Transcript The Case for Over-Voltage PdM Testing

Electric Power Utility Motor Case Study
 Diagnostics & Resolution
 Power utility contracted to perform off-line circuit and insulation testing of
critical motors during major planned outage. Tests included MCA, IR &
overvoltage hipot and impulse.
 New 4 kV motors are impulse tested to 11.4 kVpk (IEEE). Recommended
maintenance test is 9 kVpk. Impulse test discovered a 4 kV pump motor with
phase insulation breakdown voltage of 7.2 kVpk. No indication of problem
during MCA, IR and hipot tests.
 Utility opted to start the motor repeatedly to study failure of weak motor
insulation system. Insulation failed catastrophically on fifth startup.
 Motor was rewound during planned shutdown
 Cost of Diagnostics to Utility: <$10,000 including travel
 Savings to Utility
 No lost production: Potentially $100Ks
 No emergency repair costs: $10Ks
Common Off Line Electrical PdM Motor Tests
Under Voltage
{
Over Voltage
{
TEST
DESCRIPTION
EFFECTIV
ENESS
PRECAUTIONS/CONSIDERATION
S
STANDARD
Winding Resistance
DC winding resistance between phases,
compute phase balance
Trendable
Temp correction required, under voltage
IEEE 112 &
118
Winding Inductance &
Impedance
AC winding inductance between phases,
compute phase balance
Trendable
Temp correction required, effected by
rotor position, under voltage
IEEE 388 &
389
Capacitance
AC test measures capacitance winding to
ground
Trendable
Must obtain a baseline prior to surface
contamination or overheating
IEEE 286
Insulation Resistance
Measures IR winding to ground
Pass/Fail &
Trendable
Temp correction required, under or over
voltage
IEEE 43 &
others
Polarization Index/
Dielectric Absorption
Measures IR winding to ground to verify
polarizing effect
Pass/Fail &
Trendable
Temp correction required, under voltage
IEEE 43 & 95
Dissipation Factor (Tan
Δ) Power Factor
AC winding to ground, measures ratio of
power loss to total power
Pass/Fail &
Trendable
V ≤ Line voltage
IEEE 286
AC HIPOT
Line frequency over voltage winding to ground
Pass/Fail
Destructive, used primarily by
manufacturers
NEMA MG 1
IEEE 95 & 112
DC HIPOT
DC over voltage winding to ground, measures
leakage current & IR
Pass/Fail &
Trendable
Undertake after passing IR & PI, use
ramped or stepped voltage method
IEEE 95 EASA
VLF HIPOT
Low frequency (0.1 Hz) over voltage winding
to ground
Pass/Fail
Less destructive than ACH
New Std
Surge/Impulse
Over voltage impulse applied to each phase,
detects turn & phase insulation weaknesses
Pass/Fail
Undertake after passing IR, PI and Hipot
IEEE 522
EASA
Partial Discharge
PD Inception Voltage during ACH or surge
tests
Pass/Fail &
Trendable
Undertake after passing IR & PI
IEEE 522 &
1434
Core Loss
High current AC test for shorted stator core
laminations
Pass/Fail
Typically done only during rewind
IEEE 62
Electric Power Utility Motor Case Study
 Diagnostics & Resolution
 Power utility contracted to perform off-line circuit and insulation testing of
critical motors during major planned outage. Tests included MCA, IR &
overvoltage hipot and impulse.
 New 4 kV motors are impulse tested to 11.4 kVpk (IEEE). Recommended
maintenance test is 9 kVpk. Impulse test discovered a 4 kV pump motor with
phase insulation breakdown voltage of 7.2 kVpk. No indication of problem
during MCA, IR and hipot tests.
 Utility opted to start the motor repeatedly to study failure of weak motor
insulation system. Insulation failed catastrophically on fifth startup.
 Motor was rewound during planned shutdown
 Cost of Diagnostics to Utility: <$10,000 including travel
 Savings to Utility
 No lost production: Potentially $100Ks
 No emergency repair costs: $10Ks
High Voltage Surge or Impulse
• IEEE 522 & EASA
• Simulates the transients on power lines
which can cause insulation failure. Rise
time = 0.1 µsec
• Impulse test of turn and phase insulation
where test voltage is incrementally
increased in a stepped mode from 0V
voltage to target test voltage looking for
change in response frequency
• Final impulse test voltage typically at 2 x
line voltage + 1000 V (peak)
• Only test capable of complete predictive
testing of turn and phase insulation
Steps of Degrading Insulation Electrical Failure
1. New insulation has high break down voltage
2. Motor insulation experiences aging




Thermal aging
Chemical or moisture contamination
Mechanical wear
Corona/Partial Discharge
3. Turn, phase or ground insulation break down
voltage degrades resulting in arcing due to
transients, start up coil movement, etc
Steps of Degrading Insulation Electrical Failure
4. Induced arcs cause insulation to
deteriorate much faster
5. Turn to turn or ground insulation
breakdown voltage drops below operating
voltage effectively causing a short
between turns or a short to ground.
•
Shorted turns transformer action causes a large induced
current in the short which creates a hot spot
6. Heat causes rapid insulation failure
(typically minutes to hours)
Breakdown Voltage Degradation v. Time
Electric Power Utility Motor Case Study
 Diagnostics & Resolution
 Power utility contracted to perform off-line circuit and insulation testing of
critical motors during major planned outage. Tests included MCA, IR &
overvoltage hipot and impulse.
 New 4 kV motors are impulse tested to 11.4 kVpk (IEEE). Recommended
maintenance test is 9 kVpk. Impulse test discovered a 4 kV pump motor with
phase insulation breakdown voltage of 7.2 kVpk. No indication of problem
during MCA, IR and hipot tests.
 Utility opted to start the motor repeatedly to study failure of weak motor
insulation system. Insulation failed catastrophically on fifth startup.
 Motor was rewound during planned shutdown
 Cost of Diagnostics to Utility: <$10,000 including travel
 Savings to Utility
 No lost production: Potentially $100Ks
 No emergency repair costs: $10Ks
Contact Info:
Torq Engineering
Randy Keener
cell: 970-412-9337
[email protected]
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