Parallel control of hybrid propulsion aboard a fast naval combatant

Abstract

Environmental and operational challenges associated with an excessive fuel consumption have led to the need for future naval vessels to be less dependent on fossil fuels. The chosen configuration is combined diesel-electric and diesel (CODLAD). However, there is a performance gap regarding the manoeuvrability between the previously implemented gas-turbine propulsion and the diesel-electric hybrid propulsion. In order to reduce this performance gap between a gas-turbine propulsion system and a diesel-eclectic propulsion system a new optimized control system needs to be developed.
Adaptive pitch control avoids the angle of attack peak, reducing the torque peak that can potentially overload the diesel engine. Adaptive pitch control is the only control strategy that operates at a low angle of attack, and therefore is less perceptible for cavitation. However this peak in angle of attack is responsible for the peak in thrust. Adaptive pitch control therefore needs large shaft speed fluctuations to be able to create a large thrust. Adaptive pitch control is therefore either slow or forces the diesel engine to rapidly make large changes in its shaft speed. The rapid changes in shaft speed could increase the amount of wear inside the engine, reducing the time between maintenance.
During the simulations torque control for the diesel engine performs extremely well during acceleration. Using torque control for the diesel engine the static power distribution of the propeller load was varied. During these variations it was concluded that a large acceleration can be achieved with a heavy loading on the diesel engine in steady state, supported by a large torque production during acceleration by the induction machine. A heavy matching of the diesel engine was possible because the diesel engine only produced the power needed for the steady state propeller power as matched via the combinator. The induction machine is made responsible for keeping the speed and coping with disturbances. The induction machine is inherently more suited than the diesel engine, due to its fast response as well as it being able to produce full torque or maximum power without any negative adverse effects. However torque control is not available from manufacturers.
This research includes a novel implementation of a dynamic power split, which produced very promising results. The dynamic power split uses the diesel engine with speed control parallel with the electric drive in dynamic torque control. The dynamic torque control for the induction machine allow for optimal usage of the inherent characteristics of the electrical machine. Using dynamic torque control the induction machine is responsible for the high frequency load changes such as disturbances as well as the first part of the acceleration. The diesel engine governed by a slower integral dominated PI controller accounts for the low frequency load changes, including the higher load at a new steady sate. The diesel engine with speed control parallel with the electric drive in dynamic torque control produces excellent acceleration characteristics, making it a viable option to improve the acceleration characteristics of a diesel-electric hybrid system.