Generator Testing

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EAME/CIS TC Conference EP/Petroleum - Malaga, October 2007

Generator Insulation Testing

TC Conference

Malaga

October 2007




Agenda

• Purpose

• When to Test

• Safety

• Prepare for the Test

• The Tests

• Post Test

• References

• Other Tests




Insulation Quality

• Insulation Degrades Due To:

– Electrical Stress

– Mechanical Stress

– Chemical Attack

– Thermal Stress

– Environmental Contamination

– Time

• Can be Measured >> Maintenance Tool

– Insulation resistance as high as 55 GOhm




Generator Insulation Testing purpose

• Manage the generator maintenance

• Determine the condition of the insulation

• Identify insulation systems in need of drying

• Identify potential future or immediate repair need




When is it Necessary to Test the Generator Insulation?

• After receipt of generator

– Before storing

– Before initial startup

– This Is “The Base-Line”

• Prior to commissioning

– Don’t commission if results are questionable!

• Prior to running after long period of no use

• After routine maintenance

• Periodically

• KEEP RECORDS of all test results




Periodic Testing

• Annually in clean, controlled environment

• Quarterly in severe environments

– High humidity

– Water or snow ingestion

– Smoke

– Chemicals in air

– Salty air (such as near the ocean)

– Dusty environments (sand)

– Construction debris




Monitoring Operations

• Trending

– Understand the Monitored Process

– Normalized Readings

– Record on Chart

– Focus on Changes Occurring Over Time

• A plot of results over time

› Deterioration over time

› Abrupt changes from “normal”

» For example when moisture makes its way into thewindings

– Scheduled Repairs Instead of Unplanned Downtime

• Requires normalized test results




Safety




Insulation Testing Safety Preparations

• People

– Have multiple people present for testing

– Electrical safety training !

– Comply with all applicable safety standards

• For example IEEE510

– First aid training !

• Before testing

– Isolate the system to be tested

• Lock it out

– Barricade around the generator to keep people away

– Use personal protection devices – Dissipate stored energy before the test

• Short circuit the windings

› Use a grounding stick for high voltages




Insulation Testing Safety

• During testing

– External arcing can occur

• For example if bare conductor is too close to ground

• Stay clear of the test equipment, conductors, and item beingtested

• After testing

– Dissipate the energy after the test

• Short circuit the windings

› Use a grounding stick for high voltages

• Leave short circuited for 5x to 10x test duration




Pre-Test

• Understanding the test process

• Tester selection

• Testing voltage

• Temperature compensation

• Guard terminal




Test Process

• READ instruction manual of your instrument for detailed instructions

– Test safety is paramount

• Test Process

– Apply a Test Voltage

• Less Than or Equal to Generator Operating Voltage

• Test Current Will Flow

– Measure response




Test Current Components

• Different processes cause insulation current to flow

• Leakage Current

– High resistance in parallel with insulation capacitance

– Reaches constant value quickly

• Polarization (absorption) Current

– Electric field causes charges to align themselves to the field

– Moisture & contamination has large impact

– Duration in generators is usually minutes or more

• Capacitive Charging Current

– Initially large, goes to zero

– Duration relatively short but not insignificant




Insulation Tester Selection

• Tester models available from multiple vendors

– EAMC (Chauvin Arnoux)

– Megger

– Numerous others

– Newer models are very portable

• Key things to look for

– Adequate range

• At least to 100 GigaOhm (100,000 MegaOhm)

– Preprogrammed, automatic tests

– Test voltages

• 500 Vdc, 1,000 Vdc, 5,000 Vdc

– Calibration program

– Scheduled maintenance options




Testing Voltage

• Test at a voltage near the rated voltage

– Higher voltage

• Increases stress

• More likely to find weakness

• More expensive test as voltage goes up

• GM EMD study on their locomotive generators

– 5k Vdc is sufficient




Test Voltage Recommendations

• Main Stators

– Low voltage generators (100 v – 500 v)

• Test at 500 Vdc

– Low voltage generators (500 v – 1000 v)

• Test at 1,000 Vdc

– Medium voltage generators (2,400 v – 7,200 v)


– High voltage generators (10,000 v – 15,000 v)


• Can be tested at 10,000 Vdc

• Main rotors – Test at 500 Vdc

• Exciters (stator and rotor)

– Test at 500 Vdc




Three phase machine testing

• Preferred to test each phase separately

– Significantly different results for each phase are a concern

• Can test three phases at once




Interpreting the Results

• General result interpretation rules are included

• Result interpretations are device dependent

– Test history

• Other vital information – Previous history of the machine

– Careful visual inspection

– Comparison with like devices in like circumstances

– Log of previous testing

• Possible outcomes

– Machine is returned to service

– Schedule repairs – Clean, dry, retest

– Repair immediately, retest

– Replace machine




Temperature Compensation

• Insulation resistance measurements are temperature dependent

• Normalize to a standard temperature

– Typically either 25 °C or 40 °C

– Get exact temperature coefficient from manufacturer• Rough estimate

› Resistance is cut by 50% for 10 °C increase intemperature




Guard Terminal - Leakage Current

• Leakage currents in parallel with the insulation

– Voltage tracking

– Surface currents on exposed insulation

– Can give false low resistance readings• Guard terminal connection

– Part of instrument

– Eliminates undesired currents

– Must be isolated from other connections

• Guard against flashover

– Usually connected to ground through a low impedanceconnection

• Older instruments may connect guard to high potentialthrough a low impedance!




