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

Ship hybridization has received some interests recently in order to achieve the emission target by 2050. However, designing and optimizing a hybrid propulsion system is a complicated problem. Sizing components and optimizing energy management control are coupled with each other. This paper applies a nested double-layer optimization architecture to optimize the sizing and energy management of a hybrid offshore support vessel. Three different power sources, namely diesel engines, batteries and fuel cells, are considered which increases the complexity of the optimization problem. The optimal sizing of the components and their corresponding energy management strategies are illustrated. The effects of the operational profiles and the emission reduction targets on the hybridization design are studied for this particular type of vessel. The results prove that a small emission reduction target of about 10% can be achieved by improving the diesel engine efficiency using the batteries only while the achievement of a larger emission reduction target mainly depends on the amount of the hydrogen and/or on-shore charging electricity consumed. Some design guidelines for hybridization are derived for this particular ship which could be also valid for other vessels with similar operational profiles.

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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

Wind energy is becoming an important contributor in the world’s energy needs. An important trend in wind turbine design is the focus on reliability and increased availability of wind turbines. This is the aim of the study. It focusses its attention on the generator and power electronic converter due to the susceptibility of the generator system to failure. The first step is identifying the problem. This is achieved by a review of existing studies in failure rates and failure mechanisms to identify critical failures, their probabilities and their failure mechanisms. This is followed by the identification of approaches that can be used to increase the availability of wind turbine generator systems, focussing on – component reliability, active control, and fault tolerance. It identifies three aspects that are analysed in detail...@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

Modularity is promising from a view to increasing turbine availability through fault tolerant operation as well as reduced downtimes, especially for offshore wind turbines. This paper focuses on a quantitative analysis of large scale (or extreme) modularity in power electronic converters of wind turbine generator systems. It uses mathematical models to investigate the effect of the choice of module number on the availability of a converter. It further analyses the availability in conditions where increased levels of modularity lead to a reduction of failure rates in the system. The paper extends this analysis by quantifying the benefits for a 10MW case study turbine. Finally, it concludes that extreme modularity holds merit only when it is accompanied by a reduction in failure rates.

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The power electronic converter, especially the power semiconductor, is a major contributor to the failure rates of the wind turbine drivetrain. As the temperature is a major driving factor behind the failure mechanisms of these power semiconductors the choice of topology and switching strategy can have a significant effect on the reliability of the converter. This paper presents a detailed comparison of several three level converter topologies and switching strategies on the basis of loss distribution, thermal, and lifetime performance. This investigation is done through simulations on a 10MW direct drive permanent magnet drivetrain. The study shows that over-rating in the form of using overrated topologies, or the use of overrated components can result in large gains in lifetime expectancy and quantifies these gains. It concludes that the improvements offered by overrated topologies and overrated components are comparable and that use of the overrated topologies do not offer a significant advantage over the use of topologies with overrated components.

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The brushless Doubly-Fed Induction Machine, without brushes and slip-rings, is regarded as an attractive alternative to the conventional Doubly-Fed Induction Machine in terms of reliability. This paper presents a sensorless field oriented control strategy for the Brushless-DFIM, which further increases its attractiveness as a drive system.With the use of a straightforward and alternative time-dynamic Brushless-DFIM model, a complete Brushless-DFIM based drive is modelled and used for the development of the sensorless control strategy. The developed sensorless control strategy is implemented in an experimental set-up. Simulation and measurement results demonstrate that the developed sensorless control strategy can control the Brushless-DFIM in a stable and responsive manner over its full operating speed range. In this way, a fully functional sensorless control strategy for the Brushless-DFIM is demonstrated for the first time in literature.

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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

In this paper, the brushless DFIM based wind turbine drive-train topology is compared to the DFIG based and PM generator based drive-train topologies, that are most commonly applied in modern wind turbines. The comparison will be based on a 3:2MW case study wind turbine. By using FE based multi-objective optimization, optimized generator designs for the dierent topologies are generated. Then the capital expenditures of the resulting drive-train topologies are calculated and compared. Additionally, wind turbine drive-train congurations with 1, 2 and 3 stage gearboxes as well as a direct-drive conguration are taken into account. The resulting comparison shows that the brushless DFIM based drive-train with a 2 stage gearbox conguration provides a feasible alternative in commercial wind turbine drive-train applications. @en

Although the reliability of wind turbines have improved over time, there is still considerable interest in improving their availability. As the generator system has a sizeable contribution to the overall failure rates of turbines, it is important to consider methods of reducing the effects of these failures on the availability of turbines. This paper examines modular concepts for wind turbine generator systems from the point of view of increasing the availability of wind turbines. It explores the modularities possible in wind turbine generator systems at different layers, i.e. the functional and the physical layer. The paper also attempts to highlight some opportunities and challenges in including modularity in these layers.@en

Availability is an important factor to be considered when designing wind turbine generator systems. The quest for increasing availability is based on the following five design approaches - design for component reliability, active control for reliability, design for fault tolerance, prognostics, and design for maintainability. This paper reviews methods focussing on the first three, i.e. component reliability, active control, and fault tolerance. The paper further identifies some promising directions for further research.@en