ABB - Machine Maintenance

7/28/2019 ABB - Machine Maintenance

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MMaaiinntteennaannccee PPllaann

HV Machines

ABB Limited


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Level 1 - Inspection

When is Level 1 Inspection to be conducted?

1. After every 5% consumption of insulation life based on thermal degradation model,

or on alarms/deteriorating thermal or vibration conditions or any other abnormalevent as reported by the customer

or on completion of 10000 equivalent operational hours and intervals of the same,whichever is earlier.

Objective:

1. To assess the thermal life of the insulation.

2. To identify bearing related defects, and assess the impact of conditions that generateforces on the bearings

3. To identify rotor winding related and other electromagnetically related defects

4. To determine the requirement of Level 2, Level 3 or Level 4 Inspection/Maintenance

Scope of work:

The following data is to be sent by customer.

1. Recording of temperatures Winding, Core, Ambient, Bearings

2. Review of operational data Starts/stops (hot/cold), overloads, Op. Hrs.,

(See Attached Question Form in Appendix I)

3. To perform a vibration and stator line current analysis on the motor or alternatively,

To analyze data collected and submitted by the customer from vibration and stator

line current spectra.

It is to be noted that the above vibration analysis is in addition to routine vibrationmeasurements that are performed by plant personnel.

Expected downtime: 0 day (Online Analysis)

Deliverables:

ABB will analyze the data submitted and will submit a report that will include informationrelating to the following

1. Extent of the thermal life degradation of the insulation

2. Recommendations for bearing maintenance

3. Maintenance plan based on insulation condition, bearing condition or rotor windings

4. Analysis of machine losses for detection of possible thermal abnormalities (for ABBMachines only)


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Level 1 - Maintenance

When is Level 1 maintenance to be conducted?

1. During machine operation on basis of Level I inspection analysis or on information ofabnormal increase in winding temperatures from site personnel.

Objective:

1. To take necessary action based on Level-1 inspection or based on customerintimation from site.

Scope of work:

1. Cleaning of filters, where present

2. Changing of filters, where possible on-line.

(New filters will be supplied by customer)


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When is Level 2 inspection to be conducted?

1. On the basis of Level 1 analysis, for every 10 % consumption of the insulation lifebased on the thermal degradation model for the insulation.

2. or on alarms/deteriorating thermal or vibration conditions or any other abnormalevent as reported by the customer

3. or after every 20,000 equivalent hours of operation or 2 years, whichever is earlier.

Objective:

1. To assess the machine condition by performing measurements on the statorwindings.



4. To develop a maintenance plan for life extension of the machine windings.

Scope of work:

Before stooping the machine for a Level 3 inspection, all data as required for a level 1inspection are to be collected and analyzed.

After stooping and disconnection of the machine from the power supply

Testing the Machine

a) Stator Windings

i) Polarization-Depolarization Current Analysis

ii) Capacitance & Tan Delta Analysis

iii) Non-linear Analysis

iv) Partial Discharge analysis

v) Winding resistance

For detailed scope of work, see Appendix II, Part A

Expected downtime: 1 day

Pre-requisites:

1. The machine terminals should be disconnected from the main bus bars.Deliverables:

1. Condition Assessment report for the stator windings for a more accurate calculationof the insulation degradation.


3. Maintenance plan based on insulation condition, bearing condition or rotor windings.

4. Maintenance plan with tentative Level 3, 4 Inspection schedules.


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1. During the Level 2 inspection, when the machine is shutdown

Scope of work:

1. Cleaning/changing of filters (where present)

2. Bearing Maintenance

a) Sleeve Bearing

i) In position dismantling of the bearings.

ii) Checking of initial bearing clearances.

iii) Inspections of bearing housings for leaks, tightness of fastenings and guidesupport.

iv) Checking of wear and damage of shaft seals and hemp packing and cleaning

of drain holes in seals.v) Inspection of bearing seat surfaces on shaft and in bearing housings

vi) Replacement of bearing oil as per the original grade recommended by theOEM, if required.

b) Anti-friction Bearing

i) Remove external grease caps.

ii) Check tightness of lock nuts.

iii) Inspect condition of grease retainers, spacers, lock washers and otherexternal components for signs of wear and damage

iv) Replenish grease as per recommended procedures.


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When is Level 3 inspection to be conducted?

1. If Level 2 analysis indicates a 25 % reduction in estimated insulation life.


3. After first 40,000 equivalent hours of operation or 5 years, whichever is earlier.

Objective:

1. To assess the machine condition.



4. To determine the time for performing Level 4 inspection/maintenance.

5. To plan suitably for Level 4 inspection/maintenance with aim to reduce theshutdown time.

Scope of work:

Before stooping the machine for a Level 3 inspection, all data as required for a level 1inspection are to be collected and analyzed.

