condition monitoring in power electronics€¦ · underpinning research what is condition...

Underpinning Research

Condition Monitoring in Power Electronics

Dr Li RanProfessor of Power Electronics - Systems

University of Warwick

26th November 2015


Contents

1. Introduction to condition monitoring

2. Instrumentation and signal processing

3. Failure mechanisms of power modules

4. Temperature sensitive electrical parameters

5. Condition monitoring algorithms

6. Monitoring capacitors

7. Condition based maintenance

8. Summary


What is condition monitoring?

● Detect impeding fault at an early stage.● Detection is obtained without affecting

normal operation.● The objective is to prevent sudden and

catastrophic failures & cascaded damages.● Appropriate actions are suggested based

on prognosis.

● Condition monitoring is usually appliedin systems that are safety critical orexpensive to repair after faults.

● It would be ideal to use signals that arealready available for control or protectionto constrain the cost.


What is condition monitoring?

Condition monitoring vs Protection

● Condition monitoring is to prevent something really bad from happening.- It can be passive or active;- It is usually difficult of detect the weak early signatures;- It can be slow in response taking into consideration lifetime interest.

● Protection is waiting for something really bad to happen.- It is generally passive;- It usually needs to be very fast and automatic.


Incentives of condition monitoring

Safety critical

O&M cost accountfor >30% of energyProduction cost

Loss of productionis very expensive


Tasks in Condition Monitoring

● It is necessary to understand the monitored plant to decide what signals to transduce and whatsignatures to capture.

● Signal processing depends on knowing the operating condition of the monitored plant; issues ofvariable operating condition are of concern in many systems such as wind turbines and Evs.

● The change and trending of the monitored signature should be combined with prognosis to give judgement of severity of impeding fault.

● Condition based maintenance scheduling is the traditional operational strategy, but direct stressmanagement through power electronic control is becoming attractive.

(Tavner, Ran, Penman, Sedding, Condition Monitoring of Rotating Electrical Machines, IET Press, 2008)


Arrangement of Data Acquisition

● Raw signals are conditioned and processed locally, and it is usually the high level informationabout the condition monitored that is transmitted over longer distance to central points.

● Data acquisition channels are multiplexed and computational power can be shared withother functionalities in the system.

(Tavner, Ran, Penman, Sedding, Condition Monitoring of Rotating Electrical Machines, IET Press, 2008)


Contents








8. Summary


Instrumentation

What will be measured for power electronics condition monitoring?

● I am not sure… But perhaps: current, voltage, temperature, flow rating (water/oil cooling), strain gauges.● Performance requirement: appropriate bandwidth, range, precision, accuracy, stability over time, and cost.● Some measurement points can be difficult to access, e.g. internal temperature, device switching current

and voltages. Condition monitoring based on external (w.r.t. component package) temperatures,converter level voltage and current, and/or gate driver signals may be more favourable.

● Measurement is always subject to environmental impacts.

Reference junction compensation for a thermocouple


Distributed Temperature Sensing

The Raman-scattered component in the reflected lightis sensitive to the thermally influenced molecularvibration. According to the time when the signatures inthe back scattered light are received, the temperaturealong the optic fibre can be detected.


Current Sensing

● Bandwidth: DC to over 100 MHz.● Current range: 1500 A (but expensive).● The same principle is used to measure voltage.

● Bandwidth: AC up to 10’s of MHz.● Not affected by external magnetic effects.● No saturation.


Signal Processing

● At the foundation of signal processing is Fourier series.

(Lucas Barbosa, Wikipedia)


Sampling Requirement

● ‘Seeing is believing’ Is it?

● Shannon’s sampling theorem: In order for a band-limited signal with bandwidth fB to be

reconstructed fully, it must be sampled at a rate fs≥2fB. A signal sampled at 2fB is said to be

Nyquist sampled, and 2fB is called the Nyquist frequency. No information is lost if a signal

is sampled at the Nyquist frequency, and no additional information is gained by sampling faster than this rate.

● Misunderstanding of the sampling theorem has often caused aliasing.


Prevention of Aliasing

● Sampling theorem is sometimes misunderstood as to sampling at twice thefrequency of the interested component.

● Actually the signal to be sampled must be band limited to half of the sampling frequency.


