core monitoring and testing - ac machines

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.



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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



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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

core monitoring and testing - ac machines

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