After Stoppage, Disconnection and end cover removal:

1. Visual inspection of

a) Terminals/Bushings and HV connections

b) Bearing condition

c) Endwinding portion of stator winding

d) Supports/ Tie-ups/blocks along Stator

e) Rotor bars/winding/Rotor coil supports

f) Balancing weights

g) Slip rings (if present)

h) Brush rocker assembly (if present)

i) Fans

j) Air gaps

2. Testing the Machine:

a) Stator





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v) Winding resistance measurement

b) Rotor (Wound Rotor or Synchronous Machine Rotor)


ii) Winding Resistance/Impedance Measurement

c) Exciter & Exciter Windings (Synchronous Machines)

i) IR Measurement


iii) Diode Check

d) RTD & Space Heater Check

(see relevant parts of Appendix II Parts A & B for details)

Expected downtime: 3 days

Pre-requisites:

1. The machine terminals should be disconnected from the main bus bars.

2. The inspection windows should be opened where present

3. The end-covers/end cover need to be opened, where possible.

Deliverables:

1. Condition Assessment report for the stator windings for a more accurate calculationof the insulation degradation of the stator as well as, condition assessment of the

rotor and exciter.2. Recommendations for bearing maintenance


4. Maintenance plan with tentative Level 4 Inspection schedule.


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Level 3 Maintenance


1. Level 3 maintenance is to be conducted when the machine is stopped for the level 3inspection.

2. During the Level 3 maintenance the machine is shutdown and end covers areremoved.

Scope of work:

1. Cleaning of filters (where present)


a) Sleeve Bearing




iv) Checking for wear and damage of shaft seals and hemp packing andcleaning of drain holes in seals.

v) Inspection of bearing seat surfaces on shaft and in bearing housings

vi) Recording of bearing insulation resistance.

vii) Re-fitting of end covers and bearings after completion of electricalmaintenance.

viii) Replacement of bearing oil as per the original grade recommended by theOEM.

b) Anti-friction Bearingi) Clean and inspect all bearing components for signs of wear.

ii) Clean and inspect bearings to the maximum extent possible.

iii) Check lock nuts for tightness.

iv) Check fit of outer race in housing/on end cover.

v) Re-grease prior to assembly

vi) Replace bearing if warranted.

3. Brush Rocker Assembly Maintenance (where present)

i) Cleaning of brush rocker assembly using solvents

ii) Dry-out of the brush rocker assembly after cleaningiii) Replacement of brushes (if found damaged)

iv) Brush seating

v) Integrity check of connections

4. Stator Maintenance

i) Cleaning of the overhang portion of the winding.

ii) Dry-out of the stator windings


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iii) Polarization-depolarization Current Analysis (repeated post cleaning-drying toevaluate efficacy of cleaning and drying process).

5. Stator Terminals

i) Inspections of all line and neutral connections

ii) Checking of tightening torque on all connections

iii) Checking/Installation of Thermal Tags

6. Rotor Maintenance (synchronous machines)

i) Cleaning of the overhang portion of the winding, slip rings and other accessibleareas.

ii) Dry-out of the rotor windings

iii) IR PI measurement post cleaning and drying


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When is Level 4 to be conducted?

1. If Level 1, 2 or Level 3 analysis indicates a 50 % reduction in estimated life.


3. After first 80,000 hours of operation or 10 years, whichever is earlier.

Objective:

1. To assess the stresses on the machine and identify the effects of the same on theinsulation system.



4. To determine the residual life of the stator windings.5. To carry out maintenance activities as recommended by Level - 2 & Level - 3 plans.

6. To increase its life of the stator insulation system.

Scope of work:

Before stopping the machine for a Level 4 inspection, all data as required for a level 1inspection are to be collected and analyzed.

After Stoppage, Disconnection and rotor removal:

1. Visual inspection of

a) Stator windings and insulation

b) Slot portion of the winding and wedge condition

c) Supports/ Tie-ups/blocks along Stator

d) Stator core laminations

e) Rotor bars/winding/Rotor coil supports

f) Balancing weights

g) Slip rings (if present)

h) Shaft

i) Air gaps

j) Terminals/Bushings and HV connections

k) Bearing condition

l) Cooling system, ducts/tubes

m) Seals, Gaskets

n) Brush rocker assembly (where present)

o) Fans

p) Lubrication system


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q) Instrumentation and protection system associated with the unit

2. Residual Life Assessment of the Machine: (See the relevant sections in Appendix II,III and IV for details)

a) Stator





v) Winding resistance

vi) Wedge Mapping

vii) Flux Loop Test

viii) Coupling Resistance Measurement (where possible)

ix) Corona PD Probe Test

x) Stress Analysis using FEM

b) Rotor (Wound Rotor and Synchronous Machines)


iii) Recurrent Surge Oscillograph (RSO) Test (turbo rotors only)


c) Exciter & Exciter Windings (Synchronous Machines)

i) IR Measurement


iii) Diode Check

d) RTD & Space Heater Check

Pre-requisites:

1. The rotor needs to be threaded out, and suitably mounted for inspection

Deliverables:

