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8/18/2019 Core monitoring and testing - AC machines
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CORE MONITORING ANDTESTING
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Stator Lamination
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Stator CoresCores provide low reluctance paths for
working magnetic fluxesSupport stator winding
Cores must be capable of withstandingoperating forces: mechanical andmagnetic
Provides primary heat removal fromindirect cooled stator winding
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Core AssemblyCore size determined by magnetic flux
requirements and flux density
Stator frame, core and winding are usually
factory assembled Assembled stator is the heaviest generator
component for shipment and lifting, up to
500 tons
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Stator Core
Alternating
Magnetic Flux
Conductor Bars
Stator Teeth
Building Bars
Core design
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Sample Core Damage
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Core Meltdown
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Stator Core Tests
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Stator Core TestingCore tightness test
Through Bolts Insulation
Core vibration test
Core loss testRated flux test
EL CID test
“Evaluation of the condition of a core is a
major technical challenge”
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Core Tightness Testing Suspected loose areas can be confirmed by a
“Knife Test”
This involves trying to insert a knife with a0.25 mm (10 thou) tick blade into the corebore (stator) or OD (rotor)
If the knife penetrates more than 5.0 mm (0.2ins) then the core is loose
EDF “Crabe”
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Knife Test
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Core Loss Test
Core is excited and power absorbed
measured by a wattmeter
Results are expressed as loss per mass of
coreShould not exceed about 6-10W/kg
Increase from previous test should not be
more than 5%
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Rated Flux TestPurpose and Theory
Used to check the integrity of the interlaminar insulation
The excitation winding must have theappropriate number of turns and a powersource capable of inducing approximately 80-100% of rated flux in the back of the stator
core
The heat produced by circulating currents isdetected
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Low Flux Stator Core testing Weak link in the core is the lamination insulation
If it fails – creates hot spots and can lead to coremelting and stator winding failure
Traditional test is the full flux test – problems
detected by core heating at rated magnetic flux In late 1970s the CEGB invented the
electromagnetic core imperfection detection(ELCID) test which excites core to only 4% ofnormal flux, and is much easier to perform
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ELCID Evolution
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Operating Principle
Any imperfections in the core produce fault
currents Sense head coil (Chattock) detects fault
current
ELCID processor measures & displays results Each 100 mA of detected fault current (at 4%
flux), corresponds to about a 5C temperature
rise on the full flux test
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Fault Current
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Large Power Supply Required ( up to 3 MVA)
Safety Concerns with High Voltage/Current
Expensive Thermal Sensing Equipment
1) High Power Ring Flux Test - the LOOP test
2) ELectromagnetic Core Imperfection Detector - ELCID
Low Power Requirements (1-3 kVA)
No Safety Concerns due to High Voltage/Current
Accepted Test Methods
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Accepted Test Methods-ELCID
POWER
SOURCE
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Accepted Test Methods (LOOP)
POWER
SOURCE
Power
Cables
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Required Excitation Levels
Loop Test
80-100%EL CID
4%(of rated flux density)
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Understanding Fault Magnitude
4% 100mAequates to 5-10°C
on HFRT Test
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Typical Turbo-generator
EL CID Excitation System
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Typical Hydro-generator
EL CID Excitation System
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Digital ELCID - Evolution
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Method of Scanning
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Reviewing Results
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Interpretation of Data
QUAD signalfrom fault withinChattock span isalways opposite
polarity toPHASE signal.
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ELCID Signal and Thermal Response to Faults
Correlation of EL CID & HFRT results
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
0 5 10 15 20 25 30 35 40 45 50 55 60
HFRT values (Deg C)
EL CIDSignal (mA)
Correlation boundary lines
From CIGRE Questionnaire 2003
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EL CID for Rotor Bar Testing
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EL CID for Rotor Bar Testing
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CIGRÉ Report 257, 2004
“ There seems to be general consensus that ifan EL CID test is performed and no damage is
found, then the core is defect free. EL CID has
gained good credibili ty in its ability todetermine and locate the presence of faults
and to verify repairs when faults are found.”
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EL CID Summary
Low Excitation Power - 4%
Fast, Portable - Easy to Setup
Low Manpower Requirements
Significant Reduction in Safety Hazards
Portability
Instant Interpretation of Test Results
Permanent Data Storage
Minimal Risk of Further Damage
Ability to Re-Test During Maintenance Cycle
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Robotic Inspection Vehicle
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-Speeds of 2, 4 or 6 meters per minute, forward & reverse.
-Can be used on slots from 65mm wide to virtually any
width.
-Has automatic guidance system and optical encoder fordistance recording.
-Magnetically self supporting on stator surface.
-Adjustable for machine curvature.
-Can be used on some machines for Rotor-in-situ testing.
Robotic Inspection Vehicle
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CCTV Camera Module (Optional)
RIV Mounted Wedge Tightness Detector (Optional)
X-Axis Distance Encoder
Chattock Holders
for EL CID
Adjustable for width
Adjustable for
Curvature
Robotic Inspection Vehicle
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RIV SummaryBy running in the air
gap between rotor and
stator, the RIV can be
used to facilitate Rotor-
in-Place EL CID or
Wedge Tightnesstesting, or visual
inspection using a
CCTV Camera.
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Product Offering
EL CIDStator Core Evaluation
EL CID Evolution
RIV-702 Robot Inspection Vehicle
RIV-752 Video Camera
PDA/TGA/PPM/DCRStator Winding Evaluation
Portable and Continuous
ON-Line and OFF-Line
Direct Current Ramp Test Corona Probe (PPM probe)
Wedge Tightness Detection
WTD-501 Wedge Tightness
Detector with hand-held and
robotic probes
Shorted Rotor Turns Detection
RFA II S
RFA II R
Flux Trac II