Modeling, Design and Control of Hybrid Electric Power and Propulsion for Future Low-Emission and Autonomous Vessels

Researcher: Marius Ulla Hatlehol

Throughout the last decade, the focus on sustainability and reducing the environmental footprint has changed- and is still shaping the maritime industry. This transformation is driven by the motivation to combat climate change by reducing harmful greenhouse-gas (GHG) emissions. This has led to the development of hybrid power- and propulsion systems. As a result, modern vessels combine energy storage systems (ESS) with internal combustion engines (ICE) and fuel-cells (FC) as well as wind-assisted propulsion. The mix of energy sources can serve different purposes depending on the operation in order to minimize fuel-consumption, emissions, and down-time. In addition, all-electric ships (AES) are becoming more common as the quality and lifetime of energy storage systems are being improved. In light of the new era of autonomy and digitization, all-electric- and hybrid power and propulsion systems will play an integral part to enable reliable, robust, and dependable autonomous vessels. As a result, onboard power systems are becoming an essential part of ships of the future.

In the traditional diesel-electric propulsion systems with alternating current distribution, the dynamics are largely governed by the electrical- and mechanical characteristics of the machines, such as the diesel-generators, transformers, and the propulsion motors. The power- and propulsion system is controlled by a supervisory power management system (PMS), that ensures availability, integrity, and redundancy of the power system based on the vessel operation. To ensure compatibility and operational requirements of the power system components, one conventional practice is to assess the compatibility by short-circuit levels in combination with machine parameters and formulate a trade-off between harmonic content and short-circuit levels. Thereafter, the speed and voltage regulators are tuned and tested on board in order to meet class requirements for transient voltage- and frequency deviations given as a percentage of nominal values for a certain duration of time. The transient conditions can be large load steps or short-circuit scenarios determined by simulation studies. However, in DC shipboard power systems, transient behavior and harmonics are challenging to predict- as the power system dynamics are governed by the control- and modulation systems of the power electronic converters interfacing the sources and the propulsion units. Unfortunately, today's standards and practices are based on conventional propulsion systems, and they are not directly applicable to modern shipboard power systems.

The integration of the various energy sources is made possible by power electronic converters (PEC) to interface the onboard distribution and propulsion systems. Multi-converter shipboard power systems are complex systems with a high degree of flexibility. This leads to a situation where the dynamics of the power systems are determined by the structure of the low-level control systems. The low-level control layer is responsible for the behavioral dynamics; droop and current control. To elaborate further, the power converters are often off-the-shelf based, designed to operate in both terrestrial- and marine power systems with a stable predefined low-level behavior. It has been well documented that instabilities and oscillations occur when these power converters are interfaced to a common DC-bus due to their high-bandwidth controllers. This leads to an unintentional phenomena known as negative incremental impedance, which could have a deteriorating effect on power system stability. Furthermore, the low-level controllers can also cause inter-harmonics when they interact with several power converters- and subjected to mechanical oscillations propagating through the motor drives and into the power system. A solution to this problem could be to preserve stability structurally through the low-level control to enable the flexibility and connectivity of the different power sources in hybrid power and propulsion systems.

Problem formulation:

Onboard power systems are becoming increasingly complex. This complexity introduces several new challenges that are not addressed in today’s standards- and practice. Therefore, there is a need to develop more sophisticated methods and control systems to ensure the secure and reliable operation of the different components. This requires an overall system-based approach, with emphasis on the behavioral dynamics of the components and the energy efficient operational philosophy. This means building on the existing knowledge of vessel operation, combined with emerging concepts and a power system perspective. As a result, this will provide the control methods that enable flexible plug-and-play integration of energy sources in vessels with hybrid power systems.