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CONTROLS FOR MARINE POWER & PROPULSION SYSTEMS
M BENATMANE
AD CRANE
N CLARKE
ALSTOM Power Conversion Ltd
Boughton Road
Rugby
CV21 1BU
Abstract :- Modern vessels feature a high degree of power and propulsion system integration in order to gain
significant operational performance and through life cost benefits. The requirement to provide very high
system performance places heavy demands upon control technology, particularly in military applications. This
paper describes the state of the art in power and propulsion system control technology, applicable to multiMegawatt system ratings.
2.
1.
Introduction
Marine power and propulsion systems employ large
scale distributed control systems with extensive
communication structures. The control systems
must be widely distributed in order to achieve the
required dynamic performance throughout the
system, to maximise system availability, to limit
the space occupied by equipment I/O
interconnections and to support manufacturability
of system components.
Digital controllers are used in order to facilitate
system integration, which is, in turn, heavily reliant
upon serial communications and efficient
communication protocols. Digital controllers also
facilitate the provision of the compact graphical
user interfaces that operators demand in the interest
of reducing a vessel’s complement.
A high degree of control system modularity and
standardisation is typically achieved throughout a
vessel’s power and propulsion system. A
significant proportion of the control hardware and
software may be identical across the vessel, but
inevitably, the most demanding parts of the
application will require dedicated solutions.
General control system
requirements
Extremely robust systems are required. In military
applications, these systems must tolerate a
significant degree of “fighting” damage whilst
continuing to operate at specified performance.
When the extent of damage exceeds a critical level,
performance will inevitably be affected, but the
system must continue to provide a level of
performance that is consistent with the extent of
damage. This progressive performance reduction is
known as graceful degradation.
Single point failure modes must be avoided. Whilst
it is operationally convenient to group a number of
user interfaces in a single location these interfaces
must also be repeated in order to avoid single point
failure modes.
In order to support system integration, many parts
of the system communicate with one another by
means of serial links. Serial links may be
vulnerable to fighting damage and redundancy is
always employed. The system must be designed to
have the minimum of dependency upon serial links.
Integrated power and propulsion systems are
typically built using multiples of standard modules
and this supports the use of distributed intelligence.
A group of several controllers may have common
functions within a modular array, eg, converters
with parallel-connected channels. In this case, a
means of controller arbitration must be provided.
Master-slave, majority voting, hot-swappable and
duty-standby configurations with “bump-free
transitions” are commonly employed.
tank levels. The system provides centralised status
monitoring and control of machinery allowing
regular routines such as fuel oil transfer to be
performed from a machinery control centre.
3.
3.3.2 Fluid/Cargo Systems
Fluid management may vary from simple bilge
level monitoring and manual control of ballast
pumps and valves, to the more complex control of
cargo pumps, calculation of load and stability
information, etc. The AVC may be configured to
display fluid temperature, viscosity, tonnage and
percentage fill, dependent on tank shape and cargo
temperature.
Automated Vessel Controls
Systems
There is a continuing requirement to reduce the
complement of future platforms, either commercial
or naval. Automated Vessel Control systems
(AVC) increased the efficiency of day to day
running and provided a greatly enhanced capability
within the constraint of reduced manpower.
The AVC system combines shipwide supervisory
control and monitoring together into a single
system, enabling operators to have a complete
overview of all ship systems. Distributed field
stations provide local plant interface, with multifunction operator workstations providing the
Human Machine Interface (HMI).
3.1
Power management
The effectiveness of a power generation and
distribution system is heavily dependent upon
power management controls, without which it
would be impossible to achieve the full benefits of
an integrated electric propulsion system.
3.2
Alarms and monitoring
The AVC system provide a comprehensive
centralised alarm and monitoring system with audio
visual annunciators, watchcall systems and multizone deadman/patrol man systems that can alert
personnel to potentially hazardous conditions
arising within the machinery spaces.
3.3
Vessel Management Systems
3.3.1 Auxiliary Systems
Auxiliary systems cover the basic processes
necessary for vessel operation. The health and
efficiency of these systems is vital and the AVC
system provides the constant monitoring of
parameters such as pressures, temperatures and
3.4
Diagnostics, Maintenance and
trending
AVC systems are designed to have a high fault
tolerance and to incorporate self-diagnostic
routines to aid swift fault finding. Networked
solutions enable a high degree of web based
diagnostic facilities, allowing the operator to view
high level systems network information as well as
the ability to drill down to the modules at plant
level.
Systems also provide features to assist in
diagnosing plant-side faults through comprehensive
on-line chart recording techniques for monitoring
any plant signal. Recording can be triggered by
individual events or may be selected to run
continuously. Results may be downloaded and
stored for further analysis onboard or transferred to
an office-based location.
Modules typically have short circuit and loop
monitoring to assist with swift fault detection in the
field.
4.
Power generation and
distribution systems
A high performance propulsion system will consist
of a number of generating sets feeding a busbar of
one or more sections. Typically, at least 4
generators and MV AC switchboards are provided,
as shown in Figure 1.
GT1
DG 1
DG 2
GT 2
Diesel
Generators
Gas Turbine
Gas Turbine
G
G
G
G
MV AC
SB2
SB1
SSTX
1
Harmonic Filter
SB4
SB3
SSTX
2
Harmonic
Filter
Harmonic Filter
Harmonic
Filter
LV AC
PC1
PC2
LV Ships Service Distribution System
Multi-channel
propulsion converter
Multi-channel
propulsion converter
PM 1
PM 2
Propulsion motor
Propulsion motor
Figure 1. Typical Single Line Diagram
The MV AC switchboards may be linked together
using a bus tie (single island operation) or they may
be operated in an isolated manner (multi-island
operation). This main distribution system is
subsequently linked to a LV ships service
distribution system. The objective of the
architecture is to allow sufficient flexibility to
match the number and type of prime movers in
operation to the total system load, in the most
effective manner, the emphasis typically being
upon fuel efficiency.
The total installed generation capacity will depend
on the total ship electrical load and any diversity
factors that need to be applied to meet the total load
with the equivalent operating profile and the
required redundancy.
4.1
Prime mover controls
The control of generator prime mover speed and
hence generator output frequency, is performed by
the prime mover governor.
For stable control of active power, the governor has
a drooping characteristic, which causes the engine
speed to fall with an increase in active (kW) load.
Without this droop, the prime mover would either
develop maximum or minimum output depending
on the speed setting of the governor in relation to
the speed corresponding to the supply or busbar
frequency. Droop is employed to ensure load
sharing of parallel-connected generators.
4.2
Generator controls
The control of generator voltage is achieved
through the amount of excitation applied to the
generator field via the Automatic Voltage
Regulator (AVR).
The AVR, like the governor is equipped with a
drooping voltage / reactive load characteristic to
allow stable, parallel operation with other generator
sets.
The amount of influence the governor and AVR
set-points have on the system frequency and
voltage respectively will be a function of the
individual rating of the generator in relation to the
composite rating of other generators connected to
the system.
For the purposes of this paper, a simplified
generator equivalent circuit is given in Figure 2.
Machine resistance and the effects of saturation
have been neglected. Corresponding phasor
diagrams that illustrate the influence of controls are
shown in Figures 3a&b.
Xd
I
VXd
E
Vt
Figure 2: Simplified generator equivalent circuit
diagram
P
P'
E
'
VX
d
VX'd
E'
P

 '
Q
Vt
I
Q'
I'
Q
Figure 3a: Phasor diagram for an increase in input
power
P
P
E'
E
'
Xd
VXd
V
simultaneous trimming of the appropriate governor
and AVR set points following a change in system
load thus restoring system frequency and voltage
back to a pre-set, nominal value. It is, therefore,
important to recognise that the PMS must be
insensitive to any transient phenomena discussed in
the following section.
Modern electronic governors have the facility to
operate in isochronous mode. In this mode, the load
sensors on each of the governors are effectively
linked together using load share lines. Any
imbalance in active load between connected
generators will cause a change to the regulating
circuit in each governor. This will result in each
governor producing its proportional share of the
load to meet the total load demand. Under this
operating condition, equal active power sharing is
achieved at rated speed (and hence frequency) i.e.
without the need for any intervention from an
external control system. Whilst this operating mode
has many benefits such as excellent dynamic
response to load changes and tighter variation
limits, it should be noted that such a system can
only be configured for identical governors and
therefore is usually limited to power systems
containing identical prime mover / governor types.
