appears on the emergency shutdown theengine.

•Observe that output voltage of the generator falls in proportion

to the falling engine speed.



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Residual voltage checks

•The minimum residual voltage at the generator terminals required for

positive build up is about 8V ac with the generator running at rated speed.

•If the machine does not build up due to low or loss of residual magnetism,

the following procedure is adopted with the generator stationary.

The exciter field leads F1,F2 are disconnected from GCUand connected to

12Vbattery with its positive lead connected to F1and negative to F2 for fewseconds.F1,F2are then disconnected from battery and reconnected to GCU

as before.



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Rated voltage and Phase balance checks

•The generator is started and run at its rated speed

•

The generator should be excited on no load till the rated voltage isdeveloped between line to line.

•The voltage balance on three lines of the generator should be checked.

Direction of phase rotation and phase sequence

•Standard phase sequence is UVW for clockwise rotation at the driven end.

•However the operators must ensure that direction of rotation and phase

sequence are the same as stated in the name plate of the machine.



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Harmonic circulating current and neutral connection

•When two or more generators operating in parallel and having their star

point connected together.

•Chances are that they may have slightly different line to neutral voltage

waveform.

•The harmonics voltages which do not balance-out will result in the

circulation of harmonic currents.

•The sum of current in 3-phases will flow through the neutral causing

additional heating of the generator .•Such circulation of current can be avoided by connecting the star point

of one generator to to the neutral line only.



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Caution :

•Rotor should be freed before running the single bearing generator.

•The space heaters must be switched off before running the generator.

•The generator should be isolated and its prime mover should be shut down

immediately when the operator observes any abnormal operation.

•Connection diagram,dimension drawing,all instruction should be followedas per manufacturer guidelines.



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Maintenance:

•Depending on the individual site conditions,frequent ,maintenance should

be practiced.

•Before carrying out any maintenance work the generator should be stopped

and isolated electrically from all external loads or supplies.

General cleaning

•Both interior and exterior of the machine (particularly the windings,

bearing,ventilation openings,exciter etc,) should be kept free from any oil

or dust by blowing out frequently with clean dry air at a pressure not

exceeding 3.5kg/cm2.

•If the blowing is ineffective to remove oil or dust,the parts should be

•carefully cleaned with a rag lightly damped in white sprit.



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Maintenance:

Connections

•

All the external and internal connections(including rotating rectifier assembly and GCU) must be checked for tightness at joints and continuity.

•Fuse link should make a good fit in holders.

•All the hardware,including those of terminals,should be tightened if required



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Maintenance:

Loss of residual

•

If the generator residual is completely lost because of long storage or demagnetizing due to any maloperation,it will be necessary to disconnect

the exciter field terminals at the GCU and connect a 12V battery directly

across the field (with correct polarity) for few seconds, to re-established

the residual.



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Maintenance:

Rotating rectifier assembly

The health condition of the diodes can be checked when required as follows

•The pigtail terminal of each diode is disconnected.

•Using an ohmmeter with at least 1.5 Volts source, the resistance of diode

between stud and pigtail in both direction is measured.

•Diodes in good condition will show a low resistance in the forward

direction and very high resistance (upto 1mohm) in reverse direction.•A high resistance in both directions or a low resistance in both direction

indicates a faulty diode which should be replaced.

•Failure of surge suppressorcan be detected by smell and appearance.

•Failure of fuses can be detected by checking its continuity using multimeter.



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Maintenance:

Bearings

•The roller bearing or ball bearing will run for a long periods.

•The bearing s are to be relubricated with correct quantity of grease in the

interval stated on the name plate of generator,in order to replenish any

grease lost during running.

•If the grease is caked before commissioning ,it has to be completely

replaced.

•Grease of different makes and grades should not be mixed.•When the machine has to be stationary for long period,the rotor should

be turned periodically to avoid setting of grease at the bottom of housing.



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Do’s

•Maintain log book.

•Monitor the parameter.

•Check the performance.•Check IR when set is kept idle for long .

•Check mechanical and electrical joints periodically.

•Check and calibrate protective relays.

•Check earth connections.

•

Switch on space heaters ,when fitted,while set is idle.•Use correct size / rating of cables.

•Check alignment periodically.

•Use genuine spares.



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Don'ts

•Do not meggar with AVR in circuit.

•Avoid moisture entry in to the machine.

•Do not load at low lagging P.F.

•Do not run at low speed on load.

•Do not overload.

•Do not open the machine when running and live.

•Do not mix different grade and make of grease.

•Do not disturb bearing unnecessarily.•Avoid unbalance load.

•Do not field flash with AVR in circuit.

•Do not block / restrict ventilation.



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Maintenance of AC generator

Daily checks

•Vibration

• Noise•Temperature of respective parts.

•Odour

•Lubricating condition.

•

Oil leakage.•Condition of ventilation.

•Battery condition.



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Mechanical maintenance•General cleanliness

•Bearing - Sleeve Bearing

•Cooler

•Alignment

•Tightness of joints



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Electrical maintenance•Rotating rectictifier assembly.

•Generator control unit.

•Space Heater

•Residual magnetism.

•Insulation resistance.

•Protective relays.



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Fault Finding:Sr.no Fault Probable cause Action

1 No voltage build up Wrong mode selection Ensure Auto/manual

on manual mode. SW.in manual posi.



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