Illustration of Picket Fence Leakage

● This figure shows a frequency resolution of 0.1 Hz, corresponding to a sampling length of 10 seconds.● But the frequency components of interest may be at 59.83 Hz, 59.85 Hz and 59.87 Hz, in addition to

59.80 Hz and 59.90 Hz. Components not at Fourier output frequencies need to be estimated.● This can happen in converter-machine systems with complex control modulation processes.


Variable Operating Condition

● Wavelet transform may be used to analyse non-stationary time-domain signals.

dt

a

bttf

abaW )()(

1),( *

where a is the scaling factor and b the time-shift.

Mother wavelet could be chosen as

0a

)]2sin()2[cos()( 2/2

tjtet t

● As b changes, the analysis is focused on different parts of the time-domain signal.● As a changes, the mother wavelet is squeezed or stretched. Signatures of different

frequencies in the time-domain signal can be picked up.


Example of Wavelet Analysis

rotor resistance: 0.685 Ohm

rotor resistance: 0.35 Ohm


Example of Wavelet Analysis

signature contained in starting current

signature extraction from

starting current


Contents








8. Summary


Dominant Failure Mechanisms

solder

1. Bond wire lift off

2. Solder cracking

• both mechanisms are due to temperature excursion，mismatch of CTEs and

elevated temperature towards the end of lifeRoot cause:

(source: Semikron)


Stresses during Temperature Cycling

The copper substrate material properties were taken to be the following: E = 110 GPa; α = 17 x10-6 K-1. For the Si die, E is about 162 GPa and α is 4.1 x10-6 K-1, while the E and α values for the SiC die are 501 GPa and 4.3 x10-6 K-1.

E: Young‘s modulusα: CTE

ΔTj=50 K


Monitoring Capacitor Ageing

● A difficulty is to place the current sensor ina sandwiched capacitor arrangement.

● The differential mode EMI (conducted) wouldgo through the reservoir capacitor, and beaffected by ESR.

● Loss of capacitance (farad) may also be detected in slow switching converters.


Contents





(TSEPs)




8. Summary


TSEPs – On-state Characteristics

● Infineon 1700 V, 1000 A power module.● The static V-I characteristics depend on temperature.● Solder fatigue would increase junction temperature, and higher on-state loss.● Bond wire lift-off would further increase on-state resistance.


TSEPs– Diode Reverse Recovery

● Infineon 1700 V, 1000 A power module.


TSEPs – Switching Edges

Positive current

A

Vg+

Vg-

Vdc

+

-

P

N

t

td td

ton

toff

Vg+

t

t

t

Vg-

VAN*

VAN Vdc - Vsat

-Vd + - -

T1

T2

D2

D1

i

● Voltage discrepancy:

500 V x 50 ns x10 kHz = 0.25 V


TSEPs – Gate Drives

● Source of figures: Niu and Lorenz● Turn-on (left): The Miller gate plateau (amplitude and duration) is Tj dependent.● Turn-off (right): The time delay between the two pulses of veE is Tj dependent.


TSEPs – Gate Drives

● Source: Rodriguez, Claudio, Theilliol and Velan, 2007.● Miller gate plateau rises and shortens with Tj.● Also sensitive to electrical operating point.

● Luo, Chen, Sun, Li and He, 2014.● The delay increases with Tj.● Also sensitive to electrical

operating point.


TSEPs for SiC Devices

TSEP Advantages Disadvantages

Body diode forward

voltage (with low

current)

Good temperature

sensitivity

Easy to measure

Less susceptible to noise

Good linearity with

temperature

Doubtful application on line

On-state voltage

drop

It can be done on-line

No modification to the

module

Not linear in SiC

Threshold voltage Can be done on-line

Module modification required

Not very temperature

sensitive in SiC

Susceptible to noise

Miller capacitance

discharge time

Can be done on-line

No module modification is

required

High precision circuitry

required (ns)

Susceptible to noise

Small temperature sensitivity

Turn-off and Turn-on

times

Can be done on-line

No module modification

is required

Low temperature sensitivity

Very susceptible to noise

Complex behaviour in SiC

Gate current

Can be done on-line

No module modification

is required

Temperature sensitivity?

• Different TSEPs have been investigated for SiC power devices

• Temperature dependencies of different parameters have been investigated

• Based on initial measurements, condition monitoring strategy will be pursued


Contents





(TSEPs)




8. Summary


On-line Measurement of Vce

● Source: Aalborg University, Denmark.● D1 and D2 must have large voltage blocking capability.● D1 and D2 must have very close characteristics.● Results are dependent on load current.


