Insulation health monitoring in electric
drives based on PWM common mode
voltage and current
Integrated Drive Theme
Igor Tsyokhla
University of Sheffield
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Key Targets
Integrated Drive Theme
• Aim to achieve ambitious, aspirational targets
of:
 A power density of 25kW per litre for a drive of around
100kW.
 Halving the commercial cost for an industrial drive.
 One quarter reduction of the system losses of current
drives.
 Increase in reliability by an order of magnitude.
 Operation at an ambient of 300 degrees
Fault classification
 Insulation degradation
 Partial discharge
 Neutral/ground/bearing current
 Inter-turn short circuit
 Open-circuit failure
 Partial demagnetisation
 Bearing failure/eccentric
 Drive failure




Capacitor
Sensor failure
Devices/gate drive failures
Control failure
Capacitor failure
 All failure modes have links to temperature
3
Resistor
Inductor
Connections
Devices
Gate Drivers
Other
Insulation health degradation
Capacitance and dissipation factor
of motor ground-wall insulation
4
Insulation Test Methods
DC Resistance
C/DF Test
Partial Discharge
Pros:
Pros:
Pros:
•
Cheapest test
Cons:
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•
Poor diagnostic tool
Requires stoppage
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•
Overall indicator
Most problems detected
Runs Online
Cons:
•
•
Expensive
Requires stoppage
Attempted Method
•
•
Runs Online
Most problems detected
Cons:
•
•
Expensive
Subjective interpretation
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C/DF Test
Traditional Procedure:
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•
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Dissipation Model
High voltage injected between
Winding and Ground
Magnitude and phase measured
C and DF calculated
Dissipation can be represented as a
resistor
Health Indicators:
Thermal Degradation:
•
•
•
Organic insulation
is made out of
polymer chains
•
•
Vibration of chains dissipates energy
Thermal degradation splits and multiplies
chains
More particles, more vibration
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more dissipation
•
C increase
= moisture ingress
C and R decrease = thermal
degradation
6 Other degradation modes possible
•
Common mode (CM) voltage
and current in drives
Typical Servo Drive Connection
CM Current Path
•
•
CM voltage measured close to
the machine
CM current goes
through ground wall insulation
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Insulation Health monitoring
Common mode (CM) Leakage current measurement
High sensitivity current transformer
Common mode
leakage current
paths
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New Way to Measure C/DF
Sample C/DF
Traditional Measurement:
•
•
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Sine wave injected
Measurement made at 50Hz
@50Hz Ceq is 1.4nF
@50Hz Req is 500 MOhm !
Novel Measurement:
•
CM voltage used (below)
•
Voltage is rich in switching harmonics
•
This range of harmonics is used,
Req is lower than 1 MOhm
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Main objectives
• Continuously monitor winding ground-wall insulation and
provide earlier warning of degradation
• Establish degradation model for predicting remaining useful
lifetime (RUL) through on-line monitoring
First detectable Vibration
sign
(1-9)months
Insulation
(1-6)months
Thermal
degradation
(1-12)months
Good
Preventative
maintenance
Audible noise
Excessive heat
Few weeks
Predictive maintenance
detects problem early
Failed
Few days
Equipment fails
Corrective
maintenance
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Experimental setup
Machine:
• 3kW 3000 RPM 480V SPM
• Standard servo machine
• Small, low voltage,
random wound
Drive
• Standard commercial
drive, switching
frequency at 6kHz s
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Measurement Box
Sensors In the Test:
Main Sensors:
•
Differential Probe 25 MHz
Measures zero sequence voltage
via Artificial Neutral R Network
•
Bergoz current sensor 1MHz
Aux Optional Sensors:
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External digitally isolated 10MS/s ADC
Op amp Vabc summation
Total cost = £20
•
•
LEM Fluxgate sensor (£10)
LEM hall effect sensor (£10)
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Current (A)
Raw Data
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Initial Test Results
With Emulated Degradation
Raw Impedance Data
Emulated Degradation:
• Resistance added in parallel to
common mode path
• Different values used to test
sensitivity
Simulated Health
Progression
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Accelerated Test Methodology
Why Accelerated Aging ?
• Ambiguity of realistic Req Ceq
progression
• Need to clarify required sensitivity
• No end point criterion in for C/DF in
current standards
End Point Criterion
What information is
expected ?
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•
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Observation of parameter change in real
time
Is there a sharp change at end of life ?
Correlation between temperature and end
of life
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Aging curve from IEEE std 98
Accelerated Test Schedule
Sample in Oven:
•
Full Stator assembly tested
Schedule (days)
Sample 1
2
Oven 1
Sample 3
Sample 4
Oven 2
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Cold Test Results
Results Discussion:
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High consistency of data
Good agreement with offline
measurement for Ceq
Conclusion:
Ideal sensors show good
measurement of C/DF
parameters, online
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•
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Req trend is the same
Absolute value disagreement
Consistency remains,
Indicates calibration issue
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Degradation Progression Ceq
Ceq Plot:
• Progression of the 6kHz harmonic
shown
• Ceq decrease expected
• Hot sample shows faster
progression as expected
@6kHz
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Degradation Progression Req
Req Plot
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Correlated noise in
both ovens
Increase in Sample 1
Decrease in Sample 2
Trend clearly visible
@6kHz
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Summary and Future work
• Novel insulation health monitoring method
outlined
• Method verified with ideal sensors
• Aging experiment in progress
• Initial results shown
• Data analysis and prognostic model to be
established
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Thank you for your attention
Any question?
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Cheap Sensor Results
LEM Fluxgate and 10MS/s ADC Results:
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Capacitance shows good agreement
Dissipation shape is wrong
 Usable
 Not good enough
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