How well do we really understand the electrical systems powering today’s vessels? Many of us realize how important electricity is to our modern world but may be a bit shaky on the specifics. The Welsh comedian Tommy Cooper once joked, "Electricity is a wonderful thing. Do you realize that if we didn't have electricity, we'd be watching television by candlelight?"
For those who have worked on vessels for many years, the increasing complexity may be a sore point, especially for those who have been hands-on in their careers and been zapped a few times. George Carlin once joked, "Electricity is really just organized lightning."
The Growing Complexity of Marine Electrical Systems
All kidding aside, the growing complexity of marine electrical engineering is becoming a defining factor in modern vessel design and operation. At EBDG, we have been fortunate to develop significant experience in hybrid technologies over the last decade. However, if that immediately elicits thoughts of a Prius or Tesla, there is much more happening electrically onboard today’s vessels and understanding these systems is becoming increasingly important as vessel designs continue to evolve.
From Space to Sea: Expanding Electrical Capabilities
One exciting area that the US leads the world in is the marinization of space technology. Such examples include Rocket Lab's autonomous station-keeping barge Return On Investment being converted at Bollinger Shipyards in Amelia, La. Blue Origin used the advanced barge Jacklyn last year to successfully land a rocket. SpaceX led the way having landed its first rocket on an autonomous drone barge with Of Course I Still Love You dating back to 2016. And, not surprisingly, NASA has a long experience in this area. For instance, they operated two specially-designed ships for space shuttle booster recovery built in 1980 at the Atlantic Marine Shipyard, now BAE Jacksonville.
Like offshore oil and gas, autonomous landing barges usually depend on dynamic positioning (DP) to maintain precise coordinates. And, like offshore, this leads to a clear advantage for diesel-electric propulsion. Another area that we've been working on involving DP and diesel-electric propulsion is that of research vessels. For all of these diesel-electric vessel types, it can become a complicated analysis to decide between the increasingly popular DC grid propulsion system vs the conventional constant-speed variety.
DC Grid Systems: A Shift in Propulsion Architecture
A DC grid retains the use of standard AC generators and AC motors. However, it replaces both the AC propulsion switchboards and propulsion Variable Frequency Drives (VFDs) with a single DC grid "switchboard". With the main propulsion bus now DC, generators connect through rectifiers located in the DC grid line-up which also includes inverters feeding AC propulsion motors and an AC ship service switchboard.
There are several advantages. The DC grid is composed of the same rectifiers and inverters that are the front and back halves of the VFDs in a standard diesel-electric. Yet, large AC propulsion switchboards and their circuit breakers are no longer needed. The large phase-shifting transformers from older systems can also be eliminated with the potential for considerable volume and weight savings. The generators can operate at variable speed improving fuel efficiency and can also come online faster. There is no frequency to synchronize; the rectified generator voltage just needs to match the DC bus voltage.
Challenges and Considerations
The primary disadvantage with this new approach is the DC short circuit current. While DC grids can achieve significantly lower arc flash energies for personnel safety, the brief peak in short circuit current typically exceeds that of an AC propulsion bus. High-speed fuses and electronic bus ties are needed with circuit breakers comparably too slow for the much faster peak current.
Also, inverters supplying an AC ship service switchboard can’t supply the same short circuit current as an AC propulsion switchboard so coordination of downstream circuit breakers can be a bit more challenging.
The Evolution of Propulsion Motors
There's another component in electrical propulsion systems undergoing major changes. That's with the typically largest and most expensive electrical component, the propulsion motor. The DC motor was dominant until perhaps the 1980's when separately excited synchronous motors took over. Then about 20 years later, induction motors began to dominate. Nowadays, the permanent magnet synchronous motor (PMSM) is looking to take the lead.
To meet the strictest underwater radiated noise (URN) requirements ever applied to a vessel of this class, Steerprop will supply Germany’s new Polarstern 2 icebreaking research vessel with PMSM units delivering 9 MW through a 15.7-foot diameter propeller. Not to be outdone, Kongsberg developed their incredible high-efficiency, low-noise rim-drive thrusters with the permanent magnets integrated right into the propeller rim. ABB has designed and installed permanent magnet shaft generators that save up to 20% on footprint over other AC machine types. Schottel, MAN/Ramme, DRS and many others have also developed PMSM propulsion applications.
Performance Advantages of PMSMs
PMSMs have considerable advantages when it comes to power and torque density over induction motors. One can typically expect a 20-50% savings in volume and weight that can lead to installation or operating cost savings. They also generate less noise and vibration, thanks to their simpler design. Induction motors in large sizes typically have efficiencies in the 94-95% range. For the same size and RPM, PM motors will typically achieve 97-98% or an average of 3% in savings. Further, PMSMs efficiency advantages increase at slower speeds where the fall off in efficiency with induction motors is steeper.
To achieve the low output speed commonly required of propulsion motors, a higher quantity of motor poles is needed. For instance, while a four-pole machine utilizing 60Hz AC power will output about 1800 RPM, a 16-pole motor will output roughly 450 RPM. So, another advantage for PMSMs is their relative ease in increasing the pole count vs. an induction machine. The latter suffer an extra efficiency penalty with higher pole counts. If one adds a reduction gear to the lower pole induction motor, that adds its own inefficiency to the drive train. Either way, one can expect to suffer roughly an additional 3% efficiency loss for the induction propulsion motor.
Trade-Offs and Practical Considerations
All things of course have trade-offs and there are challenges with PMSMs. The biggest one may be cost. The typical magnets used are a neodymium-iron-boron type and they aren't cheap. Along with the rare earth neodymium, dysprosium and terbium are often added for heat resistance. The term rare earth is a bit of a misnomer, at least compared to the truly rare platinum group minerals. Nevertheless, they can be toxic to process. China possesses by far the large reserves in the world and so supply chain issues could be of significant concern.
Though any motor might be seriously damaged by overheating, such as with an undetected loss of the cooling medium, PM motors have the unique weakness that such an event could demagnetize the expensive permanent magnets. Finally, a safety caution is in order about the incredible strength of these magnets. At very infrequent intervals during installation or later maintenance, the rotor containing these magnets may be partially or fully exposed to personnel. It is important to avoid approaching these motors with magnetic materials, for instance having steel tools, keys and similar items in one's pockets, or if you have a pacemaker or surgically implanted metal. One must also avoid rotating the rotor relative to the stator or voltage will be generated on exposed conductors.
Making Informed Decisions in a Complex Landscape
It is hoped that the discussion here on key decision points for a diesel-electric system will make the reader a little more comfortable in tackling such issues. There are obviously advantages and disadvantages to such complex propulsion systems. But with the proper amount of time and effort, an informed decision can be made that will benefit those making such investments.
About the Author: Will Ayers is Chief Electrical Engineer at Elliott Bay Design Group with over 25 years of experience in marine electrical systems, hybrid propulsion, and vessel power system design. He supports electrification strategies, system integration, and advanced propulsion technologies across ferry and specialized marine projects.
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