of HMS Conqueror before returning to the MOD as the Head of the Nuclear Steam ... serving with the Vanguard class submarine project and then as a Project ...
The Electric Warship II *Cdr C G Hodge, BSc, MSc, CEng, FIMarE, RN and Cdr D J Mattick, BSc, CEng, MIEE, MINucE, RN MOD, Bath Integrated full electric propulsion has become a normal first choice for several commercial operators, in particular for those operating cruise ships, where it proves to be a fuel efficient transmission system. Operating profiles for warships of frigate size and above are similar to those for cruise ships, with long periods at well below full power. However, the size, weight and initial cost of electric propulsion equipment has generally precluded its selection for warships. With the application of modern technologies, equipment power densities are being increased such that integrated full electric propulsion (IFEP) has now become a serious competitor to the mechanical transmission systems traditionally adopted for warships. The Ministry of Defence has a development programme in hand aimed at enabling future warships to gain the benefits of IFEP. This paper follows on from one presented at The Institute of Marine Engineers by the same authors one year ago. It reviews the progress made over the last year towards being able to define the requirements for a future warship designed and built to the IFEP and Electric Warship concepts.
INTRODUCTION
The following is a very brief summary of the first Electric Warship paper:
ship’s service power systems are significant; even before allowing for manpower reduction the savings may be sufficient to allow purchase of one extra ship in a class of around 30 vessels. However, a frigate sized hull is small by comparison to the merchant navy vessels in which electric propulsion is employed and again, as was explained in the authors’ previous paper, the size of motors and converters needs to be reduced if they are to be installed in ships of such a small size. The earlier paper concluded that compact motors of the required power and torque density were achievable through Permanent Magnet Motor designs, utilising either the Axial Flux or Transverse Flux topologies, and that these could be fed by Pulse Width Modulated (PWM) Converters using Insulated Gate Bipolar Transistors (IGBTs).
1. The feasibility of employing a common power system for both propulsion and ship’s services has been under investigation for some considerable time. As those aware of the authors’ previous paper on this subject will know, the financial savings by integrating the propulsion and
2. The Electric Ship (ES) concept was developed from Integrated Full Electric Propulsion (IFEP) and it was aimed to reduce Unit Purchase Cost (UPC) and yet retain the IFEP reduced Running Cost (RC). Merchant Navy applications of IFEP have always had the advan-
This paper is the second on this subject presented at The Institute of Marine Engineers, the first being read here almost exactly one year ago1. The aim of this paper is to provide an update on general progress and developments in what was then, and remains now, an exciting and challenging field for the authors.
BACKGROUND
Authors’ biographies After initial training as a mechanical engineer, and service as an Assistant Marine Engineer in HMS Warspite, Christopher Hodge joined HMS Orpheus as the Marine Engineer Officer in 1982. He subsequently took an MSc in Electrical Marine Engineering and served in the MOD as the project officer for electrical ship propulsion. After promotion to Commander in 1989 he served as the Marine Engineer Officer of HMS Conqueror before returning to the MOD as the Head of the Nuclear Steam Raising Plant Electrical Design Authority. Since 1993 he has served as the head of the electrical power systems specialist group within the MOD. Cdr David Mattick initially specialised as a nuclear submarine weapon engineer. After various weapon engineering jobs both at sea and in the MOD, including Weapon Engineer Officer of HMS Warspite, he was appointed as the Marine Engineer Officer of HMS Swiftsure in 1982. After promotion to Commander in 1984 he headed the electrical power systems specialist group within the MOD, subsequently serving with the Vanguard class submarine project and then as a Project Manager with Director General Ship Refitting. In 1994 he became the Surface Ship Marine Engineering Desk Officer in Director Future Projects (Naval), charged with concept design of future naval vessels. He has recently been appointed as the Electric Ship Programme Manager.
*The views expressed in this paper are those of the authors and do not necessarily represent those of the Ministry of Defence or HM Government.
© British Crown Copyright 1996/MOD.
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Fig 1 Electric warship power system
tage of space and have thus been able to fit commercially available equipment. Perhaps a much more important advantage, however, has been the willingness of the Merchant Navy operator to accept an increased UPC in order to gain a reduced Through Life Cost (TLC). The result of this is that the Merchant Navy ship is frequently found to be fitted with a high number of prime movers; each can then be operated closer to its optimum loading because the incremental step between the change caused by a single additional prime mover is spread among a number of other engines. This brings the maximum advantage in terms of fuel efficiency, albeit at the expense of a higher UPC. The Royal Navy does not share this advantage; UPC must be minimised together with TLC. The ES has two main features in addition to those to be found in a traditional IFEP vessel. a. Minimum Generator Operation. The UPC and space constraints require fewer but more highly rated prime movers than would be found in a merchant IFEP vessel. In order to restore the fuel savings conceded by fitting fewer prime movers the ES runs under a regime of minimum generator operation; for the majority of the vessel’s operation only one prime mover may be in use at any one time. This produces a much more favourable engine loading, and brings significant gains in both fuel consumption and maintenance costs due to the minimised engine running hours. b. Electrification of Auxiliaries. Additional maintenance and manpower reductions can be achieved by using electric auxiliaries wherever possible. In addition, there will be benefits to be gained from this policy in terms of overall weight of equipment fitted (central energy storage and high reliability ship-wide electric power systems). 128
3. The authors’ earlier paper described their early vision of an Electric Ship Power System; it is shown in Fig 1. Surprisingly for the authors it has proved a remarkably robust vision; a few minor changes have occurred and these will be dealt with later in this paper. The underlying concept of using dc for the lower power ship’s service power system and ac for the high power propulsion requirement remains, as does the use of a battery for back up during the failure of a single generator.
RECENT WORK The following is a description of the recent work conducted in support of the ES programme.
Ac and dc comparison The comparison of ac and dc for power transmission was studied within the parameters of a future frigate design of the ES architecture. The principal reason for choosing dc for power transmission in the early work was that the number of power conversions was thereby minimised. This arose from the fact that the prime movers for the ES were, and are to be, compact complex cycle gas turbines. To maximise the benefits of reducing size, the generators coupled to these engines are to be directly driven. Not only does this avoid the weight and volume penalty of a gearbox but it also reduces the size of the generator itself, since at higher speed the torque requirement for any given speed is reduced in proportion. The expected speeds for the prime movers are: 1. 1.25 MW: 20 000 rev/min; 2. 6–10 MW: 7000 rev/min; 3. 21–25 MW (WR21): 3600 rev/min.
