Ultra-compact Intelligent Electrical Networks
S.A. Long#1, D.R. Trainer#1, J. Chivite#2, A.M. Cross#2, A.J. Forsyth#2, S. Smith#2
#1
Rolls-Royce plc, PO Box 31, Derby, DE24 8BJ
#2
The University of Manchester, Power Conversion Group, Sackville Street, M60 1QD
Abstract
The objective of the SEAS DTC project ’ultra-compact intelligent electrical networks’ is to
develop a novel platform for evaluating intelligent electrical power networks suitable for
application in uninhabited autonomous vehicles, having multiple loads and generation
sources with the ability to effectively re-route power during faulted conditions. This paper
presents an update of the development of the laboratory system for the evaluation of such
networks, which comprises of closed loop controlled induction machines for prime mover
representation, novel high power density generation sources, active rectifiers for load
emulation and the associated control systems.
Keywords: Electrical networks, UAV’s, Autonomous, Intelligent, Flexible.
Introduction
This paper presents an update on the
development of the electrical power system
being developed for the UAV ‘intelligent
electrical power networks evaluation
facility’ (IEPNEF) under the SEAS DTC
theme, ‘Propulsion, Power Generation and
Energy Management,’ [1]. It is being
housed at the Rolls-Royce University
Technology Centre (UTC) in ‘Electrical
Systems for Extreme Environments’ within
the School of Electronic and Electrical
Engineering at the University of
Manchester.
Having previously defined the scope of
elements to be included in the evaluation
facility, [1], the subsequent challenges
include the specification, installation and
commissioning of the elements for
application in the facility. The main pieces
of hardware have all been specified,
including, the prime movers for
representing the low pressure (LP) and high
pressure (HP) gas turbine spools, the high
power density electrical power generation
sources for representing LP and HP
embedded generation, the electrical loads,
and the control architecture. The system
under development is a novel ±270V DC
electrical power network having a
continuous electrical power generating
capability of 100kW, derived from the
combined efforts of a 30kW switched
reluctance generator and a 70kW
permanent magnet brushless AC generator.
These generators will be loaded by a
combination of resistive load banks,
unidirectional active programmable load
banks
and
bi-directional
active
programmable load banks, the latter
offering the capability to address the
benefits and energy management issues
associated with power regeneration from
load systems, such as electrical flight
actuation surfaces. The following provides
a progress review of the system
development, the aims for the remainder of
this year and the key research objectives.
System Overview
The key research objectives include
addressing the re-configurability and fault
handling ability, functionalities that form
the basis of an ultra-compact intelligent
electrical
network.
However,
to
appropriately exercise the network, the
stimulus to the prime movers and the loads
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is a pre-requisite to successful system
studies, thereby requiring significant
resource for development. In addition,
since it is housed in a laboratory
environment, and the protection and
intelligent systems are being developed as
part of a series of research programmes, a
supervisory safety strategy, operating in the
background, is required to protect the
integrity of the complete system and
personnel, regardless of decisions made by
intelligent algorithms. In order to address
these requirements, the facility architecture
takes on the form shown by the schematic,
Figure 1, the elements of which are the
focus of this paper.
induction machines to behave together as a
real engine, Figure 3b, thereby having
FaDEC functionality.
Figure 2: Prime mover hardware
(a)
(b)
Figure 3: Engine shaft emulation
Generation Sources
Figure 1: Laboratory System Architecture
Prime Movers
The induction machine drive systems,
which were identified as the most
appropriate means to act as the prime
movers within the laboratory environment,
have been installed in the laboratory and
commissioned, Figure 2. These fulfill the
criteria identified previously [1].
The conceptual arrangement of the prime
movers and the associated generators is
shown in Figure 3a, where the two prime
movers can be treated as different shafts of
a common engine. In order to implement
this conceptual arrangement, a real-time
simulation (RTS) computer system will run
a validated engine model, the control
interface being developed as part of a
collaborative work programme between
Control Systems, Rolls-Royce, Bristol and
the Manchester UTC. This will enable the
In view of the high power density of
switched reluctance (SR) and permanent
magnet (PM) technologies, and their
benefits of high tolerance, both are being
employed within the facility to address, in
more detail, their behaviour in electrical
networks of this nature.
Switched
(SRSG)
Reluctance
Starter-Generator
The SRSG, Figure 4, which represents,
electrically, a HP spool embedded machine,
has been developed by Switched
Reluctance Drives Ltd (SRDL). This has
been mechanically integrated to the HP
prime mover, thereby completing the HP
spool arrangement. The machine was
characterised on-site at Manchester UTC
by SRDL, with the support of Manchester
UTC personnel. Following this, a series of
commissioning tests, Figure 5, were
successfully undertaken using a manually
operated resistive load bank to exercise the
machine over its complete operational
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envelope, including transient load step
changes.
operational in the IEPNEF before the end
of quarter 2.
Figure 6: PMG stator winding arrangement
Figure 4: SRSG prior to installation
Figure 5: Commissioning of SRSG
Permanent Magnet Generator (PMG)
The PM machine, representing the LP
embedded generation, has been designed by
the Rolls-Royce UTC in ‘Advanced
Electrical Machines and Drives’ at The
University of Sheffield. This is a fault
tolerant 5-phase topology, with a winding
design to provide an impedance of 1 perunit, therefore having the ability to
continuously withstand full fault current.
This is based on the technology recently
developed and proven under the
Department of Trade and Industry (DTI)
funded programme ‘Fault
Tolerant
Electrical
Machines
and
Drives,’
(FaTEMD), [3], for application as a low
pressure fan driven generator. Figure 6
shows the stator assembly in its recently
completed condition; the associated rotor is
undergoing its final fabrication process.
Delivery of the completed PMG and
associated inverter, to Manchester UTC, is
imminent, allowing the system to be
Loads
The means for implementing the
fundamental
loading
characteristics,
previously defined for exercising the
system (steady-state constant loads;
continuous switching power electronic
loads; dynamic switching power electronic
loads; pulsed loads) [1], has been defined.
To provide the greatest level of flexibility
for system configuration, high bandwidth
programmable active rectifiers (current
sources / sinks) are being employed for
load emulation. These provide the facility
with a generic loading capability, such that
sourcing and/or sinking of current can be
undertaken as required to replicate the
power flow that would otherwise be
imposed by many physical loads. The
system is initially being designed to
accommodate a total of six loading
systems, a pair of three different systems
with the aim of introducing a guest load at
a later date to emulate loads of a pulsed
nature. The three loading systems and the
personality modules associated with the
active loads will be described in the
following sub-sections.
Resistive Loading Systems
There are two resistive loading banks,
Figure 7, both rated up to 25kW,
controllable in incremental steps of 5kW,
through manual or remote digital control.
These are being procured from Hillstone
Products for operation up to 750V
continuous and 1200V transiently and will
essentially
be
used
to
represent
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‘background’ loads such as the avionics,
having the combined ability to absorb 50%
of the total continuous system generation
capacity.
Figure 8: Uni-directional active load
Figure 7: Resistive Load Banks
Unidirectional Active Loading Systems
Unidirectional loads can be configured to
sink current from an electrical network in
the same manner as a real ‘physical’ load.
However, they are not capable of emulating
the effect of regenerative loading effects,
and also require a water-cooled system to
dissipate the energy. Loading systems of
this nature are available commercially
having the operational voltage levels and
bandwidths specified for the IEPNEF.
Zentro Elektronik active load systems have
been selected following a series of tests
undertaken on a sample 800V, 20A, 1kW
unit, Figure 8, to evaluate their suitability.
A range of frequency response tests were
undertaken,
which
identified
the
operational bandwidths ranging up to
20kHz, depending on the operating
conditions, such as peak and offset values
of the demand profile. Typical frequency
response measurements for the unit under
test, Figure 9, relate to a range of AC
stimuli at various DC offsets, essentially
for a range of sample load demand profiles.
Finally, an FFT of the load current
waveforms produced by the loads at
various operating points proved the loading
system imposes minimal distortion on the
desired load profile, only becoming
significant at frequencies well beyond the
bandwidth of the system.
Figure 9: Frequency response of uni-directional
active load
Bi-directional Active Loading Systems
Bi-directional loading systems have the
added benefit over unidirectional loading
systems of being able to both source and
sink power, thereby enabling the issues
associated with regenerative loads to be
addressed. In addition, these have the added
benefit of not requiring a cooling system to
dissipate the energy, as it is returned to the
grid. Unfortunately, at the voltage levels
and bandwidths specified for the IEPNEF,
commercial systems are not available.