1. Condition Assessment report for the stator windings for a more accurate calculationof the insulation degradation.



4. Maintenance plan with scope definition for Level 4 maintenance.


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1. During the outage of the machine for Level-4 inspection

Scope of work:

1. Cleaning of filters and/or replacement (condition based)


a) Sleeve Bearing




iv) Checking for wear and damage of shaft seals and hemp packing andcleaning of drain holes in seals.

v) Inspection of bearing seat surfaces on shaft and in bearing housings

vi) Recording of bearing insulation resistance.

vii) Re-fitting of end covers and bearings after completion of electricalmaintenance.

viii) Replacement of bearing oil as per the original grad recommended by theOEM.

b) Anti-friction Bearing

i) Clean and inspect all bearing components for signs of wear.

ii) Clean and inspect bearings to the maximum extent possible.

iii) Check lock nuts for tightness.

iv) Check fit of outer race in housing/on end cover.

v) Re-grease prior to assembly

vi) Replace bearing if warranted.

3. Brush Rocker Assembly Maintenance (where present)

i) Cleaning of brush rocker assembly using solvents

ii) Dry-out of the brush rocker assembly after cleaning

iii) Replacement of brushes (if found damaged)

iv) Brush seating

v) Integrity check of connections

4. Stator Maintenance

i) Cleaning of the overhang portion of the winding.

ii) Dry-out of the stator windings


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iii) Epoxy spray impregnation of windings

iv) Heating of stator for curing of applied epoxy resin

v) Providing a final coat of a moisture resistant anti tracking varnish on windings

vi) Polarization-depolarization Current Analysis (repeated post cleaning-drying toevaluate efficacy of overhauling process.

5. Stator Terminals

i) Inspections of all line and neutral connections

ii) Checking of tightening torque on all connections

iii) Checking/installing thermal tags

6. Rotor Maintenance (Wound Rotor and Synchronous machines)

i) Cleaning of the rotor winding and slip rings.

ii) Dry-out of the rotor windingsiii) IR PI measurement post cleaning and drying

iv) Epoxy spray impregnation of windings

v) Heating of rotor for curing of applied epoxy resin

vi) Providing a final coat of a moisture resistant anti tracking varnish on windings


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APPENDIX I

Namep late Details

Tag ID

Sr. Number

Type

Manufacturer

Frame

kW/HP

Voltage

Current

Frequency

Speed

Power Factor

EfficiencyInsulation Class

Enclosure

Cooling

Test Certif icate Details

No Load Current (A)

Wdg Resistance (m? )

Winding Cap (pF)

Starting Torque

Pull Out Torque

Pull Up Torque

pf at 25% load

pf at 50% load

pf at 75% load

pf at 100% load

Start-Stop Detai ls

Starting Current

Starting Time (NL)

Starting Time (Load)

Number of Starts/stops

Total OperationalHours


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Load Cycle (ON TIME)

kW-1

Current-1

kW-2

Current-2

KW3

Current-3

kW4

Current-4

Load Cycle (OFF TIME)

kW-1

Current-1

kW-2

Current-2KW3

Current-3

kW4

Current-4

Operating Details SET - 1 SET - 2

Voltage

Current

kW

SpeedFrequency

Bearing Numbers

NDE

DE

Type

Vibrations

H DE

V DE

A DE

H NDE

V NDE

A NDE


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RTD Temperatures

Winding

1

2

34

5

6

7

8

Core

1

2

Bearing

NDE

DE

Body

Maximum

Ambient

Av Day max

Av Day min

Other Detai ls

Breaker Type

Dist from MachineRelay Type

Surge Arrestor at

Machine end

Breaker end

Unusual Conditions

Last Rewound


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Capacitance and tan delta measurements will be performed using a transformer ratio arm bridge.Measurements will be performed at increments that will not exceed 0.2 VL. Maximum test

voltage will be (1/?3)*VL, r.m.s.

Resulting Data will be analyzed to obtain the following parameters:

1. Discharging void volume ratio (if discharges are present)2. Effective phase of occurrence of discharges3. Characterizing constants if variations are due to stress grading4. Effective area involved in slot discharges (if slot discharges are present).

c) Partial Discharge Test:

PRINCIPLE:

Partial discharge pulse patterns will be monitored and recorded using a transformer ratio armbridge with appropriate coupling capacitors. The p.d. pulse patterns will be analysed with regardto pulse count, pulse magnitude, polarity dependence and phase to identify the nature of

discharges which can then be classified as:

(i) Internal Discharges(ii) Surface Discharges(iii) Slot Discharges

d) Non-linear Insulation Behavior Analysis:

PRINCIPLE:

To study the non-linear behaviour and characteristics of insulation material.

The tan delta and capacitance measurements vary with voltage even in absence of partial

discharges and one of the most obvious reasons for such a behaviour is the presence of non-linear field stress grading system at slot ends. Other reasons are space charge/interfacialpolarization due to contamination, electrostatic forces on delaminated insulation surface, partialdischarges etc. It is evident that both the voltage supply across insulation and the current passingthrough the insulation have harmonics, which cause increase or decrease in the measured tandelta and capacitance values. Thus, it becomes necessary to understand this time varying effectof insulation impedance on the capacitance and tan delta measured.Non-Linear Analysis provides a detailed understanding of these non-linearities and supplementsthe tan delta analysis. The analysis provides additional insights into the aging of insulation.

The machine insulation is tested by applying a known voltage across the insulation andmonitoring the voltage and the current flowing through the insulation, by capturing severalwaveform cycles of the voltage and the current on a digital storage oscilloscope. The insulation is

tested at predetermined voltage levels upto a maximum of (1/?3)*VL, r.m.s. The instantaneousadmittance of the insulation is calculated and the admittance patterns analysed for specificharmonic patterns.The extent of harmonics, predominance of odd or even harmonics, high or low frequencyharmonics is analysed to provide information on:- The integrity of the stress grading system used at the slot ends- The contribution of the slot stress grading system, contamination and ageing to the non-linear

behaviour


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- To confirm that observed anomalous tan delta variations can be physically related to theabove non-linear phenomena

e) Winding Resistance Measurement on Stator winding :

Winding resistance is measured to identify the existence of any shorts, breaks (open circuit) or

high resistance joints in the stator winding.

f) RTD Checks:

Resistance Temperature Detectors will be checked for ohmic resistance, and the insulationresistance.

Part B

g) Corona Probe Measurements:

A probe in the form of an RF coil is mounted on the end of an insulating rod and is used forcollecting data at the D-end and the ND-end of the stator winding, with regard to partial dischargeactivity in the various slots when the end covers are removed. The data collected is viewed on aDigital Storage Oscilloscope and relevant data is stored.

Tests on Rotor:

a) IR & PI test:

IR & PI test will be carried out on rotor winding using 500 V megger.

b) AC Impedance Test:

For detecting the presence of shorted turns in the rotor winding by comparing obtainedimpedance values with earlier measured values.

c) Winding Resistance Measurement:

Winding resistance is measured to find any inter turn shorts and breaks (open circuit) or for high

resistance joints.

d) Charging Discharging Current Analysis :

This test is performed on the rotor to distinguish, in the case of suspected leakage to damage

and/or contamination in the rotor windings, whether such problems are localized (damagerelated) or global (contamination related).Test is performed same way as DC Absorption test on stator winding, at test voltages not

exceeding 500 V.


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PART C

Other stator checks

a) Flux loop core check:

A loop is toroidally wound in the stator core in order to develop a flux level in the stator cotre thatis as close as possible to the rated flux, with the power supply that is made available by thecustomer at site. The stator core temperatures are monitored after a minimum of 30 minutes afterthe loop is excited, and the surface temperature of the core noted. The increase of surfacetemperature over the avaerage temperature of the core is noted. This temperature differenceshould not exceed 10 degree C with the rated flux passing through the core.

b) Wedge Mapping Test:

Wedge looseness check will be performed by tapping method and a wedge tightness map isprepared. If ripple springs are provided in the wedges, wedge deflection fixture or other

automated techniques are used for looseness check.

c) Coupling Resistance Measurement:

Contact resistance will be measured between the Bar insulation and the Ground (Slot wall) forboth the top and bottom bars using a probe, and a chart showing the coupling resistance readingat each slot will be prepared.


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APPENDIX III

ADDITIONAL INFORMATION ON MEASUREMENTS PERFORMED

STATORa) Polarization-Depolarization Current Analysis:

While performing IR and PI measurements in machines with modern day insulation systems, it isoften noticed that good/acceptable IR and PI values are obtained in spite of the machine windingsbeing excessively contaminated. This is mainly due to the fact that IR and PI measurements arebased on leakage current detection and are largely reflective of charge transport rather thancharge storage mechanisms.

Charge storage analysis is useful since it is possible to identify whether charge is stored innormal traps within the insulation or within contaminants that are likely to be present in theinsulation. Such analysis is therefore aimed at increasing the success rate of identifying thepresence of contamination in the insulation. The PDC Analysis is used to quantify andcharacterize charge storage mechanisms, and is therefore a reliable indicator of presence of

contamination.The machine insulation (individual phase as well as three phases combined with respect toground) is charged for more than 15 min and later discharged through a resistor for a period oftime equal to the charging period. The charging and discharging currents are measured at certainfixed intervals of time. The depolarization current is mathematically split into three parts usingregression analysis. The three parts are representative of space charge and interfacialpolarization phenomenon in slot and endwinding region. These phenomena are assumed to havenegligible spatial interdependence.

These three curves are then analyzed mathematical to calculate three time constants T1, T2, T3and charge values Q1, Q2 and Q3 attributable to the respective polarization phenomena. Othercalculated particulars include:

Dispersion ratio: It is ratio of capacitance calculated due to charge storage in the insulation tothe geometrical capacitance of the winding and is reliable indicator of contamination. This isgenerally to be used in conjunction with Q3 or the charge stored due to interfacial polarization onthe endwindings.

