5-Generator Stator Testing.pdf

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Generator Stator

Testing



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Generating timely r epair soluti ons

Session Objective

• Identify Stator Tests

– Electrical

• Standard and Specialized

– Mechanical

• Standard and Specialized

• Identify Standards That Define Tests

– IEEE

– ANSI

• Periodic Test Recommendations

• Testing Logic

• Examples Of: – bad test results

– recommendation/corrective actions

– good test results



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Purpose of Stator Testing

• Identify & correct detrimentalsituations before serious/expensive

damage occurs

• Provide confidence that the windingwill not fail in-service prior to the nextoutage



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IEEE Standards

ANSI / IEEE STANDARD TITLE

95

IEEE Recommended Practice for Insulation

Testing of Large AC Rotating Machinery with

High Direct Voltage

43

IEEE Recommended Practice for Testing

Insulation Resistance of Rotating Machinery

C50.10

American National Standard General

Requirements for Synchronous Machines

56

IEEE Guide for Insulation Maintenance of

Large Alternating Current Rotating Machinery(10,000 KVA and Larger)



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Industry Standard Recommendations

Grounding time should be 4x charge time - min. 1 hour IEEE 43 - sect. 4.6.2IEEE 95 - sect. 4.7.1

Windings should be shorted during high voltage tests to protect

against surges in the event of a failure IEEE 43 - sect. 7.2.4.1

Rm = minimum 1 minute megger at 40 deg. C should be : Rm = E

+1 where Rm = megs,E = rated AC line-to-line voltage in kilo volts (KV)

For 13.8 KV machine, Rm = 13.8 + 1 = 14.8 megs should expect

10 - 100 times minimum on healthy, clean windings IEEE 43 - sect. 8.2 & 9.3 & 9.3.4

Minimum PI for class B & class F stators prior to high voltage

testing = 2.0

IEEE 43 - sect. 9.2

IEEE 95 - sect. 6.3

Machines rated above 10 MVA should have both PI & megger

values per above before performing high voltage testing IEEE 43 - Section 9.5

New stator winding hi pot = 2E & 1 KV ANSI C50.10 section 9.3.1.1

New field winding hi Pot = 10 E AC ANSI C50.10 section 9.3.1.2.1

Hi Pot voltage is held for 1 minute

ANSI C50.10 section 9.3.3

IEEE 95 - section 5.3



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• Measure with Kelvin Bridge/DLRO

• Determine if phase resistances:

– Are balanced – Reflect design / factory

readings

– Are consistent over time

Winding Resistance



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C

(LEFT)

B

(CENTER)

A

(RIGHT)

Resistance Taken @ 29OC 0.002287 0.002283 0.002284

Resistance Corrected to 25OC 0.002252 0.002248 0.002249

Factory Resistance @ 25OC 0.003120 0.003115 0.003116

Phases shou ld be balanced w ithin 1%

Winding Resistance




Winding Resistance

• Problem if: – Not balanced

• Deteriorated bolted connection

• Deteriorated braze or solder joint• Cracked/broken strands

– Significantly different from factory /historical tests

• Poor test• Global deterioration




Winding Resistance

• Recommendations:

– Search & Dissect

• Strip & inspect bolted connection(s)

• Sample strip & inspect braze/solder joints• Sample strip to expose strands

– Current & Thermo-Vision

• Not an industry standard test

• From experience

– 1000 amps/in2 of conductor cross section

– Investigate > 5o C differential




• Generally referred to as Megger test

• Per IEEE 43 & IEEE 95

• For stator windings, typically performed at either2500 VDC or 5000 VDC (megger)

• Indicates general cleanliness / dryness / condition of

insulation system

• Indicates insulation system is capable of highervoltage testing

Insulation Resistance & Polarization Index




PI - Before Drying

INSULATION RESISTANCE

0

500

1000

1500

2000

2500

0.5 1 2 3 4 5 6 7 8 9 10

MINUTES

M E G O H M S

C-Left

B-Center

A-Right




PI - After Drying

INSULATION RESISTANCE

0

1000

2000

3000

4000

5000

6000

7000

0.5 1 2 3 4 5 6 7 8 9 10

MINUTES

M E G O H M S

C-Left

B-Center

A-Right





• Suitability for service megger, per IEEE43 – @ 1 minute, > E +1 in Megohms

– Expect 10 – 100 times that for clean, dry,healthy windings

• 10 min. megger reading / 1 min. reading

• Minimum PI for high voltage testing, per

IEEE 43 & 95 – 2.0 for class B and class F insulationsystems





If megger or PI is low:

– Clean

• Solvents / rags

• Dry Ice (CO2) or corn cob blast

– Dry

• Heaters & dehumidifiers

• Current through the winding• Bake/oven




DC Leakage Test(Controlled Over Voltage)

