HighVoltageDesignTestGuidelines-SAEACGSCMeeting16October08

7/28/2019 HighVoltageDesignTestGuidelines-SAEACGSCMeeting16October08

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1.1

Design & Test Issues for High Voltage

Design of Electric Flight Control

Actuation & Power ElectronicsAmit Kulshreshtha

Moog Inc

&

Ian CottonNational Grid Senior Lecturer

SAE ACGSC Oct 2008 Meeting. NY


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1.2

Presentation Summary

Aircraft Electric Power System

Introduction to the importance of HV in electric

actuator systems

Basic review of HV design

Discussion of test methods

Summary


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1.3

HV Electric Actuation & Challenges in Design

Previous generation electric drives mostly operated with line voltage operated

at a constant frequency unlike todays PWM driven motor/drives driven by highdV/dT PWM drives and operated near or higher than Partial Discharge

Inception Voltages (PDIV)

Limited separation between high voltage signals and electrodes for (i) motor

winding, i.e. turn to turn(inter-turn

) wire separation of copper enameled wiresand, (ii) interconnects signals for power drive reduces electric discharge

voltage.

Todays adjustable motor drives use inverter driven high current PWM signals

resulting in significantly higher electric stresses than previously experienced

Limited volume/space limits the separation and spacing of high voltage

signals/power lines in electric machine windings as well as cabling and power

electronics combined with low pressure with high temperature often results in

the operation near or, higher than PDIV/CIV for electric discharge


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1.4

Hi Voltage Electric Actuator: What is Hi Voltage? Paschens curve describes electric discharge voltage as a function of

atmospheric pressure and wiring/electrode separation defining the minimumvoltage for breakdown in air to be 327V. Voltages, steady state or repeatedtransients higher than 327V are referred as high voltages

270VDC input voltage based systems, and motor windings may experiencerepeated applications of even higher than dc link/inverter voltage. It may

increase electric motor drive voltages further during 4 quadrant operation inhigh PWM/dV/dT driven electric drives with added regenerative voltages.

Apart from input electric power/voltages i.e., 270VDC or, 115VAC or, 230VAC,the internally generated DC Link Voltage to drive motor inverter andinstallation dependent motor winding voltages need considerations as it may

be higher than PDIV or, CIV even though input power voltages may be lower

Imperfections in the insulation system and/or, lack of due consideration for HiVoltage design and results in partial discharge resulting in accelerated agingof insulation and its dielectric strength and wiring that had been a subject ofintense study after the loss of TWA Flight 800 in 1996


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1.5

High Voltage Design & Testing

Guidelines For Electric Actuators The high voltage (~327V) operation of electric actuators at extended

temperature ranges, humid conditions and at altitude affects the safety

as well as reliability of the electric drive including its power electronics,

electric motor etc.

The current generation Hi Voltage PWM (pulse width modulated) drivesoperating at high altitude have higher levels of electrical and

mechanical stress compared with those encountered in the past.

Aircraft electric actuation systems have to meet certification

requirements including safety per FAR Pt 25 as well as operational

reliability, availability, continuity of service and life cycle data as perFAR Pt 90/91 & 121. This must be done with no historical data, making

them a novel design.

In general, aircraft electric power system are designed to operate below

high voltage or, corona inception voltages to avoid high voltage issues.


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1.6

High Voltage (HV) Related Definitions

Tracking Progressive formation of conducting paths, which are produced on the surface

and/or within a solid insulating material, due to the combined effects of electricstress and electrolytic contamination

Can occur at any voltage as long as conducting paths can be formed

Very dependent on pollution layer

Partial Discharges Electrical discharges which do not completely bridge gap Different forms corona, surface, cavity, electrical trees, floating parts

Substantially reduce the life of insulation

EMC Issues (?) - fast current pulses, rise times in order of nanoseconds

Very dependent on voltage type (i.e. AC/DC)

The spacing between the conductors, their geometry, and the imperfections in the

insulation materials, such as the presence of small/microscopic voids in theinsulation and motor winding enamel such as polymides, contribute to the partialdischarge

Disruptive Discharges or, Arcing Electrical discharges which do completely bridge gap

Flow of fault current follows discharge

Can permanently damage insulation


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Definitions Clearance is the shortest distance through air between two

conductors and is the path where damage is caused by short

duration maximum peak voltage

Creepage is defined as the shortest distance between two

conductive parts along the surface of any insulating material

common to both parts and the breakdown of the creepage distanceis a slow phenomenon based upon dc or, rms voltage

Clearance relates to flashover creepage relates to tracking

Mammano B, Safety Considerations in Power Supply Design, Underwriters Laboratory / TI


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Partial Discharge Types


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Partial Discharge Types


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Paschen's Curve

100

1000

10000

100000

0.01 0.1 1 10 100 1000

p.d (Pa.m)

Vbk(Volts

)

Small distance (high field)

Low pressure (high mean free path)


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1.13

Electric Actuators & High Voltage

Electric Actuators include Electronic Motor Control Unit (EMCU), Electric Drive/Motorcoupled to Mechanical Transmission for Electromechanical Actuators (EMA) or, toHydraulic Transmission for Electrohydrostatic Actuators (EHA).

