Download Power Systems for Aerospace, Marine and

Power Systems for Aerospace, Marine and Automotive Applications (H64AN1)
Power converters
The More Electric Aircraft
Motivations: Fuel burn/efficiency, reliability, maintainability, availability,
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dispachability, performance, weight/size, cost
Remove hydraulic system: reduce weight, easy maintenance, power on demand
Remove pneumatic system: improved efficiency “bleedless” engine
Advanced diagnostics and prognostics: increased availability
Lower running costs (less maintenance scheduled and unscheduled)
Challenges
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High quality electric motor drives
DC link capacitors
Power quality and stability
Weight/volume (conversion/wiring/power
distribution significant)
 Total non-thrust power more than 1MW
Generator Control Unit, GCU:
Regulates voltage,
Protects generator,
Controls & excites generator,
Manages main bus contactors
APU
LSG
2x250kVA
RSG
2x225kVA
For 2 motors, the options are:
 2 single converters powering each motor
o 2 DC capacitors, 2 heat sinks
 - large/heavy system
 Dual use power converter
o Share 1 converter using a switch to operate either motor
 - large/inefficient due to switch contacts (higher losses)
 Dual output power converter
o Shared DC capacitor, heat sink & current measurement
 + Individual power semiconductors allows system optimisation for each
motor
 + 1 Hall Effect transducer
 + shared rectifier circuit means cost/weight/size decreases
Future HVDC systems
2x250kVA
Power distribution
 Left Side Generator and RSG connected to engines
 Auxiliary Power Unit (APU) is a gas turbine
generator located at the back of the plane, used
to start engine, run ECS without running main
engines, and can also act as back up to a failed
engine/generator
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Electrical power distribution system
28V DC loads
(small aircraft)
115V AC 3phase loads
(current civil
standard)
230V AC 3- ±270V DC
phase VF
(future
loads (future military)
civil)
 Aerospace voltages quoted as line to neutral (peak): 230V peak  400V line to line
 Electro-Mechanical actuator (EMA):
Power
Motor
o Actuator is moved as motor spins
converter
Reduction
 + compact
gearbox
 – ball screw can jam (surface unmovable)
 Electro-Hydrostatic Actuator (EHA)
o No direct connection between motor and actuator arm
o 3-ph supplypower convertermotorpumphydraulic ram
 + surface is movable if there is a hydraulics failure (fluid leak)
 + familiar technology for aircraft component manufacturers
 Actuator control:
Power converter
o All controlled surfaces can be
Supply
Converter output current (300Hz control loop)
powered by more than 1
Control
Motor speed (30Hz control loop)
voltage
transducers
Ram position (2Hz control loop)
power source
Feedback
o Actuation can be controlled by
Actuator/ram
position demand
hydraulic and/or electrical circuits
 Primary: critical for flight e.g. ailerons, rudder, elevators.
 Secondary: useful for comfort and safety but not critical e.g. flaps, slats
Ball
screw
Electrical power systems
 Model based design electrical systems simulation
o Validate/verify requirements
o Model faults and analyse stability/transients
o Use distributed computing for faster results of complex (behavioural) models
 Diagnostics: what the malfunction is and the cause
 Prognostics: prediction of failure of device/system
 Power failure sequence: Main generators (duplex and quadplex systems)
 Large users of power: Environmental Control System (for cabin heating/cooling,
pressurisation during flight), WIPS (Wing Ice Protection System), actuation
o Pneumatic (WPS, ECS, engine start)
o Hydraulic (slats, flaps, landing gear)
o Electrical (galleys/lights, avionics)
 Generator Control Unit (GCU) works like a voltage regulator
DC (GRbusIloads)
vs.
AC (GbusAC-AC converterloads)
+ Distributed energy storage
+ Variable frequency systems
+ Fewer conductors required
- Power stages more complex (internal energy
storage often required – less efficient) 
- Rectifier in critical path to all loads
+ Only 1 power conversion stage
- Circuit breakers much bigger
+ Circuit breakers small & cheap
o AC power generation is conventionally sourced by constant frequency
generators fed by a Constant Speed Drive
Constant speed
shaft
Variable speed
3-phase
(CSD) which produces a fixed output
CSD
Generator
engine
400Hz,
115V
of 400z
 + allows generators to operate in parallel
 - expensive to purchase & maintain (due to patents)
o Variable Speed/Constant Frequency generation:
 - not yet proven technology, power Variable speed
3-phase
VF
Power
Generator
engine
400Hz,
converter
converter is key failure point
115V
o Variable Frequency generation:
 + generator directly connected to power
Variable speed
3-phase
bus (simple & reliable)
Generator
engine
320-900Hz,
230V or 115V
 + cheaper
 - requires motor drives (IM’s performance decrease & weight increase) 
fuel and hydraulic pumps heavier and less reliable
 Ground Power Supply:
 Rectification techniques:
3 or 6
phase
o 6-pulse diode
Aircraft 
400Hz
50Hz
Output
PWM controlled
Transformer
Rectifier
power
filter
inverter bridge
(5 leg)
bridge rectifier mains
(400Hz) 
6-pulse
Provides isolation
Attenuates sw.
Controls output
 + simple circuit
diode bridge
(& voltage gain)
frequency components
voltage waveform
 - large 5th and 7th harmonics