Fig 2 Induction motor efficiency
Clearly the 1.25 MW and 6–10 MW generators will be running at far above the standard 60 Hz maximum of 3600 rev/min. Even though the WR21 gas turbines are being designed for 3600 rev/min there may be a fuel efficiency advantage to running with a variable engine speed related to load. Therefore, some form of power conditioning must be used to interconnect these engines into a unified power system. This will not be a difficult problem to solve, power electronics already holds the answer. However, the type of power electronic solution will determine the overall efficiency of this power conditioning and distribution subsystem. The second part of the original argument in support of dc distribution is the expectation of additional user power conditioning after distribution. There are many reasons to expect such activity: 4. Variable speed motor drives. At present most mechanical pumps align their output to the system requirement by throttling the output created by constant speed motors. There will be efficiency gains to be drawn by running the pump at the correct speed for its required volumetric output. This will require power conditioning of some form. 5. Variable voltage motor drives. Apart from matching voltage to frequency within a variable speed drive there will be efficiency savings to be made by matching voltage to load for constant speed induction motor drives. The variation of induction motor efficiency with slip is shown in Fig 2. The savings arise by increasing the slip of an induction motor by reducing the applied voltage to a point where the efficiency is at its maximum. Once again this technique will require some form of power conditioning. 6. Weapon system supplies. The normal practice for weapon system equipment is to produce the specialised power supplies the equipment requires by direct conversion from the standard supply; evidently this also requires power conditioning. If the power from the generators was distributed as at present, power electronics would be used first to rectify the
non-standard frequency output of the generator and then invert it back to a fixed 60 Hz. However, as noted above, the majority of power users will also be applying power conditioning of the same form: they will be rectifying their 60 Hz input and then inverting it back to the required frequency and voltage. There is, therefore, in this power transmission train, a nugatory inversion (at the end of the generator power conditioning) and a nugatory rectification (at the start of the user power conditioning). This represents half of the total amount of power conditioning and therefore wastes, to no purpose, half of the losses it causes. Clearly a more efficient system would remove the nugatory steps in the power conditioning, distribute the power as dc and allow the users to invert directly to their requirement. The dc/ac study2 took the argument further and also showed that, in terms of the requirement to fit copper, dc held the edge over ac. In an ac distribution system three conductors transmit √3 times the simplistic line power (line RMS voltage multiplied by line RMS current). However, this is further reduced by the power factor of the loads being fed by the cables which, for example, might be 0.8. Therefore, a three phase ac distribution system transmits around 1.386 times more power than the RMS line volt amp product and uses three conductors to achieve this. Turning to dc: a pair of dc conductors transmits precisely the line volt amp product, however the limits of voltage are now different. The dc conductor can transmit power at the full limit set by the cable insulation. An ac conductor can not; it transmits at the RMS value of the same peak level and in consequence the dc cable gains a further advantage of √2. Therefore, when compared to the single line current and voltage of an ac system, the overall factors are: for three ac conductors 1.386; and for two dc conductors 1.414. This implies that less copper will be required to transmit the same power in a dc distribution system than in an ac equivalent. The disadvantage lies in the availability of dc switchgear and its cost and weight. The study showed that the choice between ac and dc for the ship’s service distribution system was finely balanced, with the final choice being dictated by considerations of system efficiency. However, modern power electronic developments may offer a solution: the heavy and expensive dc 129
Fig 3 Resonant pole converter
Fig 4 IGBT equivalent circuit
Fig 6 Emitter switched MCT equivalent circuit
circuit-breakers could be replaced by solid state breakers or, indeed, much of this fault limiting function could be incorporated within the power conditioning electronics. With this concept, the choice of dc for ship’s service power transmission becomes almost axiomatic, since it would offer reduced initial cost, reduced weight and improved transmission efficiency.
Power electronics
Fig 5 MCT equivalent circuit 130
The status of the IGBT as a rapidly maturing technology has been affirmed over the last year. Several commercial companies are using the devices routinely in their power electronic products. The voltage rating of the device is increasing rapidly, 2 kV is soon to be achieved. The difficulties of
Fig 7 ICR propulsion module
operating IGBTs in series are being overcome and the present view of the limiting power rating of a PWM IGBT Converter is already above that required for the Electric Ship. 20 MW PWM IGBT Converters are being designed and at least one is being built. Designs of converters at up to 40 MW appear feasible. Such satisfactory progress can not be reported on the p-type MOSFET Controlled Thyristor (pMCT); it remains very lowly rated in terms of current when hard switching (at significant current and voltage) without bulky snubber circuits. However, soft switching (at zero current or voltage) can allow use of a pMCT at currents up to 300A , and this suits the newly developing class of resonant converters where the switching occurs at a zero current or voltage induced by the current transient in an auxiliary resonant circuit. Resonant converters also offer improved efficiency through removal of switching losses. A diagram of one leg of a resonant PWM converter is shown in Fig 3. Two variants of the original pMCT, the emitter switched MCT and the n-type MOSFET Controlled Thyristor (nMCT), are beginning to solve the production difficulties causing the low hard switching rating of the original p-type MCT3. A robust MCT, with a rating of above 300A and 1000V when hard switching, without snubber circuits, is confidently expected to be marketed within the next year. Apart from the difficulties of manufacture there are also problems associated with operation of the MCT, which negate one of its main advantages. Thyristors have a natural positive feed-back circuit to their base which reinforces the ‘on’ state; this ‘clamps’ the thyristor in the ‘on’ state and allows it to display a forward bias volt drop of less than 1V, and significantly less than that for a bipolar power transistor which is around 2V. The difficulty with the MCT is that it can not be allowed fully to ‘clamp’ into the ‘on’ state if the MOSFET drive is to be able to turn the device off with the required speed. This means that the
conducting forward volt drop for an MCT more nearly matches that of the IGBT than might otherwise be thought, and thus the efficiency argument alone is not as strong a reason for use of an MCT rather than an IGBT. The original view that IGBTs would offer sufficient advantage in terms of volume reduction of converter sizes, and be available with adequate voltage and current ratings, has already been fully justified. Indeed, the converter associated with the Permanent Magnet Propulsion Motor (PMPM) prototype development will be based on IGBT technology. Figures 4, 5 and 6 show equivalent circuits for the IGBT, generic MCT and emitter switched MCT.
Energy storage The use of a battery for energy storage and to allow reduction to single prime mover operation was taken from the perspective of two nuclear submarine engineers. However, the decision has been reinforced by other considerations. The battery is available now, therefore no development is required. The impact on mass can be minimised by removing some of the ballast already being fitted. Vulnerability and operational effectiveness considerations gain much from having a battery fitted. The advantage of not having to run any prime movers at all can have great consequences when tracking submarines (when noise is a huge penalty), or when attempting to survive an attack involving heat sensitive missiles. The possibility of using fuel cells at a future date can not be discounted, but the long time constants associated with changing fuel cell power may well require a large energy source and sink, such as a battery. This would be particularly relevant if use were made of the hydrogen being reformed from standard hydrocarbon based fuel. The stabil131
Fig 8 Specific fuel consumptions
Fig 9 NOx emissions
ity of the dc power system will be much improved by the addition of a battery and finally, but not inconsequentially, the battery, if operated to a suitable regime, will be able to provide a sink for the regenerated power from the propulsion system during transient manoeuvring. There are other possibilities for energy storage other than a battery, however their associated development costs, lacking for a battery, will prove a difficult hurdle to overcome. A review of energy storage is to be undertaken and this will look particularly at flywheel based systems.
