9 0 1 MB
CONDITION MONITORING TESTS ON HYDRO/TURBO GENERATORS AND LARGE AC MOTORS K.Mallikarjunappa Central Power Research Institute Bangalore
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CONDITION MONITORING TESTS ON HYDRO/TURBO GENERATORS AND LARGE AC MOTORS * GENERATORS - unit rating up to 500 MW − rated output voltage up to 30 kV * MOTORS − unit rating up to 40 MW − rated terminal voltage up to 15 kV
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** Reliability Reliability and and life life ** Stator Statorwinding winding ** Stator Statorcore core ** Rotor Rotorwinding winding
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INSULATION • Operational reliability depends to a large extent on the condition of the insulation system • Insulation is the weakest link • Any unexpected failure (forced outage) in generating stations & process industries disrupt the system & cause heavy financial losses • Majority of failures have been attributed to the insulation failures 4
LIFE LIMITING FEATURES • • • • • • • • • •
Stator insulation Stator winding slot & end winding portions Tightness of stator bars in slots Stator core tightness & insulation Stator end winding bracing High levels of mechanical vibrations Frequent starts & stops Rotor winding wedging system & end winding portions Rotor end ring ( cracking, deformation ) Rotor winding insulation
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STATOR INSULATION - composite type (i) Mica (ii) Glass fabric or cellulose paper (iii) Resin [Synthetic, Non-synthetic]
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STRESSES ACTING • Stator winding is subjected to a combination * Thermal …. High operating temp. during normal & abnormal conditions
* Electrical…. Over Voltages during transient conditions * Mechanical…. High levels of mechanical Vibrations * Environmental…. Moisture, oil, dust, contaminants
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Thermal stress -
Delamination, tape separation, embrittlement, strand separation, girth cracking.
Electrical stress - Cumulative electrochemical effects of Partial discharges.
Mechanical stress - Loosening of wedges & end winding blocks, abrasion of the insulation - Erosion of stress grading paint & corona shielding paint
Coil
Corona shielding coating
Stress grading coating
Core 8
Environmental stress - Render stress grading coating ineffective. - Electrical tracking. • Slot discharges
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• End winding discharges | * Lead to rapid failure.
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CONDITION ASSESSMENT PROGRAMME # Consists of the following steps • Collection of the historical data • Visual inspection & examination • Condition monitoring tests
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HISORICAL DATA # Can indicate problems which are generic/developed due to ageing • • • • • • • •
Age of the machine Running hours Number of starts & stops Load levels Overloading Major electrical disturbances and faults Vibration & Temperature abnormality Record of repair and replacement of components etc.
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VISUAL INSPECTION & EXAMINATION • • •
Visible symptoms of deterioration Mechanical damage to stator bars & end winding, migration of edges Deformation of the end winding sections
• Deterioration due to thermal effects .. Embrittlement, change in colour • Corona damage & electrical tracking.. White/brown powdering • • • • • • •
Loose end winding blocks, ties, lashing Deposit of oil, dirt. moisture ingress, salt etc. Powdering due to abrasion Loose core laminations Core damage due to surface discharge Change in colour of core surface due to hot spots Abrasion of the slip ring and the like.
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CONDITION MONITORING (DIAGNOSTIC) TESTS # Conducted to * Assess state, condition & extent of deterioration *Assess trend in ageing • Data logged enable to initiate appropriate remedial measures to prevent forced outages > Service life could be extended
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HYDRO & TURBO GENERATORS Diagnostic tests…. Stator winding Sl. No
Tests
Detection capability
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Polarisation index test
Index of dryness,cleanliness
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Tan delta & capacitance test
Dielectric losses
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Partial discharge test
Incipient faults, slot & end winding discharges
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Wedge mapping
Loose wedges & Loose stator bars
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DC leakage current
Discontinuities & cracks
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Winding resistance measurement
Loose or bad conductor joints
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Surge comparison test
Inter turn faults
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Stator core
* ELCID Test
Imperfections & hot spots in the core
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Rotor winding * Dominating stresses > Thermal & mechanical Tests
Detection capability
IR/PI
Index of dryness, cleanliness.