Q1/Q2 Ratio: Knowledge of the distribution of charge storage is important. This ratio givesinformation on the proportion of charge storage in slot region and is an indicator of problems suchas lack of contact of coil with slot.

Aging Factor: It provides an indication of aging due to de-polymerization mechanisms of resin

that could occur in close vicinity of the electrodes.

Besides, the analysis also provides information on the extent of contact of coil with slot, andleakage current sources.

b) Tan delta and Capacitance Test:

The PDCA test described above is more sensitive to the surface condition of the insulation. Foran in depth understanding of insulation characteristics there is a need for ac measurements. Tandelta measurements have traditionally been conducted at various voltage levels up to the ratedvoltage of machine, by using a Schering or a Transformer Ratio Arm bridge. The rationale of thismethod has been to assess the extent of partial discharge activity that occurs in the air spaces inthe insulation, which is reflected in the tan delta tip-up measured with increase in the test voltage.However, correlation of the insulation condition with tip-up measurements has not been easy dueto the following:


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??Guidelines have been established for testing of coils during quality control checks. Thesetests are generally performed using guard electrodes in order to discount the losses thatoccur in the stress grading system employed at the slot ends. While performingmeasurements on machines in-service, it is not possible to use guard electrodes. Thestandard norms are therefore no longer applicable. Also, the losses that occur in the stressgrading system can be large enough to completely overshadow the losses that are generated

by the discharging of air spaces in the insulation. It is therefore not unusual that tip-up in tandelta values measured below and above discharge inception, might hardly be perceptible.

??Partial discharges that occur near the voltage peak increase the tan delta tip-up to aconsiderable extent with very little resultant variation in the capacitance measured. Partialdischarges, on the other hand that occur near the voltage zero, increase the measuredCapacitance considerably, while having little effect on the tan delta tip-up. This phasedependence of partial discharges on the measured tan delta would imply that despiteconsiderable partial discharge activity tan delta tip-up could be very low, thus defeating thevery purpose for which such measurements were intended. Unfortunately, norms generallymention only tan delta values, totally ignoring the change in the capacitance measured.

??The presence of harmonics in the power supply voltage, either due to problems in the voltagesource or due to the non-linear nature of the capacitance measured, give rise to variations inthe phase of occurrence of partial discharges and therefore have a major impact on the tan

delta measured.

??The occurrence of partial discharges in large air spaces (say between the coil and the slot)results in a tan delta curve, which peaks at twice the discharge inception voltage. Whilemeasuring machines where there is a combination of loss effects, the tan delta curve willexhibit a decrease with voltage increase, once more making interpretation difficult. Also, inthe event of interfacial polarization effects, in the stress grading systems used in the slot andat the slot ends, there will be a reduction in tan delta with voltage increase.

For these reasons, both tan delta and capacitance values are measured with increase in themeasured voltage. The maximum voltage employed is the phase to ground voltage. The curvesobtained are analyzed both below and above discharge inception voltage to reveal presence ofpartial discharges and its general location.

Unlike conventional tests, values obtained are also analyzed below discharge inception voltage toreveal physical abnormalities like lack of contact of coil with core, interfacial polarization,presence of contamination and looseness of coils/wedges. The variations in tan delta andcapacitance due to polarization effects in the stress grading systems, contaminants, and other airspaces are estimated above discharge inception and subtracted from the variations due to theoccurrence of partial discharges.

An effective maximum change of capacitance is calculated, taking into account the phase ofoccurrence of the partial discharges. The discharging air space volume is estimated at a giventest voltage and is proportional to this maximum change of capacitance calculated. Themodification of normal properties of the stress grading system, are also estimated. The test alsodoes not trend the absolute values are they are sensitive to several physical phenomenon, butcalculates certain parameters like effective phase shift, discharging void volume content, which

make the test independent of previous readings.

c) Partial Discharge Test:

Partial discharges have been known to accelerate the aging process. They cause erosion ofinsulating material and propagate through treeing mechanism. Partial discharges in stator windinginsulation could be considerably large with little damage in the insulation system, due to thepresence of mica, which has a very high discharge resistance. In present day insulation systems,the possibility of internal discharges or discharges that occur within the main insulation are rare.PD phenomenon can be found,


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?? Within the main ground-wall insulation as a result of de-lamination of voids caused by missingor incompletely cured bonding material

?? Within the slot when contact is lost between the conducting surface on the coil and the core.This is slot discharge and can cause serious burning of coil surface and slot fillers.

?? In end-winding region due to surface contamination, called surface discharge.

?? In the region where coil exits the slots due to sharp change of potential along the surface ofcoil between the portion grounded to the stator core and ungrounded portion.

Partial discharge causes erosion of the insulating material at the tip of spark, carbonization,ozone formation and even nitrogen-based acids through some chemical reactions. Hence itsdetection and effective classification is crucial to identify the above-mentioned locations of pdphenomenon.