High Voltage (>327V) can be generated within the EMCU or at the Electric Motor /

Drive Paschens Curve defines the relationship between voltage breakdown voltage as a

function of pressure (altitude) and airgap and below 327V there is no discharge andso no need for concern.

Previous generation electric drives mostly operated with line voltages lowered thanPaschens minimum operated at a constant frequency. Modern motor/drives driven

by high dV/dT PWM drives and operated near or higher than Paschens minimum.


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1.14

HV Design for Electric Motor & Electronics HI VOLTAGE ELECTRONICS CIRCUITS ASSY. SHOULD BE DESIGNED TO HAVE

ENOUGH INSULATION BY SEPERATION/AIR GAPS & INSULATING COATINGS TOAVOID ANY ELECTRIC DISCHARGE INCLUDING PARTIAL DISCHARGE/CORONA:MARGINS ON IPC-2221A?

PRINTED WIRING BOARD/BOX LEVEL CONFORMAL COATING IS GENERALLY NOTCONSIDERED ACCEPTABLE DUE TO ITS AGING/DEGRADATION

ELECTRIC CABLING/WIRING & POWER ELECTRONICS MODULES/ASSY. SUBJECTTO HI VOLTAGES SHOULD BE DESIGNED AND INDIVIDUALLY TESTED FORPARTIAL DISCHARGE TO ENSURE ANY MICROSCOPIC VOIDS/IMPERFECTIONS ININSULATION

ELECTRIC MOTOR WINDINGS THAT ARE SUBJECT TO HI VOLTAGES WHERE THE

SEPERATION BETWEEN WINDINGS IS POLYMIDE ENAMEL WITH LIMITEDSEPERATION SHOULD BE TESTED & EVALUATED FOR PARTIAL DISCHARGE OVERITS LIFE AS THE INSULATION MAY DEGRADE WITH CONTINUOUS USE

PARTIAL DISCHARGE IS DEPENDENT UPON DRIVE VOLTAGE WAVEFORM: PEAKMAGNITUDE FOR PARTIAL DISCHARGE IS LOWEST FOR SINUSOIDAL WAVEFORM,INCREASES FOR BIPLOAR-SQUARE/RECTANGULAR WAVEFORM i.e., +/- 270VDC

AND HIGHEST FOR UNIPOLAR SQUARE/RECTANGULAR WAVEFORM i.e., 0-560VDC


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1.15

Electric Motor Stator Winding & Electric Stress

Motor Windings,

Voltage Stress &

Partial Discharge

Inception Voltage

(PDIV), its Variation

with Freq & Temp.Courtsey: Kaufhold et al.:Failure

Mechanism of Low Voltage, IEEE

Electrical Insulation Mag. March 1996


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1.16

Effect of Cable Length Connecting

Electronic Converter with Motor Windings

Wiring distance between PWM/Square Wave based Power

Drive/IGBTs and Motor Winding results in higher voltages due to

reflected waveforms:

700VDC Link Voltage may create 1.2-1.4kV at motor windings

270VDC Link Voltage may create 350-420V at motor windings

Courstey: Wheeler, IEEE Insulation Magazine March/April 2005


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1.17

Overvoltage & Effects on Motor Windings

Electric Motor Windings may see significantly higher voltages thaninput power/voltages for PWM driven motors due to transientvoltages / overshoot at inverter and reflected voltages

The close spacing of winding coils dont allow traditional methodsof separation/clearances to be maintained for enhancing insulation

strength

Melfi, Low Voltage PWM Inverter Fed Insulation Issues, IEEE Trans IA, Jan 2006


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1.18

The Role Of Insulation

Insulation provides protection against voltage hazards, prevents leakagecurrent, electric discharge and short circuit current

The operation of electric drives at high altitude/low pressure coupled with

high temperature, humidity, and with high current/frequency pulse widthmodulated (PWM) drive signals lowers the strength of insulation .

The limited space & separation distances between power signals, motorwirings windings may result in designs operating in close proximity to thevoltage at which discharge will take place

Any imperfection in an insulation system may result in partial discharge(PD) which may reduce the life, reliability and integrity of the insulationand eventually result in a full disruptive discharge such as arcingdestroying the insulation altogether.