Conductor resistance
Loose or bad joints
Winding impedance
Inter turn shorts in poles
Recurring surge test
Intern turn & earth faults
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IR Measurement : - Reflects surface condition of the insulation - Indicates surface contamination & moisture content - PI is used as an index of dryness. PI = 2 R
Line
Test voltage
Y Neutral
B _ Measuring connection of stator winding 17
DC Leakage current measurement • DC voltage is increased in steps. • At each step, voltage is maintained constant for a predetermined time interval (100 sec.) and current is recorded • Max. test voltage as per guidelines • Plot current verses test voltage
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Micro amps.
(2α)
(3) (2)
(1)
DC voltage (kV) (1) - Solid homogeneous insulation in good dry condition (2) - Faulty insulation due to dirt & oil, ageing, mechanical damage or tape separation (2a) - Faulty insulation - step ladder curve due to internal voids & ionisation (3) - Insulation in wet condition
Typical curves obtained when testing insulation of large rotating machinery. Vdc = 1.6 x (AC test voltage level) 1.5 Vph
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Tan delta Test:
Represents dielectric losses
Concept Of Tan delta Insulation between two electrodes Treated as Capacitor HV
Electrodes
INSULATION
LV
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In a Perfect Capacitor current leads the voltage by 900
Perfect Capacitor
Phasor Diagram
I
Cp
900 V 21
Practically Phase angle is < 900 δ Loss angle φ Phase angle Due to dielectric losses I
I
Ic
δ φ Ir
V
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Lossy Dielectric
Ic
I
Ir
I Ic Cos φ = Ir / I = Sin δ
Tan δ = Ir/Ic
Cp Rp
δ φ
Ir
Phasor diagram 23
Measurement of Tan delta : • High Voltage Schering Bridge H.V.
Rp +(1/jωCp)
Z1
Rx
Cx
Cn
Z2 CRO
Z3
R4 R3
C4
Z4
L.V
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Tan δ measurement procedure : * Single phase testing transformer of suitable KVA rating * Equipment under test needs to be disconnected from the system * Tan δ kit to be grounded to the system grounding and test voltage is raised in steps upto the rated phase voltage Generator Stator Motor
R
HV
Y B
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Tan δ - Voltage characteristic. Test voltage is raised in steps up to a maximum of rated service voltage. Tan δ is measured at each voltage level. Plot Tan δ v/s Voltage. Wet & contaminated Deteriorated Gaseous loss
Tan δ Solid loss
Sound
0 Voltage
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Test parameters: - Tan delta & capacitance at 0.2 VL - Tan delta tip-up
tan delta Vph - tan delta (0.2VL) 2
- Capacitance tip-up
Cap. Vph - Cap. (0.2VL) Cap. (0.2VL)
* Changes in the above quantities with machine age * Statistical variation of these quantities of similar machines.
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Limiting values of tan delta for new coils/new windings Rated line voltage VL (kV)
6.6
11.0
Mica with synthetic bond tanδ at 0.2 VL
∆ tanδ
0.04 0.02 0.03 0.03 0.04 0.02 0.03 0.03 0.04
0.003 0.0025 0.0025 0.003 0.003 0.0025 0.0025 0.003 0.0025
Maximum ∆ tanδ per 0.2 VL 0.006 0.005 0.005 0.006 0.006 0.005 0.005 0.006 0.005
tanδ at 0.2 VL 0.05 0.04 0.03 0.05 0.05 0.05 0.03 0.05
Mica with nonsynthetic bond ∆ tanδ
0.006 0.003 0.0025 0.006 0.006 0.003 0.0025 0.006
Maximum ∆ tanδ per 0.2 VL 0.016 0.006 0.005 0.012 0.016 0.006 0.005 0.012
a)a) BEAMA BEAMA REM REM500, 500, 1969 1969 (b) (b) Balcombe Balcombeand andStatt Statt(CEGB), (CEGB), 1973 1973 (c) (c) CENELEC, CENELEC, 1974 1974 (d) ESI Standard 44-5, 1987 (e) VDE 0530 (d) ESI Standard 44-5, 1987 (e) VDE - 0530 28
Partial discharge Test Partial discharges
tan delta
## PD PDoccur occur due duetotothe thepresence presenceof of **voids, voids,conducting conductingparticles, particles,de-lamination de-lamination **PD PDare aredeleterious deleterioustotothe theinsulation insulation **Cause Causechemical chemical&&mechanical mechanicaldestruction destructionof ofthe thesurrounding surrounding insulation insulation
Gaseous loss Material loss
Voltage 29
Concept Conceptof ofPartial Partialdischarges discharges
- Discharge process in which the gap between two electrodes is only partially bridged. HV
Void Conductor Insulation
* Cause chemical & mechanical destruction of the surrounding medium & hence premature failure.