Prime mover development The WR21 ICR gas turbine (Fig 7), rated at 21–25 MW, is being developed on a US Navy contract with Northrop Grumann as the prime contractor. They, in turn, have subcontracted the engine development to Rolls-Royce who are marinising various elements of the RB211 and Trent engines. Development testing continues well at the RN gas turbine 132
test establishment at Pyestock, with several hundred hours of recuperated and intercooled operation completed. RollsRoyce have produced various papers4–6 describing the engine and its development. The significant point to note is the fuel economy, as shown in the graph (Fig 8) taken from the authors’ earlier paper. It shows the specific fuel consumption of the WR21 and compares it with that of an equivalently rated simple cycle gas turbine and a medium speed diesel; it clearly shows the impact of intercooling and recuperation. If to this impressive part load performance of the WR21 is added the modular design of the engine – which reduces maintenance costs compared with previous generations of gas turbine – and the lower NOx emissions (Fig 9), then this power dense prime mover is likely to be widely used in future warships. Whilst the MOD has not done extensive studies, it seems likely that the engine matches the requirements of some sectors of the commercial marine market as well. In the authors’ previous paper1 it was suggested that two other sizes of prime mover would be the optimal fit to
Fig 10 1–2 MW gas turbine alternator: typical development programme
Table I Vulnerability 1. IFEP installation flexibility enables more survivable installations. 2. Vulnerability significantly influenced by detail of installation. 3. Versatile tools needed to assist ship layout decisions.
minimise the running costs of the ES: ie, engines rated at 7 MW and a 11/4 MW engine. The MOD is currently conducting an investment appraisal to determine how much they can afford to invest in engine development and still show a return from RC savings over the service life of these equipments. What is already clear is that industrial and inter-governmental co-operation will be required if the marinisation, recuperation and intercooling (where appropriate) development, and introduction to service costs are to be paid back by RC savings. Whilst RC savings are significant, the cost discounting procedure militates against early spending on development and ship fit. With regard to the smallest engine, several potential candidates have been identified and there is the potential for commercial application in the rail traction industry. The MOD is undertaking feasibility studies of these with the Netherlands and France to identify the national contributions required and the advantages of these engines in warship applications. Figure 10 shows a possible current programme to deliver such an engine. The medium sized engine is less advanced. The MOD is undertaking a market survey, both to establish what is available and suitable for developing into a marinised complex cycle engine and to determine the breadth of potential applications for such an engine; it is close to the shaft power requirements of auxiliary vessels, as well as being the optimum cruise power for major warships. The MOD is also negotiating with other countries to assess which would wish to be involved in the co-operative development.
Ship studies A number of ship studies have now been conducted in several areas and some are outlined below: 1. Vulnerability. Two studies have been conducted on this important military facet and a combined MOD/YARD paper is to be presented at the SEE AES Conference in Paris next year. The findings of these studies (Table I) are that, not unexpectedly, the flexibility of arrangement of an IFEP installation has the potential for far better survivability than a mechanical transmission system. However, the improvement is very dependent on the detail of the installation. Areas where there is no redundancy – perhaps from the converter to the motor – are best located close together (this is similar to the ‘unit’ philosophy of earlier generations of warships). Where there is significant redundancy it is important that geographical separation is adopted, for example for parallel supplies to the converters. Whilst this is intuitively sensible, numerical analysis of an installation is needed if the best fit is to be realised. This in turn needs versatile and quick tools to allow an installation to be assessed by the naval architect, without having to wait excessive amounts of time for the return of the latest results – a typical cycle time can be as long as six months – and these might just assess a supply cable being routed forward or aft of a bulkhead. There is another facet, the number of shafts. Given that a two shaft installation is not essential – the merchant marine application of thrusters, azimuthing podded drives, tandem motors and sophisticated control systems shows this – how is survivability best ensured in the face of underwater shocks? Assessments are underway on various alternative shaft lengths and podded externally mounted propulsors, both separately and in combination. Possible options are shown in Fig 11. 133
Fig 11 Single shaft and podded propulsor
2. Whole ship studies. Whole ship studies contributing to the prime mover investment appraisal include an escort vessel fit of just two WR21 engines, with an anchor load machine of 11/4 MW. Whilst this does not realise the RC savings that a 7 MW cruise engine yields, it does reduce both the development cost and the initial ship fit costs. However, before any firm conclusions can be drawn concerning the acceptability of such a fit, the assumptions need to be re-examined (the suite of tools used in the assessment is based on historical data and does not allow for the flexibility of an electric propulsion installation) and the Availability, Reliability and Maintainability (ARM) performance of such an installation is being assessed. 3. Naval architecture. Naval architectural studies of IFEP and ES installations have been commissioned and the results are starting to be received. Whilst there is no overview available at the moment, what follows is one vision of a future aircraft carrier (Fig 12) adopting the electric ship architecture. As is commonly known, the MOD is considering a trimaran option for future escorts and intends to build a demonstrator to validate the naval architecture design tools required. From a naval architecture perspective, and when compared with a monohull, the trimaran offers the same upper deck space in a smaller displacement (and cheaper) ship. From the marine engineering perspective, the advantage of a trimaran is that less shaft power is required to achieve maximum speed albeit, as indicated in Fig 13, more power is required to propel a similar displacement trimaran below about 20 kn. However, if 134
the trimaran with an equivalent weapons fit – and weapons dictate the upper deck space – is lower displacement to the monohull, the additional low speed powering requirement is defrayed. The marine engineering challenge of a trimaran is that the centre hull is long and thin compared with a monohull, and the side hulls are narrow and of small displacement; the available ship volume to fit machinery is thus low. Certainly, IFEP is a natural partner to the trimaran and, if time is available, the full ES architecture can provide an optimum solution with low UPC and RC.
POSSIBLE UPDATED ELECTRIC SHIP ARCHITECTURE This is a fast moving area and is likely to have changed between the paper being written and presented. However, the issues are discussed below. To avoid confusion between the original (Fig 1) and updated systems, the assumption of a 4500t monohull capable of 30 kn is common to the previous paper1.
Propulsion busbar The voltage and frequency of the propulsion busbar have not yet been determined but will reflect that which is most suitable for the converter to drive the PMPM; a development contract is shortly to be placed. However, current indica-
Fig 12 Potential layout of a future aircraft carrier
Fig 13 Trimaran and monohull power speed curves
tions are that a high frequency (about 120 Hz) at 3.3 kV may prove to be the lowest cost fit, although there may be some advantage to a variable voltage on the busbar.