Algorithms Using Converter Level

Measurements, Example 1

Inside Power Module

● External (e.g. case, ambient) temperatures are to be measured.● For water cooling, flow rate and water temperatures are to be measured.● The electrical operating point is also to be monitored.


• Aging was emulated by increasing junction-case thermal resistance by 20% of the original total.

• Junction temperature is increased by about 10 K.

• Case-above-ambient temperature is increased by 1.59 K, which is detectable.

Simulation using validated Semikron model:

Ta (◦C) Tc (◦C) Tj_d (◦C) Tj_tr (◦C) P_d (W) P_tr (W) P_tot (W)

Before aging 43.3 98.1 101.7 101.9 2.04 3.57 33.66

After aging 43.3 99.69 111.5 111.7 2.12 3.65 34.63

steady-state simulation results






• Inverter is subject to a step load demand.

• Case-above-ambient temperature shows clear distinction before and after aging.

• Thermal transient due to heat sink is slow and long, making the conception application questionable under dynamic loading.

Step-input test:

experimen

tsimulation

experimen

tsimulation




●Testing adapted algorithm under variable operating condition.




Issues examined:

Demonstration by deployment into a

commercial converter.




Positive current

A

Vg+

Vg-

Vdc

+

-

P

N

t

td td

ton

toff

Vg+

t

t

t

Vg-

VAN*

VAN Vdc - Vsat

-Vd + - -

T1

T2

D2

D1

i

7th harmonic

fundamental harmonic

5th harmonic

i

ΔV

3rd harmonic

t

t

● The non-ideal characteristics are sensitive to Tj.● The delay of the turn-off edge causes discrepancy in inverter output voltage.● The discrepancy contains harmonics which can be detected. ● This is like uncompensated on-state voltage in inverter output.




● Increase the sensitivity of signature detection in closed-loop control.● This is highly dependent on the application.


Contents





(TSEPs)




8. Summary


Monitoring Capacitors

● Source: E Wolfgang.● Capacitors are another major contributor to unreliability.● Ageing is usually accompanied by loss of capacitance and increase of resisatnce.● Measuring capacitor current in a sandwiched +Vdc/-Vdc busbar arrangement

can be difficult.



● Source: Ran, Gokani, Clare, Bradley, Christopoulos, 1998.● It may be possible to measure the HF, differential mode

EMI (conducted) across the dc busbars, which depends onthe ESR of capacitor.

● The result will be highly sensitive to output cabling, loadterminal condition, and EMI filter if present, which needsto be maintained constant.



● In converters or converter modules operating at lowswitching frequency, it may be possible to detect thechange of capacitor voltage ripple, after the capacitancehas lost by a few %.

(5 ms/div)


Contents





(TSEPs)




8. Summary


Condition Based Maintenance

● Source: Held, Nicoletti, Scacco, Poech, 1997● Schedule maintenance for: completing a critical mission, capturing the next period of high production etc.● A central issue of maintenance scheduling is lifetime modelling and stressor estimation.● Accelerated lifetime testing is conducted by device manufacturers; effect of stresses during

normal operation is hard to estimate (Huang, Mawby).


Condition Based Maintenance

● Source: Lai, Chen, Ran, Alatise, Xu, Mawby, 2015.● Low ΔTj during operation would normally not causing ageing to an

unaged power module.● However, the initially same low ΔTj will cause further ageing on an aged

module.


Summary

● Power module failures could be a result of ageing (wear out) processes. Therefore the concept ofcondition monitoring can be beneficially applied to power module.

● Provided that the power modules are used in converters according to their specifications, the ageing processes are more likely to happen to the packaging structure: e.g. solder delaminationand bond wire lift-off.

● Power module ageing would cause the module to operate at higher internal temperatures. Thereforecondition monitoring very much relies on temperature sensitive electrical parameters (TSEPs) ofthe device.

● Condition monitoring could be directly detecting the TSEPs such as Vce_on, or it could detect theconsequence of the changed electrical parameters under certain operating condition, such aspower losses and external temperatures.

● Signatures in gate drive circuitry are currently of high interest in developing condition monitoringsolutions; techniques based on converter level measurements also sound attractive.

● Condition monitoring power electronics is a rapidly developing area. Therefore I have includedsome general information about instrumentation and signal processing. This may be relevant infuture development. Sincere apologies if this is not,

Thank you and have some good time at Warwick!

condition monitoring in power electronics€¦ · underpinning research what is condition...

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