Partial discharge pulse patterns will be monitored and recorded using a transformer ratio armbridge with appropriate coupling capacitors.

The p.d. pulse patterns will be analyzed with regard to pulse count, pulse magnitude, polaritydependence and phase to identify the nature of discharges which can then be classified as:

(i) Internal Discharges

(ii) Surface Discharges

(iii) Slot Discharges

d) Non-linear Insulation Behavior Analysis:

The test is supplementary test to both PDCA and Tan Delta & Capacitance Analysis. Traditionalmeasurements performed on stator winding insulation indicate variation in capacitance and tandelta values with voltage, even in absence of partial discharges. One of the most obvious reasonsfor this variation is the presence of non-linear field stress grading system employed at the slotends. Other reasons include space charge and interfacial polarization phenomenon, due tovariety of reasons including contamination of the windings, aging of the insulation, and effects ofelectrostatic forces on delaminated stator insulation. Besides, partial discharge activity results inchange of instantaneous capacitance with voltage and hence is also a contributor of such non-

linear behavior.In this test, an AC high voltage is imposed on the insulation system, and the current drawn by theinsulation is subjected to a special non-linear analysis. Due to charge storage mechanisms, thiscurrent is replete with harmonics. The relative content of harmonics in the admittance of theinsulation are estimated, predominant harmonics and the pattern of harmonic magnitudes isindicative of anomalies in the insulating system such as ionic activity in slot region, presence ofcontamination and the occurrence of partial discharges. The test also provides a clearerindication of aging of insulation (if any).

e) Winding Resistance Measurement on Stator winding:

Winding resistance is measured to identify the existence of any shorts, breaks (open circuit) orhigh resistance joints in the stator winding.

f) Wedge Mapping Test:

Wedge looseness is a dangerous condition as coils are not restricted from moving in the slot,leading to coil surface erosion due to its rubbing with core and eventually partial discharges inslots or slot discharges. While the effects of looseness namely slot discharges, and other erosionof the coil surface, can be detected by the diagnostic tests, wedge checks are performed toidentify looseness as the very initial stages, so that further damage could be precluded.


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One of the methods used is by tapping each wedge in all the slots, after dividing each wedge intothree imaginary parts, with a light hammer and listening to the emanating sound, while manuallyfeeling the wedge for minor movement. A map is prepared to represent an overall picture ofwedge tightness.

For large alternators, this method has been automated using an automated tapping hammer at 10Hz, and picking up the vibrations on the tapped wedge with accelerometers.

While the automated method is used even in cases where ripple springs are employed in statorslots, a wedge deflection test is also adopted. This check is done by applying pressure on thewedges using a known force and measuring the deflection of the wedges, thereby determiningthe spring stiffness.

g) Flux Loop Test:

The Hot spot & Electromagnetic Core Imperfection detector is a test for detection of core faultssuch as inter-laminar short circuits particularly in large generators, where it can be rathercumbersome to perform a standard loop test.

Defects in the inter-laminar insulation cause fault currents to flow locally in the core. Thesecurrents can produce dangerous local over heating or hot spots in the damaged areas and the

damage to the core may become progressively worse. In extreme cases sufficient heat isgenerated to melt small parts of the core and even modest rises in core temperature adjacent tothe winding can result in the premature failure of the winding insulation.

In this method, the stator core is excited to a flux level as close as possible to the rated flux andthe temperature rise of the core is noted to detect hot spots.

h) Coupling Resistance Measurement:

Contact resistance will be measured between the Bar insulation and the Ground (Slot wall) forboth the top and bottom bars using a probe, and a chart showing the coupling resistance readingat each slot will be prepared.

i) Corona PD Probe Measurements:In this test the contact resistance/capacitance between the coil outer surface and ground in theslot is measured for both the top and bottom bars and a chart showing the coupling resistancealong each slot is prepared. This test provides information on extent of contact of coil side withcore and therefore the extent of coil looseness due to side clearances or any other deteriorationor damage to the discharge protection coating.

This test is used to quantify the lack of contact of coil with core problem that is detected in thePDCA and C-Tan Delta Analysis and provides valuable data inputs for FEM modeling and stresscalculations.

ROTOR (if the rotor has insulated windings)

a) Polarization-Depolarization Current Analysis:Similar to the test as described above for Stator Windings, with the difference being that the testis carried out using 100 - 500 V megger.

b) Winding Resistance / Impedance Test:

For detecting the presence of shorted turns in the rotor winding by comparing obtainedimpedance values with earlier measured values or for detecting high resistance joints.


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APPENDIX IV

Life Assessment Approach by

Operating Stress Evaluation

using Fine Element [FEM]Technique

During operation, an electrical rotating machine is subjected to thermal, electrical, mechanicaland ambient stress either singly or in combination, ultimately resulting in aging of insulation. Thetensile curve determined from operational data and critical stress value are assumed for theparticular class of insulation. The tensile curve is corrected based on the diagnostic tests, whilecritical stress levels are assumed. This helps in remnant life estimation with enhanced accuracy.