o Autotransformer (ATRU) and 12-pulse rectifier
 + cancellation of 5th and 7th harmonics
 - requires inter-phase reactors

Actuators:
+ no reactive power
+ fewer wires
+ no AC losses
+ no frequency effects (like skin effect)
- safety issues
- AC supply from generators needs rectification (heavy transformers)
failure
→
failure
APU start & batteries →
Ram Air Turbine deployed & batteries
o Multiple sources of power required:
 To ensure fault tolerant systems
 To reach required failure levels for flight
Protection features:
o I2t protects wires from thermal overloads
 Simple process  I2R
o Arc Fault Protection (AFP), Delta Current (ΔC), Ground Fault Interruption (GFI),
Differential Protection (DP)
Electrical load management systems
Electro-mechanical relay
and electro-thermal
circuit breaker
+ Well understood
technology
+ Air gap isolation
+ High tolerance to over
voltages
+ High temperature relay
performance
+ Low on state resistance
- Lots of wiring
Contactors and
relays
- Requires
electronics to
provide protection
- Slow on/off times
- Lack of control
- Slow improving
technology
Solid State Power Controller (SSPC)
+ Switch DC currents
+ Fast (µs) + No arcing
+ Can detect faults other than short
circuits
+ Facilitates more efficient power
distribution technologies
+ Improved diagnostics and prognostics
+ Technology improvements expected
+ Cost coming down
The More Electric Car
Hybrids
Petrol is very good for energy storage, batteries have poor kW/kg
Series hybrid
Battery
Series
o + urban driving (stop/start)
Power
hybrid
Motor
converter
Parallel hybrid
Engine
Gen
o + can be driven by engine or battery or both
Power
o + regenerative braking
Battery
converter
Motor/
Series-parallel hybrid
Parallel
generator
hybrid
Engine
o Combination of both using mechanical
power splitter
 Hybrid terms in common use:
o Full hybrid: vehicle can drive with just batteries, engine or both
 – requires large and expensive lithium ion batteries
o Mild hybrid: engine used for primary power, motor used for start,
acceleration and regenerative braking
o Soft hybrid: similar to mild hybrid but motor not used for acceleration
12V
vs.
42V
+ Optimised technology
+ More efficient for higher power systems
+ Proven systems/reliability
+ Reduced mass and volume (motors)
- Relative short
- Potential safety issues (electrocution, customer perception)
term cost
- Cost of change (development, retraining…)
 Initial systems will have a hybrid of 12V/42V using DC-DC converter
 Peak powers of systems usually much more than average power
Wheels
Wheels
High Voltage design and testing
Marine power systems
 Modern systems have a mixture of gas turbine and diesel generators to supply
HV bus and LV bus
o + reduced fuel consumption
o + faster response + better manoeuvrability
o + reduction in volume
o + less propulsion noise + less vibration
o – increased investment costs
o – bigger variety of components
o – cost of service skills and maintenance engineers
 Voltages from 11kV (for 20MW) to 440V 60Hz (common for American low
power devices)
Inverters
LV: VSI IGBT  Cycloconverter  Multi-level VSI  CSI :HV
Switching
devices
Motor
waveforms
Control
VSI
IGBTs
+ Very high switching frequencies
+ Low switching losses
- Low voltages
- Relatively low current
- Expensive for power level
- High conduction losses
PWM waveform
+ Good motor voltage and current
waveforms
+ Smooth torque  reduces
motor losses, noise in propeller
signature and vibration
- Poor input harmonics
+ PWM control is conventional
technology
+ Can achieve very high
performance with vector control
+ Faster torque response than CSI
or cycloconverter
Cycloconverter
CSI
Thyristors
+ Cheap for power level
+ High voltage and current ratings
+ Low on state losses
- Poor supply power factor at low
speeds
- Harmonic rich supply current
Switches parts of
Quasi-square
supply voltage into
wave motor
motor
current
+ Better current
- Pulsations in
waveforms than CSI current/torque
+ High torque
- Spikes in
- Motor frequency < supply voltage
1/3 supply frequency
(due to high
 slow speed
di/dt)
- Supply PF lag
- May need
- Speed dependant
PWM at low
harmonics
speeds
- Big converter (each
+ Relatively
phase must be
simple control
