1.1

Design & Test Issues for High Voltage Design of Electric Flight

Control Actuation & Power Electronics

Amit KulshreshthaMoog Inc

&Ian Cotton

National Grid Senior Lecturer

SAE ACGSC Oct 2008 Meeting. NY

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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

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 high dV/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 wires and, (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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Hi Voltage Electric Actuator: What is Hi Voltage? • Paschen’s curve describes electric discharge voltage as a function of

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

• 270VDC input voltage based systems, and motor windings may experience repeated applications of even higher than dc link/inverter voltage. It may increase electric motor drive voltages further during 4 quadrant operation in high 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 and installation 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 Hi Voltage design and results in partial discharge resulting in accelerated aging of insulation and its dielectric strength and wiring that had been a subject of intense study after the loss of TWA Flight 800 in 1996

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) drives operating 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 per FAR 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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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 electric stress 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 the insulation and motor winding enamel such as polymides, contribute to the partial discharge

• 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 distance is 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)

Vb

k (V

olt

s)

Small distance (high field)

Low pressure (high mean free path)

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Electric Actuators & High Voltage

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

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

• Paschen’s Curve defines the relationship between voltage breakdown voltage as a function of pressure (altitude) and airgap and below 327V there is no discharge and so no need for concern.

• Previous generation electric drives mostly operated with line voltages lowered than Paschen’s minimum operated at a constant frequency. Modern motor/drives driven by high dV/dT PWM drives and operated near or higher than Paschen’s minimum.

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

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

• PRINTED WIRING BOARD/BOX LEVEL CONFORMAL COATING IS GENERALLY NOT CONSIDERED ACCEPTABLE DUE TO ITS AGING/DEGRADATION

• ELECTRIC CABLING/WIRING & POWER ELECTRONICS MODULES/ASSY. SUBJECT TO HI VOLTAGES SHOULD BE DESIGNED AND INDIVIDUALLY TESTED FOR PARTIAL DISCHARGE TO ENSURE ANY MICROSCOPIC VOIDS/IMPERFECTIONS IN INSULATION

• ELECTRIC MOTOR WINDINGS THAT ARE SUBJECT TO HI VOLTAGES WHERE THE SEPERATION BETWEEN WINDINGS IS POLYMIDE ENAMEL WITH LIMITED SEPERATION SHOULD BE TESTED & EVALUATED FOR PARTIAL DISCHARGE OVER ITS LIFE AS THE INSULATION MAY DEGRADE WITH CONTINUOUS USE

• PARTIAL DISCHARGE IS DEPENDENT UPON DRIVE VOLTAGE WAVEFORM: PEAK MAGNITUDE 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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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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Effect of Cable Length Connecting Electronic Converter with Motor WindingsWiring 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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Overvoltage & Effects on Motor Windings• Electric Motor Windings may see significantly higher voltages

than input power/voltages for PWM driven motors due to transient voltages / overshoot at inverter and reflected voltages

• The close spacing of winding coils don’t allow traditional methods of 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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The Role Of Insulation• Insulation provides protection against voltage hazards, prevents

leakage current, 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 width modulated (PWM) drive signals lowers the strength of insulation .

• The limited space & separation distances between power signals, motor wirings windings may result in designs operating in close proximity to the voltage 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 insulation and eventually result in a full disruptive discharge such as arcing destroying the insulation altogether.

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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

Par

tial

Dis

char

ge

Ince

pti

on

Vo

ltag

e / V

Relative Permittivity=3 Relative Permittivity=8

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Electric Motor Winding Insulation Material Considerations

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

• These insulation material may become brittle and develop cracks when subjected to extreme temperature variations in presence of other mechanical and vibration stresses over the life of the equipment.

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

• Manufacturing of such materials per aircraft approved quality process i.e., bonded stores 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 the equipment

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Insulation Material Selection• The use of higher grade PWM/corona resistant (CR) materials

developed for industrial applications or multiple coating of insulation over the copper enameled wire can extend the endurance of the dielectric strength when PD takes place

• However, these insulation material may become brittle and develop cracks when subjected to extreme temperature variations in presence of other mechanical and vibration stresses over the life of the equipment.

• The use of such materials or coatings for flight critical systems in aircraft requires their characterization (electrical & mechanical) at altitude/low pressure, in the presence of humidity etc. as well as when subject to aircraft containments 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 contain it for life time of the equipment

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Avoidance Of Partial Discharge• Can be achieved through very careful dielectric design

– Can reduce fields to a point below which void discharge cannot occur etc.

– Careful control of manufacturing process very important (e.g. in machine 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 of PD

• PD dependent on local pressure, temperature but a weak dependence on frequency

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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

normally require 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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Clearances To Avoid Flashover In Air

• Clearances between two conductive parts (e.g. connector pins) easily defined using Paschen’s 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

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0.0001 0.001 0.01 0.1 1 10 100 1000

Distance ( mm)

Vb

k (V

olt

s)

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

Higher Altitude

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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 of pressure on pollution (e.g. boiling point) is significant• Measurements have shown observing IPC requirements can still lead

to tracking• Conformal coating can help eliminate tracking damage but is generally

not considered in terms of long term performance due to its aging/degradation

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Particular Actuator / Power Electronic Issues

• Degradation from PD possible within winding structure• Testing of multi-phase systems / ones operating with PWM difficult (although

much can be 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 materials within the module as many of the power electronics package designs use silicone gel during packaging of electronics- presence of air molecules/voids in the gel make it susceptible to partial discharge

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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 test will not detect PD– Won’t 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

the voltage and measure corresponding leakage current)– Improves matters, particularly if appropriate

frequency is used, but still cannot detect all partial discharge or turn to turn defects (severe PD may be detected as leakage current flow)

• Surge testing– This test detects ‘turn to turn’ or, ‘coil to coil’ or,

‘phase to phase’ insulation defects by comparing the transient response

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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 and standard for all HV equipment during

manufacture

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 in real

conditions

Non-contact, applicable for all voltage types.

Allows testing of equipment of real

conditionsDisadvanta

geSensitive to electrical

noise. Cannot test circuit in operating condition in

most cases. Most commercial equipment can only test at up 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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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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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 and equipment 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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Safety & Reliability Over The Equipment Lifetime

• Any design – electrical or mechanical operating at maximum possible design 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

an electric 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 selected accordingly

• 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 the testing 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 Paschen’s Curve