the electric warship then, now and · pdf filethe electric warship then, now and later c g...

Paper on electric propulsion and the advances over the last 30 years presented at INEC 2008 in Hamburg, Germany.

April 2008 of 9 © BMT Defence Services Ltd/ Converteam UK Ltd

THE ELECTRIC WARSHIP THEN, NOW AND LATER

C G Hodge BSc MSc CEng FIMarEST FREng 1 D J Mattick BSc CEng MIET FINucE FIMarEST 2

1 BMT Defence Services Ltd & Honorary Professor of Engineering, University of Warwick Maritime House, 210 Lower Bristol Road, Bath, Somerset, BA2 3DQ,

+44 (0) 1225 473 679, [email protected] 2 Converteam UK Ltd,

Boughton Road, Rugby, Warwickshire, CV21 1BD, +44 (0) 1162 845 678, [email protected]

SUMMARY

Over the past 30 years electric propulsion has been frequently selected as the propulsion system of choice for various types of warship and auxiliary ship in several different nations’ navies. Throughout this period, the authors have been involved with Royal Navy electrical power and propulsion systems as operators, maintainers, trainers, designers, procurers and testers.

This paper will present the authors view of the advances achieved over this period, identify the drivers causing the changes and make suggestions about how future systems may develop.

INTRODUCTION

During the 1980’s the UK Royal Navy’s Type 23 Frigate and the Upholder Class submarines were being developed and built and both were in service by 1990. Whilst electrical propulsion had been previously used for surface warships and submarines, these were early applications of modern power electronics to electric propulsion of Royal Navy warships, albeit at relatively low shaft powers.

Since then Royal Fleet Auxiliary’s Auxiliary Oiler (AO) and Royal Navy’s Landing Platform Dock (LPD) have entered service with higher power electric propulsion systems and the Type 45 Destroyer is undertaking trials with yet higher shaft powers. This sequence that introduces modern electric propulsion to the Royal Navy is understood to have achieved both military advantage and reduced running costs.

Looking to the future, there are opportunities for electric propulsion to be adopted by future surface combatants and submarines but the challenge is to provide suitable power dense and fully integrated electrical power and propulsion technologies.

During the 1990’s the authors foresaw this need and in a series of papers [1], [2], [3], [4], [5] & [6] identified the technologies that they anticipated would achieve the power density objectives. In the event, the majority of the novel developments anticipated did not come to fruition but the current power density challenges have been achieved by incremental development of existing technologies, innovative approaches to the system as a whole and a focus on heat management.

Extrapolating from this the authors anticipate future power density challenges being achieved in a similar manner, at least until inevitable technology breakthroughs are achieved in non-naval arenas - such as High Temperature Superconductivity (HTS) and Silicon-Carbide Semiconductors.

THEN

Type 23 Frigate

The Type 23 Frigate has proved to be an extremely successful design illustrating to many Frigate Captains the advantages of electrical propulsion - albeit in this instance through a relatively low power hybrid installation. Figure 1 shows a simplified schematic of the power and propulsion system. The system was the first installation of an electrical power system using parallel operated generators as a standard line up in a Royal Navy warship, the importance of this development in opening up the possibility of much larger capacity integrated power and propulsion systems in later classes, such as the Type 45 Destroyer, should not be underestimated.



The system employed a half controlled - unidirectional - main AC-DC converter. This in itself starts to illustrate how power electronics has advanced over the intervening years. The choice of a half controlled bridge was taken to reduce the number of (then) very expensive Thyristors. The need to create a four quadrant electrical propulsion system able to propel in both directions and absorb regenerated power in both directions (but with a unidirectional flow of current) therefore required a bi-directional field system to enable the armature “back” emf to be reversed independently of the direction of armature rotation. Conversely the field converter was designed with a fully controlled rectifier (twice the number of Thyristors) to enable the converter to force the current to reverse more quickly; but being a lower current and lower power system, the cost penalty was reduced to acceptable levels.

