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Design & Test Issues for High Voltage
Design of Electric Flight Control
Actuation & Power Electronics
Amit Kulshreshtha
Moog Inc
&
Ian Cotton
National Grid Senior Lecturer
SAE ACGSC Oct 2008 Meeting. NY
1.1
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
1.2
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
1.3
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
1.4
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.
1.5
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
1.6
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
1.7
1.8
Partial Discharge Types
1.9
Partial Discharge Types
1.10
1.11
Paschen's Curve
100000
Small distance (high field)
Low pressure (high mean free path)
Vbk (Volts)
10000
1000
100
0.01
0.1
1
10
100
1000
p.d (Pa.m)
1.12
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.
1.13
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
1.14
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
1.15
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
1.16
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
1.17
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.
1.18
Required Insulation Thicknesses
• Insulation thicknesses must more than double to
prevent PD when voltage is doubled
Partial Discharge Inception Voltage / V
1800
1600
1400
1200
1000
800
600
400
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Cable Insulation Thickness / mm
Relative Permittivity=3
Relative Permittivity=8
1.19
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
1.20
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
1.21
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
1.22
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
1.23
Clearances To Avoid Flashover In Air
100000
Vbk (Volts)
10000
Higher Altitude
1000
100
0.0001
0.001
0.01
0.1
1
10
100
1000
Distance ( mm)
100,000ft
•
•
50,000ft
10,000ft
Sea level
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
1.24
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
1.25
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
1.26
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
1.27
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
Measures light
current pulse produced by
emission from partial
charge transfer during partial
discharges
discharge
Measures radio
Measures the acoustic
frequency interference emissions produced
generated by the
by a partial discharge.
discharge
Advantage
Non-contact,
Non-contact,
applicable for all
applicable for all
voltage types. Allows voltage types. Allows
testing of equipment in testing of equipment of
real conditions
real conditions
Depending on
Sensitive to other
equipment being
acoustic emissions.
tested, EM emissions Signals cannot always
can prevent detection propagate through
of PD
insulation / casings
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
Disadvantage Sensitive to electrical noise. Insensitive to any form
Cannot test circuit in
of internal partial
operating condition in most discharge. Sensitive to
cases. Most commercial
light and highly
equipment can only test at
directional.
up to 400Hz
1.28
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
1.29
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
1.30
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.
1.31
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
1.32