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Effects of PD PD can give rise to • Ozone • Nascent oxygen - strong oxidising agents • Nitric acid in presence of moisture • Oxalic acid in polymeric insulation • Mechanical erosion due to ion bombardment • Intense heat in the discharge channel • Power loss
* PD cause chemical & mechanical destruction of adjacent materials. 32
HV
3 2 1
Dielectric
1 - Internal partial discharge (Cavity discharge) 2 - Internal partial discharge ( between metallic & dielectric surfaces) 3 - Surface discharge (outside the insulation)
Representation of a partially defective dielectric 33
PARTIAL PARTIAL DISCHARGE DISCHARGE TEST TEST IEC-60270 IEC-60270
HV HV
Cc Cx Test object
Cb
Z1
GG
Detection impedance
Ec
Discharge detector
Cb - Blocking capacitor
Basic Partial Discharge Detection Circuit
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ANALYSIS OF PD DATA * PD are highly stochastic in nature
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Calibration • Effected by injecting pulses of known charge contents. • Calibrating pulse -- PD pulse - Magnitude & time characteristics must be comparable. • Rise time -- 50 - 100 nano sec. • PD magnitude, q = eq. Cq V 0
Calibrating pulse
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PARTIAL DISCHARGE TEST * Found to be effective * Capable of revealing incipient faults * By analysing the PD data it is possible to identify type of fault in the machine
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Partial discharge test continued…. •
Hydro & Turbo generators * Internal discharges ….. Occur in voids / cavities * Surface discharges…… Highly deleterious > Slot discharges ….. Between coil surface & iron core
• •
> End winding discharges ….. Junctions of corona shielding & Stress control coatings
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PD Analysis Continued…..
• Need to be analysed statistically • PD Quantities * Magnitude (q) * Number density (n) * Polarity * Phase angle of occurrence (ø) * Quadratic rate
# Distribution profiles * Magnitude - Number density distribution (q-n) * Magnitude - Phase angle distribution (q- ø) * Number density - Phase angle distribution (n-ø) and * 3D patterns of ( q-n- ø ) # Finger prints and temporal changes can be used to characterize defects
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Comparison of PD patterns Int. Void
Void facing the HV electrode
Void facing the grounded electrode
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Comparison of PD Patterns Int. Void
Void facing the HV electrode
Void facing the grounded electrode
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On-line On-line Condition Condition monitoring monitoring of of Turbo Turbo & & Hydro Hydro Generators Generators Using Using P.D P.D Testing Testing •• Deterioration Deterioration mechanisms mechanisms result result in in P.Ds P.Ds caused caused by by ** Voids Voids in in the the Insulation Insulation ** Electrical Electrical tracking tracking on on the the end end windings windings ** Sparking Sparking between between the the stator stator core core and and loose loose stator stator coils coils •• Insulation Insulation deterioration deterioration can can be be detected detected by by monitoring monitoring P.Ds P.Ds 42
How Howto to detect detect PD PD in in Generators Generators ?? •• Three Three Types Types of of PD PD sensors sensors ** Capacitive Capacitive couplers couplers ** HFCT HFCT ** Stator Stator slot slot couplers couplers
•• Sensors Sensors are are permanently permanently installed installed in in the the stator stator winding winding during during planned planned outage outage or or during during manufacturing manufacturing stage. stage. 43
1. 1. Capacitive Capacitive couplers couplers (80 (80 pF pF -- 1000 1000 pF) pF) •• Coupled Coupled to to the the stator stator winding winding at at ** Generator Generator bus bus bars. bars. ** Stator Stator winding winding connecting connecting rings rings at at the the overhang overhang portions portions ** Can Can be be retrofitted retrofitted to to old old generators. generators.
