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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• 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

Integrated Drive Theme

Key Targets

Fault classification

Insulation degradation Partial discharge Neutral/ground/bearing current Inter-turn short circuit

Open-circuit failure Partial demagnetisation Bearing failure/eccentric Drive failure

Sensor failure Devices/gate drive failures Control failure Capacitor failure

All failure modes have links to temperature

Capacitor

Resistor

Inductor

Connections

Devices

Gate Drivers

Other

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Insulation health degradation

Capacitance and dissipation factor

of motor ground-wall insulation

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DC Resistance Pros:

• Cheapest test

Cons:

• Poor diagnostic tool

• Requires stoppage

Partial Discharge Pros:

• Runs Online

• Most problems detected

Cons:

• Expensive

• Subjective interpretation

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Insulation Test Methods

C/DF Test Pros:

• Overall indicator

• Most problems detected

• Runs Online

Cons:

• Expensive

• Requires stoppage

Attempted Method

Traditional Procedure: • High voltage injected between

Winding and Ground

• Magnitude and phase measured

• C and DF calculated

• Dissipation can be represented as a resistor

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C/DF Test

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

more dissipation

Dissipation Model

Health Indicators: • C increase = moisture ingress

• C and R decrease = thermal degradation

• 6 Other degradation modes possible

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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

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

Traditional Measurement: • 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

Sample C/DF

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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 sign

Good

Failed

Vibration (1-9)months

Insulation (1-6)months

Thermal degradation

(1-12)months Preventative maintenance

Predictive maintenance detects problem early

Equipment fails

Corrective maintenance

Audible noise

Excessive heat

Few weeks Few days

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Machine: • 3kW 3000 RPM 480V SPM • Standard servo machine • Small, low voltage,

random wound Drive • Standard commercial

drive, switching frequency at 6kHz s

Experimental setup

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:

• 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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Measurement Box

Raw Data

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Cu

rren

t (A

)

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Initial Test Results With Emulated Degradation

Simulated Health Progression

Raw Impedance Data Emulated Degradation: • Resistance added in parallel to

common mode path

• Different values used to test sensitivity

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

What information is expected ? • 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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Accelerated Test Methodology

End Point Criterion

Aging curve from IEEE std 98

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Schedule (days)

Oven 1

Oven 2

Sample 1

2 Sample 3 Sample 4

Sample in Oven: • Full Stator assembly tested

Accelerated Test Schedule

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Cold Test Results

Results Discussion: • High consistency of data

• Good agreement with offline measurement for Ceq

• Req trend is the same

• Absolute value disagreement

• Consistency remains, Indicates calibration issue

Conclusion: Ideal sensors show good measurement of C/DF parameters, online

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Ceq Plot: • Progression of the 6kHz harmonic

shown • Ceq decrease expected • Hot sample shows faster

progression as expected

Degradation Progression Ceq

@6kHz

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Degradation Progression Req

Req Plot • Correlated noise in

both ovens • Increase in Sample 1 • Decrease in Sample 2 • Trend clearly visible

@6kHz

• 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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Summary and Future work

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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: • Capacitance shows good agreement Usable • Dissipation shape is wrong Not good enough