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1.19

Required Insulation Thicknesses

Insulation thicknesses must more than double to

prevent PD when voltage is doubled

400

600

800

1000

1200

1400

1600

1800

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Cable Insulation Thickness / mm

PartialDis

chargeInception

Voltage/V

Relative Permittivity=3 Relative Permittivity=8


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1.20

Electric Motor Winding Insulation Material

Considerations

Use of such higher grade PWM/corona resistant (CR) materials developed for industrialapplications or multiple coatings insulation over the copper enameled wire extends theendurance of the dielectric strength under PD should be analyzed for aircraftapplications

These insulation material may become brittle and develop cracks when subjected to

extreme temperature variations in presence of other mechanical and vibration stressesover the life of the equipment.

The use of such materials or coatings for flight critical systems in aircraft requires theircharacterization under altitude/low pressure, humidity etc. as well as aircraftcontainments such as fuel, hydraulic fluids, lubricants etc for operation in presence ofmechanical stress experienced by the motor windings.

Manufacturing of such materials per aircraft approved qualityprocess i.e., bondedstores with traceability should also be ensured.

It will be ideal to avoid corona by design instead of trying to contain it for life time of theequipment


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1.21

Insulation Material Selection

The use of higher grade PWM/corona resistant (CR) materials developed forindustrial applications or multiple coating of insulation over the copperenameled wire can extend the endurance of the dielectric strength when PDtakes place

However, these insulation material may become brittle and develop cracks

when subjected to extreme temperature variations in presence of othermechanical and vibration stresses over the life of the equipment.

The use of such materials or coatings for flight critical systems in aircraftrequires their characterization (electrical & mechanical) at altitude/lowpressure, in the presence of humidity etc. as well as when subject to aircraftcontainments such as fuel, hydraulic fluids, lubricants etc

Manufacturing of such materials per aircraft approved quality process, i.e.bonded stores with traceability should also be ensured.

It will always be ideal to avoid discharge by design instead of trying to containit for life time of the equipment


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1.22

Avoidance Of Partial Discharge

Can be achieved through very careful dielectric design

Can reduce fields to a point below which void discharge cannot occuretc.

Careful control of manufacturing process very important (e.g. inmachine windings vacuum application to remove voids from

encapsulation) Prevention of sharp edges to minimise field enhancement

As with flashover, ultimately a test is required to prove absence ofPD

PD dependent on local pressure, temperature but a weakdependence on frequency


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1.23

Can We Tolerate Electrical Discharges?

Tracking

Cannot be allowed as it will cause carbonisation of insulation

surfaces and could cause fire

Disruptive Discharges

Cannot be allowed to occur as a disruptive discharge will normallyrequire the operation of circuit protection to clear

Partial Discharges

Can be allowed as long as a number of questions can be answered

Does equipment remain safe, functional and reliable over the

aircraft lifetime? Is any interference caused to other systems?

In reality, answering these questions is very difficult so PD must be

designed out

Electrical utilities do not tend to allow partial discharge


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1.24

Clearances To Avoid Flashover In Air

Clearances between two conductive parts (e.g. connector pins) easily defined

using Paschens law Simple to make adjustments for temperature, pressure and frequency

Breakdown voltage very approximately proportional to pressure

Inversely proportional to temperature

Can reduce by approximately 20% with use of high frequencies / PWM

1cm gap 30kV DC @ sea level, 1.2kV @ 47000ft and 327V @ 150000ft

100

1000

10000

100000

0.0001 0.001 0.01 0.1 1 10 100 1000

Distance ( mm)

Vbk(Volts)

100,000ft 50,000ft 10,000ft Sea level

Higher Altitude


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1.25

Creepage Distance Requirements Little known (or at least published) regarding creepage distance dimensioning

(at least in scientific literature) Important in determining safe distances over insulation surfaces

While pollution is dominant in determining performance of surfaces, impact ofpressure on pollution (e.g. boiling point) is significant

Measurements have shown observing IPC requirements can still lead totracking

Conformal coating can help eliminate tracking damage but is generally notconsidered in terms of long term performance due to its aging/degradation


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1.26

Particular Actuator / Power Electronic Issues Degradation from PD possible within winding structure

Testing of multi-phase systems / ones operating with PWM difficult (although much canbe transferred from extensive work on higher voltage machines)

Much work done on power electronic switches Particularly vulnerable to impact of humidity

Difficult to test owing to presence of semiconductor element

PD leads to degradation in very short timescale

Industrial grade Power Electronics Modules with IGBTs or other power switching elements

may be a source of partial discharge (PD) due to stacking of different dielectric materialswithin the module as many of the power electronics package designs use silicone gelduring packaging of electronics- presence of air molecules/voids in the gel make itsusceptible to partial discharge