Ac versus dc for ship service ring main As discussed earlier, the current front runner is dc at 750V. A 750V battery is virtually three times the cost, volume and weight of one at 240V – the commonly used battery section in a submarine – and is thus not an economic proposition. Moreover, as battery terminal voltage varies with the state of charge, this can increase cable costs as the voltage falls, as the
battery supplies more current. One solution conceives a converter at the battery terminals – the losses are irrelevant as the battery spends most of its time on float. This could be a dc converter only, or just as easily an inverter if an ac distribution system proves advantageous. This arrangement also allows the battery sections to be split, with the advantages of improved vulnerability and less demanding protection requirements. The advantages of a dc distribution system stem from considerations of conversion losses in general and are based on the assumption that the majority of motor loads are supplied from variable speed drives. As explained earlier, there are RC advantages to variable voltage drives even for constant speed applications. 135
Fig 14 An alternative electric ship power system?
Cruise generator output There are good reasons for the cruise generator to supply the propulsion busbar; it reduces the power rating of the ac/dc converters and the current rating of the dc busbars. However, it does increase the system losses since the ship service loads are supplied from the propulsion busbar at sea. The impact of these changes is shown in Fig 14.
to confirm the detailed system architectures to support the ES. Some of the studies will yield authoritative answers to the questions posed earlier. Other studies are expected to continue through design and development into production of prototype equipment for ship trials. For example, one task will be to define the ratings for the Zone Power Supply Unit modules, assess various conversion options and then develop the prototype modules.
FUTURE WORK
Manpower
Auxiliaries MOD internal activity has commenced to assess the advantage, if any, of electrifying all auxiliaries, including steering gear and stabilisers. A first look at what is available in industry would indicate that electrification of any auxiliary is feasible – albeit the track record of electrical equipments exposed on the upper deck (such as winches and davits), or in the bilges (such as hull valve control and indication systems) is not as good as it should be. Unlike valve actuators, winches and davits, electric steering gear and stabilisers are not commonly available and a significant cost benefit will need to accrue if the MOD is to fund development alone. Work is now commencing to assess the benefits and costs in all areas and first indications are that electrification will result in larger equipments – but smaller systems when distributed hydraulic power packs are included. The comparative costing data is eagerly awaited.
As discussed in the previous paper1, it is expected that adoption of the ES architecture will lead to unmanned machinery rooms and bridge control of the propulsion system. As such, damage control in general – and firefighting in particular – has been shown to be the marine engineering manpower motivator, and studies are starting to assess where technology can assist this function and thus determine the minimum sized department. The difficulty is that all available work is based on traditional ship layouts with large machinery compartments. The advent of electric propulsion and unmanned machinery allows individual machines to be located in their own rooms, complete with fire sensing and suppression equipments, sophisticated monitoring systems and rapid shutdown facilities – for the complete room.
CONCLUSIONS IFEP studies A contract is soon to be let to undertake the studies required 136
Since last year the MOD’s plans have been refined and steps towards contracting the study and development work re-
quired to realise IFEP for warships and the ES architecture have been taken. The main achievement of the year has been the high level endorsement of the programme and support for its bold, yet realisable, aims of reduced RC by the adoption of IFEP, along with reduced UPC by the adoption of the ES architecture, without any loss of military effectiveness. Funding, inevitably, still remains an unresolved issue, albeit a good grip on the probable cost of the programme has emerged and various routes to making the finance available are being explored. Not least of these are contributions from other governments, where some progress has been made with some close allies who also recognise the validity of the aims. Less progress has been made with industrial contributions despite a widespread series of presentations to UK and foreign industry. The time has now come for industrial partners to assess their position and make firm offers to the MOD. If the audience has been convinced that the aims are realisable, then to be first into production of a low cost, warship qualified propulsion system has the potential to earn significant foreign sales in the next century. Can industry afford to miss this opportunity?
ACKNOWLEDGEMENT This paper is published with the permission of the Controller of Her Britannic Majesty’s Stationary Office.
REFERENCES 1. Cdr C G Hodge and Cdr D J Mattick, ‘The Electric Warship’, Trans IMarE, Vol 108, Part 2, pp 109–125 (1996). 2. YARD Report C4759/TR/LW8v1 (Unpublished Report). 3. D Flores, P Godignon, M Vellvehi, J Fernandez, S Hidalgo, J Rebollo and J Millan, ‘Standard and aborted anode non-punch through emitter switched thyristors’, Centro Nacional de Microelctronica (CNM) CSIC-UAB, Barcelona, Spain. 4. R North and R Dawson, ‘The design and initial development of a combustion system for the WR21 ICR gas turbine’, ASME-95-GT-361. 5. M L Parker, W Hawkins, J M Hutchinson, D A Branch and A K Broadbelt, ‘The US Navy’s intercooled recuperated gas turbine – from concept to reality’, Paper 11, INEC 94: Cost Effective Maritime Defence, IMarE Conference, Volume 106, 3, The Institute of Marine Engineers (1994). 6. E R Watson, M L Parker and D A Branch, ‘Development testing and validation of the WR21: an intercooled and recuperated marine gas turbine’, Paper 19, INEC 96: Warship Design: what is so different?, IMarE Conference, Volume 108, 3, The Institute of Marine Engineers (1996).