isolated)
+ Thyristor
- Requires 2 bridges
inverter very
per phase for bisimple
directional current
- Performance
limited by slow
switching of
thyristors
 Current Source Inverter (CSI)
o Rectifier feeds large inductor, inverter directs DC current into phases of
synchronous AC machine
2 bridges (for bidirectional current)
 Cycloconverter:
Supply
Load
o Direct AC-AC converter that switches
Transformer
parts of input voltage
for isolation
 Voltage Source Inverter (VSI)
o 3-ph supply  rectifier  DC link  IGBT inverter  Induction Motor
 Multilevel VSI
o Connect lower voltage IGBTs in series (in a 2-level inverter) or use diode
clamped inverter:
o + Better output voltage than 2-level VSI
o Use 3-level/5-level/7-level… inverters to
produce better output voltages and reduce
device voltage balancing
o T45 uses VSI drive for Advanced Induction
Motor (AIM) which has 3 channels x 5 Hbridges (= 15 phases/windings in total)
 + Fault tolerant
 + Efficient at low speeds/loads
Podded systems
 Motor and propeller are outside main ship body, power converter is still in the
ship
o Saves space in the ship
o Greater manoeuvrability (since pod can rotate ±180°)
Definitions:
 Tracking: progressive formation of a conducting path
o occurs at any voltage
 Partial discharge: electrical discharge which does not completely bridge gap
o Reduces life of insulation, fast nanosecond current pulses - EMC issues
 Disruptive discharge: completely bridges gap
Cable
o Can permanently damage insulation
Partial discharge
Conductor
Void
Corona
 Types:
Ground
Air
o Voids within cable insulation:
Ground
o Corona discharge around a conductor’s sharp point:
Surface
Air
o Surface discharge between 2 conductors:
discharge
Ground
o Partial discharge between wire and plane
 Paschen’s law defines minimum voltage at which breakdown occurs in an
insulator with uniform field (327V in air) making modifications for temperature
and operating frequency
 Breakdown occurs when accelerated electrons create additional electrons
(ionisation) upon impact with insulator molecules and is self-sustaining
o Acceleration, a = qE/m  the higher the velocity, the higher the probability of
ionising collisions
o Mean free path: electron’s average distance between collisions
 High gas density  low mean free path
o Townsend breakdown: electron avalanche  positive ions accelerate to the
cathode  secondary electrons  new avalanche
o Higher temperature  lower gas density  higher mean free path
o In non-uniform fields, partial discharge is determined by how sharp the
electrode is, breakdown is mainly determined by gap size
 Gap clearance = Pam/Pa
o 1 mbar = 100 Pa
o Vpk = √2 x Vrms
o Every 25° above room temperature adds 0.1mm clearance
o Critical frequency = 200/d (kHz) where d is gap distance (mm)
 Slow moving positive ions distort the electric field  decrease breakdown
voltage by 20%
 Above critical frequency, add 20% clearance
 Dielectric analysis of solid insulation can predict the partial discharge inception
voltage
𝑡𝜀
 The percentage of the system voltage through an air gap = 𝑟
Insulation
o εr is the relative permittivity of the cable insulator
𝑉
o 𝑉𝑠𝑎𝑓𝑒 = 𝑏𝑟𝑒𝑎𝑘𝑑𝑜𝑤𝑛 𝑜𝑓 𝑎𝑖𝑟 𝑔𝑎𝑝
% 𝑣𝑜𝑙𝑡𝑎𝑔𝑒 𝑖𝑛 𝑎𝑖𝑟 𝑔𝑎𝑝
𝑑+𝑡𝜀𝑟
Insulation
+V
Cable
d
 Electric field distribution in steady state DC system
Air
t
defined by resistivity  varies significantly with
0V
Ground
temperature
o Discharge recharges much slower in DC system
 In an aircraft, as conductor size increases  insulation decreases (due to
size/weight constraints)  voltage rating decreases
o A higher voltage rating does not necessarily compensate for a decrease in
current rating
PD
Corona/voids/surface
 PD testing:
amplitude
PD
Test
PD
AC
o Optical, RF/EMI, acoustic
object
LCR
270°
Phase
0
o Electrical:
90°
o Results:
PD detector
Tracking
 Voltage to allow tracking based on voltage required to evaporate a contaminant
o Boiling point, θliquid is lower at lower pressure (higher altitudes)
o
𝑉 2 𝑤ℎ
𝜌(𝜃𝑙𝑖𝑞𝑢𝑖𝑑 )𝑑
=
𝜃𝑙𝑖𝑞𝑢𝑖𝑑 −𝜗𝑎𝑚𝑏𝑖𝑒𝑛𝑡
𝑇
o To prevent tracking:
 Make sure system cannot be polluted (keep clean/dry)
 Use coating to restrict flow between electrodes
User interface
(steering
wheel)
Electric Power Steering
 Modernisation of power steering:

