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 multi-Megawatt system ratings.
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.
2.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.
3.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 multi-function operator workstations providing the Human Machine Interface (HMI).
3.1Power 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.
3.2Alarms and monitoring
The AVC system provide a comprehensive centralised alarm and monitoring system with audio visual annunciators, watchcall systems and multi-zone deadman/patrol man systems that can alert personnel to potentially hazardous conditions arising within the machinery spaces.
3.3Vessel Management Systems
3.3.1Auxiliary 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 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.
3.3.2Fluid/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.
3.4Diagnostics, 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.
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 issubsequently 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.
4.1Prime 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.
4.2Generator 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.
Figure 2: Simplified generator equivalent circuit diagram
Figure 3a: Phasor diagram for an increase in input power
Figure 3b: Phasor diagram for an increase in excitation for constant input power
4.3Power 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 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.
4.4Transient 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.
4.5Connection 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.
4.6Ships 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.
4.6.1Quality 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.
4.6.2Zonal 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
5.1Drive 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-variablesystems.
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.
5.2Rotating 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.
5.3Supply 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 non-linearity. 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 isentirely 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.
5.4System interfaces
5.4.1User 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.
5.4.2Plant 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.
- 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: