Power availability to preserve propulsion is a vital issue in the shipping industry which relies on persistent power generation and maintaining the stability of the power and propulsion system. Since the introduction of on-board all-electric Direct Current Power and Propulsion Systems (DC-PPS) with hybrid power generation, which are more efficient compared to direct-diesel and Alternating Current (AC) all-electric configurations, there have been extensive investigations on stabilization and power generation control to enable robust and reliable performance of DC-PPS during different ship operations. In this paper, a multi-level approach is proposed for hybrid power generation control. For this goal, first, a mathematical model is proposed for each power system component and then, the overall on-board power system is modeled in a state space format. Then, a multi-level Model Predictive Control (MPC) approach is proposed for the DC voltage control which unlike conventional droop control approaches, takes the DC current generated by power sources into account explicitly. The performance of the proposed approach is evaluated via several simulation experiments with a high fidelity model of a high voltage DC-PPS. The results of this paper lead to enabling more effective approaches for power generation and stability control of constant power loaded microgrids.

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Among different ways of harnessing tidal energy, tidal stream turbines are gaining popularity over traditional tidal dams. This is owing to the lower capital cost, and potentially lower ecological impact of tidal turbines. However, compared to more developed sources of energy, tidal energy remains expensive, which impedes its large-scale utilization. Significant reduction in the cost of energy can be achieved by reducing the maintenance expenses and improving the capacity factor. In other words, improving reliability can make tidal energy substantially cheaper. In this context, this thesis investigates a horizontal axis tidal turbine (HATT) power take-off system with a direct-drive generator. The focus of this thesis is on improving the reliability of the electrical subsystems in the HATT power take-off system. From this perspective, power converter and generator are the two most important components in the drive train. For the converter, the reliability improvement is analyzed from the objective of delaying the thermal cycling failure in the power semiconductor modules beyond the turbine lifetime. Whereas on the generator side, a flooded generator is investigated as a potentially more reliable alternative to conventional airgap generator.@en

This paper compares active and passive cooling systems in tidal turbine power electronic converters. The comparison is based on the lifetime of the IGBT (insulated gate bipolar transistor) power modules, calculated from the accumulated fatigue due to thermal cycling. The lifetime analysis accounts for the influence of site conditions, namely turbulence and surface waves. Results indicate that active cooling results in a significant improvement in IGBT lifetime over passive cooling. However, since passive cooling systems are inherently more reliable than active systems, passive systems can present a better solution overall, provided adequate lifetime values are achieved. On another note, the influence of pitch control and active speed stall control on the IGBT lifetime was also investigated. It is shown that the IGBT modules in pitch-controlled turbines are likely to have longer lifetimes than their counterparts in active stall-controlled turbines for the same power rating. Overall, it is demonstrated that passive cooling systems can provide adequate cooling in tidal turbine converters to last longer than the typical lifetime of tidal turbines (>25 years), both for pitch-controlled and active speed stall-controlled turbines. @en

Recently, tidal stream turbines have become a preferable mode of harvesting tidal energy. The main issue for low utilization of tidal energy is the high levelized cost of energy (LCoE) from tidal stream turbines. A major reason for this is the high operation and maintenance costs for submerged installations. A possible way of minimizing the LCoE and improving the availability is to use a flooded (or a wetgap) generator rather than a conventional airgap generator. Inside flooded generators, the gap between the stator and rotor is filled with the seawater. This architecture has the potential to improve cooling and reduce reliance on ancillary systems (e.g., bilge system), thereby improving reliability. The chapter begins with a brief description of the generator systems used in current tidal stream turbines. The focus of the chapter is, however, to give a basic insight into the design aspects of the flooded generators, and compares it with the currently used sealed airgap generators in tidal turbine systems.@en

This paper analyses the electromechanical dynamics of the pole-piece rotors in a pseudo direct-drive wind turbine generator. The investigated generator has a rated power of 10 MW and was designed in the INNWIND.EU project. One way coupled finite element analysis (FEA) was used to calculate the rotor dynamic defections caused by electromagnetic forces in the air gap. Results show that the deflection exceeds the air gap length which is not mechanically feasible. To solve the problem, three structural modifications are proposed and tested in FEA to reduce the pole piece deflection significantly. The results of the three solutions are compared to idenfity the most promising one. It shows that a ribbed rotor piece topology is the most suitable option because it achieves a reasonable tradeoff between the torque generation and mechanical deflection.

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Thermal cycling is one of the major reasons for failure in power electronic converters. For submerged tidal turbine converters investigating this failure mode is critical in improving the reliability, and minimizing the cost of energy from tidal turbines. This paper considers a submerged tidal turbine converter which is passively cooled by seawater, and where the turbine has fixed-pitch blades. In this respect, this study is different from similar studies on wind turbine converters, which are mostly cooled by active methods, and where turbines are mostly pitch controlled. The main goal is to quantify the impact of surface waves and turbulence in tidal stream velocity on the lifetime of the converter IGBT (insulated gate bipolar transistor) modules. The lifetime model of the IGBT modules is based on the accumulation of fatigue due to thermal cycling. Results indicate that turbulence and surface waves can have a significant impact on the lifetime of the IGBT modules. Furthermore, to accelerate the speed of the lifetime calculation, this paper uses a modified approach by dividing the thermal models into low and high frequency models. The final calculated lifetime values suggest that relying on passive cooling could be adequate for the tidal converters as far as thermal cycling is concerned.@en