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1.27

High Voltage (HV) Test Techniques HI-Pot Testing: A DC technique that will (usually) pick up

gross defects in an insulation system Many insulation systems have a frequency dependent

insulation strength (in terms of breakdown)

Partial discharge not frequency dependent but a HI-Pot testwill not detect PD

Wont detect turn to turn insulation defects in a machine /

actuator There is therefore a place for HI-Pot testing but this is

certainly not the total solution

Insulation Resistance/Simple AC Testing (i.e. raise thevoltage and measure corresponding leakage current)

Improves matters, particularly if appropriate frequency isused, but still cannot detect all partial discharge or turn toturn defects (severe PD may be detected as leakage currentflow)

Surge testing

This test detects turn to turn or, coil to coil or, phase tophase insulation defects by comparing the transientresponse


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1.28

HV Testing Complete Systems Electrical Methods as defined in IEC60270/EN60270 require application of overvoltage and can be used

for passive elements inclnding wiring/cabling, PWBs, Motor/Stator Windings etc

Overall assy can be tested using a non-intrusive i.e., calibrated RF Detection method operating in

altitude/thermal chamber. LRU/Box level testing is some times challenging with RF detection as the

box/enclosure provides shielding for Electro-Magnetic Emissions and may be masked.

Significant difficulty in testing complete systems using standard lab testing techniques

Entire systems must generally be energised with multi-phase / DC / PWM voltages

Need non-contact testing to verify if PD is present When do we test? Type test or routine test?

Electrical Optical RF / EMI Acoustic

Description Electrical circuit that picks up

current pulse produced by

charge transfer during partial

discharge

Measures light

emission from partial

discharges

Measures radio

frequency interference

generated by the

discharge

Measures the acoustic

emissions produced

by a partial discharge.

Advantage A good sensitivity andstandard for all HV

equipment during

manufacture

Non-contact, applicablefor all voltage types.

Allows testing of

equipment in real

conditions

Non-contact,applicable for all

voltage types. Allows

testing of equipment in

real conditions

Non-contact,applicable for all

voltage types. Allows

testing of equipment of

real conditions

Disadvantage Sensitive to electrical noise.

Cannot test circuit in

operating condition in most

cases. Most commercial

equipment can only test atup to 400Hz

Insensitive to any form

of internal partial

discharge. Sensitive to

light and highly

directional.

Depending on

equipment being

tested, EM emissions

can prevent detection

of PD

Sensitive to other

acoustic emissions.

Signals cannot always

propagate through

insulation / casings


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1.29

Test Conditions

It is essential that qualification and life cycle HV testing

(Hi-Pot, AC, PD etc) be carried out in an appropriate

test environment

Electronic units and electric actuators should be tested

at the appropriate altitude, with vibration and

temperature cycling.

The mechanical load will also need to be incorporated

into a test as this will affect the circuit voltages


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1.30

PWM & Impact of High Voltage on Insulation &

Bearings

Hi Voltage increases dV/dT affecting the life of insulation

and bearings current; limiting high voltage to lower value

will reduce

Bearing current & insulation affect life/reliability andequipment usually passes qualification test-need to

address mitigation

Courtesy: Muetze & Binder, IEEE Insulation 2006 Courtsey: Lipo,IEEE Ind Appl. Mag Jan/Feb 1998


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1.31

Safety & Reliability Over The Equipment Lifetime

Any design electrical or mechanical operating at maximum possibledesign stress can fail at any time. Reliability is built in the design by

ensuring that the operating stress is a fraction of maximum design stress

The life of insulation under constant electric stress varies inversely to its

applied voltage and so it is important to ensure voltage gradients.

Electronics elements should be designed to ensure that the minimum

spacing between conductors is maintained with added safety margins over

the industrial standards. Electric motor windings need careful attention to

ensure that voltage stresses remain within acceptable limits

The design should be based on any steady state or repeated transient

voltages that occur with added safety margins to ensure safety.


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Summary

Voltages higher than the nominal input voltage can be present in anelectric actuation system

These voltages can lead to tracking, partial discharge or breakdown

resulting in continual insulation degradation or arcing

Designs must be analysed to determine maximum peak/transient

voltages and insulation materials / clearances / geometries selectedaccordingly

Should always try and prevent partial discharge occurring and not

control it using materials

Testing of equipment is essential however it is difficult to

comprehensively test a complete system need to consider thetesting of components / sub-assemblies

There is a need for expanding on-line monitoring and PHM/Condition

Based Monitoring to ensure integrity of the insulation over the life of

the equipment for operation over minimum Paschens Curve

HighVoltageDesignTestGuidelines-SAEACGSCMeeting16October08

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