Similarly, the voltage droop can be overcome by
connecting the individual AVR sensing circuits in
series. This is often referred as ‘cross current
compounding’. Like governors configured for
isochronous control, it is not normally possible to
configure non-identical AVRs for such control.
 '

' I
Q
Vt
Q
Q'
I'
Figure 3b: Phasor diagram for an increase in
excitation for constant input power
4.3
Power management systems
With ship electrical propulsion systems, the
magnitude of system load can be subject to
frequent variations due to the variation in
propulsion load. Therefore, system frequency and
voltage will also vary due to governor and AVR
droop respectively. Often, an external control
system, such as a Power Management System
(PMS) is employed to supervise power system
operation. One function of the PMS is to control
the AVR and governor droop effect by
4.4
Transient performance
As a result of various limitations imposed, a ship
power system is essentially a weak power system.
It is therefore important to recognise that a load
transient whether a predicted occurrence such as a
motor start or intentional switching event, or an
unpredictable disturbance such as a fault will
influence the system voltage. Excessive transient
voltage dips on a power system can cause
undesirable effects – contactors and other items
may trip on under voltage causing a supply outage
to essential pieces of equipment. Excessive
transient over voltages may damage sensitive
equipment.
The transient voltage response of the system will be
dependent on the size of the load application in
relation to the generation capacity. As the generator
characteristic is mainly reactive, the effect on the
generator terminal voltage will be dependent on the
power factor of the combined load and fault that
may possibly occur.
During Direct On Line (DOL) starting of motor
loads, the starting current may cause a transient
voltage dip on the power system. Obviously this
sudden step change in load does not apply to
Variable Speed Drives (VSDs) as their load is
usually increased or decreased at a pre-defined rate
to minimise the disturbance to the power system.
VSDs and other loads may be fed via a step down
transformer because generation voltage for large
ship systems can be as high as 13.8kV. When a
transformer is first energised, inrush current flows.
Inrush current is unavoidable and not a fault
condition. As a consequence, transformer feeder
protection must not operate spuriously during this
transient condition.
Unfortunately, different types of prime movers
react in different ways to a step load application. It
is therefore necessary to manage load increases
according to the nature of generator reserve
capacity.
4.5
Connection of generators
The rapid connection of generators to the power
system is a common requirement on ships. The
ability of the power system to support increasing
load demand is essential if blackouts are to be
avoided and vessel manoeuvrability is to be
maintained. The power system will incorporate an
automatic synchroniser for each generator that
provides fast, accurate and reliable connection of
the generator to the power system.
Systems tend to operate with the generators
running close to their rated output power whenever
possible in order to maximise fuel efficiency and
avoid long term performance degradation. This will
obviously limit the power systems ability to take on
additional load should the need arise and any load
limitation must be temporary.
4.6
Ships service distribution systems
A wide range of auxiliary loads are supplied by
ships service distribution systems (SSDS). A
widely distributed control system is required to
support total ship power management. Whereas it
is a simple matter to reduce propulsion power when
auxiliary loads are a priority, when the propulsion
load must be prioritised, particularly when power
generation has been compromised, a significant
decision process is required in order to determine
which auxiliary loads must be sacrificed. This
decision process is further complicated by the
highly re-configurable zonal and redundant
architecture that is required in order to maximise
availability.
4.6.1 Quality of power supply
The SSDS has multiple links with the MV
distribution system and it is common for the SSDS
Quality of Power Supply (QPS) requirements to be
more stringent than that for the MV System. LV
QPS is increasingly responsible for the use of
active filters. In effect, active filters have
comparable control requirements to modern high
performance propulsion drives and high bandwidth
digital controllers are employed.
4.6.2 Zonal converters
High bandwidth digital controllers are also used to
control zonal converters associated with local
energy storage devices and high integrity power
supplies.
5.