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C J Hodge and D J Mattick
Discussion Dr G Armstrong (Three Quays Marine Services) 1. Am I right in assuming that the dc ring main does not imply dc motors driving auxiliaries? 2. With reference to Fig 12, have the implications of a substantial machinery space forward been considered? Please comment on the supply of services and, particularly, the proposed exhaust run in relation to the position of the bridge. 3. With reference to Fig 14, the harbour load in the absence of shore power will be supplied by the 1.25 MW generator. If this is out of action the 7 MW generator must be used. Please comment on the question of low load running. 4. Is the fuel cell part of current thinking? Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) The authors thank Dr Armstrong for his questions. Taking them in turn: 1. There is no implication that the proposed dc ring main will bring dc motors with it. Indeed, it is the reduction of conversion losses gained through utilising dc distribution to supply ac motors that was at the heart of the original selection of a dc distribution system. It is our view that all motors will be ac fed, with the ideal frequency and voltage for their load and duty derived by power electronic inversion from the dc distribution system. Current thinking is that all auxiliary motors will be cheap induction motors (IM) and that they will be supplied through inverters from the dc bus. Using an inverter based on the back end of a variable speed or variable voltage drive will allow the IM to be soft started (and thus does not need to be designed to withstand the starting transient), and will allow the power consumed to be minimised (either by voltage variation to achieve the optimum efficiency state for a constant speed motor, or by speed variation to achieve the optimum efficiency state for the driven pump or fan for a variable speed installation). It is hoped that the only brushes onboard will be for scrubbing out! With regard to the propulsion busbar, the current concept is that this will be ac so that transformers can be used if required to minimise cabling costs by reducing the transmitted current at the high powers required. However, the specification for this has not yet been defined as it is not yet necessary – indeed, there is the possibility that variable frequency and/or voltage might be advantageous to the design of the converter supplying the PMPM. Until the PMPM contract has been let and systems aspects of the installation assessed, finalising the supply standard on the propulsion busbar is undesirable. 2. The implications of a major machinery space forward in the ship, and the arrangements for the exhaust with respect to the bridge and other services, have not been fully considered; the intention with Fig 12, and indeed 138
the proposal, was to emphasise that radical change might be possible with respect to the Naval Architecture of the Electric Warship as a whole that might not otherwise be possible. Figure 12 is purely illustrative and was included to emphasise the possibilities that electric propulsion could open up for the naval architect when designing ship layouts. Again, the implied supply and exhaust runs are also purely illustrative. For smaller displacement ships there are certainly indications that the status quo has a lot of advantage unless there is some other driver, such as the superstructure mounted diesels in a Type 23, which bring the advantage of acoustic isolation for the ASW role. 3. The use of the 7MW GTA to supply harbour load in the absence of the 1.25 MW engine is evidently not desirable from the perspective of fuel economy, however it is certainly practical and should not impose any insuperable control difficulties. Indications are that a gas turbine that is not constrained in speed (and the expectation is that the power electronics output stages will provide the appropriate quality of power supply without constraining the speed of the machine), can support 2-5% load for extended periods without detriment to AR or M characteristics. 4. The fuel cell is under investigation and we do expect it to become a useable marine power source in time. That will not occur until the cell can be run from a strategic fuel for which an infrastructure exists, such as Dieso. The reforming process which that implies might be scientifically understood and feasible, but implementing it in a robust, compact and marinised form presents the challenge. It is worth noting that the long time constants likely to be associated with power level changes for a Dieso supplied reformer and fuel cell, mean that a battery or other energy storage device will be required. Fuel cells are seen as the generators of the future. However, they are not seen as being available within the timescales of the next generation of future warships. Significant advantages of fuel cells, if and when they can be supplied with hydrogen reformed from a strategic fuel (Dieso or aviation fuel in surface warship terms), are likely to be the following: a. their full load efficiency can be expected to be higher than that of a diesel or gas turbine and that this efficiency rises as load reduces; b. the exhaust is likely to be more environmentally friendly than from a diesel or gas turbine. Dr R W G Bucknall (University College London) I would like to thank the authors for a very interesting and an extremely far sighted paper. The presentation was also very professional. I would like to ask the following questions: 1. The 7 MW gas turbine is quoted in the paper as operating at 7000 rev/min and quoted in the presentation as
Trans IMarE, Vol 109, Part 2, pp 127–144
operating between 7000 and 10 000 rev/min. If the latter is true, is it the case that power conditioning is required at the output of the alternator or are variable voltage, variable frequency (up to 167 Hz) busbars preferred? 2. Would the authors like to comment on how the adoption of an all-electric ship architecture will affect marine engineering training in the Royal Navy. What changes are most likely to be needed? 3. High speed alternators are available at low power for the aircraft industry. Are these types of alternator being considered for the all-electric ship or are new types being developed? Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) The authors thank Dr Bucknall for his kind comments. Taking his questions in turn: 1. The 7MW GTA has yet to developed; it is understood that a rating of 7MW implies between 7000 and 10 000 rev/min to maximise the efficiency of the gas turbine. Similarly, the 1.25 MW GTA is expected to rotate at around 20 000 rev/min and the WR21 at 3600 rev/min. Integration of these prime movers into a unified power system, which lies at the heart of the Electric Warship concept, will clearly require power conditioning. If the 7 MW GTA is connected onto the dc ring main, along with the 1.25 MW GTA, then clearly controlled rectification would provide the solution. Connection of the 7 MW GTA onto the medium voltage ac propulsion busbars would be economically desirable if the majority of the engine’s power output was used for propulsion. Once again power conditioning would be required to connect the engine into the same power system as the WR21 GTAs which would, at 3600 rev/min, be generating at 60 Hz or perhaps 120 Hz. Whether the ac propulsion system would operate to advantage under a variable frequency regime is yet to be decided. However, even then there will still be a requirement for power conditioning and this would be best applied to the smaller 7 MW prime mover. It is anticipated that the gas turbine will be run at its most economic speed for the existing load (following a propulsion cube law profile is more fuel efficient than following a constant speed generator law at most powers). Accordingly, it is planned to use power electronics to condition the output and to supply to a dc busbar. 2. There will clearly be a requirement to alter the training infrastructure of the Royal Navy to cater for a change as radical as that proposed for the Electric Ship. However, there are other decisions, relating to the shape, size and sustainability of the engineering branch as a whole, that need to be taken before this can be fully identified. However, it is thought likely that a general systems level approach to engineering training will be required for those due to serve in an electric warship. Marine engineering (ME) training is likely to be widely affected by adoption of the electric ship (ES) architecture and the authors believe that a root and branch reappraisal is required. The first point to be addressed is how many
