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Failure Modes in High Voltage Systems for Automotive and Aerospace: An Overview of Partial Discharge Electronics Reliability Webinar

Dr Adam Lewisadam.lewis@npl.co.uk 09/02/2021

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Bob Willis Online Webinarswww.bobwillis.co.uk

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Effect of high voltage (up to 1000V) on Dendrite Failures and Conductive Anodic Filament (CAF) Failures of Electronic CircuitsTuesday 13th April 2021

Ling ZouNP

Book webinar online at https://register.gotowebinar.com/register/7709096448651536652

Failure Modes in High Voltage Systems for Automotive and Aerospace: An Overview of Partial Discharge Electronics Reliability Webinar

Dr Adam Lewisadam.lewis@npl.co.uk 09/02/2021

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Outline

Introduction to Electronic and Magnetic Materials Research at NPL

Motivation for Understanding Partial Discharge

Overview of Partial Discharge Damage Mechanisms

Review of Existing Standards

Antenna Characterisation Project and Effect of Air Pressure

Ongoing Partial Discharge Measurements at NPL

Conclusions5

INTRODUCTION TO ELECTRONIC AND MAGNETIC MATERIALS RESEARCH AT NPL

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The UK’s national standards laboratory

• Founded in 1900

• World leading National Measurement Institute

• 600+ specialists in Measurement Science

• State-of-the-art standards facilities

• 360+ laboratories

• The heart of the UK’s National MeasurementSystem to support business and society

World leadingmeasurement

science building

36,000 m2

nationallaboratory

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About NPL …

Electronics and Magnetic Materials Group

SIR and condensationtesting

CAF

PCB Reliability

PCB Delamination

TinWhiskers

Printed electronics

InterconnectReliability

Smart Textiles

Conformal Coatings

High Temp.Interconnects & Substrates

WEEE

Printed Sensors

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How we work: Bespoke 3rd party research and measurement

• Mission extension consultancy and testing• Sn whisker behaviour• Replacement alloy testing• SIR & CAF testing• Condensation performance of components

National Measurement System – to develop capability –in collaboration with industry• Metrology for high temperature electronics• Coatings for harsh environments• High voltage SIR and CAF testing• Reliability of embedded components• Power cycling in low vacuum

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How we work: Collaborative research

Multi-partner projects• Sn whisker mitigation• Condensation testing

UKRI (Innovate UK)• High temperature interconnect materials• Protective coatings to operate at elevated

temperatures• Cost effective high temperature substrates

Test services• Surface Insulation Resistance• CAF• Condensation 10

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Thermal shock• -70 °C to 300 °C, ramp rate: ~40 °C/min• programmable dwells • rates of change of temperature of ~40 °C/min• typically used for fatigue life testing of assemblies

Thermal cycling • -65 °C to 200 °C, ramp rate: ~10 °C/min• typically used for fatigue life testing of assemblies

Humidity bias testing • 5% to 98% relative humidity• -20 °C to 100 °C electrical bias up to 1000 V • typically used for electrochemical reliability testing

Condensation testing• Controlled condensation using custom platen to chill samples

below dew point of humidity chamber

Thermal ageing• up to 300 °C with in-situ monitoring

Power cycling• multi-channel switching and monitoring of power devices

using cooling platens to increase cycle time • including low vacuum capability

Mechanical shock testing • accurate control of acceleration rate and shock pulse width• hot mechanical shock testing – up to 250 °C

Shear testing of component attachments• degree of crack propagation and damage to solder joints or

die attach, strength of the joint, comparison of alloys, post thermal fatigue

• typically used for testing joint formation with new platings, fluxes or finishes

• hot shear testing – up to 300 °C

Pull testing• temperature controlled - up to 300 °C

Solder joint reliability testing• low cycle fatigue, driven by the mismatch of the coefficients

of thermal expansion, solder joint cracking, adhesive loss of glob tops and underfills, delamination in multilayer boards and modules

Reflow soldering• for sample build and simulated manufacturing conditioning

Conditioning, Ageing, Stressing

Monitoring(before, during & after)

Analysis

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Constant continuity monitoring• in-chamber monitoring of electrical continuity: up to 900 channels • simultaneous measurement for interrupts of >100 µs.

Electrical monitoring• in-chamber monitoring using programmable multichannel switching systems for resistance and capacitance

Surface insulation resistance (SIR) testing• in-chamber leakage current detection (~pA), resistance (up to 1013 Ohm)• continuous monitoring over extended test periods (1500+ hours)

Conductive anodic filament (CAF) testing• in-chamber monitoring of printed circuit boards for leakage current due to conductive salts forming

Solvent extract conductivity• using isopropanol and water to remove soluble contaminants• provide a measure of the ionic contamination

Solderability testing• performed on solder pads or component terminations

Tin whisker propensity• electrical monitoring of tin whisker growth• measuring wetting force and time

Adhesion testing• coating adhesion measurement using pneumatic or customised pull testing

Conditioning, Ageing, Stressing

Monitoring(before, during & after)

Analysis

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Micro-sectioning• high-quality polished micrographs for various investigations,

including crack detection and grain structure analysis

Optical microscopy and image analysis• a variety of high resolution optical imaging and measurements

systems

Scanning electron microscopy and focussed ion beam

• for analysis of surfaces with complex topography with a magnification of over x100,000

• used to locate tin whiskers, examine intermetallic layers, investigate crack failure modes and general failure analysis

Scanning acoustic microscopy• uses ultrasonic waves reflecting or transmitting at material

interfaces to study buried solid interfaces of dissimilar materials and non-destructive detection of features such as bonded interfaces, delaminated interface, voids and cracks

X-ray florescence• used to determine the atomic content of material, screening

for ROHS compliance, thickness measurements and validation of solder content

Energy dispersive X-ray spectra• electron microscopes for image and elemental analysis

X-ray radiography• nano-focus X-ray inspection

Fourier-transform infrared spectroscopy with microscope capability

• used to identify organic and polymeric materials, using infrared light to scan test samples and observe chemical properties

Electrochemical impedance spectroscopy• used for the characterisation of electrochemical systems and

to investigate mechanisms in electro-deposition, electro-dissolution, corrosion studies and the study of biosensors

Surface profiling• 3D micro coordinate and surface roughness using contact

and non-contact methods

Surface energy• drop shape analyser for measuring surface free energy

Electrokinetic analyser• automated surface zeta potential analysis of solids

Thermal analysis• differential scanning calorimetry (DSC) - rapid technique that

measures the heat flow associated with material transitions as a function of temp. and time

• dynamic mechanical analysis (DMA) - a versatile technique used for characterising time, temp. and frequency dependent mechanical behaviour

• thermo mechanical analysis (TMA) - used for measuring dimensional changes in a material as a function of time, temperature and the applied force

Conditioning, Ageing, Stressing

Monitoring(before, during & after)

Analysis

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Motivation for Understanding Partial Discharge

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The Push for Higher Voltages

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Push for Higher Voltages

Environmental push: Reducing CO2emissions, clean air, green energy

Industry/consumer pull: e.g. automotive, aerospace, …

Infrastructure:• Charging stations

Technologies:• Power electronics, compound

semiconductors (SiC, GaN, …)• Battery technology

Electric Vehicles in the News

16https://www.theguardian.com/environment/2021/jan/19/electric-car-batteries-race-ahead-with-five-minute-charging-timeshttps://www.bbc.co.uk/news/business-55728337https://www.theguardian.com/environment/2021/jan/22/electric-vehicles-close-to-tipping-point-of-mass-adoptionhttps://www.ft.com/content/8e69d4da-00d2-4ada-96f0-b5dc5c7ca40a

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OVERVIEW OF PARTIAL DISCHARGE DAMAGE MECHANISMS

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What is a Full Electrical Discharge?

Flow of electrical current requires charge carriers

All materials made up of charged particles• Conductors: high concentration of charge carriers• Insulators: negative charges (orbital electrons) tightly bound to atomic

nuclei

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MetalConduction Electrons

Plasma | ElectrolytesIons

ElectricalBreakdown

DielectricBreakdown

DielectricStrength

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What is a Full Electrical Discharge?

In solids electrical breakdown occurs when a strong electric field pulls outer valence electrons from their atoms, thereby making them mobile.

Breakdown field strength• Air ~3 MV m-1*

• PTFE ~30 MV m-1*

19Conductor –

Air

Conductor +

Conductor –PTFE

Conductor +

Example:300 V

Min. distance, d:Air: 100 µmPTFE: 10 µm

*Note: these are typical values used to illustrate an example

d d

Electrical Discharge Damage

Arcs are high temperature – this can damage solid insulation materials• Short burst of current• Creation of hollow channels

• Break through insulation layer

• Charring of organic dielectrics

Dielectric breakdown strength testing• Raise the voltage across a sample until

breakdown occurs

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What is Partial Discharge?

