From oars to reactors

Naval warfare has always benefited from advances in technology going right back through history when oars were the prime propulsors, to today?s propulsion plants such as gas turbines and nuclear reactors. Recently, the commercial sector has led the way in technological advances with the application of electric propulsion largely due to the requirements of ropax ferries, offshore supply vessels and cruise liners. Electric propulsion provides significant benefits for such vessels including increased cargo carrying capabilities, lower running costs, less maintenance, reduced manpower, greater redundancy, lower emissions and improved manoeuvrability particularly with podded or azimuth type propulsors.

In the military, however, electrical propulsion has not been a serious contender for major warships although conventional submarines and hydrographic survey vessels have had electric propulsion, usually based on DC motors. Over the last twenty years the Royal Navy (RN) tended to use gas turbines for main propulsion with DC motors and converters for slow speed operation on some ships. Since the early 1990?s naval forces have been looking seriously at full electric propulsion but it is only now, through advances in power electronics, electro-magnetics and other technologies, that it is becoming a reality.

The Royal Navy in the

vanguard

The RN has been in the forefront of the drive to electric propulsion with the new Landing Platform Dock (LPD) ?Commando Carriers?, HMS Bulwark and HMS Albion ) which are fitted with twin screw diesel electric propulsion utilising two 6MW, 12 pulse AC synchronous motors and converters resulting in a top speed of 18 knots. These vessels represent the first installations of fully integrated electric propulsion in major RN surface warships. Compared to the 1960?s built LPD ships which the newbuildings replace, engineering personnel has been reduced by almost 65 %, while overall there is a reduction in the ship?s company by almost 40 %, from 550 to 325 due solely to the introduction of automation and new technology. These ships provide the UK Joint Rapid Reaction Force with effective amphibious capabilities, including heavy lift and provide a base from which military land forces can rapidly deploy into a war zone.

Why ?electric ships??

Warships share the majority of the benefits of electric propulsion enjoyed by commercial vessels. However, some additional and, perhaps, more important benefits can be summarised as follows: –

l Reduced life cycle and procurement costs ? propulsion motors and converters usually emanate from industrial developments reducing first costs and over life cycle (i.e. replacement and upgraded systems).

l Improved survivability and reduced vulnerability ? power generators, propulsion drives and other essential equipment can be strategically located in different zones and compartments to minimise damage and disruption due to enemy action.

l Reduced signatures (acoustic, infra-red and magnetic) ? with electric propulsion noisy gearboxes are normally not required thus reducing sonar signatures. Due to optimum loading of generators, thermal emissions are lower while electromagnetic signatures are minimised by automatic power flow control.

l Increased range and endurance ? by maintaining generators at the most efficient operating point(s), fuel consumption is reduced, increasing operating range.

l The use of advanced electric weapon systems ? the installation of greatly increased power generation equipment and advanced sensors permits the use of emerging high electrical energy weapons such as ?rail guns?, ?coil guns? and pulsed lasers. In addition, these weapons do not require explosive propellant charges thereby eliminating significant safety and survivability hazards for future ships.

l Greater design flexibility ? this is especially true where podded propulsors are used. The latest LPD for the Royal Netherlands Navy, launched in 1988, uses fully integrated podded propulsion.

First into the fray

The first ever destroyers fitted with full-integrated propulsion were ordered recently by the UK?s Ministry of Defence. The Type 45 ?Daring? class destroyers, of which twelve may eventually be delivered, are to replace the aging Type 42 air-defence destroyers, fast approaching obsolescence and increasingly costly to operate and maintain.

At around 7,350 tonnes displacement and over 150 m long, the T45?s will be the largest and most powerful destroyers ever operated by the RN. They will also be the world?s first true ?electric warships? and will be fitted with the advanced UK / French / Italian PAAMS (Principal Anti-Air Missile System). The first six T45?s are due to enter service from 2007 and will have range of 7,000 nm at 18 kts and a maximum speed of 28 kts.

Powering the T45?s

Power generation onboard the T45?s is be based on a mixture of Rolls-Royce WR21 gas turbine alternators (2 x 21MW) and diesel generators (2 x 2MW). The electric propulsion system comprises of two 20MW Alstom AIM (advanced induction motors), powering two shafts and controlled by Alstom VDM 25000 4.16kV, multi-layer, 15 phase ?pulse width modulated? (PWM) sea water cooled converters with 6 pulse input bridges and dynamic braking. The kinetic energy produced during rapid stopping or speed reversal is absorbed by the converter(s) resulting in the dc bus voltage rising which, if left uncontrolled, would trip the propulsion drive(s). The dynamic braking system detects the rise in dc bus

voltage and on reaching a predetermined limit diverts the energy into a large resistance bank where it is dissipated as heat.

An issue surrounding current technology

Current drive converters of almost any type produce harmonic current distortion which is a function of the power conversion process within the converter. Harmonics are acknowledged to be a cause of damage to and disruption to electrical equipment and have to be attenuated so that any potential effect on equipment is minimised. The drives to be installed in the Daring class are no different and the initial batch will have 6 pulse SCR (thyristor rectifier) ?front ends? resulting in an estimated total voltage distortion (Vthd) of 16-17%, considerably above the recommended 5% Vthd maximum level. Passive harmonic filters (for the 5th and 7th harmonics) therefore have to be connected to the main bus bars, interfaced with the drive control system, to ensure excessive harmonic distortion does not occur. Should a ?harmonic filter fault? develop, speed may have to be reduced to minimise harmonic distortion.

There is equipment for converters around such as ?active front ends? which have very low harmonic footprints but these are expensive, complex and require additional space. So called ?harmonic free? drive technologies are now emerging including ?resonant? and ?multi-port, PFM? (pulse frequency modulation) converters.

AIM motors

Compared to standard induction motors,

AIM motors

are extremely robust and designed specifically for variable frequency supplies with windings capable of withstanding high dv/dt stress. They have increased air gaps and higher levels of power density, power factor and efficiency. The

AIM motors

for the T45s, developed by Alstom, are 15 phase, each controlled by one converter having 3 channels and 5 phases per channel. The motor ?winding? comprises five discrete windings, each electrically displaced (phase shifted) to reduce torque ripple and to provide redundancy. The motor is designed to run on five, ten or fifteen phases, fed from either one, two or three converter channels, depending on the operational requirements. Should any one coil (or converter channel) develop a fault, the remaining ?healthy? ones should not be affected, at least in theory.

Over the pond

Over the Atlantic the US Navy is currently testing Higher Temperat-ure Semi-conductor (HTS) supermotors, which claim to be 60% smaller than conventional motors with ?harmonic free? multi-port pulse frequency modulation drives. As drive and motor technologies progress new types of drive system may be installed in future T45s and other vessels, including possibly ?resonant converters?. It is interesting to note that both ?resonant converters? and the ?multi-port PFM? converters were both developed by Dr Rudy Limpaecher whose previous projects included the laser power system for the US ?Star Wars? programme.

Electric ships are the future

It is clear that the combination of electric propulsion, advanced sensors and electric weapons will give unimaginable flexibility and graduated lethality to future naval warfare both to surface ships and submarines. In January 2000 the US Secretary of the Navy stated “changes in propulsion systems fundamentally change the character and power of naval forces ? This has been shown by the movement from sail to steam ?Electric drives will open immense opportunities for redesigning ship architecture ? and allocating a great deal more power to war fighting applications”. The Type 45?s are breaking new ground in naval warfare and all the world?s navies are watching them.

The author would like to convey his thanks to the Editor of the Navy News for the provision of the illustrations used in this article.