Flashover

• Be prepared for arcing

– Not common but does happen

– Will distort results if not prevented – Careful

• Internal arcing – Test failure

• External arcing

– Bare conductors too close together or too close to ground

– Voltage tracking on insulation?




Insulation Quality Tests

• Visual Inspection

• Insulation resistance

• Polarization Index

• Dielectric Absorption Ratio• Dielectric Discharge

• Step Voltage

• Hi-pot

• Other tests (appendix)

– Isothermal Relaxation Current (KDA test)

– EDA Test

– Power Factor Test




Visual Inspection

• Quick Look at Appearance

– Remove Terminal Box Panel

– Look at Winding End Turns Using Flashlight

– Can look at internals with boroscope• Indications

– Cleanliness

– Voltage Tracking

– Visual Damage

• Black from arcing

• Discolored steel from overheating

• Cracking from multiple causes




Basic insulation resistance

• Insulation resistance reading

– Resistance

– Temperature of insulation (windings)

• Or reference temperature if normalized – Length of time voltage applied before reading

• Document instrument used

– Calibration

• This basic test should be done regularly

– Use trend charts




Insulation resistance – Test Considerations

• Insulation resistance measurements are temperature sensitive

– Resistance measurements without the winding temperature aremeaningless!

• Use the resistance reading at same point in time – Suggest at 60 sec

• Allows for dissipation of short time constant currents




Insulation resistance – 3-Phase Test Process

• 3-Phase testing – Lock out device under test

– Discharge leads

– Measure winding temperature

– Disconnect the 3 phases from the load, PTs, ground (neutral side)

– Connect the 3 phases together

– Hook up tester – See tester instruction manual

• High side to the phase leads

• Low side to the core (laminations), steel frame, or rotor shaft

• Ground the frame

– Apply voltage

– Take reading - Typically after 60 sec – Discharge windings

• Tester may have provision for discharging

– Disconnect




Insulation resistance – 1-Phase Test Process

• 1-Phase testing – Lock out device under test

– Discharge leads

– Measure winding temperature

– Disconnect the 3 phases from the load, PTs, ground (neutral side)

– Connect the 2 phases not tested to the core, frame, or shaft

– Hook up tester – See tester instruction manual

• High side to the phase lead

• Low side to the core (laminations), steel frame, or rotor shaft

• Ground the frame

– Apply voltage

– Take reading - Typically after 60 sec – Discharge windings

• Tester may have provision for discharging

– Disconnect




Insulation Resistance - Results Interpretation

• Interpretation of resistances corrected to 25 °C (or 40 °C)

– Dry, new generator

• Very high resistance

• Typically higher than 1,000 MOhm (1 GOhm)› Can be as high as 50,000 MOhm (50 GOhm)

– Used generator

• Anywhere from a few MOhm to many GOhm

– Generator in questionable condition

• Significant decrease from readings when new

• Low MOhm readings

• Significant difference between phase readings

• Significant difference between component readings

› Main stator, main rotor, exciter stator, exciter rotor, PMGstator




Polarization Index (PI)

• Measurement

– The ratio of 10 min. reading to 1 min. reading

– Valuable test if no historical readings are available

– Sensitive to moisture in or contamination of Insulation• Indicates

– Moisture Content of the Insulation

– Good Indicator of the Drying Process

• Will not indicate the failure of an insulation layer in a multi-layerinsulation

• Does not work if there is no absorption current




PI Results Interpretation

• PI > 6

– Very good

– Usually only seen when

winding system is new• Newer designs

– 1.5 < PI < 6

• Normal range

– PI < 1.5• Significant moisture in

the windings

• Failure may occur at

any time• Recommend to clean

and dry immediately

• Older designs

– 2 < PI < 6

• Normal range

– PI < 2• Significant moisture in

the windings

• Failure may occur at

any time• Recommend to clean

and dry immediately




Dielectric Absorption Ratio (DAR)

• Similar to Polarization Index

• Time at measurements different

– One typical combination is 60 sec divide by 30 sec reading

• > 1.4 is good• Interpretation more difficult because currents may not have

stabilized

• Far less common than Polarization Index




Dielectric Discharge (DD)

• Measurement

– Insulation is Charged to Stable Condition

– Insulation Discharge Current is Measured

• Voltage & Capacitance also measured – Dielectric Discharge = I1 min / (V * C)

• Indicates

– Moisture Content

– Internal Damage to a Single Insulation Layer




DD Results Interpretation

• DD > 7

– Layer of multilayer insulation is damaged

• 4 < DD < 7

– Questionable, damage may be possible• 2 < DD < 4

– Poor

• DD < 2

– Good




Step Voltage

• Measurement

– Response to Increasing Voltage Steps

– Differences in Response Between Increasing Voltage Steps

• Indicates – Local Weak Spots

– Response Changes as Electrical Stress is Increased

• Comments

– Use trend analysis.