The next accuracy stage is to determine the actual stresses within the machine. These stressescan be evaluated using finite element techniques. Testing engineer collects the data regardinggeometry of the coil, materials used, operating temperatures, arrangement of blocks and ties etc.These are later modeled using FEM software that calculates the Von-Mises stresses. From theknowledge of the operating stress, machine health and aging extent, a weak link in the insulation

can be identified. Thus, the life can be calculated with highest accuracy.

Based on design data and actual measurements of the key dimensions of the machine, thesestresses can be evaluated using finite element techniques. Figure 2 shows thermal stress on acoil and figure 3 shows high electric stress in regions around a crack in the electrical insulation.From the knowledge of the operating stress, machine health and aging extent, a weak link in theinsulation can be identified and plotted.

Figure 2. Finite element analysis (thermal) of generator stator coil

In this approach also, failure is defined in terms of a specific loss of an insulation property, analternative, - seemingly obvious approach, - would be, to define the life of stator winding

insulation directly in terms of its electrical breakdown in operation.

Stator winding insulation is exposed to a combination of stresses, - electrical, mechanical,thermal and environmental - which act upon the insulation and result in a global or localweakening of the insulation structure.


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Figure 3. Finite element analysis of a crack in insulation

The dominant nature of mechanical stresses have been observed:

?? during multi-factor aging experiments

?? from operational data, - starts/stops have found to correlate well with failures at the slot ends.

?? from studies/surveys on the causes of failures of stator winding insulation.

Defects that form and develop under the influence of mechanical stresses have been known tohave an ability to be modeled.

STRESS ANALYSIS:

THERMO MECHANICAL STRESSES:

The change in the winding temperature from a cold to hot condition and from a hot to coldcondition constitutes a thermal cycle. Thermosetting insulation systems generally are stabledimensionally with increase/decrease in temperature. However, the copper of the bars tends toexpand on application of heat. The restraining force is generally offered by the end-windingbracing supports that are used to limit winding movement due to electro-magnetically generatedforces. These constraints result in a mechanical strain at certain bracing support locations. Also,the conductor bends and twists due to a change in the direction of expansion of the coil, resultingin additional development of stresses.

The developed stresses are computed using a three-dimensional finite element (FEM) package.

STEP 1: A 3D model of the coil is constructed based on the geometry measurements made. Theties are modeled at the end-windings as per their location from the slot ends.

Air

InsulationInsulation

Core


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STEP 2: The temperatures are then incorporated in the model. A 700C is assumed for the portion

of coil in the slot region (straight portion) and a temperature of 600C is assumed for the end-winding

portion of coil.

Modeling of

ties

3D model of coil based on

Geometrical data


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STEP 3: Based on diagnostic testing earlier we make use of 4 parameters(i) Charge storage Q1 and Q2 in slot region(ii) Void Volume Content(iii) Coupling Resistance(iv) Partial Discharges

To arrive at a Contact Index number.

STEP 4: The boundary conditions are then fixed for the coil model.The contact index number determines the percentage area of the coilthat is not making proper contact with the stator core and is modeledas without friction, while the remaining portion is modeled as withfriction. The coil motion is restricted in radial and transverse direction(i.e. along X and Y axis) but coil movement can take place axially(along Z direction). The coil motion is also restricted at the region ofties. The following figure shows the coil with boundary conditions.

STEP 5: The FEM Stress analysis is then done and the material dataregarding Poissons ratio and Modulus of elasticity, the differential co-efficient of expansion are fed during the analysis. The FEM plot asshown in Fig.2 above is made and the magnitude and location ofmaximum and minimum stresses are calculated and displayed in theplot.

STEP 6: Based on the operational data such as the operationalhours and starts-stops data we calculate the critical stress. Thevalues are then calculated for aged insulation at 90 deg C.

STEP 7: The Arrhenius curve as discussed in first method is thenconverted into the Ultimate Tensile Strength curve and is plotted against the operationalhours. This is done by calculating the percentage life used up as per the Arrhenius curve andthen assuming a 50% reduction in tensile strength for the life that is used up. The ultimate tensilestrength curve also takes into account the thermal cycle fatigue caused by fluctuations in day and

night temperatures.

STEP 8: LIFE ESTIMATION

From the operational data, we have the present operating hours, which is marked on the ultimatetensile strength curve. We then plot the developed stresses as constant on this curve. The stresscurve (constant) line intersects the ultimate tensile strength curve at some point. This point givesus the operational hour beyond which there is a high risk of failure.

The difference between the operational hours at intersecting point and the present operatinghours determines the residual life in terms of operational hours. (as shown in figure below)


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UNIT 4

50

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60

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66

68

70

1

4

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0

0

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Operating hours

DEVELOPED STRESS

ULTIMATE

TENSILESTRENGTH

PRESENTS

EQUIVALENT

OPERATIONAL

HOURS

EQUIVALENT

OPERATIONAL

HOURS AT

INTERSECTING

POINTREMAINING LIFE

We are not satisfied by just calculating the residual life of the machine. We use our expertise andexperience to understand the root cause of problem and recommend suitable actions toeliminate/reduce the problems and increase the life.