Improving availability of tidal turbines has been identified as a key area to lower the levelized cost of energy from tides. One of the approaches suggested to achieve high reliability is to use a flooded permanent magnet (PM) generator. Flooded PM generators are designed to operate with a seawater filled stator-rotor gap in the generator. Using such generators will prevent frequent maintenance required for the high pressure rotary mechanical seals between the turbine shaft and the nacelle enclosure. Flooded generators are conventional generators with additional stator and rotor-can material to protect the active parts of the generator against water ingress. Normally, stator-can is made of an electrically nonconductive material to prevent excessive losses from the PM field. On the other hand, choice of the rotor-can is more flexible. If the rotor-can of the flooded generator is made of stainless steel or any other electrically conductive material, this may give rise to substantial losses in the rotor-can due to the space and time harmonics of the stator magneto-motive force. The time harmonics arise from the pulse-width modulated inverter used to control the torque/speed of the generator. This paper aims at quantifying losses in the rotor-can of the flooded generator due to the time harmonics of the stator current. Results show that the rotor eddy current losses due to the space harmonics are not always negligible. However, losses due to the time harmonics, under normal operating conditions, can be assumed to be negligible.@en

A novel controller design procedure is proposed for a 5-degree-of-freedom (DOF) active magnetic bearing (AMB) system, based on sliding mode control (SMC) and neural network (NN). The SMC is used to achieve high robustness and fast response while the NN can compensate unmodeled uncertainty and external disturbance by on-line tuning algorithm. The proposed controller is compared with the well-tuned PID controller by simulations. The simulation results show the superior performance of the proposed controller.@en

Permanent magnet machines with fractional slot windings are popular because of their shorter end windings, low cost and ease of manufacturing. On the other hand, fractional slot machines have higher eddy current losses in the rotor magnets and sleeve. An accurate calculation of these losses usually involves a time-stepped finite-element analysis with moving rotor, which is time consuming. This paper compares different methods of evaluating these losses using finite element analysis based on their time-cost. The models are mainly divided into two types: time-domain and frequency-domain models. The paper proposes an equivalent frequency-domain analysis in the stator frame for rotor loss calculations, using the Lorentz force term. This method yields similar results to the time-stepped finite element model with a significant time saving. The time saving can be significant in a machine optimization study.@en

This paper proposes a simplified approach to model the thermal behavior of the insulated gate bipolar transistors (IGBTs) in a subsea power electronic converter. The models are based on empirical relations for natural convection in water, and IGBT datasheet values. The proposed model can be used in the design of subsea converters and in the reliability analysis of their IGBTs. Experimental results are provided to validate the proposed thermal model. Suggestions are made to minimize the net thermal resistance by introducing a high conductivity thermal material as a mounting plate between the IGBT and the cabinet walls. Impact of the mounting plate dimensions, and material properties on the junction temperature of the IGBTs is studied. A case study analysis is made on a 100 kVA converter. Results indicate that the thermal spreading resistances in the mounting plate and the cabinet walls contribute significantly to the overall thermal resistance. Spreading resistances can be mitigated by appropriate design measures. Furthermore, it was observed that the passive cooling in water is not as effective as the forced water cooling. However, the low cost, simple design and higher reliability of passive cooling systems might make them a favorable choice for subsea systems.@en

Flooded Generators have been proposed as a possible solution to eliminate the risks due to the seal failure, and improve cooling in tidal stream turbine generators. In a flooded generator, the stator-rotor gap is filled with the seawater; this poses a serious corrosion risk to the material inside the generator. For this reason, a can or a protective shield is used on the stator and the rotor surfaces in the watergap. This paper compares different rotor-can materials based on the eddy current losses in the rotor-can, and its impact on the temperature inside the generator. Due to the flooded gap, the temperature rise in magnets, because of rotor-can eddy current losses is much lower than in a corresponding airgap machine. Thus from the thermal point of view, the choice of the rotor-can material is less critical in a flooded generator than in an airgap generator.@en

Tidal energy is predictable, which makes it highly attractive from the grid integration point of view. Recent years have seen the growth of the tidal current turbines over the traditional tidal range technology. This is primarily because of the success of the wind turbines, which are in principle similar to the tidal current turbines, and the high cost and environmental impact of the tidal range systems. This paper provides a brief review of the available and proposed solutions for the tidal current turbines. The idea is to give an overview of the different types of technologies involved in the tidal current energy conversion systems, rather than giving a summary of the different market solutions. The paper concludes by highlighting the major challenges currently faced by the tidal current turbine industry, and an indication of the present trends in the industry.@en

Modular design of wind turbine generators is a promising means to increase the generator availability by reducing downtimes, especially for offshore turbines. With such physically modular designs, the structural support for the generator is an important consideration as it needs to overcome the loss of rigidity in the generator yoke. This makes the structural support design of the generator an important criterion when comparing modular topologies. This paper presents a comparison of modular generator topologies, both from an electrical and a structural performance perspective. It concludes that the 6/4 (6 slots in each stator module corresponding to 4 rotor poles) module is promising solution from the standpoint of maintenance, electrical performance, and structural performance.@en

Brushless Doubly-fed Induction Machines (BDFIMs) have been envisaged as a potential candidate for generators in offshore wind farms.@en