Figure 1: Type 23 Frigate Power & Propulsion System

To limit the propulsion currents as much as possible the voltage of the main propulsion AC power system was set at 600 V rms which provided a maximum DC system voltage of around 900 Volts and was set at his level in order not to exceed the LV voltage limitation.

The presence of the propulsion rectifiers imposed significant harmonic distortion on the ships electrical power system and this was isolated from the ships service sub-system through the use of rotating motor generators. However the use of induction motors for the ships service Motor Generators (MGs) required an amount of slip to be allowed for and so the 650 V systems was designed to be operated at 61.5 Hz.

Type 2400 Class Submarine

At the same time as the Type 23 the propulsion systems for the Type 2400 conventional submarine was also being developed and a simplified schematic is shown at Figure 2. The main motor was a double armature single frame DC machine as in previous classes of conventional submarine motor. But for the first time the pole face series connected compensating windings were removed to improve noise performance. Again use was made of the available power electronic technology but not for motor armature voltage control due to the very high currents to be controlled. Instead coarse motor voltage control was implemented through the traditional conventional submarine arrangement of batteries in series and parallel and (for batteries in parallel) motors in parallel and series. This was accomplished through automated cam operated off-load contactors in the propulsion switchboard.



Figure 2: Type 2400 Submarine Power & Propulsion System

Fine speed control within each speed group was accomplished using a fully controlled field converter which also reversed field voltage to reverse motor rotation. With the Type 2400 only having a single shaft there was no longer the opportunity to connect four motor armatures in series (previously termed “shafts in series”) and so a dedicated motor generator set was installed to apply a reduced voltage to one of the propulsion motor’s two armatures.

AO & LPD

The AO and LPD were the next classes of ship to be developed after the Type 23 and Type 2400. Although being only somewhat later the rapid developments in power electronic capability allowed a much greater use of power conversion. Other ships had been electrically propelled in the past, for example the H Class survey ships and RMAS NEWTON used a very impressive DC propulsion system utilising a constant current loop and even further back in time HMS ADVENTURE had been electrically propelled by DC motors in the 1920s. But the AO and the LPD were the first navy ships to use full electrical propulsion integrated with the ship’s service power system and to fully control AC propulsion motors with power electronics.

Figure 3: AO & LPD Propulsion Converter



However, the technology was still Thyristor based and these ships use a synchroconverter to control the propulsion motors as is illustrated in Figure 3. Synchroconverters, and indeed cycloconverters, use naturally commutated thyristors to generate coarse motor drive waveforms that are rich in harmonics. On the other hand, pulse width modulated converters generate a much more sinusoid-like waveform that is less demanding on the motor windings and creates less rotor torque pulsation. These differences are covered in greater detail in [1] to [6].

The Original Vision

The original electric warship power and propulsion system advocated by the authors was a mixed mode system as described at [1] and is illustrated at Figure 4.

Figure 4: Original UK Electric Warship System

The system is self-explanatory from Figure 4, but note that the generator ratings are quoted in MVA rather than MW (and assume a nominal power factor of 0.8). This conceptual system resulted in the topology that was used for testing by Converteam at the Electric Ship Technology Demonstrator at Whetstone for which [7] and [8] present Government and Industry perspectives respectively.

Hindsight illuminates some interesting aspects to this original system.

At the time that the UK concept was first detailed the USA were engaged in their Integrated Power Systems (IPS) Programme and were, then, considering an all DC solution. Conversely the United Kingdom initially thought an all AC system was to be preferred because the fault level would be too difficult to manage on an “all DC” system. However the enthusiasm for DC in the USA - and the technical arguments they made in its favour - resulted in the UK considering the mixed mode system as illustrated at Figure 4. The bulk high power (for propulsion) was delivered as AC but the ships service system used DC. Interestingly, when further developed, the USA IPS Programme adopted a similarly segregated mixed mode system with AC propulsion and DC ships service distribution.