• Somewhat over-voltage test

• Observe insulation characteristics upon

increasing voltage

• Can possibly see failure prior to

occurrence & abort test





• Apply initial DC voltage

• Increase voltage in steps

• Hold at each step for a determined period• Monitor leakage current at each step

– In a perfect world, E = I*R yields astraight line when plotted

– If current takes off exponentially, canpossibly stop test prior to insulationfailure




DC LEAKAGE

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

10 12 14 16 18 20 22 24 26 28

KV

M I C

R O A M P S

A - Left

B -Center

C -Right

DC Leakage - Bad




DC LEAKAGE

0.00

5.00

10.00

15.00

20.00

25.00

10 12 14 16 18 20 22 24 26 28 30 32

KV

M I C

R O A M P S

C - Left

B -Center

A -Right

DC Leakage - Good





• Graded method (IEEE 95 Appendix A) – Initial voltage approximately DC equivalent

of 0.3 E or 10 KVDC

– Each step increase = 20% of initial voltage – Time intervals per calculated “N” number

and IEEE 95 Table A1

– Max. Voltage typically E x 2 (2E) DC with no

conversion multiplier

Typically, 10 KVDC for < 18KV / 15 KVDC for > 18KV




High Voltage (Hi Pot) Testing

• Proof test

• DC or AC?

– DC via IEEE 4, 95 & 56, ANSI C50.10

– AC via IEEE 43 & 433 (.1 Hz.)

– DC has proven adequate for decades

– Some feel AC is “better”, but can notquantify

• DC value is 1.7 x AC value per IEEE 95

• Recommended test value per IEEE 56125% to 150% of rated line to line voltage

• 60 second duration




Power Factor/Tip Up

(Doble Test)

• IEEE 286

• Indicates voids/delamination of insulation

• Sensitive on short conductors

• Loses sensitivity as conductors are connected in

series

• Track/trend data over-time for analysis




Power Factor/Tip Up

(Doble Test)

• Measure Power Factor at low AC voltage

(25% phase to ground, below PD levels)

• Measure power factor at higher voltage(100% phase to ground)

• Calculate difference in % = % Tip Up




Power Factor/Tip Up (Doble Test)




Resistance Temperature Detection

Devices (RTD’s)

• Isolate from

instrumentation

• Measure resistance of

legs – Compare to each other

– Compare to expected

• Megger (500 volts)




RTD’s




Inner Gas Cooled Windings

Westinghouse and GE

• Tube-to-TubeResistance

• Tube-to-StrandResistance

• Tube-to-Copper 5,000Ohm Resistor Test

• Gas Flow• Core Torque – Thru

Bolts and Building Bolts




Electro Magnetic Core Imperfection

Detector (EL CID)

– Performed in lieu of Ring

Test @ 85% rated flux – Applies 4% rated flux

density

– Measure current between

laminations – High current indicates

shorted laminations




Electro Magnetic Core Imperfection Detector

(EL CID)



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Core Ring (Loop) Test

– Performed @ 80% - 95%

rated flux

– Thermovision Camera

used to detect

temperature differential

– W – during rewind 5 C,

otherwise 10 C

– GE - ~18 C



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Core Ring Test Set Up



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Core Ring Test Set Up



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Thermo-Vision Analysis



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“Hot” Spots Detected via Thermo-Vision



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Core Ring Test – Prior

to Core Restack



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Core Ring Test – After

Core Restack




Bottom of Slot Before Unstacking Core Iron




Bottom of Slot After Unstacking Core Iron




End Winding Resonance Frequency

Test (Bump Test)

• Excite end winding with calibrated,accelerometer equipped hammer

• Monitor response via accelerometer

• Investigate amplitude & frequency ofresponse

• Modify as required: – Add stiffness

– Add mass – Reduce stiffness




End Winding Resonance Test




Bump Test - Tangential Excitation






Initial Bump Test Results

Tangential Radial




After Nose Ring Modification

B T t R lt




Bump Test Results(After End Winding Stiffening)

Tangential Radial




Other Tests - Specialized

• Surge

– Performed during rewind of multi turnbars/coils

– Tests turn insulation

– Comparison test

– Not sensitive on long conductors

• Partial Discharge

– Monitors quantity & magnitude of highfrequency/partial discharges

– On-line/off-line – Indicator of winding condition

• AC Hi Pot – 60 Hz and .10 Hz



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Stator DC Testing Summary

• Phase (Winding) Resistance

• 2.5 KV or 5 KV Insulation Resistance (Megger) &

Polarization Index (PI)

• DC Leakage Test

• High-potential Test(Hipot)

• RTD Resistance & Megger

• EL CID Test

Documents

5-Generator Stator Testing.pdf