Therefore, as part of the research being
undertaken at the Manchester UTC, such
loading systems are being developed to
provide the IEPNEF with greater loading
options. Essentially, two of these bidirectional loading systems will be
implemented in the third quarter of this
year, as shown by the schematic in Figure
10. Essentially the systems will comprise of
an-off the shelf sinusoidal active front end
(SAFE) connected to the grid via an
isolation transformer. It is intended that the
DC:DC converter be an off-the-shelf
module, although this depends on the
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evaluation of existing commercially
available solutions; alternatively, a DC:DC
converter will be deigned at Manchester
UTC. However, the key development will
focus on the interface of the hardware subsystems, (i.e. between the SAFE, the
DC:DC converter; DC network), and the
control of the DC:DC converter. The load
model block shown in Figure 10 is the
‘personality module,’ which is discussed in
the following section.
L
SAFE
GRID
DC:DC
ifbk
id
Isolation
Transformer
Load
Model
Controller
L
SAFE
GRID
Isolation
Transformer
DC:DC
ifbk
id
Load
Model
-270V
0V
+270V
Controller
Figure 10: Proposed implementation of bidirectional active loads
Active Load System Personality Module
Development
Load Model
Figure 11: Personality module for active loading
systems
The development of the personality
modules is well under way, having the
ability to successfully track load profiles, as
observed by the measurements given in
Figure 12. The ‘X’ (red) waveform is data
produced by an off-line simulation, which
essentially sets the baseline for the desired
tracking response to a current set point. The
‘Y’ (blue) waveform is an oscilloscope
measurement of the current set point being
output from a dSpace system, (an Applied
Dynamics International (ADI) rtX PCbased RTS is being procured for final
implementation), executing a real-time
simulation model of a ‘real’ load.
‘Y’
The personality modules provide the
intelligence for the loading systems,
enabling their behaviour to be tailored to
any specific ‘real’ system load. It is
intended that an RTS computer system will
execute the desired software model to
emulate a specific load system, thereby
generating the desired load current demand
profile. As shown in the schematic of
Figure 11, a ‘real’ load, for example an
electrical surface actuator, would receive
demands of position and load as its inputs.
In order to meet these demands the ‘real’
load would extract power from the
electrical network, which is performed in
the IEPNEF by an active load system,
represented in Figure 11 as a current sink
drawing Iload, as demanded by the current
set-point. This current set point is the
output from the load model in response to
the position and load demand signals. The
models employed for emulating surface
actuator systems for the IEPNEF will use a
similar approach to that described in [4, 5].
‘X’
‘Z’
Figure 12: Load profile tracking ability of
unidirectional active load
The ‘Z’ (green) waveform is an
oscilloscope measurement of the load
current being drawn by the sample
unidirectional active load system in
response to receiving the dSpace current set
point, i.e. again this is real-time data. It can
be observed that ‘Z’ (green) tracks ‘Y’
(blue) extremely well, and produces the
desired waveform defined by the
simulation, waveform ‘X’ (red).
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Control Architecture
Plant control
system HMI
Flight control
system
Rig stimulus network
The control architecture [2] comprises of
three main systems; the test platform (TP)
controller; the autonomous electrical
generation and distribution network
(AEGDN); and the data acquisition system
(DAQ), as shown in Figure 1. This
configuration enables the integrity of the
system and safety of the personnel to be
maintained whilst the development of the
required system ‘intelligence’ is undertaken
as part of the research programme.
Test Platform (TP) Controller
This element of the control architecture
undertakes two key roles within the facility.
Firstly, it can be viewed as the vehicle
platform, for example a UAV and its
mission, delivering the stimulus, via the
‘stimulus network’ (Figure 1), to the
platform elements, such as the prime
movers and loading systems. The other role
provides the fundamental functionality of
the facility, including a safe means to start
and stop the system, the latter under both
normal and faulted conditions. It also has
the authority to over-ride the intelligent
system, (essentially the AEGDN), which
due to being developed as part of the
research programme offers an element of
risk, such as malfunction or misinformed
decision making. The schematic of Figure
13 shows the structure of the TP controller,
which comprises two networks. The
protection scheme utilises a PLC fieldbus
communications protocol, which is
accessed via a plant control system
operated by a human machine interface
(HMI). It is intended that this be developed
by Rolls-Royce Data Systems and
Solutions, a Rolls-Royce company that
specialises in PLC and safety critical
systems. The ‘stimulus network’ is used to
operate the hardware over defined mission
cycles, which are input via the flight
control system HMI.
