Generator Testing

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

Generator Insulation Testing purpose

• Manage the generator maintenance

• Determine the condition of the insulation • Identify insulation systems in need of drying

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

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)

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 the windings

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

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 being

tested • After testing

– Dissipate the energy after the test

• Short circuit the windings

› Use a grounding stick for high voltages

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

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

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

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

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)

• Test at 5,000 Vdc

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

• Test at 5,000 Vdc

• Can be tested at 10,000 Vdc

• Main rotors

– Test at 500 Vdc

• Exciters (stator and rotor)

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 in temperature

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 impedance connection

• Older instruments may connect guard to high potential

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

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

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 are meaningless!

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

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

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

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, PMG stator

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-layer insulation

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

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

Dielectric Discharge (DD)

• Measurement

– Insulation is Charged to Stable Condition – Insulation Discharge Current is Measured

• Voltage & Capacitance also measured

– Dielectric Discharge = I_{1 min} / (V * C) • Indicates

– Moisture Content

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

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.

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

Post-Test

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

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 the insulation resistance and polarization index

• Repairs must be undertaken by qualified generator / motor repair (or rewind) 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

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

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

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

EDA Test

• Measurement

– Combines Insulation Resistance, Polarization Index, and Dielectric Discharge Tests

• Indicates

– Same as Above Mentioned Tests

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 to Determine Time Constants

• Indicates