personnel will be required in an ES ME department. Assuming that unmanned machinery spaces and bridge control are adopted (both proven feasible by the merchant marine), and that good diagnostics are provided for the power electronic suites, then a much reduced department can cope with the purely ME aspects of the installation – both in numbers and skill levels. Other considerations such as damage control, branch structures, experience for future design teams or operations other than war may drive the required manpower. 3. High speed alternators are indeed used in the aircraft industry, however none use the advanced cycle which brings the fuel savings on which much of the electric warship concept is predicated. Other generic forms of high speed machine, besides those used in the aircraft industry, have been identified and are being investigated. Certainly, the intention is to navalise existing technologies rather than to develop a new one. G G Reid (YARD Ltd, BAeSEMA) The authors are to be congratulated on a paper which continues to develop the concepts of the Electric Warship. One of the areas in which I am pleased to see progress is the isolation of the battery from the dc ring main by a dc-(ac)-dc converter. This isolates the main from the high battery fault level and the wide range of battery voltage. It was also stated that dc distribution would probably require ‘solid state switchgear’. Dc distribution has been justified on the basis of energy savings. My question is: Do the added losses associated with the battery dc converter and the solid state switchgear tip the balance back in favour of ac distribution? The choice of an ac or dc auxiliary system distribution scheme should not be seen as central to the choice of an FEP concept. Developments in the area of inverters and semiconductors, particularly higher frequency PWM switches and, perhaps, matrix converters, may make this debate more and more academic, tipping the balance towards traditional ac distribution. Whichever distribution system is used, the system advantages and cost savings of energy storage, and particularly the battery, should be explored more deeply. Whether the battery is coupled through a dc-(ac)-dc converter to a dc distribution system, or through a dc-ac converter to an ac system should have only a marginal impact. Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) The authors thank Mr Reid for his comments which, together with his question, relate to the choice of ac or dc for the distribution of ship’s service load. This area has yet to be fully explored, however as our investigations of the system’s architecture have progressed little has arisen to challenge the early selection of dc for this function. Indeed, more reasons become evident for staying with the original choice of dc; for example, in addition to minimising power electronic conversion losses the total weight of copper required for the distribution system is reduced. To answer his question, the battery is envisaged as spending the majority of the ship’s operational time at float, hence the use of a dc/dc converter to isolate the battery from the dc power system should have little impact on the overall system power losses. The detailed investigations required to answer Mr Reid’s 139
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question await the placement of an Integrated Full Electric Propulsion Enabling Arrangement (IFEPEA) contract, which will enable the necessary studies. However, at this stage it is envisaged that the battery will normally be on float and thus the losses in the converter will be dominated by ‘housekeeping’ – power for cooling and protection circuits – rather than conduction and switching losses. Whilst the ac versus dc debate is in its infancy, work to date has by no means ruled out dc. K E Jordan (DERA) In an answer to an earlier question the possibility of using variable frequency, together with variable voltage, on the propulsion busbars was mentioned. This suggests the possibility of prime mover speed variation to optimise running conditions and efficiency. Could you tell us more about your intentions in this area, or if this possibility has received any significant consideration? Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) As indicated in a previous answer, there is an optimum power/ speed characteristic for prime movers and where machine output is processed by power electronics this optimal characteristic can be adopted where it is cost effective to do so. In the particular case of the WR21, our initial investigation shows no significant fuel economy advantages to counterbalance the increased control complexity. However, the option will be investigated further with respect to each of the prime movers to be used in the Electric Warship. With regard to the propulsion bus, the need and value of variable frequency and voltage is expected to emerge once the PMPM development commences. Again, it is intended that study work under the IFEPEA contract, referred to previously, will address the options and determine the most cost effective solution. Lt Cdr B Dullage (Royal Navy) 1. In earlier work one of the main concerns was the harmonic distortion of the waveforms in the distribution system caused by the power converters. There is no mention in the paper of this potential problem or the associated losses. Does this mean that the increased switching speed of the devices discussed in the paper, compared to thyristors, has effectively eliminated harmonic distortion? 2. As discussed at the meeting, the current standards for quality of power supply would not seem to be applicable to a dc distribution system. Nevertheless, will the wide variety of equipment that will be connected to the dc system mean that some type of standard will be necessary? Is it planned to follow up the (unpublished) work by Mr A Blane on the subject, to produce a specification for the quality of power supply on the dc system? 3. Effective cooling of power electronic components is essential if they are to operate efficiently. What cooling methods are envisaged for the various converters discussed in the paper and has the size, weight and cost that will be inevitably associated with providing this equipment been considered? 140
4. The cooling system is an area which is potentially vulnerable to action damage. Will the system be able to operate without cooling for a limited period and will this period be specified? 5. The reduction in the number of prime movers obviously removes the redundancy that the current generation of Royal Navy engineer officers are used to. Will the engines have the maximum number of common components, for example control systems and acoustic enclosure ancillaries, to minimise the stores’ inventory and improve availability? 6. The prospect of not only having unmanned machinery spaces but also having no dedicated marine engineer watchkeepers (as intimated in the discussion) would be a very radical change to the way our warships are operated. The paper is probably correct in that damage control drives the manpower requirement from a planning viewpoint, but, in addition, if the ship is to be selfsufficient away from base port for many months, then the level of manpower necessary to maintain and repair the plant becomes significant. On the watchkeeping side, in my experience, the majority of watchkeeping effort is expended in looking after auxiliaries, especially such unglamorous systems as sewage and sullage. If the types of manpower reductions proposed in the paper are to be realised, then much effort will need to be directed at the specification and design of the auxiliary systems. Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) The authors thank Lt Cdr Dullage for his comments and questions, and, as expected, he identifies a number of challenging areas. We shall address them one at a time as follows: The operating frequency of the conversion systems which caused us some concern in the early full electric ship studies was, in effect, 60 Hz. The synchroconverter and cycloconverter both switch in synchronism with the input supply frequency. This brings two problems, both of which are negated by using higher frequency switching and the PWM conversion strategy offered by the new range of selfcommutating devices such as IGBTs. Because the switching frequency of a synchroconverter or cycloconverter is low, the harmonics they produce contain a high level of energy. This makes any filter designed to remove these harmonics likely to be bulky. Further, because the switching frequency and output frequency are of similar value, there is a wide modulation spread among the harmonic frequencies created. This means that separate filters will be required for each harmonic; at least among the higher energy lower frequency harmonics any one filter’s bandwidth will be unable to cope with more than one harmonic. Again, this will lead to large filter systems. However, PWM switching immediately decouples the input and converter operating frequencies; in addition, IGBTs could, if necessary, switch at up to 25 kHz. Although this would impose further losses, it would limit both the energy in the harmonics and their overall frequency spread since the modulation ratio would be small. This advantage still remains when the PWM bridge is operated at a more realistic switching frequency of 2-3 kHz, and lower amplitude and much lower energy harmon-