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2. 2. HF HF CTs CTs :-:- Can Can be be incorporated incorporated at at •• Neutral Neutral end end •• Frequency Frequency range range 0.3 0.3 -- 100 100 MHz MHz •• can can be be retrofitted retrofitted to to old old generator generator
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3. 3. SSC SSC :-:•SSC •SSC isis aa broad broad band band antenna antenna (UHF (UHF Band) Band) •SSCs •SSCs are are installed installed under under the the wedges wedges in in the the stator stator •• Coaxial Coaxial cables cables are are routed routed to to aa point point outside outside the the generator. generator.
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Interpretation of PD quantities * Still a challenging task * Often subjective * Depends on experience and expertise * Subject of intense research
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WEDGE TIGHTNESS TEST > Important test specified for RLA studies on Generators # Stator wedges may be slackened due to * Shrinkage of slot packing materials * High mechanical stresses * Vibration ^ Loose wedges cause * Loosening of stator bars * Excessive vibrations * Erosion of corona shielding & stress grading coatings * Abrasion of insulation
$ EVENTUALLY LEAD TO FAILURE OF STATOR WINDING.
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ELECTRONIC WEDGE TIGHTNESS EVALUATION # Electronic method * Sophisticated * Provides map of wedge tightness * Data can be stored for accurate trending of WT data
* Hand tapping method with a hammer > Crude method > Highly subjective > No data can be generated > Trend analysis is not possible.
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WTD Methodology * Each wedge is tapped automatically by a magnetic hammer > Tapping force is constant * Accelerometer picks up the signals * Signals are processed & stored * Software provides a map of relative tightness of the wedges.
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STATOR STATOR CORE CORE Made Madeup upof ofthousands thousandsof ofthin thinsteel steelLaminations Laminations (typically (typically0.5 0.5mm) mm) Laminations Laminationsare arecoated coatedwith withaathin thinlayer layerof ofelectrical electricalinsulation insulation to toprevent preventeddy eddycurrents. currents. Laminations Laminationsare arefrequently frequentlyshorted shortedtogether togetherat atthe theback backby by support supportbars bars
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DEGRADING DEGRADING FACTORS FACTORS ** Mechanical Mechanicaldamage damageto tothe thestator statorbore boresurface surfaceor ortop topslot slotwalls walls ** Vibrations Vibrationsin inthe thecore coremay maycause causeabrasion abrasionof ofinter inter laminar laminar insulation insulation& &short shortcircuits circuits ** Shorts Shortsbetween betweenadjacent adjacentlaminations laminationscause causeeddy eddycurrents currentsto tobe be induced inducedby bythe therotating rotatingmagnetic magneticflux. flux. ** These Thesecurrents currentscan canproduce producedangerous dangerouslocal localoverheating/hotspots overheating/hotspots in inthe thedamaged damagedareas areas ** In Inextreme extremecases casessufficient sufficientheat heatisisgenerated generatedto tolocally locallymelt meltsmall small parts partsof ofthe thecore core ** Hot Hotspots spotsmay maylead leadto topremature prematurefailure failureof ofstator statorwinding winding insulation insulation
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DEGRADING FACTORS
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Vibrations
Erosion of corona shielding coating
Erosion of stress control coating
Slot Discharges
End winding Discharges
* Pittings on stator bar insulation * Fusion of core laminations (short circuiting)
CPRI
Abrasion, fretting of core laminations
Short circuiting of adjacent laminations
* Damage to core end portion * fusion of core lamination
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CPRI
Eddy currents induced circulate (Hot spots) Local burn out of the core Extensive damage to the core
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CPRI
TESTING TESTING OF OF STATOR STATOR CORE CORE
Conventional Test on Core * Core loop test - to detect hotspots in the core Cable loop
High current source
Water Rheostat
Core CT
Schematic diagram of Core Loop Test
Voltmeter
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Core Coreloop looptest test…. ….continued continued
CPRI
* A no. of turns of heavy cable is wrapped toroidally around the core & frame. * Very high AC current (hundreds of amps.) sufficient to produce flux density almost equal to operating level. * Core gets heated up. * Temp. is measured at several points on the core surface. * Infra red scanning to detect hotspots.
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ELCID ELCID TEST TEST ** Induce Induceonly onlyabout about44% % of ofthe theflux fluxin inthe thecore coreby bypassing passingan anAC AC current current(5 (5--15 15Amps) Amps)through throughaaexcitation excitationwinding windinglooped looped toroidally toroidallyaround aroundthe thestator statorframe. frame. ••Small Smallpick pickcoil coilsenses sensesthe thefault faultcurrent currentinduced inducedat atthe thedefective defective core coresection section >>Excitation Excitationcurrent current **Single Singleturn turnvoltage voltageof ofthe thegenerator,V generator,Vpp VVpp== VVphph//((kt ktpp)) where whereVVphph=Phase =Phasevoltage voltage kk ==pitch pitchfactor,0.92 factor,0.92 ttpp ==Number Numberstator statorbars barsper per phase phase ##For ForELCID ELCIDtest, test,single singleturn turnvoltage=4%V voltage=4%Vpp
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Schematic diagram of ELCID test ing
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ELCID Test on 27Mw Hydro generator
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A view of ELCID test set up
A view of ELCID testing in progress
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TYPICAL TYPICALELCID ELCIDDATA DATA
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DEFECTIVE DEFECTIVE CORE CORE
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Rotor winding
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Rotor Winding (Turbo generator)
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ROTOR ROTOR WINDING WINDING ** Dominating Dominatingstresses stresses ** Thermal Thermal ** Mechanical Mechanical
TESTS TESTS ** IR/PI IR/PI
Deterioration, Deterioration,dampness, dampness,Contamination Contamination(cleanliness) (cleanliness)
** Field FieldImpedance Impedance ** Conductor Conductor
Interturn Interturnfaults faults Resistance Resistance Bad Badconductor conductorjoints joints 67
SURGE SURGE TEST TEST ** Rotor Rotorwinding winding ----RLC RLCcircuit circuit ** LV LVSurge Surge((≈≈250 250V) V)isisapplied applied ** Resultant Resultantwaveform waveformisisrecorded recorded ** Both Boththe thewaveforms waveformsare aresuper superimposed imposed ** Waveform Waveformcoincide coincideeach eachother otherand andappear appearas asaasingle single waveform waveformififthere thereisisno nointerturn interturnfault fault 68
CASE STUDIES • 1) 11 kV, 144 MVA Hydro generator Stator winding: • • • • • • •
IR = 700 MΩ Ω tan δ = 0.87% ∆T = 0.29% ∆C = 0.36% IDE = 1.01 µJ/pF/cycle Vi = 5.3 kV Qc = 3600 pC
• Assessment: * Low dielectric losses * Low void content * Insulation condition of stator winding healthy
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Rotor winding • Pole impedance • Varied from 4.82 Ω to 10.67 Ω • Visual inspection revealed migration of turn insulation of 02Nos. of poles
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Migration of turn insulation at the top of the poles
Migration of turn insulation
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Migration of turn insulation at the bottom of pole Migration of turn insulation
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13.8kV 100Mw Hydro Generators * Operating in a Hydro power station • Age varying from 19 years to 26 years • Conducted Tan delta & PD tests
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PD Patterns
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13.8kV 100Mw Hydro Generators Generator
Tan ∂ (%)
∆T (%)
∆C (%)
PD mag.(pC)
G1
2.81
0.68
1.78
5300
3.96
G2
1.40
0.18
0.41
7363
3.22
G3
1.30
0.31
0.76
11,629
3.31
G4
1.4
0.37
0.93
11,755
3.21
G5
1.48
0.29
0.74
9,385
3.26
G6
1.14
0.17
0.27
860
3.96
G7
1.89
0.14
0.28
11,502
3.25
G8
1.25
0.14
0.26
8,943
3.44
**Low Lowdielectric dielectriclosses losses **Low Lowvoid voidcontent content **Stator Statorwindings windingsare areininhealthy healthycondition condition
Vi (kV)
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11kV, 115Mw Hydro Generators Salal Power Station Generator
PI
Tan delta (%)
∆T (%)
∆C (%)
PD Level (pC)
DIV (kV)
1
4.15
0.41
0.23
0.22
1100
3.21
2
2.54
1.02
0.18
0.19
2500
4.0
3
4.84
1.049
0.058
0.025
2000
4.0
4
3.74
0.88
0.021
0.053
4000
4.4
5
2.69
0.925
0.035
0.064
1500
3.96
6
4.21
0.876
0.0315
0,0844
4000
4.4
**Low Lowdielectric dielectriclosses losses **Low Lowvoid voidcontent content **Stator Statorwindings windingsare arein inhealthy healthycondition condition
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2) 11kV, 7.2Mw Turbo generator • 18 years old class-F machine installed in a Polyfibres industry • • • • • • •
PI = 2.9 tan δ = 1.85% ∆T = 0.22% ∆C = 0.40% IDE = 0.92 µJ/pF/cycle Vi = 4.8 kV Qc ~ 40,000 pC. Discharges of very high magnitude in R & Y phase sections
* Slot / end winding discharges were suspected.