Master PLC
PLC fieldbus
PLC
PLC
PLC
PLC
Active load
control
Active load
control
PLC
Active load
control
PLC
Active load
control
Load emulators / Energy storage emulators
HP prime mover LP prime mover
and generator and generator
Figure 13: Configuration of the test platform
controller
Autonomous Electrical Generation and
Distribution Network (AEGDN)
This forms the intelligence of the electrical
system, utilising field data to monitor its
health and status. Through real-time
analysis of this data, it is intended that the
system will determine any action that
should be taken. For example, in the case of
monitoring
currents
and
voltages
throughout the network, it will be able to
distinguish between normal operating
transients and those resulting from faulted
conditions. Similarly, it is intended that the
system distinguish between different types
of faults; earth leakage, line-to-line; line-toearth, etc, and locate these with sufficient
precision such that the faulted hardware can
be isolated [1].
Data Acquisition System (DAQ)
The DAQ continuously monitors all signals
in the IEPNEF, such as voltages, currents,
temperatures, etc, and communicates this
data to the TP and AEGDN as required.
Future Plans
There are three main streams of research
being undertaken throughout this year. The
first key objective is the development and
implementation of the bi-directional active
loads, to enable, in the longer term, the
implications of load regeneration to be
addressed. The second key objective is to
enable parallel operation of the generators
onto a common DC bus, thereby offering a
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rudimentary degree of system reconfigurability. Finally, some faulted
operation of the system will be
demonstrated, most likely in the form of the
loss of one or more phases of the PMG, to
demonstrate, to some degree, the fault
handling ability of the system.
In support of the key objectives, the
modelling and simulation activities will
continue throughout the remainder of the
SEAS DTC programme. At present the
modelling and simulation comprises of two
main parallel activities. One activity is
concerned with modelling the operational
performance of the complete IEPNEF,
addressing issues such as system start and
stop, generator load sharing, voltage
regulation and power quality, etc, which is
being undertaken through a collaboration of
Manchester UTC and the new Rolls-Royce
Electrical Systems Operating Business Unit
(OBU). The other activity is focused
specifically on the protection and fault
handling requirements for the facility,
which is being primarily undertaken by the
Rolls-Royce UTC at The University of
Strathclyde.
Conclusions
This paper has presented a progress update
on the development of the IEPNEF, which
is being housed at the Rolls-Royce
Manchester UTC. The system architecture,
including the prime movers, the loading
systems and the controller architecture, has
been presented, together with the aims for
the remainder of the programme. The HP
spool has been completed following the
characterisation and commissioning of the
SRSG. The LP spool is nearing completion
given the imminent delivery of the PMG
from Sheffield UTC. The resistive loads
and unidirectional active programmable
loading systems have been selected and are
on order. Finally, the personality modules
for replicating any desired loading system
have been successfully developed and,
subsequently, work has commenced in
developing
systems.
the
bi-directional
loading
References
[1] Long, S.A., Trainer, D.R., “Ultra-compact
intelligent electrical networks,” 1st SEAS DTC
Technical Conference, Edinburgh 13th-14th
July 2006, Paper C5.
[2] Cross, A.M., Chivite, J., Forsyth, A.J., Smith,
S., Shuttleworth, R., Long, S.A., Trainer, D.R.,
‘Electrical system evaluation platform for
uninhabited autonomous vehicles,’ accepted
for SAE 2006 Power systems Conference.
[3] Cullen, J., Mitcham, A., ‘Permanent magnet
modular machines: new design philosophy,’
UK Magnetics Society meeting on electrical
drive systems for the more electric aircraft,
14th April 2005, Bristol University.
[4] Cross, A.M., et al., ‘Modelling and simulation
for the evaluation of electric power systems of
large passenger aircraft,’ Royal Aeronautical
Society Conference on the More-electric
Aircraft, London, April 2004, pp. 6.1-6.11.
[5] Cross, A.M., et al., ‘Modelling and simulation
of high-voltage power systems for civil
aircraft,’ Society of Automotive Engineers
Conference on Power Systems, No. 2004-013182, Reno, 2004.
Acknowledgements
The work reported in this paper was funded
by the Systems Engineering
for
Autonomous Systems (SEAS) Defence
Technology Centre, established by the UK
Ministry of Defence. The authors would
also like to acknowledge the continued
financial support of Rolls-Royce plc
throughout this work programme, and the
support of personnel both within RollsRoyce plc and the Rolls-Royce electrical
UTCs at the Universities of Manchester,
Sheffield and Strathclyde.
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