o (Currently) electrically assisted hydraulic  electric

With
power system  steer-by-wire (no mechanical

feedback
motor
connection)
Encoder
 Challenges/requirements:
o High power density
Microprocessor
o High efficiency
(controller)
o High reliability (10ppm failure rate)
o Silent operation
Power
Power
converter
converter
o Ultra low torque ripple

o Very high safety integrity
OS motor/
NS motor/
o 12V supply
actuator
actuator
 Types:
o Pinion drive
o Rack drive: motor placed on rack

o Column drive: motor near steering wheel

 Diagram of main steer-by-wire system:
 Microprocessor controller uses shunt resistor in return path (high reliability due
to low component count) and vector control

 Power considerations:
o Peak power ≈ 1.2kW, 60-80A
o Battery harness (important) ≈ 10-20mΩ, 1-2V drop
 Torque ripple components:

o Fundamental ripple
o Harmonics due to manufacturing deviations
o Harmonics due to errors n current measurements
 Solution: introduce cancellation harmonics in software
 Safety and reliability
o Failure Mode and Effects Analysis (FMEA)
o Fault Tree Analysis

o Diagnostics
o Eliminate failure modes by design
 Voltage boost system:
HV bus
DC-DC
3-phase
12V Battery
EMC filter
Motor
converter
inverter
o By using HV, it:
Motor position
 + eliminates supply
Supercapacitor
and current
Controller
fluctuations at input of drive
 + reduces current rating
T
 + increases performance
 + decreased torque ripple
28V
12V
 + cost/size of motor decreases
Speed
 + torque-speed envelope is extended:
o By interleaving, the following are achieved:
 + low input and DC capacitor current ripple
 + low device current ratings
 + possible redundancy
 + separation of heat generating components
 + smaller capacitor, thus improved transient response
 - more complex control
 - increased component count
 Future
o New motor designs
o Moving EPS technology to other automotive applications (e.g. clutch,
braking…)
 + reduces development costs
o Sensorless control
 + lower component count  lower cost + higher reliability
 - low speed
 - more complex control
 - audible noise (from carrier signal injection)
 - additional sensor needed for di/dt techniques
(for short periods
of energy boost)
Reliability calculations
Power quality
Motor current harmonics  T ripple and motor overheating
Supply voltage harmonics may cause malfunction of equipment
Supply current harmonics due to rectifiers  voltage harmonics due to
supply/cable/transformer impedance
o – thermal overloads in cables/transformers
o – extra transmission losses
Transformer
o – can cause interference to
secondary side
Input reactor
Rectifier
telecommunication systems
PF correction
M
o – false tripping of circuit breakers
capacitor bank
(improves operation
at low speed)
Example harmonic current source:
o Severe voltage distortion by PF correction capacitor interacting with supply
impedance
o PF correction capacitor is low impedance at high frequency thus will draw
harmonic currents which can get quite big
Harmonics occur at frequencies given by 6k±1 (k=1,2,3…)
𝑇𝐻𝐷 =
2
√∑∞
𝑛=2 𝐼𝑛
𝐼𝑓
where If is the fundamental magnitude
Mitigate harmonics by:
Adding supply or DC link inductance
o + makes output current continuous
o + passive approach
o - DC voltage decreases (voltage drop across L)