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ics are produced which are contained within a frequency spread which would allow their removal by one slightly detuned filter. Hence, and to answer the question directly and in Lt Cdr Dullage’s own slightly modified words: this means that the increased switching speed of the devices discussed in the paper compared to thyristors, has significantly reduced the problems normally associated with harmonic distortion. However, as distortion frequencies rise, radiated interference and thus EMI becomes more important. Cable installation rules and screening of radiating equipment is a mature technology, thus no unexpected problems are anticipated. The concept of the Electric Warship is not suitable for application of the existing Naval Electric Power System Standards to it. The use of dc and medium voltage ac both require new standards to be produced. Currently, only a standard for medium voltage ac is being produced; a dc standard will be produced once that work is complete and, although the work of Mr A W Blane will serve as a foundation of the new dc standard, it can not be its complete embodiment since that work relates to submarines and to systems with the majority of the loads being rotating machines or passive linear loads. In fact, little equipment is expected to be connected directly to the dc busbar other than Zonal Power Supply Units. However, specifications for the dc bus will be required and Mr Blane’s unpublished work, as well as the US IPS Specs, will be inputs to the exercise. Combining questions 3 and 4, which both relate to cooling of the new power electronic equipment, we would like to state that the cooling of the equipment is thought likely to use standard fresh water. The use of IGBTs, with their insulated base plates, does not require the normal demineralised water for cooling. However, using salt water as the normal water is not thought sensible since this would lead to secondary problems of marine growth and fouling, although its use as an emergency supply has not been discounted. The impact of providing cooling for the power electronic equipment of the Electric Warship has been considered. Indeed, it is the authors’ view that the vital supply for the ship now becomes cooling water; there will be no possibility of operating the power electronic equipment in some reversionary mode without cooling water. However, in the authors’ view, cooling water supplies should be limited to each zone, which will then retain its high level of autonomy and the associated damage control advantages. The approach envisaged is to provide a high integrity cooling supply (perhaps with a natural circulation capability) and not to specify an uncooled rating. The reduction of the total number of prime movers does seem to imply some loss of the redundancy which we have grown used to and fond of. However, the integration of all prime movers into the IFEP system allows their use for both ship’s power system and propulsion; overall, therefore, redundancy is only reduced as a second order effect since one engine failure affects both functions. The engines themselves are unlikely to have many components in common. However, the total number of engine types, three, remains the same as in a COGOG Frigate, but with less of each to support. Ideally, commonality of components between prime movers – like commonality between classes – is a very desirable attribute that will be striven for. However, without
widespread mandating of designs in detail – which restricts competition and risk transfer – it is unlikely to be achieved as widely as is desirable. The reduced manning possible for the Electric Warship should be taken whenever and wherever possible. The potential savings in this field dwarf the initial and through-life cost savings on which, to date, the whole programme has been predicated. However, to gain the required reductions in manpower will probably take a major revision of the Royal Navy’s Engineering Branch structure. Within the totality of marine engineering development a significant number of items are related to auxiliaries and for all of these stringent ILS targets are set. In the particular case of sewage and sullage – a subject dear to every marine engineer’s heart – completely new systems and equipment are being developed to meet the MARPOL legislation and MOD’s environmental policy. It is anticipated that these will much reduce the high maintenance burdens we see at present. In addition, and as Lt Cdr Dullage points out, we will need to address the design of all the auxiliary systems, so that they can attain the levels of reliability they are known to be capable of and match those to be achieved by the main equipment. In the view of the authors, the effort necessary to achieve these levels of reliability in the auxiliary systems will at least match those currently being expended on the development of the prime movers and electrical systems and equipment. H Rush (Independent Consultant and Associate, ASA Consulting Engineers Ltd) The authors have produced a wide ranging and interesting paper which is an excellent insight to the developments since their first paper about this subject. In the conclusions they comment about the need for funding and its possible sources. There is a hint of hopefulness for more industrial contributions and the carrot of the potential to earn significant foreign sales. Commercial projects in which there is a risk contribution, tend often to have commensurate potential rewards through a profit sharing element. Military projects have been notorious for change and delay. In numerical terms, the returns on some higher capital items do not seem to offer the equivalent opportunities as, say, aircraft. Do the authors agree? What is, or are, the nature(s) of the contributions sought from industry? What will make industrial partnerships a profitable and therefore realistic proposition? Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) The authors thank Mr Rush for his kind comments and hope to rise to the challenge of his penetrating questions. Funding does, of course, lie at the very heart of any development project and, as he rightly says, military projects do suffer from change and delay inspired, to be fair, not only by indecision but also, and perhaps mainly, by changing threats and technologies – warships have long lives and a class can span 4-5 decades. Combined with this programme of turbulence and longevity, the small number of expensive ships that the RN requires does not sit well with a desire for industrial co-operation, part funding and profit sharing. Nor does current procurement policy allow the additional contractual flexibility that this radical arrangement would require. However, all possible methods and arrangements 141
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to enable what we see as a mutually beneficial arrangement are being investigated. To address the questions specifically: 1. The authors do indeed understand and agree that the returns from ships do not seem to offer industry the same opportunities as aircraft. 2. The contributions sought from industry mainly come down to part funding. However, non-funding related contributions include partnership, with MOD and industry personnel working side by side to derive the optimum way ahead, both commercially, militarily and technically – in part this is starting with industry providing ‘secondees’ to MOD’s Electric Ship Programme management team. 3. MOD’s Electric Ship Programme management team is just embarking on a route that will lead to a better understanding of these issues by undertaking a ‘Private Finance Initiative Feasibility Study’. However, at this stage, the incentive to provide funds to undertake development is expected to reflect the level of involvement and the commitment by the MOD. For example: a. At equipment level, industry might be keen to partfund developments that can provide a commercial revenue stream outside the military arena – perhaps power electronics that can provide trackside power to a railway system, as well as meet an electric ship objective. b. At system level, industry might partly or fully fund development of a propulsion system if there was a high probability of sales to either the commercial marine sector or to other navies – particularly if this was supported by some limited guarantees of MOD sales. Whilst such a commitment is against current MOD procurement policy, this would need to be tested if a suitable deal seemed likely. c. At ship level, industry might fully fund the development against guaranteed ship orders from MOD. Again, such a commitment is against current MOD procurement policy but would need to be tested if a suitable deal seemed likely. M Murphy (Cegelec Projects Ltd) A question was raised relative to the harmonic distortion present on the propulsion busbar (Fig 14). It is not necessarily the case that an IGBT ‘front end’ (supply side) converter would be appropriate for the propulsion converter – though clearly this is the preferred technology for the motor side converter. It might be envisaged that a diode or thyristor bridge would be adequate for the supply side bridge, giving probable savings in size and weight for the whole converter. In this case, classic harmonic effects would be seen on the propulsion busbar – but, given the nature of the system architecture (noting the power conversion isolation between propulsion and dc ring mains), little impact is anticipated in the context of meeting ship electrical supply quality standards. It is indeed questionable how STANAG 1008 or its equivalent applies, or is relevant to the Electric Warship. 142
Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) The authors thank Mr Murphy for his comments with which they are in entire agreement. S K Firth (Horizon Joint Project Office) Can the authors please comment on the Electric Warship programmes that are also running in the US and French navies. In particular, to what extent is there a consensus of opinion as to the types of technology to be employed and the means of solving the design difficulties that the authors have presented. Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) There is a wide consensus among the NATO navies that the Electric Ship is a necessary and urgent development objective. However, only two other nations are actively pursuing the dream. The USA is further advanced than the UK, France less so. The USA is developing an ‘Integrated Power System’ for both its future surface combatant and submarine vessels. The philosophy is identical though the detail topology of the electrical power system varies. They are developing compact permanent magnet motors, although in their particular case the need is not so pressing since their expected future escort is significantly larger than ours. France is now starting to develop equipment and systems suitable for an Electric Warship, system design studies are in hand and they are searching for a contractor to develop a permanent magnet propulsion motor. P J Best (Frazer-Nash Consultancy) I would like to thank the authors for an interesting and enlightening paper. It is my personal experience that Integrated Full Electric Propulsion has a major impact on the fighting capability of large warships, especially aircraft carriers. Could the authors comment on how the Electric Ship programme is influencing the work on future aircraft carrier studies. I notice that in the discussion of vulnerability the authors state that they are considering various shaft lengths and podded propulsors. Has any work been done on the use of pump-jet thrusters rather than the relatively complex retractable thrusters they mention? Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) The work of the Electric Ship Programme is applicable to all three ship types of the Royal Navy’s future building programme. The Future Escort, Future Carrier and Future Attack Submarine could all use the technologies being developed if desired by the project themselves. The applicability of the Electric Warship concept to the Future Carrier and Future Attack Submarine is under constant review and some outline investigations and design studies have been conducted. S J Short (Whipp & Bourne, Rochdale) Our interest is in the electric power systems which are being considered for the Electric Warship, with particular reference to the switchgear which will be used for the propulsion system and for the ship’s services system. We have already concluded that the propulsion system should be at 6.6 kV, 120 Hz which rates the busbars and the highest current rating circuit at 2500A. Running at 3.3 kV, as suggested in the paper, would double the normal current rating to 5000A. Cables associated with this level of current
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Fig 1 A dc zone switchboard
pose a problem at the terminations due to their size and number. Single core cables, in line with commercial practice, have to be used to minimise the numbers and to assist with the terminations (8–630A cables per phase for 5000A). Even at 6.6 kV the situation is only just manageable. The commercial solution is to use 11 kV where the maximum current rating is 1600A and cables can be reduced to three 630A cables per phase. There will be a considerable weight saving in cables, terminal boxes, switchgear, as well as reduction in sizes in a 11 kV selection. However, 11 kV must be acceptable for the rotating equipment, the converter and the inverter equipments. It will certainly be acceptable for their cable terminations for the same reasons as the switchgear. We are in agreement with the authors that the correct location for the 7 MW generator should be on the ac propulsion switchboard. Another reason is that it is too large to be accommodated on the 750V dc switchgear. It requires a 10 000A at 750V circuit breaker. Since the authors’ preference is for it to be on the dc system, there is a simple way to achieve this by supplying the generator output via two 5000A at 750V circuit breakers, each into the opposite side of a 5000A
ring main, as shown in the Fig above. Switchgear of this rating is achievable. Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) The authors thank Mr Short for his most helpful comments. It is most gratifying to hear reassurance that their confidence is well placed that their conceptual power system is feasible with today’s technology. To hear it from one practising in the crucial area of power system protection and distribution is doubly pleasing. The choice of medium ac voltage level has yet to be made. The 3.3 kV was only an indication that it was not to be low voltage. The authors have long harboured a suspicion that such a voltage would be too low and that 6.6 or even 11 kV would be more practical. P L Vosper (MOD, Procurement Executive) In relation to Fig 12 and the carrier concepts, it was stated that the inclusion of a lead acid battery low in the vessel would counter the rise in C of G caused by placing the gas turbine generators in the superstructure, hence maintaining stability. This is partly true. For the carrier designs considered by the UK MOD 143
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(ie to carry a certain Air Group), the design is driven by the flight deck and hangar. This leads to an excessive GM value and a ‘stiff’ vessel. Ballast placed beneath the flight deck is used to raise G and reduce GM. The placing of the gas turbine generators in the island will tend to raise G and reduce the requirement for ballast. In relation to Fig 13 and the trimaran concept, the authors quite correctly identified that the improvements in the resistance characteristics of a trimaran compared to a monohull of similar duty are apparent at high speeds (>25 kn). Also, the advantages afforded by a relatively large deck area were mentioned. What was not stressed was the inherent flexibility that both the IFEP and the trimaran concepts give. Many of the characteristics of the designs can be tailored to suit a specified set of requirements and IFEP used in conjunction with a trimaran (for an escort sized ship), is a combination likely to show many advantages over more conventional designs. Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) The authors are grateful for Mr Vosper’s comments with which they are in complete agreement. R Simpson (YARD Ltd) One of the advantages claimed for electric propulsion over conventional mechanical propulsion is the fuel saving achieved by matching the number of prime movers running to the power demand and hence always operating the engines at maximum efficiency. As shown in Fig 4 the sfc curve for the WR21 gas turbine is very flat over a wide operating power range, and hence always operating the engine at high loads will not provide significant fuel savings over an engine operating at variable load. Indeed, the lower overall propulsion train efficiency of electric propulsion, compared with mechanical transmission, is likely to result in a higher propulsion fuel consumption for the electric scheme. On the other hand, IFEP will provide ship’s service electric power more economically than the small diesel generators in the mechanical drive ship. When this is taken into account the overall fuel consumption of both the conventional mechanical propulsion and IFEP ships is likely to be very similar. The IFEP scheme permits fewer prime movers to be installed and requires fewer running hours on these prime movers to provide similar performance to a conventional mechanical propulsion scheme, with separate diesel generators for ship’s service power. Fewer machines and fewer running hours results in significantly reduced maintenance costs. It is this reduced mainte-
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nance cost rather than claims of reduced fuel consumption which should be used to justify IFEP. Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) The authors thank Mr Simpson for his comments. Once again they agree with his views. The use of an engine such as the WR21 does indeed gain most of the fuel advantages from the baseline of a Type 23 frigate, but not all, and our investigations show that advantage remains in employing a WR21 in an IFEP propulsion system, even from the perspective of fuel economy alone. The cost benefits which we have claimed for the Electric Warship are pessimistic, however they do already take into account the reduced maintenance requirement attributable to the operation of an IFEP system. Prof Y N Kireev and Assistant Prof A A Wurshevsky (Marine Technical University, St Petersburg) The Electric Ship concept must take into consideration electromagnetic compatibility (EMC) of all ship equipment and systems. It is important to check EMC (harmonics, surges, bursts, ESD, noise problems) for every proposal. EMC costs (noise suppression, protection techniques, special cable runs, etc) can raise the total cost of some new proposals. Unmanned machinery spaces and bridge control of the propulsion system need a high electromagnetic immunity for this control system. We would like to ask the following questions: 1. How do the authors take into consideration the electromagnetic compatibility of electrical and electronic equipment? 2. What future work do the authors plan in this area? Cdr C G Hodge and Cdr D J Mattick (MOD, Bath) The authors thank Professors Kireev and Wurshevsky for their helpful comments. They agree entirely that EMC must be considered early in the project, with resilience and reduction measures designed into the system from the outset. The questions have a simple answer, EMC is being considered now, with the design of the Permanent Magnet Propulsion Motor and associated converter. It is defined as a central characteristic of the design and it will be subject to extensive testing on completion of the representative motor and converter. The authors anticipate considerable future work in this area, not only within the Permanent Magnet Propulsion Motor but also within the many design studies to be initiated under the IFEP Enabling Arrangement, which is to be the subject of contract action in 1997.