Propulsion motor systems
5.1
Drive controller architecture
The controller for a propulsion motor drive system
can be conceptually split into three main areas of
functionality; the machine controls, the supply
interface and system interfacing. In practice these
three areas are highly integrated and are supported
by a large number of housekeeping functions.
Virtually all modern controllers employ digital
technology for the following reasons;
a)
Programmability.
b)
Repeatability and stability of performance.
c)
Ability to control complex multi-variable
systems.
d)
Compatibility with modern control theory
and modelling techniques.
e)
Ease of installation and commissioning.
f)
Ease of system integration.
h)
Advanced communication capability.
i)
Advanced diagnostic capability.
Possibility of standardised modular solutions for a
range of applications.
It is possible to partition the hardware and software
in a number of ways, depending on the
performance requirements. In principle, the above
three main areas of functionality could each
employ independent hardware and software, but the
communication overhead would be problematic in
all but the most simple power systems. At the
opposite end of the scale, a single hardware
platform might be used, but a very structured
programming approach would be required, system
development would be extremely time consuming
and the system would be difficult to support in
service. In practice, hardware and software
partitioning is employed so the most appropriate
solutions are used for each area of functionality,
while communication structure complexity is
limited.
5.2
Rotating machine controls
The nature of machine control is heavily dependant
upon the machine and converter topology, but the
objective is to ensure that the machine and
converter are both used most effectively in order to
satisfy performance requirements. The propulsion
motor control strategy typically influences the
following system attributes;
a)
Efficiency.
b)
Availability.
c)
Structure-borne noise and vibration
signature.
d)
Power system stability.
e)
Machine size.
f)
Converter size.
It is common place to use the drive controller as a
means of optimising both converter and the
controlled machine performance.
5.3
Supply interface
Propulsion converters exert a heavy influence upon
their supply network because they are typically a
large fraction of the installed power generation
capacity and because of their inherent nonlinearity. This non-linearity affects supply
harmonic content, power factor, voltage regulation
and stability. The supply side controls must
minimise this impact and the ability to do this is
entirely dependant upon the type of power
converter employed.
Harmonics may be reduced by classical phase
shifting techniques in thyristor supply converters.
Harmonics, power factor and voltage regulation
may be actively controlled by pulse width
modulated power converters.
By virtue of their power regulating functionality,
most modern converters present a negative
impedance load to the supply network. Actively
controlled synthetic damping must be provided by
the controller, particularly in converters that
employ energy storage devices, if power system
instability is to be avoided.
5.4
System interfaces
5.4.1 User interfaces
Redundant serial links are provided to enable the
drive to communicate with the vessel’s user
interfaces. Typically, a fixed message structure is
employed and this incorporates mobility control
referencing, operating mode selection, status
monitoring and diagnostics.
A primitive form of the remote user interface is
often provided on a local control panel to support
operation in reversionary modes. A similar serial
messaging structure to that of the main external
user interface is employed and this may contain
additional diagnostic data.
5.4.2 Plant interfaces
Redundant serial links and dedicated fail safe hard
wired links are provided for the local plant
interfaces. Hard wired links are employed for
critical interfaces, eg, motor protection. Serial
links are used to communicate with less critical
functions, eg, motor instrumentation.
6.
Conclusions
This paper has summarised the challenges
presented by modern power and propulsion systems
and has emphasised the need for system
integration. In future, system integration will be
extended to include new technologies:
Fuel cells, micro-turbine generators and bulk
energy stores will be incorporated. These power
sources present a high supply impedance and
therefore represent a significant challenge with
respect to power system stability.
Electric weapons and launchers will present heavy,
intermittent demands and will therefore represent a
significant challenge with respect to power system
transient response.
Acknowledgements
The authors are grateful to the Management of
ALSTOM Power Conversion Ltd for their
permission to publish this paper.
References:
1. NJ Clarke, Marine Electrical Power Systems,
2nd IEE Conference on Power Electronics
Machines and Drives,2004.
2. M Benatmane, All Electric Drill Ship,
AES 2000, Paris, France
3. M Benatmane/RE Maltby, Electric Propulsion
Architectures in Capital Warships, INEC 2004
4. M Murphy/JA Buckley, Machine Engineering
Challenges for the 21st Century, Electric
Propulsion the next Phase. INEC 2000