Generator Testing

Generator Insulation Testing TC Conference Malaga October 2007

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

Agenda

• • • • • • • •

Purpose When to Test Safety Prepare for the Test The Tests Post Test References Other Tests

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

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

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

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

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

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

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

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

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

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 – Scheduled Repairs Instead of Unplanned Downtime • Requires normalized test results

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

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

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

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

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

Pre-Test • • • • •

Understanding the test process Tester selection Testing voltage Temperature compensation Guard terminal

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

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

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

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

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

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

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

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

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

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) – Test at 500 Vdc EAME/CIS TC Conference EP/Petroleum - Malaga, October 2007

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

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

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

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

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

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 through a low impedance!

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

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?

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

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

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

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

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

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

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

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 • Allows for dissipation of short time constant currents

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

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

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

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

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 • Does not work if there is no absorption current

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

Post-Test • • • • •

Discharging Cleanup Record keeping Insulation drying Insulation repairs

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

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

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

Cleanup • Clean up the test site • Remove the lockout only after test location is completely safe

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

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

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

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

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

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 • Rewind quality is critical

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

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

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

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

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

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”

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

References - Standards • IEEE 43 • IEEE 510

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

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

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

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

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

EDA Test

• Measurement – Combines Insulation Resistance, Polarization Index, and Dielectric Discharge Tests • Indicates – Same as Above Mentioned Tests – Software is Used to Calculate Additional Indicators

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

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 – Time Constants Correlate to Cable Insulation Aging

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