The prime movers split, broadly, into low power (1.5 and 8 MW DC) and high power (2 of 20 MW) but the selection of the mid-power range prime mover as a low voltage unit was not a clear cut decision and was made for reasons of distributing the prime movers across the two systems and throughout the ship. Further consideration however illustrated that the best system on which to locate any of the prime movers was the system that would consume the majority of the prime mover’s power (and thereby minimise conversion losses). Use of the Prime Mover Running Cost Model developed by the authors [2] showed that in fact - with a typical surface ship operating profile - the mid-range prime mover would use most of its power for propulsion rather than ships service and thus the unit was better located on the high voltage high power AC system (as was the case on the ESTD [7] & [8]).



Much early thought was expended on the option of incorporating an energy storage device and a putative option of a submarine battery was chosen. This was not because the authors were prejudiced from their submarine background but because when the discounted fuel savings arising from single prime mover operation (at the time energy storage was considered a necessary precursor for single prime mover operation) were assessed there was enough money available to justify purchase of a battery but not to develop any other form of energy storage.

NOW

Type 45

The Type 45 Destroyer is notable as the first implementation of a voltage source Pulse Width Modulated (PWM) main propulsion converter in a warship. The advantages of PWM conversion were discussed in [5] and the technique, although not without some disadvantage (such as high dV/dt and high switching losses), has become all but ubiquitous in its applications.

The system as implemented in the Type 45 Destroyer uses an H bridge for each individual phase of the Advanced Induction Motor (AIM) employed as the main propulsion motors. This leads to a requirement for 15 different H bridges arranged as 3 sets of five (balanced) phase groups.

Figure 5: Type 45 Power and Propulsion System

The power density necessary to achieve the required installed power for the Type 45 has been achieved by adopting advanced air cooling of the propulsion motor (hence AIM), sea water and midel oil cooling of the converter, The HV and LV filters work in consort to reduce the harmonic distortion caused by the propulsion converter; while the HV filter is an assembly of passive components the LV filter is an active filter that can also compensate for harmonics generated by the LV system; both filters are cooled by midel, air and chilled water

Later - The Vision Updated

References [1] to [6] identified the two following principal technologies that it was thought would prove of value in reducing the size of the propulsion motor and converter to allow them to fit in future warships:



Transverse Flux Machine (TFM): This was the novel permanent magnet propulsion motor developed, to an early stage, by Rolls-Royce and ALSTOM Power Conversion (now Converteam) on behalf of the UK MoD. It suffered from a key disadvantage; its power factor was low (around 0.7) and dropped further (to around 0.4) if the torque density of the machine was maximised. A reduced power factor implied higher load currents (as the reactive currents increase). For example at a power factor of 0.4 the overall current is 2.5 times greater than that required simply to transmit the power. This additional current still needs to circulate in the output stage of the converter, increasing its size and conduction and switching losses (all by a factor of 2.5). At a power factor of 0.7 the machine is reasonably balanced between machine and converter sizes but at that point it is no smaller, overall, than a system based on the AIM. Hence the development was suspended even though the basic physical principal itself was proven.

MOS Controlled Thyristor (MCT): This was the next generation power electronic device thought most likely to supplant the Integrated Gate Bi-Polar Transistor (IGBT). There were also others - though less well funded or developed - such as the Dual Gate Device; and devices based on monolithic chips (Thyristors) such as the MOS Turn Off Thyristor (MTO) and Emitter Turn Off Thyristor (ETO). These were all thought promising at differing stages and were reported variously at [1, 2, 3, 4, 5 & 6]. All came to nought as the massive funding behind IGBTs, which at one stage represented over 80% of the world funding for power electronic devices, allowed incremental device improvements and enabled IGBT based power conversion to exceed the requirement of an electric warship.

