# A Review in Fault Diagnosis and Health Assessment for Railway Traction Drives
## 5 Discussion and Conclusions  
CBM is relatively new in Railway applications but an increasing importance in availability, as well as maintenance cost reduction, has made it a key point. The importance of traction drive in Railway system has led to develop fault diagnosis approaches for the Health Assessment of the traction drive. Although the literature is not so broad in Health Assessment in Railway traction drives, compared to aerospace systems, during the last decade a variety of research works have been done. Figure 14: Power electronics for a traction drive connected to DC catenary. Despite there are interesting reviews in Health assessment for on-board mechanical systems, such as wheel, suspension or bogie frame, there is a need to summarize all the research work carried out, both in fault diagnosis and health assessment, in Railway Traction drives, as part of a CBM framework. The framework for CBM implementation is based on Standard ISO13374, initially developed for diagnosis of machines. As moving vehicle, different topologies are presented for CBM implementation, depending on the location Health Assessment module of Standard ISO13374. Thus, it is possible to implement a remote or on-board diagnosis. Furthermore, the diagnosis can be distributed or centralized. In case of remote diagnosis, a huge quantity of data should be transferred to off-board maintenance centre, due to variety of variables and high sample time, with the constraints of a moving vehicle, such as connectivity or data storage on-board. Then, fault diagnosis approaches, mainly data-driven, can be used for Health Assessment. It is not possible to implement a real-time fault detection but it can be a suitable solution for prognosis or for faults with low dynamics. In the near future, due to the advances in communications between on-board and remote systems, data would be transferred with a latency of 100 ms, which could be enough to detect faults in systems with low dynamics. Nowadays, main rail equipment manufacturers provide supervision systems, which allow transferring data from on-board traction unit to remote maintenance centre, for a further analysis. On the other hand, in case of on-board diagnosis, the fault detection can be in real-time and there is no need of huge quantity of data to be transferred to off-board maintenance centre, as Health Assessment is done on-board. A faster fault detection and isolation is done, allowing a further reconfiguration for a fault tolerant system. Model-based or signal-based approaches, which do not need to store huge quantity of data, could be suitable. As a possible drawback, the computational cost to implement the diagnosis algorithms in the on-board control traction unit. Among the fault diagnosis approaches implemented for Railway traction drives, signal-based approaches are mainly used in electrical machines, in order to detect rotor bars breakage or eccentricities. Wavelet transform has been implemented to detect open-circuit faults in power switches. Wavelet transform is used in combination with classification techniques too. Among the model-based approaches implemented for sensor fault detection, observer-based approaches are mainly utilized. Moreover, fault reconstruction by mean of a sliding mode observer is implemented too. Fusion of different model-based approaches are presented too in order to detect and isolate the faults in all the sensors located in a traction drive. Finally, data-driven approaches PCA and ICA are used for sensor fault detection, applied to incipient fault detection. First, an offline stage for threshold setting is needed, based on historical fault-free data and finally the online detection is done by processing the actual data. The main drawback of these statistical analysis based approaches for on-board implementation could be the data storage capacity, until data can be transferred to remote maintenance centre and the high computational cost. Due to the fact that apart from score vectors calculation, normally PDF and statistics tests need to be calculated.