AC Drives in marine service

The all-electric ship is making progress.

In both naval and commercial operations, the drive towards the all-electric ship, quite literally, is taking place on the back of advances in high-power semiconductors, and Integrated Full Electric Propulsion (IFEP) systems. The UK?s Type 45 for the Royal Navy will feature induction motor drives, whilst the successful deployment of innovative drives, such as the “Swath Ferries”, equipped by Bakker Sliedrecht is being followed by a further vessel, already under construction. Alstom already has delivered electric power to the new Queen Mary II and a range of naval vessels, including an auxiliary from BAE Systems Marine fitted with synchronous motors operated in tandem, so some exciting new developments are in prospect.

There are many benefits to the all-electric ship, notable amongst these is the environmental gain from use of electronic diesel engine management and emission control systems. Combinations of fuel cells, intelligent power management systems and electric propulsion are reducing operating costs, and delivering a cleaner environment.

The underlying trend though is that all forms of electric propulsion are able to be taken forward and installed, as a direct result of improvements in the performance of the high-power semiconductor technology, irrespective of whether these are thyristor or transistor based systems. In addition, the provision of digital based control, signal processing, analysis and monitoring systems, enables reduced complexity in the rotating machinery.

The more recent use of PWM/VSI inverter drives has also spearheaded improvements in the overall power density, and with the benefits of lighter weight, modular designed electrical systems provide greater reliability and availability. Examples of such drives include those from ABB, where existing medium voltage AC drive provides a challenge for drive technologies used in azimuthing propulsion systems with direct motor torque control and using IGCT power semiconductor technology.

While it is correct to say that for the majority of PWM VSI?s, induction motors are normally used, technically, today, for propulsion. Drive motors can be synchronous types or permanent magnet synchronous motors. ABB have a number of examples of these different types in both the Azipod and Compact Azipod propulsion drives.

Increased drive motor power density offers potential additional benefits to the maker, builder and vessel operator, in that the cost of building and installing the drive motors is reduced, with less complex rotor windings, leading to less maintenance. This reduction of complexity in drive motor designs effectively means more power, less weight and space occupied by the machinery, although it is not removed totally from the drive, but relocated from the rotating machinery, to the control systems, where the high-power components in the electronic switching systems take on the roles formerly carried out by the drive motors and circuits. The relocation of this complexity has only been made possible as a result of the introduction of high-power IGBT and IGCT technology.

In the design of the motors themselves, the emphasis is on drives that produce low noise and vibration signatures are reliable, with redundant elements, have optimal life-cycle costs, and high availability. The reduced complexity in drive motor design with synchronous motors, typically used in cruise applications, and state of the art induction motors are helping increase power density. This is where the future lies, where more power, less weight helps to reduce the space occupied by the machinery.

Recent developments

Recent developments

in the UK, have resulted in even further increases in power density in the use of transverse flux, permanent magnet motors, with a project initiated by the Defence Evaluation and Research Agency, and supported by Rolls Royce. In Germany, similar work was being progressed with Siemens, whilst both ABB and Siemens have developed radial flux machines. With its Advanced AC Induction Motor, Alstom delivered a compact, high power machine that met all of the criteria for the Electric Ship Technology Demonstrator project in the UK. This 20MW motor occupies a space of no more than 3 metres square, and at 100% of its rated speed is 97% efficient.

Through the power plant solutions, use of variable speed drives with either synchronous or asynchronous propulsion motors, together with fixed pitch propellers operate at their optimum conditions, and maximum torque is always available. The solid-state controls provide some intriguing and exciting new opportunities.

Older forms of drive control have been replaced with newer systems, where the overall focus is on integration and control ? ICM (Integrated Control & Management) of the power generation and propulsion controls. The systems required to monitor and control the power plant, propulsion and thruster systems is of increasing importance, to ensure the power plant?s reliable and optimum use.

Examples of current control systems include ABB?s Marintronics platform, SISHIPCIS IMAC from Siemens (developed from the well-known SIMATIC platform used by industry), and Alstom?s Integrated Alarm Control Monitoring System (IACMS)

Manufacturers such as Bakker Sliedrecht have provided turnkey solutions for both specialist vessels ? such as the Celtic Explorer, and more recently, the ?Swath? RoRo ferries introduced for the Vlissingen ? Breskens service. The designs for these ferries, built at one of the Damen shipyards, feature a pair of 1585kVA diesel generators, with redundancy built in to the propulsion and control systems. The main drive motors are double rotor units, in a single housing, each developing 1320kW and incorporating thrust bearings. The redundancy in the system design uses a brushless assembly used to reduce both space and weight, whilst the control of power generation and propulsion is achieved through four Bakker designed and built static AC frequency inverters.

Future Challenges

So what?s just around the corner? Effective power plant management systems are key to the successful evolution of the next steps in all marine vessels, including the move toward the all-electric ship. Choice of electric drive is clearly down to the operator/owner ? business need ? and the designer/builder ? availability of components that meet the criteria, and whether the final drive to a fixed pitch propeller is mounted on a pod, or as part of the shaftline, PWM, Voltage Sourced Inverter drives are likely to increase in number. Today, the Royal Navy has PWM VSI drives that have a high degree of integration, whilst in both commercial and naval use developments are looking into a number of areas, including:

l Reducing the problems of supply side harmonics

l Energy storage techniques

l Solid state circuit breakers

l Converter design improvements

Of course, these are not the only developments that are coming through, but all of these are underpinned by the availability of the essential high-power electronics, good power density in the induction motors for propulsion, low noise and vibration signature, high reliability, cost-effective, and integration with monitoring and control systems. For marine AC drives, the harshest of operating environments for electrical equipment and drives in particular, the technology transfer from the electrical power sector is rapidly maturing, with all of the major manufacturers and builders committed to an electric future.