An electrical discharge (or spark) which occurs within a section of the insulation between two conductors.

PD can occur at any point within the insulation where the electric-field exceeds the local dielectric breakdown strength.

PD occurs multiple times and can gradually reduce dielectric breakdown strength

Solids: it can occur within defects within the insulation or across the insulation surface.

Liquids and gases: it can occur across gas bubbles.21

Factors that Affect Partial Discharge?

22E. Sili, J. P. Cambronne, N. Naude, and R. Khazaka, ‘Polyimide lifetime under partial discharge aging: effects of temperature, pressure and humidity’, IEEE Transactions on Dielectrics and Electrical Insulation, vol. 20, no. 2, pp. 435–442, Apr. 2013, doi: 10.1109/TDEI.2013.6508745.

Temperature

Humidity

Moisture Content

Contamination

Voltage waveform

Pressure

Complex interactions between factors

Drawing independent relationship between PD and factor not simple

Typically:• Increasing temperature increases

PD damage

• Increasing dV/dt (frequency and/or amplitude) increases PD damage

• Reducing pressure increases PD damage

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Waveform

PD occurs where the rate of change of voltage is highest

Gradual damage mechanism:PD damage can build up over time, reducing insulation breakdown voltage23

Voltage Time

Regions where PD occursPush to use high frequencies, this results in higher number of opportunities for PD damage to occur

Square waves: very sharp dV/dt

Paschen’s Law

Breakdown voltage is a function of the product of gas pressure and distance between electrodes.

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��� = � � × � Conductor –

Conductor +

gap, dgas pressure, p

� × �

���

Paschen’s minimum

Low pressure few collisions high e-field required to increase

probability of ionization

High e-field high voltage

High pressure short mean free path high e-field required to reach ionisation

High e-field high voltage

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REVIEW OF EXISTING STANDARDS

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Standards

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Standards BS EN 60270:2001+A1:2016, IEC

60270:2000+A1:2015 High-voltage test techniques –partial discharge measurements

IEC 61934:2011 Electrical insulating materials and systems – electrical measurement of partial discharges (PD) under short rise time and repetitive voltage impulses

ASTM D1868-20 Test Method for Detection and Measurement of Partial Discharge (Corona) Pulses in Evaluation of Insulation Systems

IEC 60034-27 Rotating electrical machines

Samples: as manufactured (unaged)

Standards give clear guidance on the test method and important considerations• Inception voltage: minimum impulse voltage

at which PD pulses occur• Extinction voltage: maximum impulse voltage

at which PD pulses do not occur• Electrical methods: current transformers,

wide and narrow band PD instruments, oscilloscopes (coupling capacitors), and antenna systems.

• Non-electrical: acoustic, optical, chemical.

Opportunity for expanding guidance for:• Long term ageing (lifetime) tests • Evaluating effect of factors affecting PD

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ANTENNA CHARACTERISATION PROJECT AND EFFECT OF AIR PRESSURE

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SamplesCircular Monopoles

A4I Project in Collaboration with AerospaceHV

Project aim: Evaluate range of antennas for detecting PD at ambient and lower pressures.

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COTS PCB ShortLong Twisted Insulated Wire Pair

https://aerospacehv.comC. Zachariades, R. Shuttleworth, R. Giussani, and T. Loh, ‘A Wideband Spiral UHF Coupler With Tuning Nodules for Partial Discharge Detection’, IEEE Transactions on Power Delivery, vol. 34, no. 4, pp. 1300–1308, Aug. 2019, doi: 10.1109/TPWRD.2018.2883828.

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Initial Antenna CharacterisationResponses in Anechoic Chamber

Anechoic chamber

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-50

-40

-30

-20

-10

0

500 1000 1500 2000 2500

Long PEEK monopoleShort PEEK monopole

MHz

dBi

Response from Monopoles

Note: whilst the PD response is in the GHz region, the switching frequency is significantly lower

Initial Antenna CharacterisationResponses in Anechoic Chamber

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-50

-40

-30

-20

-10

0

10

500 1000 1500 2000 2500

RHCPLHCP

MHz

dBiC

-50

-40

-30

-20

-10

0

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500 1000 1500 2000 2500

RHCPLHCP

MHz

dBiC

Response from COTS Response from PCB

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Test Setup

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Sample: Twisted insulated wire pair

Primary coil voltage: 30 VRMSSecondary coil voltage: ~652 VRMSFrequency 50 Hz

Pressure: Atmospheric to 0.3 bar

All measurements made with glass lid in place

Measurement equipment:

