Electric Propulsion and High Voltage Practice (Adapted from:D.T. Hall:Practical Marine Electrical Knowledge)

The earliest electric propulsion for ships was demonstrated in Russia in 1832 with a d.c. motor powered from a battery. In 1886 an electrically propelled vessel called the Volta crossed the English Channel. By 1888 the improvements to batteries and motors led to the first commercial applications in passenger launches on the River Thames in London. As with road transport, electric river boats were soon eclipsed by the arrival of the internal combustion engine.

Electric propulsion for many new ships is now re-established as the popular choice where the motor thrust is governed by electronic switching under computer control. The high power required for electric propulsion usually demands a high voltage (HV) power plant with its associated safety and testing procedures.

Electric Propulsion Scheme Electric propulsion of ships has a long O '' vbut somewhat chequered history. There have been periods when it has enjoyed popularity, with a significant number of installations being undertaken, whilst at other times it has been virtually ignored as a drive system.

Passenger ships have always been the largest commercial vessels with electric propulsion and, by their nature, the most glamorous. This should not, however, obscure the fact that a very wide variety of vessels have been, and are, built with electric propulsion. Early large passenger vessels employed the turboelectric system which involves the use of variable speed, and therefore variable frequency, turbo-generator sets for the supply of electric power to the propulsion motors directly coupled to the propeller shafts. Hence, the generator/motor system was acting as a speed reducing transmission system. Electric power for auxiliary ship services required the use of separate constant frequency generator sets.

A system that has generating sets which can be used to provide power to both the propulsion system and ship services has obvious advantages, but ' this would have to be a fixed voltage and frequency system to satisfy the requirements of the ship service loads. The provision of high power variable speed drives from a fixed voltage and frequency supply has always presented problems. Also, when the required propulsion power was beyond the capacity of a single d.c. motor there was the complication of multiple motors per shaft.

Developments in high power static converter equipment have presented a very convenient means of providing variable speed a.c. and d.c. drives at the largest ratings likely to be required in a / marine propulsion system. The electric propulsion of ships requires electric motors to drive the propellers and generator sets to supply the electric power. It may seem rather illogical to use electric generators, switchgear and motors between the prime-movers (e.g. diesel engines) and propeller when a gearbox or length of shaft could be all that is required.

There are obviously sound reasons why, for some installations, it is possible to justify the complication of electric propulsion: Flexibility of layout Load diversity between ship service load and propulsion Economical part-load running Ease of control Low noise and vibration characteristics

Flexibility of layout The advantage of an electric transmission is that the prime-movers, and their generators, are not constrained to have any particular relationship with the load as a cable run is a very versatile transmission medium. In a ship propulsion system it is possible to mount the diesel engines, gas turbines etc., in locations best suited for them and their associated services, so they can be remote from the propeller shaft. Diesel generator sets in containers located on the vessel main deck have been used to provide propulsion power and some other vessels have had a 10 MW generator for ship propulsion duty mounted in a block at the stern of the vessel above the ro-ro deck. Another example of the flexibility provided by an electric propulsion system is in a semi-submersible, with the generators on the main deck and the propulsion motors in the pontoons at the bottom of the support legs.

Load diversity Certain types of vessels have a requirement for substantial amounts of electric power for ship services when the demands of the propulsion system are low. Tankers are one instance of this situation and any vessel with a substantial cargo discharging load also qualifies. Passenger vessels have a substantial electrical load which, although relatively constant, does involve a significant size of generator plant. There are advantages in having a single central power generation facility which can service the propulsion and all other ship loads as required.

Economical part-load running Again this is a concept that is best achieved when there is a central power generation system feeding propulsion and ship services, with passenger vessels being a good example. It is likely that a typical installation would have between 4-8 diesel generator sets and with parallel operation of all the sets it becomes very easy to match the available generating capacity to the load demand. In a four engine installation for example, increasing the number of sets in operation from two that are fully loaded to three partially loaded will result in the three sets operating at a 67% load factor which is not ideal but also not a serious operating condition, It is not necessary to operate generating sets at part-load to provide the spare capacity to be able to cater for the sudden loss of a set, because propulsion load reduction may be available instantaneously, and in most vessels a short time reduction in propulsion power does not constitute a hazard.

The propulsion regulator will continuously monitor the present generator capability and any generator overload will immediately result in controlled power limitation to the propulsion motors. During manoeuvring, propulsion power requirements are below system capacity and failure of one generator is not likely to present a hazardous situation.

Ease of control The widespread use of controllable pitch propellers (cpp) has meant that the control facilities that were so readily available with electric drives are no longer able to command the same premium. Electric drives are capable of the most exacting demands with regard to dynamic performance which, in general, exceed by a very wide margin anything that is required of a ship propulsion system.

Low noise An electric motor is able to provide a drive with very low vibration characteristics and this is of importance in warships, oceanographic survey vessels and cruise ships where,/-for different reasons, a low noise signature is required. With warships and survey vessels it is noise into the water which is the critical factor whilst with cruise ships it is structure borne noise and vibration to the passenger spaces that has to be minimised.

For very high power, the most favoured option is to use a pair of high efficiency, high voltage a.c. synchronous motors with fixed pitch propellers (FPP) driven at variable speed by frequency control from electronic converters. A few installations have the combination of controllable pitch propellers (CPP) and a variable speed motor. Low/medium power propulsion (1-5 MW) may be delivered by a.c. induction motors with variable frequency converters or by d.c. motors with variable voltage converters.

The prime-movers are conventionally constant speed diesel engines driving a.c. generators to give a fixed output frequency. Gas turbine driven prime- movers for the generators are likely to challenge the diesel option in the future. Conventionally, the propeller drive shaft is directly driven from the propulsion electric motor (PEM) from inside the ship. From experience obtained from smaller external drives, notably from ice-breakers, some very large propulsion motors are being fitted within rotating pods mounted outside of the ship’s hull. These are generally referred to as azipods , as the whole pod unit can be rotated through 360° to apply the thrust in any horizontal direction, i.e. in azimuth. This means that a conventional steering plate and stern side-thrusters are not required.

Ship manoeuvrability is significantly enhanced by using azipods and the external propulsion unit releases some internal space for more cargo/passengers while further reducing hull vibration. Gradual progress in the science and application of superconductivity suggests that future generators and motors could be super-cooled to extremely low temperatures to cause electrical resistance to become zero.