The various ways in which remaining life of the machine can be improved are illustrated below:

Improvement

in life by restoringstrength droop

Ultimate tensile strength

Developed stressStress/strain

Years

Improvement in life

by improvingstrength rate droop


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The problems such as contamination, machine running hot, lack of contact of coil with core andpartial discharges etc. cause reduction in ultimate tensile strength. The early detection ofproblems and appropriate maintenance actions, like cleaning/overhauling of the machine canhelp in restoring the tensile strength. By improving the cooling eff iciencies, better heat dissipation,removal of blockages in ventilating ducts, the tensile strength can be improved as shown by blueline above.

The problems such as looseness of coils and partial discharges cause acceleration of agingproblem and hence the developed stresses may not be constant but increasing as shown by solidred line. Placing the coils tight in slot by inserting adequate side packers, rewedging etc canarrest the looseness and prevent increase in developed stresses, causing improvement inremaining life. A further improvement can be achieved by reducing the developed stress (blueline) by rewinding/revarnishing options.

Improvement

in life by restoring

original stressdeveloped

Ultimate tensile strength

Developed stressStress/strain

Years

improvement in life

by reducing stress

through redesign


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APPENDIX V

GENERAL TEST REQUIREMENTS TO BE PROVIDEDBY THE CUSTOMER AT SITE AND OTHER NOTES

1. Machines will have to be offered for test (by the customer) with the terminals made available,and with cables/bus bars disconnected at the machine end. Disconnection and reconnectionwherever required will have to be carried out by the customer.

2. Any dismantling / assembling of Rotors or decoupling required for testing is not in ABBscope.

3. To comply with safety requirements, the customer will have to ensure that one othercompetent person of the customer in addition to our test engineer is present during testing,and that there is adequate lighting in the test area.

4. Stators will be tested up to a maximum of line to ground voltage, which is generallyconsidered to be a safe test voltage level. If the insulation of the machine fails during test, itcould only be attributed to a major defect in the insulation of the machine, and as such ABB

will not be held responsible for such a failure.

5. Power supply board with a minimum of three domestic 30 amp sockets and switches (singlephase, 3-pin, 230 V / 3 phase 440 V) at work site.

6. Suitable/bench or any other adequate arrangements to set up the test equipment as close asis possible to the machine to be tested.

7. A suitable trolley or other adequate lifting and shifting arrangements to move the testequipment to the test sites.

8. Space for the safe storage of test equipment while not in use.

9. Prior permission/gate pass for the testing team and equipment to be arranged by thecustomer.

10. Manpower for shifting of equipment to be arranged by the customer.

11. The customer will assist in providing ABB with all available data requested on the machinesto be tested, including previous tests, drawings etc., if required.

12. Customer shall provide following facilities free of cost at the site where required:

1.1 Machine parts placed at repair site, free from any incumberance1.2 Workshop facilities1.3 Crane facilities with operator

1.4 Oxy-acetylene plant1.5 DC welding generator1.6 Adequate power and lighting with switching arrangement, as required1.7 Stands for placement of components, especially rotating arrangement for rotor

with roller1.8 Clean tarpaulins as required


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COMMERCIAL CONDITIONS

1 Placement of purchase order

The purchase order shall be placed on

ABB LimitedB.S. Devshi Margoff Sion - Panvel Road, DeonarMumbai: 400 088

2 Basis of the offer

We have made our price bid in Indian Rupees for scope of work as given only. Any otherwork shall be carried out after discussion and approval of the customer at extra charge.The price quoted is for job to be done at your plant. Any duties and taxes will beadditionally charged for at rates applicable at the time of execution of the contract.

3 Time

The time generally taken for all data collection is 4 days at site. A detailed report issubmitted within 20 days of the receipt of all relevant data for performing the analysis.

4 Validity

Our offer is valid for 90 days from the date of our offer.

5 Terms of payment

?? 20 % advance payment along with placement of the order?? Balance 80 % on the of submission of the final report/ completion of the job

6 Mobilization Charges for Resources

The customer shall pay for the complete mobilization charges for the equipment fromABB Mumbai to site.

The customer shall pay all the expenses incurred for the necessary clearances andpermits if any, besides the actual charges that are involved in the to and fro transport.The charges for insurance of the equipment shall also be bourne by the customer.

7 Accommodation and local conveyance

Customer shall bear the costs of the transport of the personnel and the equipment fromMumbai to the site and back by AC 2T and shall arrange for the local conveyance.

Customer shall also arrange accommodation for the testing team at site and in transit.

8 Security of ABB Personnel

The customer shall provide and undertaking to ensure safety of ABB personnel andequipment during the period of stay during the stay in the premises.

ABB - Machine Maintenance

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