Hence these technologies were not adopted or even fully developed because they were superseded by other less expensive and less risky options.

DC Power Distribution

The same is not true for DC distribution which was also one of the central planks of the initial Electric Warship vision. A DC distribution system was proposed as a key technology, albeit it was recognised in [1] that the power demands of the propulsion sub-system would require an AC system. DC was preferred for the ships service sub-system not only because it was felt that it might be smaller but also because it was thought that it could be more efficient where power conversion dominated at both the supply and the consuming ends. The only change in the authors’ view now is that the limitation of power control that led to the selection of AC for the propulsion sub-system has been circumvented. DC can now, and should, be chosen as the single mode of power supply throughout the ship.

The critical aspect of DC power distribution is the strategy used for the system protection. If conventional circuit breakers are used then the overall system could be unmanageable in terms of volume weight and probably cost. The answer is to embed the protection function within the power electronic converters that are present in the systems for other (power conversion) purposes. This is, in a sense, the convergence of the control and protection function - the unification of power converters and solid state circuit breakers. But the concept allows much system simplification and cost reduction with breakers being replaced by off-load isolators (retained for system reconfiguration and galvanic isolation) and the protection function being abstracted to a control issue for the converters that are already interrupting their load current with a switching frequency in the kHz range.

DC System Stability

DC power systems, when feeding constant power loads, can exhibit stability issues and this has been extensively reported at References [9], [10] & [11]. When feeding a constant power load - without capacitive compensation - the source impedance needs to be greater than the load impedance. However this makes the power supply very inefficient with greater power dissipated (as losses) in the supply than transmitted usefully to the load. This is clearly not a practical power supply.

The answer is to compensate the power converter with capacitance and when this is done the stability criterion reverses and under this condition the source impedance needs to be less than the load impedance and a practically useful power supply results. Without quoting the, possibly, arcane mathematics, when capacitance is used in this way there is a second stability requirement that arises which establishes a separate stability limit dependant on the load resistance and its magnitude in comparison to the converter’s capacitance.

Therefore applying source capacitance does not guarantee stability under all load conditions, instability will still occur if the converter power exceeds the level determined as the stability boundary in comparison to the amount of capacitance installed. Thus overload can cause instability in practical converters feeding constant power loads.

Establishing stability through the use of capacitance is the normal procedure; indeed a voltage source capacitor uses the capacitance across its DC link as much to produce stability as to provide a voltage source for conversion. The amount of capacitance used is generally limited as much as possible to limit the size and cost of the converter and there will not be much margin of stable operation beyond full power and of course a failure in the capacitors used will affect converter stability and this should be borne in mind at the design stage especially if centralised bulk capacitance is chosen.



Figure 6: Stable (0.44F) and Unstable (1mF) DC Power Systems

Figure 6 illustrates the effect that capacitance has on DC power systems stability. Another approach is to apply active control to the converters to, in essence, ensure stability by establishing the correct relationship between source and load impedances. Under this strategy the amount of capacitance may be able to be reduced.

A significant difficulty with establishing DC power system stability (or indeed stable operation of DC link converters in an AC power system) is that many loads are bi-directional in nature and when the source and load inter-change then the relationship between the actual hardware impedances must also reverse. It is likely therefore that even when establishing stability through capacitance that some active control compensation will also be required for bi-directional loads.

Nevertheless, and despite all the design difficulty, the advantages of DC power distribution as initially identified at [1] and confirmed at [9] make its adoption in a future Integrated Full Electric Propulsion (IFEP) surface ship or submarine highly advantageous and it remains a key element of the current electric warship vision.

The Challenge Remains

For higher displacement platforms the current generation of IFEP systems and their derivatives will continue to offer effective solutions as evidenced by the United Kingdom Type 45 and Future Carrier.

