WP1: Electrical generator systems for reliability

Background: Generator systems (generator + power electronic converter) of wind turbines fail far too often, especially offshore, which leads to significant down time and repair cost. Large R&D programmes have been executed to analyse the failure mechanisms of offshore wind turbines. These programmes target blade monitoring, support structure, drive train, etc. Although there has been significant research in this area, in practise, the electrical systems are relatively problematic.

This work packages aims at significantly increasing the turbine availability using fault tolerance in the electrical system of the turbine. An example is to increase the number of phases. Generators normally have three phases, consisting of strings of parallel coils. Converters normally have three phases, consisting of parallel modules. If the three phases of the generator would be split into, for example, 4 sets of three phases and the converter would be split into 4 three-phase converters, the system could continue operation at a reduced power level if one of the sets of three phases fails. This could improve the availability and the energy yield without adding additional cost.

Objective: Improve availability by using modular conversion system concepts that are fault tolerant, reconfigurable and self-healing.

This Work package is organised in the following tasks.

Task 1.1: Analysis of failures in generator systems

The objective of this task is to be investigate which failures occur so often that it would be wise to change the design to reduce corrective maintenance and hence to increase the availability.
The following research questions have to be answered.

  • Which faults play a role in generators and converters?
  • What is the probability of these faults?
  • For which of these faults does it make sense to make the system fault tolerance?

Earlier studies suggest that it is worthwhile to consider making the system tolerant to failures in the power electronic switches because these components fail rather often and unexpectedly. These studies also suggest that it might not make sense to make a generator system fault tolerant to short circuit faults in the generator because these failures do not occur so often. This task will be carried out by doing a literature study but especially by creating a strong collaboration with the university of Aalborg.

Task 1.2: Fault avoidance

Avoiding faults may be more useful than making generator systems fault tolerant. Therefore, it should be investigated if faults can be avoided by using different components or by designing components in a different way. In order to design components in a more reliable way, knowledge of the failure mechanisms is very valuable.
The main research question in this task is: Is it possible to design the system in such a way that these failures are avoided, and if yes, how?
The objective of this task is not to study failure mechanisms in detail, but to create an overview of the possibilities for fault avoidance. The reason to do this is to avoid that extensive work on fault tolerance is done on types of failures that could easily be avoided. This task will be carried out in strong collaboration with the university of Aalborg.

Task 1.3: Inventory and analysis of fault tolerance

The objective of this task is to make an inventory of forms of redundancy, re-configurability and fault tolerance in the generator system. This includes modular generator systems, multilevel converters and multi-phase systems. Analysis of these systems may result in proposals for new topologies. Fault tolerance in aircraft systems can serve as inspiration, but should not be copied because of the different requirements: in aircraft system, the objective is uninterrupted operation and the cost may be high; in wind turbines the objective is a high availability at low cost.
Based on this initial analysis, some case studies will be selected that will be worked out in detail in the next phase.
The following research questions have to be answered.

  • Which forms of redundancy and re-configurability in the generator system are possible?
  • Which of these would be most useful of wind energy conversion systems resulting in an increase in availability and reduction of COE?

This task will be carried out by doing a literature study and by doing an initial analysis of the different systems.

Task 1.4: Analysis of the improvement of the availability for case studies

The objective of this task is to determine the expected increase in availability for a few case studies in more detail. The availability will be calculated using statistical methods and using statistical information about failures. The availability of these fault tolerant systems will be compared to the availability of a standard base line generator system. Mostly, fault tolerant systems consist of higher numbers of components. By definition, adding components leads to a reduction of the mean time between faults. The intention of fault tolerance is to increase the mean time between failures that lead to a system shut down. A careful analysis is necessary to see the effect of the different measures of fault tolerance.
The overarching research question in this task is: how much does the availability increase by using a fault tolerant generator system?
This task will be carried out by modelling the reliability of the selected generator systems.
The performance of the fault tolerant generator system under normal and under fault conditions must be analysed: when faults occur, they have to be detected, the control system has to adapt, and the system will operate in a different way.
The following research questions have to be answered.

  • How can failures be detected (system identification)?
  • How can the system be controlled during faults (fault tolerant control)?

This task will be carried out by modelling an simulating the system in a simulation program.

Task 1.5: Writing PhD thesis

The objective of this task is that the PhD student writes a PhD thesis.

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