Análisis de la probabilidad de transmisión concurrente Caso real

Reliability Issues

There are different strategies for improving the reliability of the power electronic converter. They can be divided into the following categories:

Improving the designed reliability of the converter. This includes improving the designed reliability of the separate converter components such as semiconductor power modules, capacitors, electrical connections, etc. or using the same components, but with improved converter topology. It can also refer to over- rating – i.e. achieving a more reliable overall design by using components of higher current and voltage ratings than the levels expected in normal converter operation, implementing a better cooling system, capable of dissipating much more heat than expected, etc.

Prognostic approach of calculating the end of the useful life of various converter elements and organising scheduled maintenance to replace them before they fail catastrophically. Component manufacturers often provide lifetime curves for their devices and techniques to calculate their expected lifetime. These techniques generally involve cycle counting and damage accumulation. Varying loading is also a factor that should be considered in the remaining lifetime prediction. For this reason, prognosis techniques can prove hard to apply to wind turbine converters – because of the unpredictability of their generator mission profiles (which is weather dependent and hard to estimate long term). Aside from

Ways of Addressing the Converter Reliability Issues

this, the most popular ones using linear damage accumulation methods, may predict longer than actual component lifetimes. Attempts have been made to achieve better lifetime prognosis [20], [21].

Adding in-built redundancy. This can also be described as a specific case of improving the system’s designed reliability. For components expected to fail more frequently, redundant counterparts are included, so that when a working component does fail, its function will be automatically transferred to its (until then idle) double ensuring uninterrupted operation. Although it cannot be used for mechanical components such as blades, it is highly applicable for electrical circuits and therefore can be used in wind turbine power electronic converters - an example is discussed in [22]. Including multiple levels of redundancy can indeed prevent failure over a given period of time. It has been successfully used in a number of critical applications (e.g. in the aviation industry) However, it is questionable whether this is best way forward for offshore wind converters, as redundancy can only delay converter breakdown and it does not prevent the need for reactive maintenance. Simply adding more redundant components is not a smart way to go, as it will increase the cost and bulk of the converter without optimising its design. The main problem with relying on only on in-built redundancy is that if the original component fails the operators may not become aware of this, as the redundant component will simply take on its function. They may be lulled into sense of false security and fail to schedule essential preventative maintenance before the redundant device fail. There is no sure guarantee how much time of

Ways of Addressing the Converter Reliability Issues

uninterrupted system operation the redundant device (or devices) will buy. Therefore, redundancy by itself will not address effectively the converter reliability issues in offshore wind turbines. For such systems it is far more beneficial to have remote visibility as offered by the next strategy for improving converter reliability.

Condition or health monitoring. This is process of keeping track of the state of health of a given component/system through supervisory data acquisition and early detection of failure mechanism’s initial stages. For effective condition monitoring a set of sensitive parameters has to be identified whose change outside given limits will signify in consecutive order the beginning of their deterioration, their degradation in process and end of life. Condition monitoring employs prognostic and diagnostic techniques, but there is a difference between prognosis, diagnosis and condition monitoring in terms of how each views failure. Prognostic approaches to lifetime estimation are interested in the probability of the failure cause and the respective time of the failure effect. Diagnostic approaches focus on the failure mode trying to identify the nature of the problem or the failure cause. Condition monitoring techniques are interested in the whole process of failure cause, development and effect, but its predominant focus is the failure mechanism [23], [24]. Condition monitoring is the only one of the means discussed so far which allows remote visibility over the state of the system and/or its components. Remote visibility is the cornerstone of remote control. By using remote control offshore the operators can act far more quickly and cost effectively to avoid catastrophic failure (e.g. by de-rating or shutting down the system) than reactive maintenance as it is

Condition Monitoring Potential Benefits

done onshore. Condition monitoring can also be an invaluable tool optimising existing routine maintenance practices by planning in advance and scheduling maintenance according to the need for it and the weather conditions.

It is important to note that all strategies of improving the reliability of power electronic converters in offshore wind applications are important and the best outcome can be expected, if all are pursued in parallel. However, as no design is 100% fault proof and as no prognosis can claim 100% accuracy, the focus this research project is specifically upon condition monitoring for power electronics, which apart from being a novel concept itself and still in its infancy stages of development, is also the only strategy that aims to provide visibility over the operation of the remote offshore equipment. Developing means for such remote visibility can help improve significantly both the designed reliability of the power electronic converters and the strategies for their operation and maintenance. It is also necessary to allow remote control of the system which may further optimise the power electronic components useful life and the availability of offshore wind turbines.