Multi-pulse techniques: connect “n” 6-pulse bridge rectifiers in series/parallel
supplied from seperate 3-phase supplies phase shifted by 60°/n
o + results in cancellation of odd k harmonics (when
n=2)
Supply
o - requires extra transformers and bridge
2 bridges (30°
Transformer
rectifiers
phase shift)
o Harmonic orders still present are 6nk±1
Add passive (shunt) filters with low impedance connected in parallel to
harmonic source
o + most harmonic current flows into filter instead of source
o - can introduce resonance in supply
o - can be large/bulky
o Single tuned notch filter (LC circuit) most common
1
o Where the harmonic filter impedance, 𝑍ℎ = 𝑋𝐿 + 𝑋𝐶 = 𝑗𝜔𝐿𝑓 +
=
Load
𝑗𝜔𝐶𝑓
𝑗𝜔𝐿𝑓 (1 −
𝜔𝑠 = √
𝐿
1
𝜔2 𝐿𝑓 𝐶𝑓
) thus a zero impedance (series resonance) at frequency,
1
𝑓 𝐶𝑓
 However, there is parallel resonance (high impedance) due to supply
impedance just below notch frequency, 𝜔𝑝 = √(𝐿
1
𝑠 +𝐿𝑓 )𝐶𝑓
so must make sure
this isn’t near any significant harmonics, though generators don’t tend to
generate even harmonics
𝑉2
 Reactive power, 𝑄 = 𝐿𝐿 (VAr) [x transformer %]
𝑋
 Rated IC = reactive power per phase (Q/3) / phase voltage (415/√3)
 Current drawn due to supply distortion, Ish = phase voltage distortion / filter
impedance at harmonic frequency
 Current drawn at hth harmonic due to load = Ilh
 Worst case for total harmonic current when Ish and Ilh in phase (thus sum)
 + efficient way to reduce harmonic distortion
 + can connect several different filters for different frequencies
 - complex to design, need to take into account change in resonant
frequency, voltage distortion harmonic currents from other loads, no limit
on harmonic current
o Add Active Shunt Filter (ASF): a power converter (controlled current source)
connected in parallel to harmonic source that inject cancellation harmonics at
point of common coupling
 + 1 filter can compensate for many frequencies (wideband harmonic
mitigation)
If+Ih
If
Non-linear
 + reactive power compensation
load
-Ih
 - requires very fast current control loops
Shunt filter
 - expensive
o Use a sinusoidal (PWM) rectifier: that acts as a controlled voltage source
 + has switching harmonics that can be easily reduced with a passive filter
 - very expensive
 Reliability: uses MTBF and λ and is determined by individual components in
system. Optimised in actuation and ECS systems.
o Mean Time Before Failure, MTBF = 1/λ (hours) where λ is failure rate
 λ of individual components are split into sections/levels
 λ sum for next level and multiply in same level with duplex systems
o MTBF method used because aircraft operators would like to predict time to
failure for preventative maintenance and reduced operating costs. But it does
not take into account:
Power system protection
 Reason for failure
 Must distinguish between transients and faults to prevent false trips
 Build quality
 Transients (voltage swells) caused by sudden removal of a large load 
 Actual operating environment
generator control cannot act quickly enough  control causes change in system
 The impact of a failure on other components
voltage (and frequency)
 Availability: how likely a component will work. Optimised with duplication of onidentical components for redundancy at expense of reliability e.g. fuel pumps