Oscilloscope: LeCroy 6100 A Current transformer: HVPD HFCT75 to confirm PD

Example of PD WaveformsPCB Antenna – Video

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Early experimental data showed all antenna systems able to detect PD, PCB antenna give largest max, but also largest SD

PCBCOTS

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Analysing the PD Waveforms in the Frequency Domain | PCB Antenna

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0.3 bar (cruising altitude)

1 bar (sea-level)

Fast Fourier Transform (FFT) applied to PD waveforms Each graph shows responses from 200 waveforms

Analysing the PD Waveforms in the Frequency Domain | Short Monopole Antenna

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0.3 bar (cruising altitude)

1 bar (sea-level)

Fast Fourier Transform (FFT) applied to PD waveforms Each graph shows responses from 200 waveforms

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Analysing the PD Waveforms in the Frequency Domain | Long Monopole Antenna

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0.3 bar (cruising altitude)

1 bar (sea-level)

Fast Fourier Transform (FFT) applied to PD waveforms Each graph shows responses from 200 waveforms

Analysing the PD Waveforms in the Frequency Domain | COTS Antenna

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0.3 bar (cruising altitude)

1 bar (sea-level)

Fast Fourier Transform (FFT) applied to PD waveforms Each graph shows responses from 200 waveforms

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Data Analysis

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Summary of Antenna Tests

With our measurement method (a high resolution oscilloscope) we were able to detect PD with all antenna systems.

PCB antenna gave largest signal, closely followed by COTS – however the monopoles have the benefit of simplicity, size, cost and very robust.

Measurements identified regions of interest in frequency spectrum.

Data collected shows PD is occurring but the following are not know:• PD location(s),• Extent of damage.

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ONGOING PARTIAL DISCHARGE MEASUREMENTS AT NPL

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Ongoing Partial Discharge Project

Need for robust, reliable datasets

Supporting development of test methods

Designing test vehicles and methods for failure analysis

• Evaluation/comparison:• PD resistant materials• Coating systems

• Validation of design

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Partial Discharge Multichannel Testing System

HV Switching System

Measurement:(e.g. insulation

resistance)

Sample#1

Sample #2

Sample#3

Sample#4

Conditioning:HV Source

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Pressure Hum

idity

TemperatureW

avef

orm

Moi

stur

e co

nten

t Contam

ination

Development of HV switching system• Link conditioning source and measurement technique to large number

of samples• Control and monitoring of wide range of environmental and

experimental factors• Combinational testing – understanding interplay between key factors• Evaluation of detection methods for quantifying evolution of PD

damage• Comparison of PD protection/prevention methods

Machine Learning and Deep Learning for PD Classification

Correlation between nature of PD source and measured PD response

Pattern recognition can be used to evaluate measured responses

Two parts to this:1. Extracting information from noisy data2. Processing of extracted data

a) Statistical analysis (e.g. of the statistical moment)b) Principal component analysis (PCA)c) Extraction of image featuresd) Derivation of Weibull parameters

Neural networks and support vector machines are popular techniques which have been used for PD source classification

These approaches need large volume of reliable data 42

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Summary

Technical, socio-economic and policy drivers behind increasing voltage

Theory behind partial discharge, including key factors and damage mechanisms

Standard test methods and short comings

Case study A4I project: antenna characterisation and effect of air pressure on PD

On-going PD test plans at NPL

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Concluding Remarks

Increase in uptake of high voltage electronics will result in more PD damage with reduction in dielectric breakdown over lifetime

Testing high voltage electronics for aerospace at sea level air pressure not appropriate (need understanding of suitable test methods)

Need to understand effect of acceleration factors, and determination of which are suitable

Need for test methods for generating data to support understanding and development of HV solutions

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Acknowledgements

The National Physical Laboratory is operated by NPL Management Ltd, a wholly-owned company of the Department for Business, Energy and Industrial Strategy (BEIS).

Colleagues:Martin Wickham, Ling Zou, Joe Beeby and Berjaeu Officer and David Knight

External collaborators:Professor Ian Cotton and David Chambers at AerospaceHV

Funding:

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Feel free to contact me:adam.lewis@npl.co.uk

Effect of high voltage (up to 1000V) on Dendrite Failures and Conductive Anodic Filament (CAF) Failures of Electronic CircuitsTuesday 13th April 2021

Ling ZouNP

Book webinar online at https://register.gotowebinar.com/register/7709096448651536652

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