However the current United Kingdom Naval Programmes, beyond Type 45 and the aircraft future carrier, now include submarine platforms as well as the future surface combatant. In these programmes IFEP and general electrification of auxiliary systems is being taken seriously. However these United Kingdom future platforms, and especially the submarine, are more constrained in terms of volume than either Type 45 or, more certainly, the Future Carrier. Therefore, when looking to the future, if electric propulsion is to be available for the United Kingdom’s near term (low displacement) naval projects, both surface and submarine, then a further step increase in power density of the electrical generation hardware and the propulsion motor and its converter are required.

For the surface fleet where volume is marginally less restrictive, then existing prime movers rotating at a synchronous speed may be acceptable. However, in the constrained volume of a submarine, an asynchronous high speed turbo generator is probably required if high power IFEP or hybrid propulsion is to be adopted.

The Electric Warship Vision Updated

Figure 7 shows the current power and propulsion systems of the electric warship concept as currently proposed by the authors and at [12]. It is shown in a nominal surface ship configuration although little change is required for a submarine topology.



Figure 7: The Electric Warship Vision - Power & Propulsion

The system includes dual wound asynchronous generators, to maximise power density, with a high voltage propulsion supply feeding propulsion motors through integrated motors and converters and also includes a low voltage DC ships service zonal distribution system with some measure of localised zonal energy storage and zonal power supply units. In addition the ability to cross-connect the LV system to the high voltage system (but at the lower voltage) to allow low power propulsion in emergency after loss of the HV system has been included.

CONCLUSION

The authors remain convinced of the opportunities offered by electrification of the power, propulsion and auxiliary systems of warships; not just to reduce through life costs of ownership but also to allow significant improvement in the fighting capability of electric warships. The vision remains as enticing as ever but the applicability to submarines offers even more cost effectiveness through shared surface and submarine power and propulsion technology

REFERENCES

1 Hodge, C.G. and Mattick, D.J. The electric warship, Transactions IMarE, Vol 108, part 2, pages 109 to 125, 1996

2 Hodge, C.G. and Mattick, D.J. The electric warship II, Transactions IMarE, Vo1 109, part 2, pages 127 to 137, 1997

3 Hodge, C.G. and Mattick, D.J. The electric warship III, Transactions IMarE, Vol 110, part 2, pages 119-134, 1998

4 Newell, J, Mattick, D J & Hodge, C.G. The electric warship IV, Transactions IMarE, Vol 111, part 1, pages 25-26, 1999

5 Hodge, C.G. and Mattick, D.J. The electric warship V, Transactions IMarE, Vol 112, part 1, pages 27-37, 2000

6 Hodge, C.G. and Mattick, D.J. The electric warship VI Transactions IMarE, Vo1 113, part 2, pages 49-58, 2001

7 F. Danan, A. Weston B. Longepe, The Electric Ship Technology Demonstrator Or How Two MoDs Are Derisking The All Electric Warship Concept, IMarEST All Electric Ship Conference 2005, Versailles, France

8 D. Mattick, M. Benatmane, N. Mcvea, R. Gerrard, The Electric Ship Technology Demonstrator: 12 Inches To The Foot, IMarEST All Electric Ship Conference 2005, Versailles, France

9 Flower, J.O., Hodge C G “Stability and transient-behavioural assessment of power-electronics based dc-distribution systems. Part 1: the root-locus technique”, Trans IMarEST Part A Volume 3 August 2004 pp 13 to 29.



10 Flower, J.O., Hodge C G “Stability and transient-behavioural assessment of power-electronics based dc-distribution systems. Part 2: the frequency domain approach”, submitted to Trans IMarEST.

11 Flower, J.O., Hodge C G “Stability and transient-behavioural assessment of power-electronics based dc-distribution systems. Part 3: application of Kharitonov Polynomials”, submitted to Trans IMarEST.

12 J Buckley and A Crane, “Electric Propulsion Architectures in (non) Capital Warships”, All Electric Ship Conference 2007, IMarEST, London.

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