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11kV, 7.2Mw Turbo generator continued…..
• • •
Decision was taken to visually inspect the stator winding and Conduct inductive probe test to locate the sites slot/end winding discharges Results of visual inspection & inductive probe test
• • • • •
Presence of white powder at the end winding regions of several bars Visible sparking was observed at the end winding regions two bars bearing No.2 & 22 ( Line end of R & Y phases ) Deposits of white powder were found both on exciter & turbine ends. Deposits of white powder are a symptoms of end winding discharges Inductive probe test indicated presence of slot discharges—900mV
•
Recommended for re-wedging. 78
11kV, 7.2Mw Turbo generator continued…..
• Company accepted the recommendation & initiated action • Tests were conducted after re-wedging with side packing materials & varnishing • • • • • •
PI = 2.8 tan δ = 0.93% ∆T = 0.087% ∆C = 0.26% IDE = 0.21 µJ/pF/cycle Vi = No discharges up to 6.35 kV
• Generator is in healthy condition.
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1). 1). 11kV, 11kV, 2700kW 2700kWSynchronous Synchronousmotor motor ** Class Class--F, F, 10 10years yearsold old ** Fertilizer FertilizerCompany Company tan tan δδ ∆∆TT ∆∆CC IDE IDE Vi Vi
== 2.51% 2.51% == 2.64% 2.64% == 9.28% 9.28% == 6.96 6.96µµJ/pF/cycle J/pF/cycle == 33kV kV
** Indicate Indicatehigh highlevel levelof ofdeterioration deterioration ## Recommended Recommendedfor forrewinding rewinding ## Failed Failedafter afteraayear year 80
2). 2). 6.6kV, 6.6kV, 5.1MW 5.1MWSynchronous Synchronousmotor motor ** Class ClassB, B, 12 12years yearsold old ** Petrochemical PetrochemicalPlant Plant ∆∆TT ∆∆CC Vi Vi
== 3.38% 3.38% == 11.6% 11.6% == 2.10 2.10kV kV
**High Highvalue value
** DLA DLApattern patternindicated indicatedpresence presenceof ofend endwinding windingdischarges discharges (unstable (unstablepattern) pattern) ## Recommended Recommendedfor forrewinding rewinding ## Failed Failedafter aftertwo twomonths months
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3). 3). 6.6kV, 6.6kV,1750KW, 1750KW,Induction Inductionmotor motor ** Class Class--F, F, 11year yearold old ** Cement CementIndustry Industry IR IR tan tan δδ ∆∆TT ∆∆CC IDE IDE
== 700 Ω 700MΩ MΩ Ω == 2.81% 2.81% == 0.39% 0.39% == 1.29% 1.29% == 0.175 0.175µµJ/pF/cycle J/pF/cycle
** Due Dueto tointense intenseslot slotor orend-winding end-windingdischarges discharges ## loop looptrace tracedistorted distorted& &unstable unstable ## wavy wavyunstable unstablepattern patternappeared appearedbeyond beyond2kV 2kV **Failed Failedafter afteraaweek week 82
CONCLUSIONS • Condition monitoring tests are Non-destructive type • * Defective components can be identified • * Premature failures can be avoided • * State & condition of the equipment can be assessed • * Impending problems or deteriorating factors can be detected • Systematic diagnosis programme and periodic monitoring enable life extension
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THANK YOU
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11kV/220kV, 43.33MVA Generator Transformers (18 Nos.) GT1 Insulation section
IR
PI
Tan delta (%)
Moisture level (%)
PD Level (pC)
HV/LV+G(R)
1680
2.19
0.367
3.18
2400
LV/HV+G(R)
2100
2.07
0.357
HV/LV(R)
3580
1.76
0.362
HV/LV+G(Y)
6200
1.56
0.36
2.54
3000
LV/HV+G(Y)
9750
1.57
0.353
HV/LV(Y)
6400
1.51
0.358
HV/LV+G(B)
4580
1.81
0.338