– Used for medium or high voltage windings




Step Voltage Results Interpretation

• Traces for subsequent voltage steps overlay each other

– Good, no layer damage

• Significant differences between traces

– One or more damaged layers




Hi-Pot Test

• NOT RECOMMENDED

– Not recommended due to high stress placed on windings

– Already Hi-Pot tested at the factory

– Wet insulation system increases likelihood of failure• Potentially destructive test!

– Pass / Fail

• Indicates condition under high stress conditions

• Test failure means repairs are mandatory! And may be expensive.

• Test

– DC Voltage, between rated and twice rated, slowly ramped up

– Applied for typically 1 min

– Turned off immediately in case of arcing (failure)




Post-Test

• Discharging

• Cleanup

• Record keeping

• Insulation drying• Insulation repairs




Discharging

• Tester may have provision for discharging the windings – Use it

• Recommend using a ground-stick for discharging

– Has built-in resistance to limit surges and voltage/current

reflections – Should be used if testing at higher voltages

• Short circuit for 5x to 10x test time




Cleanup

• Clean up the test site

• Remove the lockout only after test location is completely safe




Record Keeping

• Keep a permanent record of all testing

• Use charts to uncover trends

• Records can be used as reference for other tests

• Will help in diagnosing failures




Insulation Drying

• Follow manufacturer drying instructions

• Apply heat to raise generator temperature above ambient

• Heating insulation too fast will lead to steam formation

– Keep winding temperature below 100 °C – Steam expands Causes cracks in insulation

• Retest to verify drying success

• Drying may take days




Insulation Repairs

• Repair will be required if drying does not sufficiently raise theinsulation resistance and polarization index

• Repairs must be undertaken by qualified generator / motor repair (orrewind) shop

• Repairs

– Deep clean & drying

• Repair shop can frequently do a more thorough cleaning at

their facility – Local insulation replacement

• MAIA uses a rewind shop with true repair artists in Sardinia

– Rewind

• Be sure the rewind shop can handle the type of generator• Rewind quality is critical




Summary

• Insulation testing is a predictive maintenance tool

• Tracking results over time helps identify problems before they occur

• Good tests are available even if no history exists




References - AEMC

• AEMC Instruments Inc.

» www.aemc.com

» Chauvin Arnoux Inc

– “Understanding Insulation Resistance Testing” 02/06 rev 02 – “Dielectric Discharge (DD) Testing” 08/02

– “Poor Man’s Step Voltage Testing” 09/03

– “Megohmmeter 5050/5060 User Manual” 03/07

– “Megohmmeter 5070 User Manual” 06/07




References – Megger®

• Megger Group Ltd

» www.megger.com

» www.biddlemegger.com (distributor site with information)

– “5kV Testing. How Much Is Enough?”

• Utility Products Showcase © 2002

– “Insulation Testing Large Rotating Machines”

– “Advanced Insulation Analysis Using Megger Insulation Testers”

– “Salvaging Flood Damaged Electrical Equipment”

– “Choosing a Megger Insulation Tester”

– “The Lowdown On High-Voltage DC Testing” November 2002 Rev B

– “A Guide To Diagnostic Insulation Testing Above 1kV” June 2002

– “’A Stitch in Time…’ The Complete Guide to Electrical Insulation

Testing” June 1992 – “MIT1020 10kV Digital Insulation Tester User Manual”




References - Standards

• IEEE 43

• IEEE 510




Other Tests

• Not commonly performed in the field

• Power factor test

– Also Tan Delta Test

– Used on generators, generally not in the field• EDA

– Advanced instruments

• Isothermal relaxation current

– Used on cables

P F T




Power Factor Test

• Also Known as Tan Delta Test

• Measurement

– Changes in Power Factor or Loss Angle

– Bulky Test Equipment

• Difficult for Field Testing

• Indicates – Small Changes in Insulation Condition

EDA T t




EDA Test

• Measurement

– Combines Insulation Resistance, Polarization Index, andDielectric Discharge Tests

• Indicates

– Same as Above Mentioned Tests

– Software is Used to Calculate Additional Indicators

I th l R l ti C t




Isothermal Relaxation Current

• KDA test

• Applied to Cables

– Use with Generators is Unknown

• Measurement – Measures Discharge Current

– Using Modeling, Standard Discharge Shapes Are Curve Fit toDetermine Time Constants

• Indicates – Time Constants Correlate to Cable Insulation Aging

Documents

Generator Testing