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CORE MONITORING AND TESTING
Stator Lamination
Stator Cores ¾ Cores
provide low reluctance paths for working magnetic fluxes ¾ Support stator winding ¾ Cores must be capable of withstanding operating forces: mechanical and magnetic ¾ Provides primary heat removal from indirect cooled stator winding
Core Assembly ¾ Core
size determined by magnetic flux requirements and flux density ¾ Stator frame, core and winding are usually factory assembled ¾ Assembled stator is the heaviest generator component for shipment and lifting, up to 500 tons
Core design Stator Core Alternating Magnetic Flux Conductor Bars Stator Teeth
Building Bars
Sample Core Damage
Core Meltdown
Stator Core Tests
Stator Core Testing ¾ Core
tightness test ¾ Through Bolts Insulation ¾ Core vibration test ¾ Core loss test ¾ Rated flux test ¾ EL CID test “Evaluation of the condition of a core is a major technical challenge”
Core Tightness Testing ¾
Suspected loose areas can be confirmed by a “Knife Test”
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This involves trying to insert a knife with a 0.25 mm (10 thou) tick blade into the core bore (stator) or OD (rotor)
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If the knife penetrates more than 5.0 mm (0.2 ins) then the core is loose
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EDF “Crabe”
Knife Test
Core Loss Test ¾ Core
is excited and power absorbed measured by a wattmeter ¾ Results are expressed as loss per mass of core ¾ Should not exceed about 6-10W/kg ¾ Increase from previous test should not be more than 5%
Rated Flux Test Purpose and Theory ¾
Used to check the integrity of the interlaminar insulation
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The excitation winding must have the appropriate number of turns and a power source capable of inducing approximately 80100% of rated flux in the back of the stator core
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The heat produced by circulating currents is detected
Low Flux Stator Core testing ¾ ¾ ¾ ¾
Weak link in the core is the lamination insulation If it fails – creates hot spots and can lead to core melting 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% of normal flux, and is much easier to perform
ELCID Evolution
Operating Principle ¾ ¾ ¾ ¾
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
Fault Current
Accepted Test Methods 1) High Power Ring Flux Test - the LOOP test z
Large Power Supply Required ( up to 3 MVA)
z
Safety Concerns with High Voltage/Current
z
Expensive Thermal Sensing Equipment
2) ELectromagnetic Core Imperfection Detector - ELCID z
Low Power Requirements (1-3 kVA)
z
No Safety Concerns due to High Voltage/Current
Accepted Test Methods-ELCID
POWER SOURCE
Accepted Test Methods (LOOP) Power Cables
POWER SOURCE
Required Excitation Levels
Loop Test
EL CID
80-100%
4%
(of rated flux density)
Understanding Fault Magnitude
4% 100mA equates to 5-10°C on HFRT Test
Typical Turbo-generator EL CID Excitation System
Typical Hydro-generator EL CID Excitation System
Digital ELCID - Evolution
Method of Scanning
Reviewing Results
Interpretation of Data QUAD signal from fault within Chattock span is always opposite polarity to PHASE signal.
ELCID Signal and Thermal Response to Faults Correlation of EL CID & HFRT results 700 650 600
EL CID Signal (mA)
550 500
Correlation boundary lines
450 400 350 300 250 200 150 100 50 0 0
5
10
15
20
25
30
35
HFRT values (Deg C)
From CIGRE Questionnaire 2003
40
45
50
55
60
EL CID for Rotor Bar Testing
EL CID for Rotor Bar Testing
CIGRÉ Report 257, 2004 “There seems to be general consensus that if an EL CID test is performed and no damage is found, then the core is defect free. EL CID has gained good credibility in its ability to determine and locate the presence of faults and to verify repairs when faults are found.”
EL CID Summary z
Low Excitation Power - 4%
z
Fast, Portable - Easy to Setup
z
Low Manpower Requirements
z
Significant Reduction in Safety Hazards
z
Portability
z
Instant Interpretation of Test Results
z
Permanent Data Storage
z
Minimal Risk of Further Damage
z
Ability to Re-Test During Maintenance Cycle
Robotic Inspection Vehicle
Robotic Inspection Vehicle -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 for distance 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 Adjustable for Curvature Chattock Holders for EL CID
Adjustable for width
RIV Mounted Wedge Tightness Detector (Optional) X-Axis Distance Encoder CCTV Camera Module (Optional)
RIV Summary By running in the air gap between rotor and stator, the RIV can be used to facilitate Rotorin-Place EL CID or Wedge Tightness testing, or visual inspection using a CCTV Camera.
Product Offering EL CID
PDA/TGA/PPM/DCR
Stator Core Evaluation z z z
EL CID Evolution RIV-702 Robot Inspection Vehicle RIV-752 Video Camera
Stator Winding Evaluation z z z z
Wedge Tightness Detection WTD-501 Wedge Tightness Detector with hand-held and robotic probes z
Portable and Continuous ON-Line and OFF-Line Direct Current Ramp Test Corona Probe (PPM probe)
Shorted Rotor Turns Detection z z z
RFA II S RFA II R Flux Trac II
